Table of Contents

• What Are .NET Proxies for Java?
• What Are Java Proxies for .NET?
• When to Use .NET Proxies for Java vs Java Proxies for .NET
• How Proxy Generation Works
• Creating .NET Proxies from Java Classes
• Creating Java Proxies from .NET Classes
• Overriding Java Proxies in .NET
• Overriding .NET Proxies in Java
• Comparison: Proxy Generation vs. Manual Interop
• Decision Framework: When to Use Proxy Generation
• Advanced Proxy Patterns
• Troubleshooting Proxy Issues
• FAQ

What Are .NET Proxies for Java?

.NET proxies for Java are C#/.NET classes that mirror the API of Java classes. When you call a method on a .NET proxy, the call is forwarded to the corresponding Java object running in the JVM. The proxy handles type conversion, memory management, and exception translation transparently.

Think of it as a contract: the proxy presents a .NET interface while the implementation lives in Java. Your C# code never knows it’s talking to a Java object.

JNBridgePro generates these proxies automatically from Java bytecode, producing strongly-typed .NET assemblies that integrate with Visual Studio IntelliSense, debugging, and refactoring tools.

What Are Java Proxies for .NET?

Java proxies for .NET work in the reverse direction. They are Java classes that mirror .NET class APIs. When Java code calls a method on such a proxy, the call crosses to the CLR, executes on the real .NET object, and returns the result to Java.

This bidirectional capability is essential for scenarios where Java applications need to consume .NET libraries — for example, calling a C# business logic DLL from a Java application server.

When to Use .NET Proxies for Java vs Java Proxies for .NET

Use .NET proxies for Java when your primary application is C#/.NET and you need direct calls into Java libraries.

Use java proxies for .NET when your primary application is Java and you need direct calls into .NET assemblies.

In many enterprise systems you need both: .NET proxies for Java for one workflow and java proxies for .NET for another.

How Proxy Generation Works

The proxy generation process has three stages:

1. Introspection. The tool reads Java bytecode (from JAR files or class directories) or .NET assemblies (DLLs). It extracts class hierarchies, method signatures, field types, interfaces, and annotations.

2. Code generation. For each source class, the tool generates a proxy class in the target language. Methods are mapped to their cross-runtime equivalents. Types are converted: java.lang.String becomes System.String, java.util.List becomes System.Collections.Generic.IList, and so on.

3. Compilation. The generated source is compiled into a .NET assembly (DLL) or a Java JAR, ready to reference in your project.

The resulting proxies are lightweight — they contain only the forwarding logic, not the implementation. All execution happens on the original runtime.

Creating .NET Proxies from Java Classes

To create .NET proxies for Java using JNBridgePro:

// After adding the proxy DLL
var parser = new com.example.XmlParser();
var result = parser.parse("<doc><item>test</item></doc>");
Console.WriteLine(result.getNodeCount()); // Calls Java, returns to .NET

The generated proxy preserves the Java package structure as .NET namespaces.

Creating Java Proxies from .NET Classes

The reverse flow works the same way:

// After adding the proxy JAR
var calculator = new com.mycompany.dotnet.TaxCalculator();
double tax = calculator.computeTax(50000.0, "CO");
System.out.println("Tax: " + tax); // Calls .NET, returns to Java

This enables Java applications to leverage .NET libraries without rewriting them in Java.

Overriding Java Proxies in .NET

Sometimes the default generated proxy doesn’t fit your needs. You might want to:

• Add caching to reduce cross-runtime calls.
• Convert types differently (e.g., return a .NET DateTimeOffset instead of the default mapping).
• Add logging or instrumentation around every call.
• Simplify the API — expose a smaller surface than the full Java class.

To override java proxies in .NET, you subclass the generated proxy:

public class CachedAnalytics : com.example.Analytics
{
    private readonly Dictionary<string, double> _cache = new();

public override double computeScore(string input)     {         if (_cache.TryGetValue(input, out var cached))             return cached;         var score = base.computeScore(input); // Calls Java         _cache[input] = score;         return score;     } }

The base.computeScore() call still goes to Java. Your override wraps it with .NET-side logic.

In practical terms, teams override java proxies in .NET to centralize cross-cutting concerns (caching, logging, retry, metrics) without touching upstream Java source.

Overriding .NET Proxies in Java

The same pattern works in reverse. To override .NET proxies in Java, extend the generated Java proxy class:

public class LoggingCalculator extends com.mycompany.dotnet.TaxCalculator {
    @Override
    public double computeTax(double income, String state) {
        System.out.println("Computing tax for " + state);
        double result = super.computeTax(income, state); // Calls .NET
        System.out.println("Result: " + result);
        return result;
    }
}

This is powerful for adding Java-side concerns (logging, retry logic, fallback behavior) without modifying the .NET source code.

Many teams override .NET proxies in Java when they need Java-native resilience behavior while keeping .NET libraries unchanged.

Comparison: Proxy Generation vs. Manual Interop

Decision Framework: When to Use Proxy Generation

Use this checklist:

Use proxy generation when:

• ✅ You’re integrating with a Java/NET library (not a service)
• ✅ You need to call many classes or methods
• ✅ Type safety and IntelliSense matter to your team
• ✅ Low latency is important
• ✅ You want to override or extend cross-runtime behavior

Use REST/gRPC when:

• ✅ The target code is already a deployed service
• ✅ Teams are in different organizations and can’t share a classpath
• ✅ You only need 1–2 operations
• ✅ You need independent scaling and deployment

Avoid raw JNI/P-Invoke unless:

• ✅ You have deep native interop expertise
• ✅ You need a single, simple function call
• ✅ You can accept the maintenance burden

For proxy-based integration, explore the JNBridgePro developer demos which show both .NET-to-Java and Java-to-.NET proxy scenarios.

Advanced Proxy Patterns

Callback proxies. JNBridgePro supports callbacks — Java code can call back into .NET through proxy interfaces. This enables event-driven patterns where a Java library notifies .NET listeners.

Partial proxy generation. You don’t have to proxy an entire library. Generate proxies only for the classes you need, reducing assembly size and compilation time.

Proxy regeneration on API change. When the underlying Java or .NET library updates, regenerate proxies. The tool detects API changes and updates the proxy layer. Your consuming code only needs modification if method signatures changed.

Multi-JAR proxy sets. Generate proxies from multiple JARs in a single pass, maintaining cross-JAR type references.

For general background on cross-runtime type systems, the ECMA-335 CLI specification defines the .NET type system, while Oracle’s JVM specification defines Java’s.

Troubleshooting Proxy Issues

Problem: Proxy generation fails with “class not found.”
Ensure all transitive dependencies are on the classpath. Java classes often depend on other JARs not included in your initial set.

Problem: Type mismatch at runtime.
Check that you’re using the correct proxy version. Regenerate proxies if the underlying library was updated.

Problem: Performance degrades with many small calls.
Batch operations where possible. Instead of calling getItem() in a loop 10,000 times, call a method that returns a collection.

Problem: Callbacks throw “proxy not registered.”
Ensure callback interfaces are included in the proxy generation step. Both sides must have proxy awareness of the callback types.

FAQ

Why would teams override java proxies in .NET or override .NET proxies in Java?

They do it to add local behavior (caching, logging, retries, policy checks) close to the consuming runtime without changing the source library in the other runtime.

How long does proxy generation take?

Seconds to minutes, depending on the number of classes. A typical library with 50–100 classes generates in under 30 seconds.

Can I generate proxies for third-party Java libraries I don’t have source code for?

Yes. Proxy generation works from bytecode (JAR files), not source code.

Do proxies work with Java generics?

Yes. JNBridgePro maps Java generics to .NET generics where possible, and uses raw types as a fallback for complex cases.

Can I version proxy assemblies independently from the source library?

Yes, but it’s recommended to regenerate proxies when the source library changes to avoid runtime errors from API mismatches.

What happens if I override a proxy method and the underlying Java API changes?

Your override still compiles if the method signature didn’t change. If the signature changed, the compiler will flag it.

Does proxy generation support Java annotations and .NET attributes?

Annotations/attributes are preserved in metadata where applicable. Custom annotations can be mapped via configuration.

Get started with proxy generation. Download JNBridgePro and generate your first proxy in minutes. See the overview or contact us for guidance.

Related Articles

Continue with these related Java/.NET integration guides:

• .NET to Java integration guide
• How to connect Java to C#
• Java .NET integration options compared
• JNBridgePro overview