Archive for category: JNBridgePro

> TL;DR / Key Takeaways
>
> – There are four ways to migrate Java to C#: automated conversion, manual rewrite, full rebuild, or runtime bridging.
> – Automated converters handle syntax but miss libraries, concurrency, and frameworks — expect 40–60% manual cleanup.
> – Runtime bridging with JNBridgePro lets .NET call Java (and vice versa) in-process, with zero code conversion and setup in days.
> – The safest path for large codebases: bridge first, then migrate incrementally — module by module.
> – Download JNBridgePro free trial to test bridging against your actual Java code.

Table of Contents

• Why Teams Migrate Java to C#
• The Four Approaches to Java-to-C# Migration

– Automated Code Conversion
– Manual Rewrite
– Full Rewrite (Greenfield Rebuild)
– Runtime Bridging

• How Do I Convert Java Code to C#?
• Comparison: Migration Approaches at a Glance
• Decision Framework: When to Migrate vs. When to Bridge
• Common Pitfalls When Migrating Java to C#
• Is It Better to Port Java to C# or Rewrite from Scratch?
• J2EE and Legacy Java Migration Considerations
• Performance Considerations for Cross-Runtime Calls
• FAQs
• Next Steps

You’ve been handed the mandate: migrate Java to C#. Maybe your organization is consolidating on .NET. Maybe you’re acquiring a company whose core product runs on Java. Maybe your team knows C# better, and maintaining a Java codebase has become a tax on every sprint.

Whatever the reason, you’re facing one of the most consequential architectural decisions in your project’s lifecycle. Get it right, and you unlock a unified stack, shared libraries, and streamlined hiring. Get it wrong, and you’re looking at months of rework, subtle bugs, and a system that’s worse than what you started with.

This guide walks through every major approach to convert Java to C# — from automated code converters to manual rewrites to a third option many teams overlook: bridging Java and .NET at runtime without migrating at all.

Why Teams Migrate Java to C#

Before diving into the how, it’s worth understanding the common why. Here are the top drivers behind Java migration to C#:

• Stack consolidation. Running both JVM and CLR in production means double the ops expertise, double the CI pipelines, and double the security patching surface.
• Talent availability. Your team knows C# and .NET. Maintaining Java code means context-switching or hiring specialists.
• Ecosystem alignment. If your core product is on .NET, a Java module creates friction — different build tools, different package managers, different deployment models.
• Licensing changes. Oracle’s evolving Java licensing has pushed organizations to reconsider their JVM commitments.
• End-of-life dependencies. Legacy Java apps on J2EE application servers (WebLogic, WebSphere) may be expensive to maintain when a .NET equivalent exists.

All valid reasons. But “valid reason to want migration” doesn’t automatically mean “code conversion is the best path.” Sometimes the answer is to bridge instead.

The Four Approaches to Java-to-C# Migration

There’s no single way to port Java to C#. The right approach depends on codebase size, timeline, risk tolerance, and how much of the original Java functionality you need to preserve.

1. Automated Code Conversion

What it is: You feed Java source code into a Java to C# conversion tool that outputs equivalent C# code, translating syntax, class structures, and (to varying degrees) library calls.

Major tools include:

What automated conversion handles well:

• Class and interface declarations
• Basic control flow (loops, conditionals)
• Simple type mappings (String → string, ArrayList → List)
• Method signatures and overloads

What it doesn’t handle:

• Java-specific libraries (javax.*, Apache Commons)
• Concurrency models (java.util.concurrent → .NET Task/async patterns)
• Reflection-heavy code and annotation-based frameworks (Spring, Hibernate)
• Build system and dependency management (Maven/Gradle → MSBuild/NuGet)

Code example — before and after automated conversion:

“java
// Java source
import java.util.HashMap;
import java.util.Map;
import java.util.stream.Collectors;

public class OrderService {
private Map prices = new HashMap<>();

public OrderService() {
prices.put("widget", 9.99);
prices.put("gadget", 24.95);
}

public double calculateTotal(List items) {
return items.stream()
.filter(prices::containsKey)
.mapToDouble(prices::get)
.sum();
}

public Map getItemsAbove(double threshold) {
return prices.entrySet().stream()
.filter(e -> e.getValue() > threshold)
.collect(Collectors.toMap(
Map.Entry::getKey, Map.Entry::getValue));
}
}
`

`csharp
// C# output (cleaned up)
using System.Collections.Generic;
using System.Linq;

public class OrderService
{
private Dictionary prices = new();

public OrderService()
{
prices["widget"] = 9.99;
prices["gadget"] = 24.95;
}

public double CalculateTotal(List items)
{
return items
.Where(item => prices.ContainsKey(item))
.Sum(item => prices[item]);
}

public Dictionary GetItemsAbove(double threshold)
{
return prices
.Where(kvp => kvp.Value > threshold)
.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
}
}
`

This is a best-case scenario — LINQ maps cleanly to Java Streams for straightforward data structures. Real-world codebases are rarely this neat.

Verdict: Automated conversion is a useful starting point for porting Java to C#, but plan for 40–60% of total effort to be manual review and fixes. Works best for utility libraries with minimal framework dependencies.

2. Manual Rewrite

What it is: Developers read the Java code, understand its intent, and write equivalent C# from scratch using idiomatic .NET patterns rather than translating line by line.

When it makes sense:

• The Java codebase is small to medium (under ~50K lines of business logic)
• The code is poorly structured and you'd want to refactor anyway
• You want to leverage .NET-specific features (async/await, LINQ, Entity Framework)

The hidden cost: Manual rewrites look cheaper on paper. In practice, they're the most expensive option for large codebases. Every rewritten module needs new tests, new debugging, and new performance tuning. Bugs fixed years ago in the Java version get reintroduced. Edge cases that took years to discover get lost.

Verdict: Best for small codebases or when you're intentionally redesigning the system. Dangerous for large, mature applications where correctness matters.

3. Full Rewrite (Greenfield Rebuild)

What it is: Rather than converting code line by line, you treat the existing Java application as a specification and build a new .NET application from scratch — rethinking architecture, data models, and APIs.

The risk: Greenfield rebuilds are the most common source of catastrophic project failure. The "second system effect" is real. You lose years of accumulated fixes, institutional knowledge, and battle-tested behavior.

Verdict: Only justified when the original system is truly end-of-life. Not a "migration" — a "replacement."

4. Runtime Bridging (No Migration Needed)

What it is: Instead of converting Java code, you keep the Java code running as-is and call it directly from .NET (or vice versa) through an interoperability bridge. This is the approach many teams don't consider, but for certain scenarios it's the most pragmatic option — and it eliminates technical debt from hasty rewrites.

How it works with JNBridgePro:

JNBridgePro creates a runtime bridge between the JVM and the CLR, allowing .NET code to instantiate Java objects, call Java methods, and handle Java exceptions — all in-process, with no web services, no REST APIs, and no serialization overhead. Java objects appear as .NET proxies with full IntelliSense support.

The reverse direction works too: Java code can call into .NET assemblies. This supports teams needing bidirectional Java/.NET interoperability.

Code example — calling Java from C# via JNBridgePro:

`java
// Java — existing code, unchanged
package com.example.pricing;

public class PricingEngine {
public double calculateDiscount(String customerTier, double orderTotal) {
switch (customerTier) {
case "gold": return orderTotal * 0.15;
case "silver": return orderTotal * 0.10;
default: return 0.0;
}
}
}
`

`csharp
// C# — calling Java via JNBridgePro proxies
using com.example.pricing;

public class OrderController
{
public OrderSummary ProcessOrder(string tier, List items)
{
double total = items.Sum(i => i.Price * i.Quantity);

// Java object, called as if native .NET
var engine = new PricingEngine();
double discount = engine.calculateDiscount(tier, total);

return new OrderSummary
{
Subtotal = total,
Discount = discount,
Total = total - discount
};
}
}
`

No REST endpoints. No serialization. The Java PricingEngine runs in the same process as the C# code.

When bridging makes sense:

• The Java code is stable, complex, and battle-tested — rewriting would introduce risk
• You need Java and .NET to coexist during a gradual migration
• You need a Java library with no .NET equivalent (scientific, financial, or legacy enterprise libraries)
• You want to migrate incrementally — bridge first, replace Java components one at a time

Verdict: Bridging isn't a compromise — it's a strategy. It eliminates migration risk and gives you time to migrate incrementally (or not at all).

> Ready to try it? Download the JNBridgePro free trial and test bridging with your actual Java code. Setup takes hours, not weeks.

How Do I Convert Java Code to C#?

Converting Java code to C# involves translating Java syntax to equivalent C# syntax, mapping Java standard library calls to their .NET Framework counterparts, and adapting patterns that differ between the two runtimes. Here's a practical step-by-step process that works for most projects, whether you use automated tooling or manual conversion.

For modules that are too risky or complex to convert, use JNBridgePro to bridge them while you convert the rest.

Comparison: Migration Approaches at a Glance

Decision Framework: When to Migrate vs. When to Bridge

Migrate (convert or rewrite) when:

Bridge instead of migrate when:

The Hybrid Approach: Bridge + Incremental Migration

For many teams, the best answer is both:

This gives you the speed of bridging with a clear path toward full migration. You always have a working system.

Common Pitfalls When Migrating Java to C#

Even experienced teams hit these traps during Java-to-C# code conversion:

1. Underestimating library differences. Java and C# syntax are similar; the ecosystems are not. java.time vs. System.DateTime, java.nio vs. System.IO, JDBC vs. ADO.NET — every library boundary is a potential source of subtle bugs.

2. Ignoring concurrency model differences. Java's ExecutorService and synchronized blocks don't map cleanly to .NET's Task, async/await, and lock`. Naive translation of threaded code is a recipe for deadlocks and race conditions.

3. Forgetting the build pipeline. Migrating code is half the battle. You also need to migrate Maven/Gradle to MSBuild/NuGet, CI/CD pipelines, deployment scripts, monitoring, and logging.

4. Losing test coverage. Your Java test suite is one of your most valuable assets. Migrating production code without migrating tests removes your safety net at the worst possible time.

5. The 90/90 rule. The first 90% of the migration takes 90% of the time. The last 10% — edge cases, threading bugs, performance tuning — takes the other 90%.

Is It Better to Port Java to C# or Rewrite from Scratch?

Porting (translating existing code) preserves business logic and reduces the risk of introducing new bugs. It maintains the behavioral fidelity that years of production use have refined — every edge case handled, every race condition fixed, every subtle workaround tested in the field.

Rewriting from scratch lets you design a better architecture, adopt modern .NET patterns, and eliminate legacy design decisions. But it carries a high risk of losing subtle behaviors and edge-case handling that accumulated over years. The second system effect is well-documented.

The pragmatic middle ground: Use a runtime bridge vs REST vs gRPC approach comparison to evaluate your interoperability options. For most teams, porting or bridging is safer than a full rewrite unless the original codebase is small or architecturally unsound.

If you’re migrating from IKVM (which is no longer maintained), JNBridgePro provides a supported, production-ready alternative for Java/.NET interop.

J2EE and Legacy Java Migration Considerations

Legacy J2EE applications (EJBs, JMS, JNDI) are particularly challenging because they depend heavily on the application server runtime. There’s no .NET equivalent of an EJB container.

For teams needing C# J2EE support, bridging is often the most practical option — J2EE components continue running in their native container while .NET code interacts with them through JNBridgePro. See the JNBridgePro developer demos for examples of this pattern.

Performance Considerations for Cross-Runtime Calls

Automated conversion and manual rewrite produce native .NET code, so performance depends on your implementation. Runtime bridging adds a small overhead for cross-runtime calls.

JNBridgePro uses in-process communication (shared memory), so overhead is typically microseconds per call, not milliseconds. For most applications, this is negligible.

For extremely hot paths (millions of calls per second), batch operations or consider migrating that specific component to native C#. The bridge handles the rest.

FAQs

How long does it take to migrate a Java app to C#?

It depends on codebase size and complexity. A small utility library (under 5K lines) can be converted in a few days with automated tools plus manual cleanup. A medium application (50K–100K lines) typically takes 4–8 months with a dedicated team. Large enterprise applications (500K+ lines) can take years. Runtime bridging with JNBridgePro can be set up in days, providing immediate interoperability while you plan longer-term code conversion.

Can .NET applications use Java libraries directly?

Yes. JNBridgePro lets .NET applications instantiate Java objects, call methods, handle exceptions, and subscribe to events — all in-process without REST APIs or message queues. Java libraries appear as .NET assemblies with full type information. This is useful when a Java library has no .NET equivalent, or when you want to call Java from C# while migrating incrementally.

What tools exist for Java-to-C# code conversion?

The main options are Tangible Java to C# Converter (commercial), Microsoft’s discontinued JLCA, open-source projects like JavaToCSharp, and AI-assisted conversion using LLMs. All handle syntax-level translation but require significant manual work for library mappings, threading, and framework code.

How do I transfer Java code to C# without breaking functionality?

The safest approach is incremental. Start by bridging your Java code into the .NET environment so both runtimes coexist. Then migrate individual modules, verifying each against the original Java behavior. Maintain comprehensive integration tests running against both implementations. Never migrate everything at once — big-bang migration is the highest-risk option.

What about migrating C# to Java?

Everything in this article works in reverse. If you need to migrate .NET to Java, the same approaches apply: automated converters exist (though less mature), and JNBridgePro bridges in both directions. Java code can call .NET assemblies just as .NET can call Java.

Next Steps

If you’re evaluating how to migrate Java to C#, here’s where to go:

• Assess your codebase. Inventory Java modules by size, complexity, dependency count, and change frequency. This tells you which approach fits each component.
• Try JNBridgePro. Download the free trial and test it against your actual Java code. Setup takes hours, not weeks.
• Talk to us. The JNBridge team has helped hundreds of organizations navigate Java/.NET interoperability. Contact us for a technical consultation.
• Explore demos. The Developer Center has working examples of bridging, including J2EE integration and bidirectional scenarios.

JNBridge has been building Java/.NET interoperability tools since 2001. JNBridgePro is used by Fortune 500 companies, government agencies, and development teams worldwide to bridge Java and .NET without rewriting code.

You have a Java monolith. Management wants .NET. Rewriting everything at once is a fantasy — it’s a multi-year, high-risk project that rarely succeeds. The strangler fig pattern offers a proven alternative: wrap the old system, incrementally replace components, and eventually decommission the monolith — all while the system stays in production.

This guide walks through applying the strangler fig pattern specifically to Java-to-.NET migrations, with practical strategies for routing, data synchronization, bridge integration, and the organizational challenges that determine whether your migration succeeds or stalls halfway.

What Is the Strangler Fig Pattern?

Named after strangler fig trees that grow around a host tree and eventually replace it, the pattern works in three phases:

1. Wrap: Place a routing layer in front of the existing Java monolith. All traffic flows through this layer.
2. Replace: Build new functionality in .NET. Route specific requests to the new .NET services while the rest continues to hit the Java monolith.
3. Remove: As .NET services prove stable, decommission the corresponding Java components. Eventually, the monolith is empty and can be shut down.

Phase 1: Wrap                    Phase 2: Replace               Phase 3: Remove
┌──────────────────┐          ┌──────────────────┐          ┌──────────────────┐
│    API Gateway    │          │    API Gateway    │          │    API Gateway    │
│   (routing layer) │          │   (routing layer) │          │   (routing layer) │
└────────┬─────────┘          └───┬──────────┬───┘          └────────┬─────────┘
         │                        │          │                        │
         ▼                        ▼          ▼                        ▼
┌──────────────────┐   ┌─────────────┐ ┌─────────┐          ┌──────────────────┐
│  Java Monolith   │   │Java Monolith│ │.NET Svc │          │  .NET Services   │
│  (100% traffic)  │   │(shrinking)  │ │(growing) │          │  (100% traffic)  │
└──────────────────┘   └─────────────┘ └─────────┘          └──────────────────┘

Why Strangler Fig for Java-to-.NET Specifically

The strangler fig pattern is particularly well-suited to Java-to-.NET migrations because:

• Both platforms are enterprise-grade. Unlike migrating from a legacy language to a modern one, Java and .NET are both actively maintained with large ecosystems. The migration is about strategic alignment, not escaping a dead platform.
• The business logic is the hard part. Java and .NET share similar paradigms (OOP, garbage collection, strong typing). The challenge isn’t translating syntax — it’s migrating years of accumulated business rules without breaking them.
• Bridge tools enable coexistence. Tools like JNBridgePro allow Java and .NET components to call each other directly during the migration, avoiding the need to build temporary APIs for every migrated component.
• Teams can upskill gradually. Java developers can learn C# while working on new features in .NET, rather than stopping all feature work for a big-bang rewrite.

Planning a Java-to-.NET migration? JNBridgePro serves as the bridge layer during strangler fig transitions, enabling incremental migration without big-bang risk — try a free evaluation.</p

What Is the Strangler Fig Pattern?

The strangler fig pattern is an architectural migration strategy that incrementally replaces a legacy system (such as a Java monolith) with a new system (such as .NET microservices) by routing traffic through a facade layer. Instead of a risky big-bang rewrite, you “strangle” the old system one component at a time — new features go to the new system, while existing functionality migrates gradually over 12–30 months. The pattern is named after strangler fig trees, which grow around a host tree and eventually replace it entirely.

Strangler Fig Migration Steps

1. Map the monolith — identify domain boundaries, dependency graphs, and migration candidates
2. Build the routing layer — deploy a facade (API gateway or reverse proxy) that routes requests to old or new code
3. Migrate one component — rewrite or bridge the highest-value, lowest-risk module first
4. Bridge shared state — use a shared database, event sourcing, or in-process bridge (like JNBridgePro) to keep old and new code synchronized
5. Route traffic incrementally — shift traffic from old to new, validate with metrics, roll back if needed
6. Decommission the old component — once all traffic routes to the new system, remove the legacy code

>

Step-by-Step Migration Strategy

Step 1: Map the Monolith

Before writing any .NET code, understand what you’re dealing with:

• Domain boundaries: Identify logical modules within the Java monolith. These become candidates for independent .NET services. Look for natural seams: packages with minimal cross-references, distinct database tables, separate business workflows.
• Dependency graph: Map which Java classes depend on which. Tools like JDepend, Structure101, or a simple jdeps analysis reveal the coupling. Highly coupled areas migrate together; loosely coupled areas can migrate independently.
• Data ownership: Determine which module “owns” which database tables. Shared tables are migration blockers that need special handling.
• Traffic patterns: Identify which features handle the most traffic and which are most critical. Migrate low-risk, low-traffic features first.

# Analyze Java dependencies
jdeps --summary --multi-release 21 myapp.jar

# Output helps identify migration boundaries:
# com.company.billing -> com.company.core (tight coupling)
# com.company.reporting -> com.company.core (loose coupling) ← migrate first
# com.company.auth -> java.base (self-contained) ← migrate first

Step 2: Install the Routing Layer

The routing layer is the foundation of the strangler fig. It sits in front of the Java monolith and will eventually route traffic to .NET services:

# NGINX routing layer example
upstream java_monolith {
    server java-app:8080;
}

upstream dotnet_reporting {
    server dotnet-reporting:5000;
}

server {
    listen 80;
    
    # Phase 1: Everything goes to Java
    # Phase 2: Reporting migrated to .NET
    location /api/reports {
        proxy_pass http://dotnet_reporting;
    }
    
    # Everything else still goes to Java
    location / {
        proxy_pass http://java_monolith;
    }
}

Alternatives to NGINX: API gateways (Kong, AWS API Gateway), service meshes (Istio, Linkerd), or a custom .NET reverse proxy (YARP). Choose based on your existing infrastructure.

Step 3: Migrate the First Feature

Pick a feature that is:

• Low risk (non-critical if it fails temporarily)
• Low coupling (minimal dependencies on other Java code)
• Well-understood (team knows the business rules)
• Has good test coverage (you can verify the migration)

Common first candidates: reporting, notifications, search, user preferences, or audit logging.

// .NET implementation of the migrated reporting service
[ApiController]
[Route("api/reports")]
public class ReportsController : ControllerBase
{
    private readonly IReportingService _reportingService;
    private readonly IJavaLegacyBridge _legacyBridge; // For data still in Java
    
    [HttpGet("{reportId}")]
    public async Task<IActionResult> GetReport(string reportId)
    {
        // New .NET logic for report generation
        var report = await _reportingService.GenerateReport(reportId);
        
        // Some data might still come from the Java monolith
        // Bridge call for legacy data during transition
        var legacyData = _legacyBridge.GetHistoricalData(reportId);
        report.AppendLegacyData(legacyData);
        
        return Ok(report);
    }
}

Step 4: Handle the Hard Part — Shared State

The biggest challenge in strangler fig migrations isn’t building new services — it’s managing shared state between the old Java system and the new .NET services. Three approaches:

Approach A: Shared Database

Both Java and .NET read/write the same database. Simple but creates tight coupling.

// Both sides access the same tables
// Java (existing):  SELECT * FROM orders WHERE status = 'pending'
// .NET (new):       SELECT * FROM orders WHERE status = 'pending'

// Risk: Schema changes must be coordinated across both platforms
// Mitigation: Use database views as stable interfaces

Approach B: Event-Driven Synchronization

Java publishes domain events to a message broker; .NET subscribes and maintains its own data store.

// Java monolith publishes events
public class OrderService {
    @Autowired private KafkaTemplate<String, OrderEvent> kafka;
    
    public void processOrder(Order order) {
        // Existing business logic
        order.setStatus(Status.PROCESSED);
        orderRepo.save(order);
        
        // NEW: Publish event for .NET consumers
        kafka.send("order-events", new OrderProcessedEvent(order));
    }
}

// .NET service consumes events
public class OrderEventConsumer : IHostedService
{
    public async Task ProcessMessage(OrderProcessedEvent evt)
    {
        // Maintain .NET's own read model
        await _reportingDb.UpsertOrder(evt.ToReportingModel());
    }
}

Approach C: Bridge-Based Data Access

Use an in-process bridge to call Java data access methods directly from .NET, avoiding data duplication entirely:

// .NET service calls Java DAO via JNBridgePro bridge
public class BridgedOrderRepository : IOrderRepository
{
    private readonly com.company.dao.OrderDAO _javaDao;
    
    public BridgedOrderRepository()
    {
        // Bridge loads the Java DAO in-process
        _javaDao = new com.company.dao.OrderDAO();
    }
    
    public Order GetOrder(string orderId)
    {
        // Call Java method directly — no REST, no Kafka, no data sync
        var javaOrder = _javaDao.findById(orderId);
        return OrderMapper.ToDotNet(javaOrder);
    }
}

// When ready to fully migrate, swap the implementation:
public class NativeOrderRepository : IOrderRepository
{
    private readonly AppDbContext _db;
    
    public Order GetOrder(string orderId)
    {
        return _db.Orders.Find(orderId);
    }
}

The bridge approach is uniquely powerful for strangler fig migrations. It lets .NET services access Java data and logic without duplicating databases or building event pipelines. When a component is fully migrated, you simply swap the bridged implementation for a native one.

Step 5: Expand and Repeat

With the first feature proven, expand the migration systematically:

1. Migrate by domain boundary, not by technical layer. Don’t migrate “all controllers” or “all repositories” — migrate “all of billing” or “all of reporting.”
2. Update the routing layer for each migrated feature. The API gateway configuration becomes your migration progress tracker.
3. Run parallel verification: For critical features, run both Java and .NET implementations simultaneously, compare outputs, and alert on discrepancies.
4. Track metrics per feature: Response time, error rate, and resource usage for each migrated feature. Ensure .NET performs at least as well as Java.

Step 6: Decommission

When a Java component has zero traffic for 30+ days:

1. Remove the routing rule (fail-safe: 404 instead of routing to dead code)
2. Archive the Java source code (don’t delete — you may need it for reference)
3. Drop the Java database tables (after confirming .NET has its own data store)
4. Remove the Java JARs from the deployment
5. Update documentation and runbooks

Real-World Timeline

Based on typical enterprise migrations:

Total: 18-30 months for a medium-complexity monolith (~100K-500K lines of Java). Larger systems take longer. The key advantage over a big-bang rewrite: the system is in production throughout, and value is delivered incrementally.

Common Pitfalls

1. Migrating bottom-up (infrastructure first). Don’t start by migrating the database layer or shared libraries. Start with user-facing features that deliver visible value. Infrastructure changes follow when needed.
2. Not investing in the routing layer. A brittle routing layer blocks the entire migration. Invest in proper API gateway configuration, health checks, and circuit breakers from day one.
3. Attempting to migrate shared libraries early. Cross-cutting concerns (logging, auth, config) are the hardest to migrate because everything depends on them. Use the bridge to keep calling Java shared libraries from .NET until late in the migration.
4. Losing momentum. Strangler fig migrations stall when teams get pulled to other priorities. Set quarterly milestones and track migration percentage as a KPI.
5. Underestimating data migration. Moving business logic is straightforward. Moving data with zero downtime — especially with referential integrity and running transactions — is the hard part. Plan for it early.
6. Skipping parallel verification. “It works in staging” isn’t enough for critical features. Run both implementations in production and compare results before cutting over.

How JNBridgePro Accelerates Strangler Fig Migrations

JNBridgePro addresses the two hardest problems in strangler fig migrations:

• Temporary integration without temporary APIs: During migration, .NET services need to call Java components that haven’t been migrated yet. Instead of building REST APIs for every temporary touchpoint, JNBridgePro lets .NET call Java methods directly. When the Java component is eventually migrated, swap the bridge call for a native .NET call.
• Incremental data access: Instead of duplicating databases or building event pipelines, .NET services can use JNBridgePro to call Java data access objects directly. This eliminates the most complex part of the migration until you’re ready to move the data store.
• Risk reduction: If a .NET migration of a specific feature doesn’t work out, rolling back is trivial — just update the routing rule. The Java code is still there, still working, still callable via the bridge.

Conclusion

The strangler fig pattern transforms a high-risk, multi-year Java-to-.NET rewrite into a series of manageable, low-risk incremental migrations. Each migrated feature delivers value immediately, teams learn .NET progressively, and the system never goes offline.

The key enabler is a bridge that lets Java and .NET coexist during the transition. JNBridgePro provides that bridge — letting .NET services call Java methods directly while you migrate at your own pace, without building throwaway APIs or duplicating data infrastructure.

Related Articles

• Java .NET Interop Architecture Patterns for the Enterprise
• Migrating from IKVM to JNBridgePro
• Java .NET Integration: All Options Compared
• Java .NET Integration in Docker and Kubernetes
• Java .NET Integration Testing Guide

Both .NET and Java had landmark releases in the past year. .NET 9 shipped in November 2024 with major performance gains, native AOT improvements, and enhanced cloud-native tooling. Java 21, the latest LTS release, introduced virtual threads, pattern matching for switch, and record patterns — features that fundamentally change how Java applications are designed.

For teams running mixed Java/.NET environments, these updates matter more than just keeping up with versions. New features on both sides affect how the two platforms integrate, what performance characteristics to expect, and which architecture patterns make the most sense going forward. This guide breaks down the key changes and what they mean for cross-platform Java/.NET integration in 2026.

What’s New in .NET 9 That Matters for Integration

Performance Improvements Across the Board

.NET 9 continues Microsoft’s aggressive performance push. The highlights that directly impact Java/.NET integration:

• Dynamic PGO (Profile-Guided Optimization) is now default: The JIT compiler automatically optimizes hot paths at runtime. For integration workloads that repeatedly call Java methods through a bridge like JNBridgePro, this means the .NET side of bridge calls gets progressively faster as the application warms up.
• Improved GC (Garbage Collection) for server workloads: .NET 9’s DATAS (Dynamic Adaptation To Application Sizes) GC mode reduces memory overhead for services that handle bursty workloads — common in integration middleware that buffers data between Java and .NET components.
• ARM64 performance improvements: Significant SIMD and intrinsics improvements on ARM64. Teams running integration services on AWS Graviton or Azure ARM instances will see better throughput without code changes.

Native AOT Compilation Maturity

.NET 9 expanded Native AOT (Ahead-of-Time compilation) support to more application types, including ASP.NET Core Web APIs. For integration scenarios, this means:

• Sub-second startup: Native AOT compiled .NET apps start in under 100ms, compared to 1-3 seconds with JIT. This eliminates the cold-start problem in serverless or container environments where integration services need to spin up quickly.
• Smaller memory footprint: AOT-compiled apps use 50-70% less memory than JIT equivalents. When sharing a container or pod with a JVM, every MB saved on the .NET side is available for Java’s heap.
• Trade-off for bridging: Native AOT has limitations with runtime reflection and dynamic code generation. If your integration relies on dynamically proxied Java types, verify compatibility with your bridging tool before switching to AOT.

System.Text.Json Improvements

.NET 9 added contract customization, JsonSchemaExporter, and improved polymorphic serialization to System.Text.Json. For integration architectures using REST or message queues between Java and .NET, this means:

• Better handling of Java-style class hierarchies when serialized to JSON
• Schema generation for API contracts shared between Java (Jackson/Gson) and .NET
• Faster serialization of complex object graphs — useful when a bridge-based approach isn’t appropriate and data must be marshaled between platforms

What’s New in Java 21 That Matters for Integration

Virtual Threads (Project Loom) — The Big One

Virtual threads are the most significant addition to Java since generics. They’re lightweight threads managed by the JVM rather than the OS, enabling millions of concurrent threads without the overhead of platform threads.

What this means for Java/.NET integration:

• Higher concurrency for bridge calls: When .NET calls Java through a bridge like JNBridgePro, each call traditionally occupies a platform thread on the Java side. With virtual threads, the Java side can handle vastly more concurrent bridge calls without thread pool exhaustion.
• Simplified async patterns: Java developers no longer need reactive frameworks (Project Reactor, RxJava) for concurrent integration workloads. Straightforward synchronous code on virtual threads achieves the same concurrency as reactive streams, making integration code simpler to write and debug.
• Better resource utilization in containers: Virtual threads use significantly less memory per thread. In Kubernetes pods with limited resources, this means the Java side of your integration can handle more concurrent requests within the same memory limits.

// Java 21: Handling bridge callbacks with virtual threads
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    // Each .NET callback gets its own virtual thread
    // Can handle 100,000+ concurrent calls with minimal memory
    for (var request : incomingBridgeCalls) {
        executor.submit(() -> processIntegrationRequest(request));
    }
}

Pattern Matching Enhancements

Java 21 finalized pattern matching for switch and added record patterns. For integration code, this simplifies the common task of handling different types of cross-platform messages:

// Java 21: Pattern matching for integration message handling
sealed interface IntegrationMessage permits
    DataRequest, StatusQuery, ConfigUpdate {}

record DataRequest(String entity, Map<String, Object> filters) 
    implements IntegrationMessage {}
record StatusQuery(String serviceId) 
    implements IntegrationMessage {}
record ConfigUpdate(String key, String value) 
    implements IntegrationMessage {}

// Clean switch over message types from .NET caller
String handleMessage(IntegrationMessage msg) {
    return switch (msg) {
        case DataRequest(var entity, var filters) -> 
            queryDatabase(entity, filters);
        case StatusQuery(var id) -> 
            checkServiceHealth(id);
        case ConfigUpdate(var key, var value) -> 
            updateConfiguration(key, value);
    };
}

Sequenced Collections

Java 21 introduced the SequencedCollection, SequencedSet, and SequencedMap interfaces. For integration scenarios where collection ordering matters (think: ordered data sets passed between Java and .NET), these new interfaces provide guaranteed encounter order and reverse-view operations that map cleanly to .NET’s IList<T> and IOrderedEnumerable<T>.

Foreign Function and Memory API (Preview)

While still in preview in Java 21 (and maturing through Java 22+), the Foreign Function and Memory API (Project Panama) enables Java to call native code without JNI. While this doesn’t directly replace Java/.NET bridging (Panama targets C/C++ libraries), it signals Java’s direction toward better cross-language interop. For now, dedicated bridging tools like JNBridgePro remain the practical choice for Java/.NET integration.

How These Updates Change Integration Architecture

1. Concurrency Model Alignment

.NET has had async/await since C# 5, giving it a natural concurrency model for I/O-bound integration work. Java 21’s virtual threads finally bring Java to parity — but with a different philosophy:

Impact on integration: When using an in-process bridge, the Java side no longer needs reactive programming to handle high-concurrency calls from .NET’s async model. Virtual threads absorb the concurrency naturally. This simplifies both the integration layer and the debugging story.

2. Container and Cloud-Native Optimization

Both platforms made significant container improvements:

• .NET 9: Better container image defaults, smaller base images, improved cgroup v2 support, and the dotnet publish command now generates optimized container images out of the box.
• Java 21: Improved container detection, better cgroup memory limit handling, and the Leyden project (early access) promises ahead-of-time compiled Java for faster container startup.

For teams running Java/.NET integration in Docker and Kubernetes, both runtimes are now first-class container citizens. The resource tuning challenges of 2-3 years ago (JVM ignoring container memory limits, .NET defaulting to wrong GC modes) are largely solved.

3. Serialization and Data Exchange

Both platforms improved their serialization capabilities:

• Java 21 records + .NET 9 records: Both platforms now have lightweight, immutable data carriers. Records on both sides are ideal for integration DTOs — they’re automatically serializable, comparable, and have clear semantics.
• Pattern matching convergence: Both C# and Java now support pattern matching in switch statements. Integration code that maps between Java and .NET types can use the same logical patterns on both sides.

4. Security Updates

Both .NET 9 and Java 21 updated their TLS and cryptography stacks:

• .NET 9: TLS 1.3 improvements, post-quantum cryptography exploration, and better certificate handling.
• Java 21: Key Encapsulation Mechanism (KEM) API for post-quantum crypto preparation, updated TLS defaults.

For integration architectures using TCP channels between Java and .NET components (including JNBridgePro’s TCP channel), both sides now support the latest security standards without third-party libraries.

Migration Considerations

Upgrading .NET in an Integration Environment

If you’re running Java/.NET integration on .NET 6 or .NET 8 and planning to upgrade to .NET 9:

1. Check bridge compatibility first: Verify your bridging tool supports .NET 9’s runtime changes. JNBridgePro regularly updates for new .NET versions — check the system requirements page for the latest compatibility matrix.
2. Test GC behavior: .NET 9’s new DATAS GC mode may change memory allocation patterns. Run integration load tests to verify memory usage stays within container limits.
3. Leverage Dynamic PGO: Enable Dynamic PGO (it’s on by default in .NET 9) and monitor bridge call performance — you may see 10-20% improvement in throughput after warmup.
4. Consider Native AOT selectively: For integration components that are simple (API gateway, message router), Native AOT gives great startup and memory benefits. For components with complex bridging, stick with JIT for full runtime flexibility.

Upgrading Java in an Integration Environment

If you’re moving from Java 11 or Java 17 to Java 21:

1. Virtual threads are opt-in: Existing code works fine — virtual threads don’t change the behavior of platform threads. Adopt them incrementally in your integration layer where concurrency is a bottleneck.
2. Module system impact: If your integration uses deep reflection (common in bridge scenarios), verify that required packages are opened. Java 21 enforces strong encapsulation by default. You may need --add-opens flags for some bridging tools.
3. Garbage collector selection: Java 21’s ZGC is now generational by default, providing sub-millisecond GC pauses even with large heaps. For latency-sensitive bridge calls, ZGC can replace G1GC with better results.
4. Structured concurrency (preview): If adopting virtual threads, consider structured concurrency for managing groups of bridge calls. It provides automatic cleanup and error propagation across concurrent operations.

Performance Benchmarks: .NET 9 + Java 21

Based on community benchmarks and our analysis, here’s how the latest versions compare for integration-relevant workloads:

Note: Benchmarks are approximate and workload-dependent. The key takeaway is that both platforms improved significantly, and Java 21’s virtual threads close the concurrency gap that .NET’s async model previously dominated.

What’s Coming Next

Looking ahead to .NET 10 (expected November 2026) and Java 25+ (September 2025 release cadence):

• .NET 10 (LTS): Expected to be the next long-term support release. Anticipated improvements include further Native AOT expansion, better AI/ML integration primitives, and continued performance gains.
• Java 22-25: String templates (finalized), statements before super() (preview), and continued Project Leyden work for ahead-of-time compilation. The Foreign Function and Memory API is expected to finalize, improving native interop.
• Integration implication: Both platforms are converging on better native interop, faster startup, and lower memory usage. The gap between them in terms of raw capability continues to narrow — which makes the choice of integration tool matter more than the choice of language version.

Practical Recommendations for 2026

1. Upgrade both sides together. Don’t run .NET 9 against Java 11. Modern versions of both platforms have container-aware GC, security fixes, and performance improvements that compound when both sides are current.
2. Adopt virtual threads on the Java side. If your integration handles concurrent .NET-to-Java calls, virtual threads are a free performance win. Configure your thread executor with Executors.newVirtualThreadPerTaskExecutor().
3. Test with container memory limits. Both .NET 9 and Java 21 respect cgroup limits out of the box, but test your specific integration under realistic memory constraints before deploying.
4. Use records for DTOs on both sides. Java records and C# records have near-identical semantics. Using them for all data exchange types makes the integration boundary cleaner.
5. Pick the right integration approach for your call volume. For high-frequency calls (1,000+/sec), use an in-process bridge like JNBridgePro. For low-frequency calls, gRPC with Protobuf contracts works well. For async workflows, use message queues (Kafka, RabbitMQ).
6. Monitor both GCs independently. .NET’s DATAS mode and Java’s ZGC have different tuning knobs. Set up separate Grafana dashboards for each runtime’s GC metrics in your integration pods.

Conclusion

.NET 9 and Java 21 represent the most capable versions of both platforms yet. For cross-platform integration, the practical impact is clear: better performance, simpler concurrency models, and improved container support on both sides. Virtual threads alone transform what’s possible for high-concurrency Java/.NET integration.

The best news for integration teams? You don’t need to wait for both platforms to converge further. Tools like JNBridgePro already bridge the gaps between Java and .NET, letting you take advantage of the latest features on each side while maintaining a stable integration layer. Upgrade both runtimes, adopt the new features incrementally, and let your bridge handle the cross-platform complexity.

Running a mixed Java/.NET environment? Try JNBridgePro with .NET 9 and Java 21 today. Our compatibility team verifies each new platform release so you don’t have to.

Frequently Asked Questions

Does .NET 9 work with Java 21 for integration?

Yes. .NET 9 and Java 21 are fully compatible for integration through tools like JNBridgePro, REST APIs, gRPC, and message queues. In fact, both releases include features that improve cross-platform integration: Java 21’s virtual threads reduce resource consumption for concurrent bridge calls, and .NET 9’s improved Native AOT and Dynamic PGO enhance runtime performance.

Can Java 21 virtual threads improve Java/.NET bridge performance?

Virtual threads can significantly improve throughput for integration workloads that involve blocking operations. Instead of dedicating an OS thread per bridge call, Java 21 virtual threads allow millions of lightweight threads — reducing memory overhead and improving concurrency. This is especially beneficial for ASP.NET Core applications that make many concurrent calls to Java services or libraries.

Should I upgrade to .NET 9 or stay on .NET 8 for Java integration?

.NET 8 is a Long-Term Support (LTS) release with support through November 2026. .NET 9 is a Standard-Term Support release. For production Java/.NET integrations, .NET 8 is the safer choice unless you specifically need .NET 9 features (improved DATAS GC, enhanced Native AOT). Both versions work with current Java/.NET bridge tools. Plan your upgrade around your organization’s .NET support policy.

What happened to IKVM with .NET 9 and Java 21?

IKVM remains limited to Java SE 8 API compatibility regardless of the .NET version you’re running. While community forks may run on .NET 9, they still cannot execute Java 21 features (virtual threads, pattern matching, record patterns, sequenced collections) because IKVM translates bytecode rather than running a real JVM. For Java 21 features, you need a solution that uses an actual JVM.

Is GraalVM Native Image a good alternative to JNBridgePro for .NET 9?

GraalVM Native Image can compile Java code to native shared libraries callable from .NET via P/Invoke, but it imposes severe restrictions: no dynamic class loading, limited reflection, no Java agents, and requires explicit configuration for every reflective access. This makes it impractical for most existing Java codebases. JNBridgePro runs a standard JVM with no restrictions on Java code, making it compatible with any Java library or framework.

Related Articles

• Java .NET Integration in Docker and Kubernetes
• Java .NET Integration: All Your Options Compared
• Bridge vs REST vs gRPC for Java/.NET Communication
• How to Use Java Libraries in .NET Core/.NET 8/9
• Migrating from IKVM to JNBridgePro

Why VB.NET Developers Need Java Integration

VB.NET powers millions of enterprise applications — from financial trading platforms to healthcare record systems. But many organizations also maintain critical Java components: Apache Kafka for messaging, Elasticsearch for search, Hadoop for big data processing, or proprietary Java libraries built over decades.

Need to call Java from VB.NET in production? JNBridgePro generates .NET proxy classes from Java libraries, enabling direct method calls from VB.NET with native syntax — download a free evaluation to get started.

The challenge? VB.NET and Java run on completely different runtimes. The .NET CLR and the Java Virtual Machine don’t share memory, type systems, or calling conventions. Bridging them typically means choosing between REST APIs (with their latency and serialization overhead), message queues (adding infrastructure complexity), or rewriting code in another language entirely.

There’s a better approach: direct in-process bridging that lets VB.NET code call Java classes as if they were native .NET objects. This guide shows you how — with real code examples, performance considerations, and production architecture patterns.

Your Options for Calling Java from VB.NET

Before diving into the recommended approach, let’s understand the landscape. Each integration method involves different trade-offs in performance, complexity, and maintenance burden.

1. REST/HTTP APIs

The most common approach: wrap Java functionality in a Spring Boot or Jakarta EE web service, then call it from VB.NET using HttpClient.

Pros: Language-agnostic, well-understood, works across networks.
Cons: HTTP overhead adds 1-10ms per call, JSON serialization costs, no shared state, requires running a separate Java service. For tight integration loops making thousands of calls per second, this becomes a bottleneck.

2. Message Queues (RabbitMQ, Kafka)

Asynchronous communication through a message broker. VB.NET publishes requests; Java consumes and responds.

Pros: Decoupled, scalable, resilient to service outages.
Cons: Not suitable for synchronous request/response patterns. Adds infrastructure (broker), increases latency, and makes debugging harder. Overkill when you just need to call a Java method and get a result.

3. gRPC

Binary protocol using Protocol Buffers. Faster than REST, supports streaming.

Pros: Lower latency than REST (~0.5-2ms), strongly typed contracts, bi-directional streaming.
Cons: Still requires a separate Java process, protobuf schema management, more complex setup than REST. VB.NET gRPC support exists but is less mature than C#.

4. In-Process Bridge (Recommended)

A bridge like JNBridgePro loads the JVM directly into your .NET process and creates .NET proxy classes for Java objects. Your VB.NET code calls Java methods with native syntax — no HTTP, no serialization, no separate service.

Pros: Sub-millisecond latency, native VB.NET syntax, shared-memory communication, no network infrastructure, supports complex object graphs.
Cons: Requires both runtimes in the same process (or shared-memory TCP mode for separate processes), commercial license needed for production.

Setting Up JNBridgePro for VB.NET

JNBridgePro works by generating .NET proxy assemblies from Java classes. These proxies handle all the JVM communication transparently. Here’s the setup process:

Step 1: Install JNBridgePro

Download JNBridgePro and install it on your development machine. The installer includes the proxy generation tool, runtime libraries, and Visual Studio integration.

Step 2: Generate Proxy Assemblies

Open the JNBridgePro Proxy Generation Tool and point it at your Java JARs or class files. Select the classes and methods you need to access from VB.NET. The tool generates a .NET assembly containing proxy classes that mirror the Java API.

' After proxy generation, your Java classes appear as .NET types
' Example: Java's HashMap becomes available as a .NET class
Imports java.util

Dim map As New HashMap()
map.put("key", "value")
Dim result As String = CStr(map.get("key"))

Step 3: Configure the Runtime

Add the JNBridgePro runtime references to your VB.NET project. Configure the JVM path and classpath in your application’s configuration file:

<!-- app.config or web.config -->
<appSettings>
  <add key="jnbridge.javaHome" value="C:\Program Files\Java\jdk-21" />
  <add key="jnbridge.classPath" value="lib\myapp.jar;lib\dependencies.jar" />
  <add key="jnbridge.mode" value="sharedmem" />
</appSettings>

Real-World VB.NET + Java Code Examples

Let’s walk through practical scenarios VB.NET developers commonly face when integrating with Java.

Example 1: Using Apache PDFBox for PDF Generation

Apache PDFBox is a widely-used Java library for creating and manipulating PDF documents. With JNBridgePro, you can use it directly from VB.NET:

Imports org.apache.pdfbox.pdmodel
Imports org.apache.pdfbox.pdmodel.font

Public Sub CreatePDF()
    Dim document As New PDDocument()
    Dim page As New PDPage()
    document.addPage(page)
    
    Dim contentStream As New PDPageContentStream(document, page)
    contentStream.setFont(PDType1Font.HELVETICA_BOLD, 14)
    contentStream.beginText()
    contentStream.newLineAtOffset(50, 700)
    contentStream.showText("Generated from VB.NET via JNBridgePro")
    contentStream.endText()
    contentStream.close()
    
    document.save("C:\output\report.pdf")
    document.close()
End Sub

Example 2: Connecting to Apache Kafka

Many enterprises use Apache Kafka for event streaming. Instead of using a third-party .NET Kafka client, you can use the official Java client directly:

Imports org.apache.kafka.clients.producer
Imports java.util

Public Sub SendKafkaMessage(topic As String, message As String)
    Dim props As New Properties()
    props.put("bootstrap.servers", "kafka-broker:9092")
    props.put("key.serializer", "org.apache.kafka.common.serialization.StringSerializer")
    props.put("value.serializer", "org.apache.kafka.common.serialization.StringSerializer")
    
    Dim producer As New KafkaProducer(props)
    Dim record As New ProducerRecord(topic, "key", message)
    producer.send(record)
    producer.flush()
    producer.close()
End Sub

Example 3: Calling a Custom Java Business Library

The most common use case: your organization has a proprietary Java library for pricing calculations, risk models, or business rules that took years to build. Rewriting in VB.NET isn’t practical.

' Assuming Java class: com.acme.pricing.PricingEngine
Imports com.acme.pricing

Public Function CalculatePrice(productId As String, quantity As Integer) As Decimal
    Dim engine As New PricingEngine()
    engine.loadRules("pricing-rules-2026.xml")
    
    Dim request As New PriceRequest()
    request.setProductId(productId)
    request.setQuantity(quantity)
    request.setCurrency("USD")
    
    Dim result As PriceResult = engine.calculate(request)
    Return CDec(result.getTotalPrice())
End Function

C# Equivalents (For Mixed .NET Teams)

If your team uses both VB.NET and C#, the same JNBridgePro proxies work with both languages. Here’s the Kafka example in C# for comparison:

using org.apache.kafka.clients.producer;
using java.util;

public void SendKafkaMessage(string topic, string message)
{
    var props = new Properties();
    props.put("bootstrap.servers", "kafka-broker:9092");
    props.put("key.serializer", 
        "org.apache.kafka.common.serialization.StringSerializer");
    props.put("value.serializer", 
        "org.apache.kafka.common.serialization.StringSerializer");
    
    var producer = new KafkaProducer(props);
    var record = new ProducerRecord(topic, "key", message);
    producer.send(record);
    producer.flush();
    producer.close();
}

The proxy assembly is language-agnostic — generate once, use from any .NET language.

Performance: Bridge vs. REST vs. gRPC

For VB.NET applications making frequent Java calls, performance matters. Here’s how the approaches compare in typical enterprise scenarios:

The in-process bridge is 100-1000x faster than REST for individual calls. This difference is negligible for occasional calls, but transformative when your VB.NET application needs to make thousands of Java method invocations per second — think real-time pricing, batch data processing, or streaming analytics.

Architecture Patterns for VB.NET + Java

Pattern 1: Wrapper Service Layer

Create a VB.NET service class that wraps your Java library calls. This isolates the bridge code and gives the rest of your application a clean .NET-native API:

Public Class JavaPricingService
    Private _engine As PricingEngine
    
    Public Sub New()
        _engine = New PricingEngine()
        _engine.loadRules("pricing-rules-2026.xml")
    End Sub
    
    Public Function GetPrice(productId As String, qty As Integer) As Decimal
        Dim request As New PriceRequest()
        request.setProductId(productId)
        request.setQuantity(qty)
        Return CDec(_engine.calculate(request).getTotalPrice())
    End Function
    
    Public Sub Dispose()
        _engine.shutdown()
    End Sub
End Class

Pattern 2: Background Worker with Java Processing

For batch operations, use a VB.NET background worker that delegates heavy processing to Java:

Public Class JavaBatchProcessor
    Public Async Function ProcessBatchAsync(items As List(Of String)) As Task(Of Integer)
        Return Await Task.Run(Function()
            Dim processor As New com.acme.batch.BatchEngine()
            Dim count As Integer = 0
            
            For Each item In items
                processor.process(item)
                count += 1
            Next
            
            processor.commit()
            Return count
        End Function)
    End Function
End Class

Common Pitfalls and How to Avoid Them

1. JVM Memory Configuration
The JVM runs inside your .NET process. Set appropriate heap sizes to avoid out-of-memory errors. Configure -Xmx in your JNBridgePro settings based on your Java library’s memory requirements.

2. Thread Safety
Java objects accessed from VB.NET follow Java’s threading model. If your Java library isn’t thread-safe, wrap access with SyncLock blocks or use per-thread instances.

3. Exception Handling
Java exceptions are translated to .NET exceptions by JNBridgePro. Catch com.jnbridge.jnbcore.JNBException for bridge-specific errors, and the original Java exception types for application-level errors.

4. Classpath Issues
The most common setup problem. Ensure all JAR dependencies are listed in the classpath configuration. Use a tool like jdeps to identify transitive dependencies you might be missing.

Getting Started

Whether you’re integrating a single Java library or building a comprehensive VB.NET + Java architecture, the approach is the same:

1. Download JNBridgePro and install the evaluation version
2. Generate proxies for your target Java classes
3. Add references to your VB.NET project
4. Write code using Java classes with native VB.NET syntax
5. Test and deploy — both JVM and CLR runtimes must be present on the deployment target

For a deeper technical walkthrough, see our guides on calling Java from C# and shared memory bridge architecture. The same patterns apply to VB.NET with minimal syntax differences.

Frequently Asked Questions

Can VB.NET call Java code directly?

VB.NET cannot call Java code directly — the two languages run on different runtimes (CLR and JVM). However, tools like JNBridgePro generate .NET proxy classes that let VB.NET call Java methods with native syntax, as if they were regular .NET classes. The proxy handles all cross-runtime communication transparently.

Is calling Java from VB.NET different from calling Java from C#?

The underlying integration mechanism is identical — both VB.NET and C# run on the .NET CLR, so the same proxy classes and bridge infrastructure work for both. The only difference is syntax: VB.NET uses Dim, Imports, and Sub/Function instead of C#’s var, using, and method signatures. All code examples in this guide include both VB.NET and C# versions.

What is the performance overhead of calling Java from VB.NET?

With in-process bridges like JNBridgePro, the overhead per call is in the microsecond range — negligible for most applications. The JVM and CLR run in the same process (or communicate via shared memory), avoiding network latency entirely. For batch operations, you can minimize overhead further by reducing the number of cross-runtime calls and processing data in bulk on the Java side.

Does Java-VB.NET integration work with .NET Core and .NET 8/9?

Yes. JNBridgePro supports both .NET Framework and .NET Core/.NET 5+/8/9. If you’re migrating a VB.NET application from .NET Framework to .NET Core, your Java integration code carries over with minimal changes. VB.NET is fully supported in .NET 8/9 for console applications, class libraries, and ASP.NET Core projects.

Can I use Java Swing or JavaFX GUI components in a VB.NET Windows Forms application?

While technically possible through JNBridgePro, mixing Java and .NET GUI frameworks in the same application introduces complexity — threading models differ, and visual integration requires careful handling. A better approach is to use Java libraries for backend logic (data processing, algorithms, file handling) and keep the GUI entirely in VB.NET Windows Forms or WPF.

Related Articles

• How to Call Java Code from C# — Complete Guide (2026)
• How to Call C# Code from Java — Complete Guide (2026)
• Call Java from .NET: Anatomy of a Shared Memory Bridge
• Calling Java from .NET Made Easy with JNBridgePro

Continue Reading

• Java .NET Integration: All Your Options Compared
• How to Run a Java JAR File from C# and .NET
• Java C# Bridge: Best Tools for 2026

Table of Contents

• Why Use Java Libraries in .NET Core?
• The Challenge: JVM and CLR Are Different Worlds
• Your Options for Accessing Java Libraries from .NET Core
• Option 1: Find a .NET Alternative or Rewrite
• Option 2: Wrap the Java Library in a REST Service
• Option 3: In-Process Access with JNBridgePro
• Step-by-Step: Calling a Java Library from .NET Core with JNBridgePro
• Popular Java Libraries Developers Access from .NET
• Considerations Specific to .NET Core/.NET 8/9
• FAQ

The .NET ecosystem is powerful, but some capabilities only exist in Java libraries. Whether it’s Apache PDFBox for PDF manipulation, Apache Tika for content extraction, Stanford NLP for natural language processing, or Lucene for full-text search — sometimes the best tool for the job was written in Java.

This guide shows you how to use Java libraries in .NET Core and modern .NET (8/9) applications without rewriting them, wrapping them in microservices, or compromising on performance.

Why Use Java Libraries in .NET Core?

Developers working in .NET Core frequently need Java libraries for several reasons:

• No equivalent .NET library exists — Java’s 25+ year ecosystem includes mature, battle-tested libraries with no .NET counterpart of equal quality
• Rewriting is too expensive — Porting a complex Java library to C# could take months or years, and you’d lose community updates and bug fixes
• Organizational requirements — A Java team built internal libraries that the .NET team now needs to consume
• Migration in progress — You’re moving from Java to .NET Core incrementally and need access to Java code during the transition
• Regulatory or certification — A specific certified Java implementation must be used (common in finance and healthcare)

The Challenge: JVM and CLR Are Different Worlds

Java libraries run on the Java Virtual Machine (JVM). .NET Core applications run on the Common Language Runtime (CLR). These are completely separate runtime environments with different type systems, memory management, and execution models.

You can’t simply reference a .jar file from a C# project the way you’d reference a .dll. The two runtimes don’t share a binary format, object model, or memory space. Bridging them requires either a network boundary (out-of-process) or a runtime bridge (in-process).

Your Options for Accessing Java Libraries from .NET Core

There are three practical approaches, each with different trade-offs:

Option 1: Find a .NET Alternative or Rewrite

Before bridging runtimes, check whether a .NET-native alternative exists:

When this works: The .NET alternative is feature-complete for your needs and actively maintained.

When it doesn’t: You need specific Java APIs, exact behavioral compatibility, or the .NET port is abandoned or incomplete.

Option 2: Wrap the Java Library in a REST Service

Package the Java library in a Spring Boot (or similar) web application, expose the functionality you need as REST endpoints, and call them from .NET Core using HttpClient.

// .NET Core calling a Java library via REST
var client = new HttpClient();
var response = await client.PostAsync(
    "http://java-service:8080/api/extract-text",
    new ByteArrayContent(pdfBytes)
);
string extractedText = await response.Content.ReadAsStringAsync();

When this works: You need the Java library infrequently (a few calls per request), your team already runs microservices, and you can accept the operational overhead of a separate Java service.

When it doesn’t: You need to make many calls per operation (latency adds up), you need access to complex object graphs (serialization becomes a nightmare), or you don’t want to deploy and monitor a separate service.

Option 3: In-Process Access with JNBridgePro

JNBridgePro runs the JVM inside your .NET Core process, generating C# proxy classes that map directly to Java classes. From your .NET Core code, Java objects look and feel like native .NET objects.

// .NET Core accessing Apache PDFBox via JNBridgePro
using com.aspose.pdf; // Java classes appear as .NET namespaces

PDDocument doc = PDDocument.load(new java.io.File("document.pdf"));
PDFTextStripper stripper = new PDFTextStripper();
string text = stripper.getText(doc);
doc.close();
// Direct method calls — no HTTP, no serialization

When this works: You need frequent access to Java APIs, complex object interactions, low latency, or you’re using the Java library as a core part of your application’s functionality.

Advantages over REST wrapping:

• No separate service to deploy — the Java library runs inside your .NET Core app
• Microsecond latency — shared-memory communication vs. millisecond HTTP round-trips
• Full API access — call any public Java class, method, or field, not just what you’ve wrapped in endpoints
• Complex object graphs — pass and receive Java objects directly without serialization
• Exception propagation — Java exceptions become .NET exceptions with full stack traces

Step-by-Step: Calling a Java Library from .NET Core with JNBridgePro

Step 1: Install JNBridgePro

Download JNBridgePro and install it. The package includes the proxy generation tool, runtime libraries, and documentation.

Step 2: Generate Proxy Classes

Use JNBridgePro’s proxy generation tool to create .NET proxy classes from the Java library’s JAR file. Point the tool at the JAR (and any dependencies), select the classes you need, and generate a .NET assembly containing the proxy classes.

The generated proxies mirror the Java API exactly — same class names, method signatures, and inheritance hierarchy, translated to .NET conventions.

Step 3: Reference Proxies in Your .NET Core Project

Add the generated proxy assembly and JNBridgePro runtime DLLs to your .NET Core project. Configure the bridge in your application’s startup:

// Configure JNBridgePro in .NET Core
DotNetSide.init();
// Now Java classes are available as .NET types

Step 4: Call Java APIs Like Native .NET Code

Use the Java library’s classes directly in C#. JNBridgePro handles all type marshaling, memory management, and cross-runtime communication transparently.

Step 5: Choose Transport Mode

JNBridgePro offers two transport modes:

• Shared memory — JVM runs in the same process. Lowest latency, simplest deployment. Best for most scenarios.
• TCP — JVM runs in a separate process or on a different machine. Useful for isolation or when the Java code requires a different OS. See our TCP configuration guide.

For detailed setup instructions, see the JNBridgePro Developer Center.

Popular Java Libraries Developers Access from .NET

Based on common use cases in enterprise environments:

Document Processing

• Apache PDFBox — PDF creation, manipulation, text extraction
• Apache POI — Excel, Word, PowerPoint file processing
• Apache Tika — Content detection and extraction for 1,000+ file formats
• iText (Java edition) — Advanced PDF generation and digital signatures

Search and NLP

• Apache Lucene — Full-text search engine library
• Apache Solr client — Solr search platform integration
• Stanford CoreNLP — Natural language processing, entity recognition, sentiment analysis
• OpenNLP — Machine learning-based NLP toolkit

Enterprise Integration

• JMS client libraries — IBM MQ, ActiveMQ, RabbitMQ JMS clients (also see JMS Adapter for BizTalk)
• JDBC drivers — Access databases with Java-only drivers
• Apache Kafka client — Native Kafka producer/consumer APIs

Security and Cryptography

• Bouncy Castle (Java) — Comprehensive cryptography library
• Keycloak client — Identity and access management
• SAML/OAuth libraries — Java-specific identity provider integrations

Considerations Specific to .NET Core/.NET 8/9

Cross-Platform Deployment

.NET Core runs on Windows, Linux, and macOS. When using Java libraries via JNBridgePro, ensure a compatible JRE/JDK is available on the target platform. JNBridgePro supports cross-platform deployment — the same proxy assemblies work on any OS where both .NET Core and a JVM are present.

Dependency Injection and Hosting

JNBridgePro integrates with .NET Core’s dependency injection container. Register Java services in Startup.cs or Program.cs and inject them like any other .NET service:

// Register Java-backed services in .NET Core DI
builder.Services.AddSingleton<IPdfProcessor, JnBridgePdfProcessor>();

Docker and Containerization

When containerizing a .NET Core application that uses Java libraries, your Docker image needs both the .NET runtime and a JRE. A multi-stage build keeps the image lean:

FROM mcr.microsoft.com/dotnet/aspnet:8.0 AS base
# Add JRE
RUN apt-get update && apt-get install -y default-jre-headless
COPY --from=build /app/publish .
ENTRYPOINT ["dotnet", "MyApp.dll"]

Get Started with Java Libraries in .NET

Ready to use Java libraries in your .NET applications? Download a free evaluation of JNBridgePro to start calling Java libraries from .NET Core with native syntax — no REST wrappers, no microservices overhead. Contact the JNBridge team for help with your specific integration scenario.

.NET Core vs .NET Framework

If you’re migrating from .NET Framework to .NET Core, existing JNBridgePro integrations carry over. The proxy classes and runtime are compatible with both. Check the system requirements for specific version support.

FAQ

Can .NET Core use Java JAR files directly?

No. .NET Core cannot directly reference or load Java JAR files — they use incompatible binary formats (Java bytecode vs. .NET CIL). You need either a runtime bridge like JNBridgePro to access Java classes in-process, or a network-based approach (REST, gRPC) to access Java functionality as a service.

What Java versions work with .NET Core integration?

JNBridgePro supports Java 8 through the latest LTS releases (Java 21+). The Java library you’re accessing must be compatible with a supported JDK version. Most enterprise Java libraries support Java 8+ for backward compatibility.

Does using Java libraries in .NET Core affect performance?

With in-process bridging (JNBridgePro), the performance overhead per method call is microseconds — negligible for most applications. The Java library itself runs at native JVM speed. The main consideration is JVM memory allocation — plan your container or process memory to accommodate both the CLR and JVM heaps.

Can I use Java libraries in ASP.NET Core web applications?

Yes. JNBridgePro works with ASP.NET Core, including Razor Pages, MVC, Web API, and Minimal API projects. Java library calls can be made from controllers, services, middleware, or background services. Thread safety follows the Java library’s own thread-safety guarantees.

Is IKVM an alternative for using Java libraries in .NET Core?

IKVM translates Java bytecode to .NET CIL, allowing Java code to run directly on the CLR. This works for simple, self-contained libraries but has limitations with complex Java code that depends on JVM internals, dynamic class loading, or specific JVM behaviors. JNBridgePro runs an actual JVM, so all Java APIs and behaviors work as documented.

Related Articles

More Java-.NET integration guides:

• How to Call Java from C# — Complete Guide
• Java .NET Integration: All Options Compared
• Anatomy of a Shared Memory Bridge
• How to Call C# from Java

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• Java .NET Integration in Docker and Kubernetes
• .NET 9 and Java 21: Cross-Platform Integration