Table of Contents

• Why .NET to Java Integration Is a Recurring Problem
• Four Ways to Run Java Code in .NET
• 1. In-Process Bridging (Proxy Generation)
• 2. REST or HTTP API Wrapper
• 3. gRPC with Shared Proto Definitions
• 4. Raw JNI via Native Interop
• Comparison Table: .NET to Java Approaches
• Decision Framework: Choosing the Best Way to Run Java from .NET
• “.NET run java code”: What This Search Usually Means
• How In-Process Bridging Works Under the Hood
• Running Java Code in .NET: A Practical Walkthrough
• Performance Considerations
• Deployment Topologies
• Common Mistakes
• FAQ

Why .NET to Java Integration Is a Recurring Problem

.NET to Java integration is a challenge that never fully goes away. Enterprises accumulate Java libraries over decades — business rules engines, cryptographic modules, analytics pipelines — while modernizing their front-end and service layers in .NET. The result is a forced marriage between two runtimes that were never designed to talk to each other.

You can rewrite the Java code in C#. But rewrites are expensive, introduce bugs, and take months. The practical alternative is to run Java code in .NET directly, keeping the original Java logic intact while calling it from your .NET application.

This article compares the four main approaches, gives you a decision framework, and shows you the best way to run Java from .NET for different scenarios.

Four Ways to Run Java Code in .NET

The approaches range from tight in-process coupling to fully distributed communication.

1. In-Process Bridging (Proxy Generation)

In-process bridging connects the JVM and CLR within the same process or via a high-speed binary channel. A proxy generation tool reads your Java JARs and creates .NET classes that mirror the Java API. Your .NET code calls these proxies as if they were native .NET objects.

JNBridgePro is the established solution for this. It supports shared memory and TCP channels, handles type conversion between Java and .NET type systems, and manages cross-runtime garbage collection.

Strengths: Sub-millisecond latency, strong typing, no API layer to maintain, works with existing JARs.

Limitations: Requires JVM and CLR on the same host (or network for TCP mode). Both runtimes share resource constraints.

2. REST or HTTP API Wrapper

Deploy Java logic as a web service (Spring Boot, Micronaut, Quarkus) and call it from .NET using HttpClient. This is the default approach for teams already running microservices.

Strengths: Language-independent, well-understood, easy to scale independently.

Limitations: HTTP overhead, serialization cost, one endpoint per operation, contract maintenance.

3. gRPC with Shared Proto Definitions

Define your interface in Protocol Buffers, generate client and server stubs for both .NET and Java. gRPC provides binary serialization, HTTP/2 multiplexing, and bidirectional streaming.

Strengths: Faster than REST, typed contracts, streaming support.

Limitations: Protobuf schema management, HTTP/2 requirement, still network-bound.

4. Raw JNI via Native Interop

The Java Native Interface (JNI) lets native code call into the JVM. You can invoke JNI from .NET via P/Invoke, manually loading the JVM, finding classes, and calling methods through C-level function pointers.

Strengths: No third-party dependencies, maximum control.

Limitations: Extremely complex, error-prone, no type safety, manual memory management, no garbage collection coordination. Oracle’s JNI documentation is the reference, but it’s notoriously difficult to use correctly from managed runtimes.

Comparison Table: .NET to Java Approaches

Decision Framework: Choosing the Best Way to Run Java from .NET

Answer these questions to find your approach:

Question 1: Is the Java code a library (JAR files) or a running service?

• Library → In-process bridging. You don’t need to wrap it in an API.
• Service → REST or gRPC, depending on performance needs.

Question 2: How many Java classes do you need to access from .NET?

• 1–3 classes → Any approach works. REST is simplest.
• 10+ classes → Proxy generation saves significant development time.

Question 3: What’s your latency budget?

• < 1 ms → In-process bridging or JNI.
• 1–50 ms → gRPC or REST.
• > 50 ms → Message queues (not covered here — see async patterns).

Question 4: Do you have JNI expertise on the team?

• Yes → JNI is an option (but still risky for complex object graphs).
• No → Use a bridge tool or service layer. JNI is a maintenance liability without deep expertise.

Question 5: Can both runtimes run on the same machine?

• Yes → In-process bridging is the performance winner.
• No → Network-based approaches (REST, gRPC) are required.

The best way to run Java from .NET for most enterprise scenarios is in-process bridging when the Java code is a library, and gRPC when it’s already a service.

“.NET run java code”: What This Search Usually Means

When people search for .NET run java code, they usually mean one of two implementation paths:

If your requirement is “.NET run java code with low latency,” in-process bridging is typically the strongest fit because there is no HTTP serialization layer per call.

How In-Process Bridging Works Under the Hood

JNBridgePro operates in two modes:

Shared memory mode: The JVM and CLR run in the same process. Method calls cross the runtime boundary through shared memory, avoiding any network overhead. This is the fastest option.

TCP mode: The JVM and CLR run in separate processes (potentially on different machines). Calls cross a binary TCP channel. Latency is higher than shared memory but much lower than HTTP.

In both modes, the proxy layer handles:

• Type marshalling: Java String → .NET string, java.util.List → System.Collections.IList, etc.
• Exception translation: Java exceptions become .NET exceptions with preserved stack traces and messages.
• Garbage collection: When a .NET proxy is collected, the corresponding Java object is released.
• Thread management: Calls can be synchronous or asynchronous.

For implementation details, see the JNBridgePro developer center.

Running Java Code in .NET: A Practical Walkthrough

Suppose you have a Java PDF library (pdf-engine.jar) with class com.corp.PdfGenerator and method generate(String template, Map data).

Without bridging: You’d build a Spring Boot app, create a REST controller, serialize the map to JSON, call it from .NET, deserialize the response, handle errors. That’s a week of work plus ongoing maintenance.

With JNBridgePro:

JNBridgePro.DotNetSide.Init();
var generator = new com.corp.PdfGenerator();
var data = new java.util.HashMap();
data.put("name", "Report Q4");
byte[] pdf = generator.generate("quarterly-template", data);
File.WriteAllBytes("report.pdf", pdf);

That’s it. No REST layer, no serialization, no contract to maintain.

Performance Considerations

In-process bridging adds roughly 5–50 microseconds per method call in shared-memory mode. For comparison:

• A local REST call to localhost adds 1–5 milliseconds.
• A gRPC call adds 0.5–2 milliseconds.
• Raw JNI adds 1–10 microseconds, but without type safety or GC coordination.

For applications making thousands of cross-runtime calls per second, the difference between microseconds and milliseconds is significant. A batch job calling Java 100,000 times will finish in seconds with bridging versus minutes with REST.

Microsoft’s .NET interop guidance provides background on how managed-to-native transitions work in the CLR.

Deployment Topologies

Single-machine (shared memory): .NET app and JVM co-located. Simplest, fastest. Suitable for desktop apps, batch processors, and monolithic services.

Single-machine (TCP): Both runtimes on the same host but in separate processes. Useful for isolation and independent restarts.

Two-machine (TCP): .NET on one server, Java on another. The TCP channel traverses the network. Suitable for environments where the JVM must run on a dedicated host.

Hybrid: Use in-process bridging for latency-critical paths and REST/gRPC for loosely-coupled integrations.

Common Mistakes

FAQ

Is “.NET run java code” different from “run Java code in .NET”?

They’re the same intent. Teams using the phrase .NET run java code are typically looking for practical options to run Java code in .NET without a rewrite.

Can I run Java code in .NET Core / .NET 8+?

Yes. JNBridgePro supports .NET Framework and modern .NET (.NET 6, 7, 8+). REST and gRPC are also fully supported on modern .NET.

Does .NET to Java integration work on Linux?

Yes. JNBridgePro’s TCP mode works on Linux. REST, gRPC, and JNI are all cross-platform.

How do I handle Java dependencies (transitive JARs)?

JNBridgePro loads the full classpath, including transitive dependencies. You specify the JARs or classpath directories in configuration.

What about Java 17+ module system compatibility?

JNBridgePro supports Java 8 through Java 21+, including the module system. You may need to add --add-opens flags for reflective access to internal modules.

Can I use this with ASP.NET web applications?

Yes. Initialize the bridge at application startup and call Java from controllers, services, or middleware. Thread safety is handled by the bridge layer.

Is there a performance difference between .NET Framework and .NET 8?

Modern .NET is generally faster due to JIT improvements, but the cross-runtime bridge overhead is consistent across .NET versions.

Start integrating. Download JNBridgePro for a free trial, explore the demos, or contact us for architecture guidance.

Related Articles

Continue with these related Java/.NET integration guides:

• C# to Java integration guide
• Best way to run Java from C#/.NET
• How to use Java libraries in .NET Core
• How to call Java from C#