5 Ways to Call Java from C# — Honest Comparison from Someone Who Does This Daily

I work at JNBridge, where Java/.NET integration is literally all we do. That means I have a bias — but it also means I've seen every approach to this problem succeed and fail across hundreds of enterprise deployments. I'll be upfront about where our product fits and where it doesn't.

Here's an honest breakdown of your options when your C# app needs to talk to Java.

The Problem: Two Runtimes, One App

C# runs on the .NET CLR, Java runs on the JVM. Completely separate runtime environments with different memory models, type systems, and garbage collectors. They weren't designed to talk to each other.

But enterprise software is messy, and mixed stacks are everywhere. Here's how teams actually solve this.

Option 1: REST APIs (The Default)

Wrap your Java code in a REST service and call it from C#.

using var client = new HttpClient();
var response = await client.GetAsync("http://localhost:8080/api/calculate");
var result = await response.Content.ReadFromJsonAsync<CalculationResult>();

When it works: Your Java and C# systems are naturally separate services that communicate occasionally. This is the right answer more often than people think.

When it doesn't: You need frequent, fine-grained method calls. 20 HTTP round-trips for one business operation adds up fast — 5-50ms per call means you're looking at hundreds of milliseconds of pure overhead.

My honest take: If your call pattern is coarse-grained (a few calls per user request), start here. Don't over-engineer it.

Option 2: gRPC

Binary serialization, HTTP/2, strongly-typed contracts. REST's faster, more structured cousin.

var channel = GrpcChannel.ForAddress("http://localhost:5000");
var client = new CalculationService.CalculationServiceClient(channel);
var result = await client.CalculateAsync(new CalculationRequest { Value = 42 });

When it works: You need better performance than REST and want strongly-typed contracts between teams.

When it doesn't: Still network-bound. Better than REST (5-15ms vs 25-75ms per call), but the latency still adds up with frequent calls. Plus you're maintaining Proto definitions across two codebases.

Option 3: JNI (Seriously, Don't)

Java Native Interface lets you bridge through C++. Theoretically you could write C++ that calls into .NET via COM or hosting APIs...

I've watched teams spend weeks on this approach. You need expertise in C#, C++, Java, JNI, P/Invoke, and multiple memory management models simultaneously. The debugging experience is brutal — one wrong reference and the whole process crashes with no useful stack trace.

Unless you have a very specific low-level requirement and a team of systems programmers, skip this.

Option 4: Process Execution

Run Java as a subprocess and communicate through stdin/stdout.

var process = new Process
{
    StartInfo = new ProcessStartInfo
    {
        FileName = "java",
        Arguments = "-jar MyJavaApp.jar --input data.json",
        RedirectStandardOutput = true
    }
};
process.Start();
string result = await process.StandardOutput.ReadToEndAsync();

When it works: You need to call a Java tool occasionally — batch jobs, scheduled tasks, once-per-hour operations.

When it doesn't: JVM startup cost is 100ms+ per invocation. Fine for batch work, terrible for anything interactive.

Option 5: In-Process Bridging

This is where I work, so I'll be transparent about my perspective. Tools like JNBridgePro (ours) and Javonet run both the JVM and CLR in the same process, generating proxy classes that make Java objects look like native C# objects.

// Calling Java looks like native C#
var calculator = new JavaCalculationEngine();
var result = calculator.Calculate(inputData);

When it works: You need tight, frequent integration — dozens or hundreds of cross-runtime calls per operation. Latency drops from milliseconds to microseconds.

When it doesn't: If you only make a few cross-platform calls per request, this is overkill. You're adding complexity (two GCs in one process) that isn't worth it for simple integration patterns.

Being honest: This is a commercial tool with enterprise pricing. If REST or gRPC solves your problem, use those — they're free and simpler.

The Landscape: Other Tools You'll Find

• jni4net — Dead project (last updated ~2015). Don't use it despite the Stack Overflow mentions.
• IKVM — Converts Java bytecode to .NET assemblies. Interesting approach, but has compatibility gaps with complex libraries and reflection-heavy code.
• Javonet — Commercial competitor to JNBridgePro. Different architecture, worth evaluating alongside us.

Performance Numbers

From real deployments we've measured:

Approach	Latency per call	Best for
REST API	25-75ms	Coarse-grained, occasional calls
gRPC	5-15ms	Moderate frequency, structured contracts
Process exec	150ms+	Batch/scheduled operations
In-process bridge	<1ms	High-frequency, fine-grained calls

For an operation requiring 50 cross-runtime calls, that's the difference between 2.5 seconds (REST) and 50 milliseconds (in-process). Whether that matters depends entirely on your use case.

How to Choose

The decision tree is simpler than it seems:

1. How often do you cross the runtime boundary per user request? A few times → REST/gRPC. Dozens of times → consider in-process.
2. What's your latency budget? Seconds are fine → REST. Milliseconds matter → gRPC or in-process.
3. How coupled are the Java and C# components? Loosely coupled services → REST/gRPC. Tightly integrated library calls → in-process.

Most teams should start with REST or gRPC. Seriously. Only reach for in-process bridging when the performance numbers force your hand.

What's your setup?

I'm curious what combinations people are running in production. Are you doing Java + C# integration? What approach did you land on, and would you choose differently if you started over?

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Disclosure: I work at JNBridge, which makes JNBridgePro — one of the tools discussed in this article. I've tried to give an honest comparison of all approaches, including when ours isn't the right fit.