I've tested all 5 approaches to running Java JARs from C# in production — from simple Process.Start() to in-process bridges. Each has real trade-offs that benchmarks alone won't tell you.
Originally published on the JNBridge blog.
Why Run Java JAR Files from .NET?
You’ve got a Java JAR file. Maybe it’s a proprietary library your organization built over years. Maybe it’s an open-source tool like Apache Tika (document parsing), Bouncy Castle (cryptography), or Stanford NLP (natural language processing). Whatever it is, you need its functionality in your C# or .NET application — without rewriting it.
Want the fastest, most reliable approach? JNBridgePro lets you call Java JARs from C# with native method syntax — no process spawning, no HTTP overhead. Download a free evaluation to compare.
This is more common than you’d think. According to Stack Overflow’s developer survey, organizations running both Java and .NET account for over 40% of enterprise environments. The question isn’t whether you’ll need cross-platform interop — it’s how you’ll implement it.
This guide covers every practical method for running Java JAR files from .NET applications, from quick-and-dirty process execution to high-performance in-process bridging. Each approach includes working C# code you can adapt for your project.
Method 1: Process.Start — The Quick and Dirty Approach
The simplest way to run a JAR file from C# is to launch it as a separate process using System.Diagnostics.Process:
using System.Diagnostics;
public class JavaProcessRunner
{
public string RunJar(string jarPath, string arguments)
{
var process = new Process
{
StartInfo = new ProcessStartInfo
{
FileName = "java",
Arguments = $"-jar \"{jarPath}\" {arguments}",
RedirectStandardOutput = true,
RedirectStandardError = true,
UseShellExecute = false,
CreateNoWindow = true
}
};
process.Start();
string output = process.StandardOutput.ReadToEnd();
string errors = process.StandardError.ReadToEnd();
process.WaitForExit();
if (process.ExitCode != 0)
throw new Exception($"Java process failed: {errors}");
return output;
}
}
When to use this: One-off batch processing, CLI tools, scripts where latency doesn’t matter.
Limitations: JVM startup takes 500ms-2s per invocation. Communication is limited to stdin/stdout/stderr (text only). No shared objects, no callbacks, no streaming. If you’re calling this in a loop, you’re paying that startup penalty every time.
Method 2: Persistent Java Process with stdin/stdout
Avoid repeated JVM startup by keeping a long-running Java process and communicating through stdin/stdout:
public class PersistentJavaProcess : IDisposable
{
private Process _process;
private StreamWriter _input;
private StreamReader _output;
public PersistentJavaProcess(string jarPath)
{
_process = new Process
{
StartInfo = new ProcessStartInfo
{
FileName = "java",
Arguments = $"-jar \"{jarPath}\"",
RedirectStandardInput = true,
RedirectStandardOutput = true,
UseShellExecute = false,
CreateNoWindow = true
}
};
_process.Start();
_input = _process.StandardInput;
_output = _process.StandardOutput;
}
public string SendCommand(string command)
{
_input.WriteLine(command);
_input.Flush();
return _output.ReadLine(); // Assumes line-delimited responses
}
public void Dispose()
{
_input?.Close();
_process?.Kill();
_process?.Dispose();
}
}
Better than Method 1 because you pay the JVM startup cost once. But you’re still limited to text-based communication, and you need to design a request/response protocol (usually JSON lines or a custom delimiter).
Method 3: REST API Wrapper
Wrap the JAR’s functionality in a lightweight HTTP server (Spring Boot, Javalin, or even a simple com.sun.net.httpserver.HttpServer), then call it from C# with HttpClient:
// C# client side
public class JavaApiClient
{
private readonly HttpClient _client;
public JavaApiClient(string baseUrl = "http://localhost:8080")
{
_client = new HttpClient { BaseAddress = new Uri(baseUrl) };
}
public async Task ProcessDocumentAsync(string filePath)
{
var content = new MultipartFormDataContent();
content.Add(new ByteArrayContent(File.ReadAllBytes(filePath)),
"file", Path.GetFileName(filePath));
var response = await _client.PostAsync("/api/parse", content);
response.EnsureSuccessStatusCode();
return await response.Content.ReadAsStringAsync();
}
public async Task AnalyzeTextAsync(string text)
{
var response = await _client.PostAsJsonAsync("/api/analyze",
new { text });
return await response.Content
.ReadFromJsonAsync();
}
}
Pros: Well-understood pattern, works across machines, easy to scale horizontally.Cons: HTTP overhead (2-15ms per call), JSON serialization costs, need to run and manage a separate Java service, complex object graphs require custom DTOs.
Method 4: gRPC Bridge
For higher performance than REST, use gRPC with Protocol Buffers:
// proto definition
syntax = "proto3";
service JavaLibrary {
rpc ProcessData (DataRequest) returns (DataResponse);
rpc StreamResults (StreamRequest) returns (stream ResultChunk);
}
message DataRequest {
string input = 1;
map parameters = 2;
}
message DataResponse {
string result = 1;
int32 processingTimeMs = 2;
}
// C# gRPC client
var channel = GrpcChannel.ForAddress("http://localhost:50051");
var client = new JavaLibrary.JavaLibraryClient(channel);
var response = await client.ProcessDataAsync(new DataRequest
{
Input = "sample data",
Parameters = { { "mode", "detailed" } }
});
Console.WriteLine($"Result: {response.Result} ({response.ProcessingTimeMs}ms)");
Pros: Binary protocol (faster than JSON), strongly typed contracts, supports streaming.Cons: Still requires a separate Java process, schema management overhead, more complex setup.
Method 5: In-Process Bridge with JNBridgePro (Recommended)
The highest-performance option: load the JVM directly into your .NET process and call Java classes as native .NET objects using JNBridgePro.
How It Works
JNBridgePro generates .NET proxy assemblies from your Java JAR files. These proxies handle all the JVM communication — method calls, object creation, type conversion — transparently. Your C# code doesn’t know it’s talking to Java.
Step-by-Step Setup
1. Download and install JNBridgePro
2. Generate proxies from your JAR: Open the Proxy Generation Tool, add your JAR to the classpath, select the classes you need, and generate a .NET assembly.
3. Reference the proxy assembly in your C# project and start coding:
// Example: Using Apache Tika for document parsing
using org.apache.tika;
using org.apache.tika.metadata;
using org.apache.tika.parser;
public class DocumentParser
{
private readonly Tika _tika;
public DocumentParser()
{
_tika = new Tika();
}
public string ExtractText(string filePath)
{
var file = new java.io.File(filePath);
return _tika.parseToString(file);
}
public Dictionary ExtractMetadata(string filePath)
{
var metadata = new Metadata();
var file = new java.io.File(filePath);
var parser = new AutoDetectParser();
var handler = new org.xml.sax.helpers.DefaultHandler();
using var stream = new java.io.FileInputStream(file);
parser.parse(stream, handler, metadata, new ParseContext());
var result = new Dictionary();
foreach (string name in metadata.names())
{
result[name] = metadata.get(name);
}
return result;
}
}
Notice the code reads like standard C#. The Tika, Metadata, and AutoDetectParser objects are actually Java classes — but JNBridgePro makes them look and behave like native .NET types.
Real-World Example: Stanford NLP from C
using edu.stanford.nlp.pipeline;
using edu.stanford.nlp.ling;
using java.util;
public class NlpProcessor
{
private readonly StanfordCoreNLP _pipeline;
public NlpProcessor()
{
var props = new Properties();
props.setProperty("annotators",
"tokenize,ssplit,pos,lemma,ner,parse,sentiment");
_pipeline = new StanfordCoreNLP(props);
}
public List ExtractEntities(string text)
{
var document = new CoreDocument(text);
_pipeline.annotate(document);
var entities = new List();
foreach (CoreEntityMention em in document.entityMentions())
{
entities.Add((em.text(), em.entityType()));
}
return entities;
}
}
Performance Comparison
We measured each method calling a simple Java computation method (Fibonacci calculation) 10,000 times from C#:
The gap is dramatic. JNBridgePro’s shared-memory mode completes 10,000 calls in under a second — the same workload takes minutes with Process.Start and tens of seconds with REST. For latency-sensitive applications, there’s no comparison.
Which Method Should You Use?
The right choice depends on your specific requirements:
Use Process.Start when you need to run a JAR once (batch job, deployment script, CI pipeline). Don’t use it for anything performance-sensitive.
Use REST/gRPC when the Java and .NET components run on different machines, or when you need the Java service to scale independently. Good for microservice architectures where loose coupling matters more than raw speed.
Use JNBridgePro when performance matters, when you’re calling Java methods frequently, when you need to work with complex Java object graphs, or when you want the simplest possible C# API. It’s the right choice for most enterprise integration scenarios.
Common JAR Integration Scenarios
Scenario 1: Legacy Business Logic
Your company has a Java library that implements complex business rules — pricing engines, risk calculations, regulatory compliance checks. These were developed over years and would cost millions to rewrite. With JNBridgePro, your new .NET front-end calls the existing Java logic directly. No rewrite. No risk of introducing bugs in translation.
Scenario 2: Open-Source Java Libraries with No .NET Equivalent
Some Java libraries have no quality .NET equivalent:
Apache Tika — Document content extraction (supports 1,000+ file formats)
Stanford NLP — Natural language processing
Apache POI — Advanced Excel/Word manipulation
Deeplearning4j — Deep learning on the JVM
JasperReports — Enterprise reporting engine
Lucene — Full-text search (yes, there’s Lucene.NET, but the Java version leads by 2+ years)
<!– /wp:list —
What Is the Best Way to Run a Java JAR File from C#?
The five main methods to run a Java JAR file from C# are: (1) Process.Start — spawns java.exe as a child process, simplest but slowest; (2) REST API wrapper — wraps the JAR in a web service; (3) gRPC service — high-performance RPC with Protobuf; (4) In-process bridge (JNBridgePro) — direct method calls with microsecond latency; (5) IKVM bytecode translation — converts JAR to .NET assembly, limited to Java 8.
Quick Comparison: All 5 Methods at a Glance
Rather than wait for a .NET port or settle for a lesser alternative, use the Java library directly.
Scenario 3: Gradual Migration
Migrating from Java to .NET (or vice versa) doesn’t have to be all-or-nothing. Start new development in your target platform while keeping existing Java components running through a bridge. Migrate individual components when it makes sense, not when you’re forced to.
Deployment Considerations
Regardless of which method you choose, remember:
JDK/JRE must be installed on the deployment target (or bundled with your application)
Match architectures — if your .NET app is x64, your JVM must be x64
Classpath management — all JAR dependencies must be accessible at runtime
Memory planning — the JVM needs its own heap allocation on top of your .NET process memory
Docker considerations — use a base image with both .NET SDK and JDK installed, or a multi-stage build
Getting Started
Ready to integrate a Java JAR into your .NET application? Start here:
Identify your JAR — what classes and methods do you need to call?
Choose your method — Process.Start for simple cases, JNBridgePro for everything else
Generate proxies (if using JNBridgePro) from your JAR’s classes
Write C# code against the generated .NET types
Test and benchmark — verify correctness and measure performance
For detailed setup instructions, see our complete guide to calling Java from C#.
Frequently Asked Questions
Can you run a Java JAR file directly from C# without installing Java?
No — running a Java JAR requires a JVM (Java Runtime Environment or JDK) installed on the machine, regardless of which method you use. Even in-process bridges like JNBridgePro load the JVM within the .NET process. The only exception is IKVM, which translates Java bytecode to .NET CIL — but it has significant limitations with modern Java versions and complex libraries.
What is the fastest way to call Java from C# and .NET?
In-process bridges like JNBridgePro offer the lowest latency — microsecond-level method calls between C# and Java. Process.Start (spawning java.exe) is the slowest at 100ms–2s per invocation due to JVM startup overhead. REST APIs and gRPC fall in between at 1–50ms depending on network and serialization.
Is Process.Start a good way to run JAR files from C# in production?
Process.Start works for simple, infrequent calls but is not recommended for production systems that need performance or reliability. Each call spawns a new JVM process (100ms+ overhead), you lose type safety, error handling is limited to exit codes and stdout parsing, and you must manage process lifecycle manually. Use it for scripts and utilities, not for tight integration.
Can I pass complex objects between C# and a Java JAR?
With Process.Start, you’re limited to strings (command-line arguments, stdin/stdout). REST and gRPC support structured data via JSON/Protobuf. In-process bridges like JNBridgePro provide full object marshaling — you can pass .NET objects to Java methods and receive Java objects back with automatic type conversion, including collections, custom classes, and exceptions.
Does running a Java JAR from .NET work in Docker containers?
Yes. All five methods work in Docker, but your container needs both the .NET runtime and a JDK/JRE installed. Multi-stage Docker builds help keep image sizes manageable. JNBridgePro supports Linux containers with .NET Core/.NET 8/9. See our Docker and Kubernetes guide for container architecture patterns.
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