Calling out to C code from Java

public class CallCFromJava {
    static { System.loadLibrary("strlen-jni"); }
    public static native long strlen(String s);
    public static void main(String[] args) {
        long ret = strlen("Bambi");
        System.out.println("Return value " + ret); // 5
    }
}

// Run javac -h to generate the .h file, then write C:
// #include "CallCFromJava.h"
// #include <string.h>
// JNIEXPORT jlong JNICALL Java_CallCFromJava_strlen(
//     JNIEnv *env, jclass clazz, jstring str) {
//     const char* s = (*env)->GetStringUTFChars(env, str, NULL);
//     jlong len = (jlong) strlen(s);
//     (*env)->ReleaseStringUTFChars(env, str, s);
//     return len;
// }

void main() throws Throwable {
    try (var arena = Arena.ofConfined()) {
        // Use any system library directly — no C wrapper needed
        var stdlib = Linker.nativeLinker().defaultLookup();
        var foreignFuncAddr = stdlib.find("strlen").orElseThrow();
        var strlenSig = FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS);
        var strlenMethod = Linker.nativeLinker() .downcallHandle(foreignFuncAddr, strlenSig);
        var ret = (long) strlenMethod.invokeExact(arena.allocateFrom("Bambi"));
        System.out.println("Return value " + ret); // 5
    }
}

// Your own C library needs no special Java annotations:
// long greet(char* name) {
//     printf("Hello %s\n", name);
//     return 0;
// }

👁

C code stays plain C

The C function requires no JNI annotations or JNIEnv boilerplate — any existing C library can be called as-is.

⚡

More flexible

Directly call most existing C/C++ libraries without writing adapter code or generating header files.

🛠️

Easier workflow

No need to stop, run javac -h, and implement the interface defined in the generated .h file.