Table of Contents

• Can You Call Java from C#?
• Overview of Java-C# Interoperability Approaches
• Approach 1: REST APIs and Web Services
• Approach 2: gRPC with Protocol Buffers
• Approach 3: Java Native Interface (JNI) via C++
• Approach 4: Process.Start — Running Java as a Subprocess
• Approach 5: In-Process Bridging with JNBridgePro
• Comparison Table: All Java to C# Integration Methods
• Step-by-Step: Calling Java from C# with JNBridgePro
• Performance Considerations
• What About jni4net, IKVM, and Javonet?
• When to Use Each Approach
• FAQ

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If you need to call Java from C#, you’re not alone. Enterprise development teams frequently face scenarios where Java and .NET code must work together — whether integrating legacy Java systems, accessing Java-only libraries, or bridging teams that work on different platforms. This guide covers every viable approach to Java-C# interoperability, with code examples, performance analysis, and recommendations for production environments.

Can You Call Java from C#?

Yes, you can call Java from C# using several methods. The most common approaches include REST APIs, gRPC, the Java Native Interface (JNI) through C++ wrappers, running Java as a subprocess, and dedicated bridging tools like JNBridgePro that enable direct in-process communication between the .NET Common Language Runtime (CLR) and the Java Virtual Machine (JVM).

The right approach depends on your performance requirements, how tightly coupled the Java and C# code needs to be, and whether you need the Java code to run in-process or as a separate service. Let’s examine each option in detail.

Overview of Java-C# Interoperability Approaches

When bridging Java and .NET, you’re fundamentally connecting two different runtime environments — the JVM and the CLR. Each approach handles this differently:

• Network-based approaches (REST, gRPC) keep Java and C# in separate processes and communicate over the network
• Native interop (JNI + P/Invoke) bridges through native C/C++ code at the system level
• Subprocess execution (Process.Start) runs Java programs as external commands
• In-process bridging (JNBridgePro) runs both the JVM and CLR in the same process with automatic type marshaling and proxy class generation

Each has trade-offs in performance, complexity, and maintainability. Understanding these trade-offs is critical for choosing the right architecture for your Java-.NET integration project.

Approach 1: REST APIs and Web Services

The most widely discussed method for calling Java from C# involves wrapping Java functionality in HTTP endpoints. Your C# application sends HTTP requests to a Java web service (typically built with Spring Boot, Jakarta EE, or a similar framework) and receives JSON or XML responses.

How It Works

1. Package your Java code as a web application with REST endpoints
2. Deploy the Java service (e.g., on Tomcat or as a Spring Boot JAR)
3. Use HttpClient in C# to call the Java endpoints

// C# client calling a Java REST service
using var client = new HttpClient();
client.BaseAddress = new Uri("http://localhost:8080");

var response = await client.GetAsync("/api/books?author=Smith");
var books = await response.Content.ReadFromJsonAsync<List<Book>>();

Pros

• Language-agnostic — works between any two tech stacks
• Well-understood architecture with abundant tooling
• Independent deployment and scaling of Java and .NET components

Cons

• Network latency on every call (typically 5–50ms per request)
• Serialization and deserialization overhead for every interaction
• Requires running and maintaining a separate Java web service
• Complex error handling across network boundaries
• Not suitable for fine-grained method-level integration

REST APIs work best when Java and C# systems are genuinely separate services with occasional, coarse-grained communication. For tight integration requiring frequent method calls, the overhead becomes prohibitive.

Approach 2: gRPC with Protocol Buffers

gRPC offers better performance than REST by using Protocol Buffers for binary serialization and HTTP/2 for transport. It’s a solid choice when you need structured cross-language communication with strong typing.

How It Works

1. Define service contracts in .proto files
2. Generate Java server stubs and C# client stubs from the contract
3. Implement the service in Java, call it from C#

// book_service.proto
service BookService {
  rpc GetBooks (AuthorRequest) returns (BookList);
}
message AuthorRequest {
  string last_name = 1;
}

// C# gRPC client
var channel = GrpcChannel.ForAddress("http://localhost:5000");
var client = new BookService.BookServiceClient(channel);
var books = await client.GetBooksAsync(new AuthorRequest { LastName = "Smith" });

Pros

• Faster than REST (binary serialization, HTTP/2 multiplexing)
• Strongly typed contracts prevent interface drift
• Streaming support for large data sets
• Good tooling in both Java and .NET

Cons

• Still network-bound — latency per call, though lower than REST
• Contract management overhead (keeping .proto files in sync)
• More complex setup than REST
• Not suitable for in-process, method-level Java calls

Approach 3: Java Native Interface (JNI) via C++

The Java Native Interface is a framework built into the JVM that allows native code (C/C++) to interact with Java classes. By combining JNI with .NET’s P/Invoke mechanism, you can build a C++ bridge layer that connects C# to Java.

How It Works

1. Write C++ functions that use JNI to load the JVM, find Java classes, and invoke methods
2. Compile the C++ code into a native shared library (.dll / .so)
3. Use P/Invoke in C# to call the C++ wrapper functions

// C++ JNI wrapper
JNIEXPORT void JNICALL callJavaMethod(JNIEnv *env) {
    jclass cls = env->FindClass("com/example/BookService");
    jmethodID mid = env->GetStaticMethodID(cls, "getBooks", "(Ljava/lang/String;)[Lcom/example/Book;");
    jobjectArray result = (jobjectArray)env->CallStaticObjectMethod(cls, mid, env->NewStringUTF("Smith"));
}

// C# P/Invoke declaration
[DllImport("JavaBridge.dll")]
static extern IntPtr CallJavaMethod(string className, string methodName);

Pros

• True in-process communication — no network latency
• Direct access to the JVM from native code
• Good raw performance once set up

Cons

• Extreme complexity — requires expertise in C#, C++, and Java
• Manual memory management across three runtime environments
• Difficult to debug when things go wrong (native crashes, memory leaks)
• Platform-specific native code must be maintained per OS
• No automatic type marshaling — every type conversion is manual
• Not recommended for teams without deep systems programming experience

JNI is the technically “correct” low-level approach, but the development and maintenance cost makes it impractical for most enterprise teams. The JNI layer in the JVM was designed for extending Java with native code, not for wholesale cross-language integration.

Approach 4: Process.Start — Running Java as a Subprocess

The simplest approach: run a Java program as a separate process from C# using Process.Start, passing data through command-line arguments, standard I/O, or files.

// Launch Java process from C#
var process = new Process
{
    StartInfo = new ProcessStartInfo
    {
        FileName = "java",
        Arguments = "-jar MyJavaApp.jar --author Smith",
        RedirectStandardOutput = true,
        UseShellExecute = false
    }
};
process.Start();
string output = await process.StandardOutput.ReadToEndAsync();
await process.WaitForExitAsync();

Pros

• Dead simple to implement
• Complete isolation between Java and C#
• No additional libraries or frameworks needed

Cons

• Massive overhead per invocation (JVM startup, process creation)
• Limited to string-based data exchange
• No shared state between calls
• Impractical for frequent or real-time interactions
• Error handling is rudimentary

This approach only makes sense for infrequent batch-style operations where a Java command-line tool needs to be triggered from a .NET application.

Approach 5: In-Process Bridging with JNBridgePro

JNBridgePro takes a fundamentally different approach: it runs both the JVM and CLR in the same process, generating .NET proxy classes that let you call Java methods as if they were native C# code. This eliminates network latency, serialization overhead, and the need for complex wrapper code.

How It Works

JNBridgePro’s proxy generation tool analyzes your Java classes (from JAR files or classpath) and creates corresponding .NET assemblies containing proxy classes. These proxies handle all type marshaling between .NET and Java types automatically — including complex objects, collections, exceptions, and callbacks.

At runtime, JNBridgePro manages an embedded JVM within your .NET process, routing method calls through an optimized shared-memory channel. Both garbage collectors are coordinated to prevent memory leaks.

Step-by-Step: Calling Java from C# with JNBridgePro

Step 1: Install JNBridgePro

Download JNBridgePro and install both the .NET and Java side components.

Step 2: Generate Proxy Classes

Use the JNBridgePro proxy generation tool to point at your Java JAR files. It creates .NET assemblies containing proxy classes for each Java class you want to access.

Step 3: Configure the Runtime

Add a simple configuration to your .NET application:

using com.jnbridge.jnbcore;

// Initialize the JNBridgePro runtime
DotNetSide.init(".\\jnbcore_tcp.properties");

Step 4: Call Java Methods from C#

With proxies generated, calling Java from C# looks like standard .NET code:

using BookService; // Generated proxy namespace

// Create Java objects and call methods — just like native C#
Author author = new Author("John", "Smith");
Book[] books = Book.getBooks("John", "Smith");

foreach (Book book in books)
{
    Console.WriteLine($"Title: {book.Get_title()}");
    Console.WriteLine($"Publisher: {book.Get_publisher()}");
    Console.WriteLine($"In Stock: {book.Get_inStock()}");
}

No HTTP calls, no serialization code, no C++ wrappers. Java objects behave like .NET objects.

Key Capabilities

• Bidirectional calls: Java can call back into .NET code, enabling event-driven architectures
• Automatic collection conversion: Java ArrayList ↔ .NET List, HashMap ↔ Dictionary
• Exception propagation: Java exceptions become .NET exceptions with full stack traces
• Coordinated garbage collection: No manual memory management across runtimes
• Shared-memory transport: In-process communication with microsecond-level latency

For more details, visit the JNBridgePro Developer Center.

Comparison Table: All Java to C# Integration Methods

JNBridgePro delivers in-process performance similar to JNI but without the complexity of maintaining C++ bridge code, manual memory management, or platform-specific native libraries.

Performance Considerations

When choosing how to call Java methods from C#, performance implications vary dramatically:

Network-based approaches (REST, gRPC) add milliseconds of latency per call. For a business operation requiring 10 Java method calls, you’re looking at 50–500ms of pure network overhead. This compounds under load and introduces failure modes (timeouts, connection exhaustion, service discovery issues).

Process.Start is the slowest option. Each invocation pays the full JVM startup cost (typically 100ms–2s), making it viable only for very infrequent calls.

JNI offers excellent raw performance but introduces memory management risks. Incorrectly managed JNI references cause memory leaks that are extremely difficult to diagnose in production. Cross-runtime debugging is also challenging.

In-process bridging (JNBridgePro) provides the best balance: microsecond-level method call latency with automatic memory management and type safety. Because both runtimes share the same process, there’s no serialization or network overhead — data is passed through optimized shared-memory channels.

For applications requiring frequent Java-C# interaction (hundreds or thousands of calls per operation), in-process approaches are the only viable option.

What About jni4net, IKVM, and Javonet?

Developers researching Java-.NET interoperability often encounter several other tools:

jni4net is an open-source library that bridges Java and .NET using JNI under the hood. While it appears in many Stack Overflow answers, the project has been largely unmaintained since 2015 and lacks support for modern .NET (Core/.NET 5+). It can work for simple prototypes but isn’t suitable for production enterprise systems.

IKVM converts Java bytecode to .NET assemblies (CIL). While clever, this approach has significant limitations: it doesn’t support all Java APIs, has compatibility issues with complex Java libraries, and the project has had periods of dormancy. It works best for simple, self-contained Java libraries without complex runtime dependencies.

Javonet is a commercial tool similar to JNBridgePro. It provides cross-runtime method invocation, though with a different architecture and licensing model. Worth evaluating alongside JNBridgePro for your specific requirements.

GraalVM Native Image is a newer approach where Java code is compiled to a native shared library using GraalVM’s ahead-of-time compiler. The .NET application then loads this shared library via P/Invoke. This is technically interesting but imposes severe restrictions on the Java code (no dynamic class loading, limited reflection) and requires significant expertise to set up.

JNBridgePro remains the most mature and widely deployed solution for enterprise Java-.NET integration, with active development, professional support, and a comprehensive knowledge base.

When to Use Each Approach

Choose REST APIs when Java and C# are separate services that communicate infrequently, and you’re already invested in a microservices architecture.

Choose gRPC when you need better performance than REST with strong contract typing, but your Java and C# components can run as separate services.

Choose JNI only if you have a team with deep C/C++ and systems programming expertise, and you need maximum control over a very narrow integration surface.

Choose Process.Start for simple, infrequent batch operations where a Java CLI tool needs to be triggered from .NET.

Choose JNBridgePro when you need:

• Direct method-level access to Java classes from C#
• In-process performance without network overhead
• Automatic type marshaling between .NET and Java types
• Bidirectional communication (Java calling .NET and vice versa)
• Production-grade reliability with professional support
• Integration with legacy Java systems or third-party Java libraries

For most enterprise Java-.NET integration scenarios — especially legacy system integration, gradual migration, and cross-platform library access — JNBridgePro provides the most productive and reliable path forward. Contact JNBridge to discuss your specific integration needs, or download a free trial to evaluate it with your own codebase.

FAQ

Can you call Java from C#?

Yes. You can call Java from C# using REST APIs, gRPC, the Java Native Interface (JNI) with C++ wrappers, subprocess execution, or in-process bridging tools like JNBridgePro. The best approach depends on your performance requirements and how tightly coupled the integration needs to be. For direct method-level calls without network overhead, in-process solutions like JNBridgePro are the most efficient option.

Can .NET be used with Java?

Yes, .NET and Java can interoperate through several mechanisms. Network-based approaches (REST, gRPC) allow .NET and Java applications to communicate as separate services. For tighter integration, bridging tools run both the JVM and CLR in the same process, allowing direct method calls between the two platforms with automatic type conversion.

What is the fastest way to call Java from C#?

The fastest way to call Java from C# is using in-process bridging, which runs the JVM and CLR in the same application process. This eliminates network latency and serialization overhead, reducing per-call latency to microseconds rather than the milliseconds required by REST or gRPC. JNBridgePro and JNI both offer in-process performance, but JNBridgePro does so without requiring C++ wrapper code or manual memory management.

Is jni4net still maintained?

jni4net has not been actively maintained since approximately 2015. While it still appears in many online discussions and Stack Overflow answers, it does not support modern .NET versions (.NET Core, .NET 5/6/7/8/9) and is not recommended for new projects or production systems. For actively maintained Java-.NET bridging, consider JNBridgePro, which supports current versions of both .NET and Java.

What is the Java Native Interface (JNI)?

The Java Native Interface (JNI) is a framework built into the Java Virtual Machine that allows Java code to call and be called by native applications written in C, C++, or other languages. JNI is part of the JDK and is documented in Oracle’s JNI specification. While powerful, using JNI directly for Java-C# integration requires writing C++ wrapper code and managing memory across three runtime environments.

How does JNBridgePro differ from IKVM?

IKVM converts Java bytecode to .NET CIL, essentially translating Java code to run on the CLR natively. JNBridgePro takes a different approach: it runs an actual JVM alongside the CLR in the same process, generating proxy classes that bridge the two. JNBridgePro’s approach supports the full Java ecosystem (including dynamic class loading and all Java APIs), while IKVM has limitations with complex Java libraries and APIs that depend on JVM internals.

Related Articles

Continue exploring Java-.NET integration:

• How to Call C# Code from Java
• Anatomy of a Shared Memory Bridge
• JNBridgePro + Claude Code Demo
• JNBridge TCP Configuration Guide