> TL;DR / Key Takeaways
>
> – You cannot directly reference a JAR in C# or import a .NET DLL in Java — the runtimes are incompatible.
> – Process wrapping is the fastest to set up but slowest at runtime (~50–200 ms per call).
> – REST/gRPC wrappers work well for loosely coupled, low-frequency calls.
> – IKVM converts Java bytecode to .NET IL but is stuck on Java 8 with no commercial support.
> – JNBridgePro provides in-process bridging at ~8 µs per call with full object access and enterprise support.

Using a JAR in C# is one of the most common cross-platform interop challenges developers face. Whether you need to call a Java library from a .NET application or load a .NET DLL in a Java project, the two runtimes — JVM and CLR — don’t speak the same language. This guide covers every production-ready method for Java/.NET interoperability, with real code, performance benchmarks, and honest trade-offs.

Table of Contents

• Why JARs and DLLs Are Incompatible
• Method 1: Process Wrapping (Quick and Dirty)
• Method 2: REST/gRPC Service Wrapper
• Method 3: JNI — Low-Level Native Bridge
• Method 4: IKVM (Java on .NET — Legacy)
• Method 5: JNBridgePro (In-Process Runtime Bridge)
• How Do JAR-to-DLL Interop Methods Compare?
• Which Method Should You Choose?
• Common Pitfalls: Classpath, Type Mapping, and Memory
• FAQ
• Get Started

Why JARs and DLLs Are Incompatible

Java JARs contain bytecode compiled for the JVM. .NET DLLs contain MSIL compiled for the CLR. These are fundamentally different execution environments with separate memory management, type systems, and native interop models.

You can’t “Add Reference” to a JAR in Visual Studio or import a .NET assembly in Java. You need a runtime bridge, a service wrapper, or a bytecode translator. Here are your five options — from simplest to most powerful.

Method 1: Process Wrapping (Quick and Dirty)

The simplest approach to using a JAR in C#: launch a separate java process and capture its output.

Running a Java JAR from C\#

“csharp
using System.Diagnostics;

public class JavaRunner
{
public static string RunJar(string jarPath, string args)
{
var process = new Process
{
StartInfo = new ProcessStartInfo
{
FileName = "java",
Arguments = $"-jar {jarPath} {args}",
RedirectStandardOutput = true,
RedirectStandardError = true,
UseShellExecute = false,
CreateNoWindow = true
}
};

process.Start();
string output = process.StandardOutput.ReadToEnd();
process.WaitForExit();

if (process.ExitCode != 0)
throw new Exception($"Java process failed: {process.StandardError.ReadToEnd()}");

return output;
}
}
`

For a deeper walkthrough, see How to Run a Java JAR from C#.

Running a .NET DLL from Java

`java
import java.io.*;

public class DotNetRunner {
public static String runDotNet(String dllPath, String args) throws Exception {
ProcessBuilder pb = new ProcessBuilder("dotnet", dllPath, args);
pb.redirectErrorStream(true);
Process process = pb.start();

BufferedReader reader = new BufferedReader(
new InputStreamReader(process.getInputStream()));
StringBuilder output = new StringBuilder();
String line;
while ((line = reader.readLine()) != null)
output.append(line).append("\n");

if (process.waitFor() != 0)
throw new RuntimeException("dotnet process failed");
return output.toString();
}
}
`

Pros: Simple, no dependencies, works everywhere.

Cons: Slow (~50–200 ms startup per call), limited to string I/O, no direct object access, no marshaling of complex types, crude error handling.

Best for: One-off batch jobs, scripts, quick prototypes.

Method 2: REST/gRPC Service Wrapper

Wrap your Java or .NET code as a microservice and call it over HTTP or gRPC. This avoids the per-call process startup overhead but adds network serialization latency.

Exposing a Java JAR as a REST API

`java
@RestController
public class AnalyticsController {
private final AnalyticsEngine engine; // from your JAR

@PostMapping("/analyze")
public AnalysisResult analyze(@RequestBody DataSet data) {
return engine.process(data);
}
}
`

Calling It from C\#

`csharp
var client = new HttpClient { BaseAddress = new Uri("http://localhost:8080") };
var result = await client.PostAsJsonAsync("/analyze", dataset);
var analysis = await result.Content.ReadFromJsonAsync();
`

Pros: Clean separation, language-agnostic, independently scalable.

Cons: Network latency (~0.3–5 ms even on localhost), serialization overhead, operational complexity of running a separate service.

For a detailed comparison of bridging vs. REST vs. gRPC, see Bridge vs REST vs gRPC for Java/.NET Integration.

Best for: Loosely coupled systems, infrequent calls, existing microservice architectures.

Method 3: JNI — Low-Level Native Bridge

The Java Native Interface (JNI) lets Java call native code. You can chain it: Java ↔ JNI ↔ C/C++ ↔ P/Invoke ↔ .NET. Technically possible, practically painful.

`
Java ←→ JNI ←→ Native C/C++ ←→ P/Invoke ←→ .NET CLR
`

`c
// Native bridge (C) — simplified
JNIEXPORT jstring JNICALL Java_Bridge_callDotNet(
JNIEnv *env, jobject obj, jstring input) {
// Load CLR, locate assembly, invoke method, marshal result
// ... hundreds of lines of platform-specific boilerplate ...
}
`

Pros: Near-native performance, no network overhead.

Cons: Extremely complex — manual memory management, type marshaling nightmares, platform-specific builds, debugging across three runtimes.

Best for: Almost nobody. Only justified when you need absolute maximum performance and have deep C/C++ expertise.

Method 4: IKVM (Java on .NET — Legacy)

IKVM was an open-source project that translated Java bytecode to .NET IL, letting you reference JARs directly as .NET assemblies.

`bash
# Convert JAR to DLL (IKVM)
ikvmc -target:library analytics.jar -out:Analytics.dll
`

`csharp
// Then use Java classes like C# classes
using com.example.analytics;
var engine = new AnalyticsEngine();
var result = engine.process(data);
`

Current status: IKVM is stuck on Java SE 8 (2014 APIs). Libraries using Java 11+ features — records, virtual threads (Java 21), modern java.time — won't work. A community fork (ikvm-revived) has made progress but lacks production readiness and commercial support.

If you're currently on IKVM and hitting limitations, see Migrating from IKVM to JNBridgePro.

Best for: Legacy situations with Java 8 libraries only — and tolerance for risk.

Method 5: JNBridgePro (In-Process Runtime Bridge)

JNBridgePro runs both the JVM and CLR in the same process, creating proxy classes that let you use Java objects from C# (and vice versa) as if they were native. No serialization. No network. Direct reflection-based type mapping with full access to properties, methods, fields, exceptions, and callbacks.

Using a Java JAR in C\#

`csharp
// After generating .NET proxies for your JAR:
using com.example.analytics;

var engine = new AnalyticsEngine();
var config = new AnalysisConfig();
config.setThreshold(0.95);
config.setMode("comprehensive");

// Direct method calls — no serialization, no network
AnalysisResult result = engine.process(dataset, config);
Console.WriteLine($"Score: {result.getScore()}");
Console.WriteLine($"Items: {result.getResults().size()}");
`

Using a .NET DLL in Java

`java
// After generating Java proxies for your .NET assembly:
import system.io.FileStream;
import system.io.StreamReader;

FileStream fs = new FileStream("data.bin", FileMode.Open);
StreamReader reader = new StreamReader(fs);
String content = reader.ReadToEnd();
reader.Close();
`

For step-by-step tutorials, see How to Call Java from C# and How to Call C# from Java.

Pros: ~8 µs per call, direct object access, full type mapping, supports modern Java (11, 17, 21) and .NET (6, 7, 8), commercial support with SLAs.

Cons: Commercial license required, proxy generation step, JVM + CLR in same process uses more memory.

Best for: Enterprise integration — high-frequency calls, complex object graphs, regulated industries (finance, healthcare).

How Do JAR-to-DLL Interop Methods Compare?

> For 10,000 calls: Process wrapping takes 8–33 minutes. REST takes 3–50 seconds. JNBridgePro takes 0.08 seconds.

Which Method Should You Choose?

Use Process Wrapping if you need a quick one-off integration, calls are infrequent (batch processing), and you don't need direct Java object access from .NET.

Use REST/gRPC if your systems are loosely coupled, you're already in a microservices architecture, or calls happen fewer than 100 times per second.

Use JNBridgePro if:

Avoid JNI unless you have deep C/C++ expertise and a very specific performance requirement.

Avoid IKVM unless you work exclusively with Java 8-era libraries and accept the risk of an unmaintained project.

Common Pitfalls: Classpath, Type Mapping, and Memory

1. Classpath Issues When Loading JARs

When loading JARs from .NET — via Process wrapping or JNBridgePro — ensure all dependency JARs are on the classpath:

`csharp
// ❌ Wrong — missing dependencies
process.StartInfo.Arguments = "-jar analytics.jar";

// ✅ Right — include all JARs on the classpath
var classpath = "analytics.jar;lib/commons-math3.jar;lib/slf4j-api.jar";
process.StartInfo.Arguments = $"-cp {classpath} com.example.Main {args}";
`

2. Type Mapping Gotchas Between Java and C\#

Java and .NET have subtly different type systems. Watch these carefully during interop:

3. Memory Management Across Runtimes

When bridging Java and .NET, each runtime has its own garbage collector. Objects on one side won't be collected until proxy references on the other side are released. In long-running applications, failing to dispose cross-runtime references can cause memory leaks.

FAQ

Can I Directly Reference a JAR File in Visual Studio?

No. Visual Studio doesn't natively understand Java JAR files. JAR files contain Java bytecode, which the .NET CLR cannot execute. You need either a bytecode translator like IKVM (limited to Java 8) or a runtime bridge like JNBridgePro that generates .NET proxy classes wrapping the JAR's contents. The proxies let you call Java methods from C# with full IntelliSense and type safety.

Can Java Load and Call Methods in a .NET DLL?

Not directly. Java's classloader only understands Java bytecode in .class` format. To load a .NET DLL in Java, you need one of three approaches: a service wrapper (REST/gRPC), a native bridge through JNI, or JNBridgePro which generates Java proxy classes for .NET assemblies — giving you direct access to .NET objects from Java code.

What’s the Performance Difference Between REST and In-Process Bridging?

REST calls add network overhead even on localhost — typically 0.3–5 ms per call including serialization and marshaling. In-process bridging with JNBridgePro operates at ~8 microseconds per call with zero serialization. For 10,000 calls, that’s 3–50 seconds via REST versus 0.08 seconds via bridge — a 60× to 600× improvement.

Is IKVM Still Maintained?

The original IKVM project ended in 2017. A community fork (ikvm-revived) exists and has made progress, but it’s not production-ready for Java 11+ and has no commercial support. For production workloads requiring modern Java, consider migrating from IKVM.

Can I Use a Java JAR in .NET Without Installing the JDK?

With IKVM (Java 8 only), yes — it converts Java bytecode to .NET IL, so no JVM is needed. With all other approaches (Process, JNBridgePro, JNI), you need a JRE/JDK. JNBridgePro can use an embedded JRE, simplifying deployment.

What About Javonet or jni4net?

Javonet offers a reflection-style API for calling .NET from Java (and vice versa). jni4net is an open-source JNI-based bridge but hasn’t been updated since 2015. JNBridgePro differs by providing compile-time proxy generation with full type mapping, IntelliSense support, and enterprise-grade reliability.

Get Started

Ready to integrate Java JARs in your .NET project — or .NET DLLs in Java?

👉 Download the JNBridgePro free trial and see how in-process bridging compares to your current approach. Most teams have a working integration within a day.

📚 Explore the JNBridgePro Developer Center for demos, tutorials, and sample projects.

💬 Have questions? Contact the JNBridge team for architecture guidance or licensing details.