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Title: Add "Calling out to C code from Java" pattern: JNI vs FFM API by Copilot · Pull Request #130 · javaevolved/javaevolved.github.io · GitHub

Open Graph Title: Add "Calling out to C code from Java" pattern: JNI vs FFM API by Copilot · Pull Request #130 · javaevolved/javaevolved.github.io

X Title: Add "Calling out to C code from Java" pattern: JNI vs FFM API by Copilot · Pull Request #130 · javaevolved/javaevolved.github.io

Description: Adds a new advanced language pattern contrasting the legacy JNI approach with the modern Foreign Function & Memory (FFM) API introduced in Java 22, highlighting how FFM eliminates the need for C-side Java awareness entirely. New pattern: content/language/call-c-from-java.yaml Both the old and modern sides call strlen("Bambi") and print "Return value 5", making them directly comparable: Old (JNI): declare native long strlen(String s) → run javac -h → write a C wrapper using JNI conventions (JNIEnv*, GetStringUTFChars, ReleaseStringUTFChars, etc.) → compile and link a .so Modern (FFM): call strlen directly from the C stdlib via Linker, Arena, and MethodHandle — no C code required at all The comparison highlights that JNI can't even call a simple stdlib function without a C wrapper, while FFM calls it directly: void main() throws Throwable { try (var arena = Arena.ofConfined()) { 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")); IO.println("Return value " + ret); // 5 } } Navigation chain compact-canonical-constructor → call-c-from-java → enterprise/servlet-vs-jaxrs Proof script: proof/language/CallCFromJava.java Calls strlen via FFM; uses //JAVA_OPTIONS --enable-native-access=ALL-UNNAMED Translations Added for all 12 non-English locales: de, es, pt-BR, zh-CN, ar, fr, ja, ko, bn, it, pl, tr Technical terms (JNI, FFM, javac, JNIEnv, MethodHandle, Arena, JDK 22) are preserved untranslated in all locales Original prompt This section details on the original issue you should resolve [Pattern] ### Category language Slug call-c-from-java Title Calling out to C code from Java Difficulty advanced Since JDK 22 Summary Simplifies C/C++ code to be called from Java, allowing most existing libraries to be used without adapters or cumbersome C code modification. Old code label Java 1.1+ Old code public class CallCFromJava { static { System.loadLibrary("hello-jni"); } public static native int greet(String name); public static void main(String[] args) { int ret = greet("Bambi"); System.out.println("Return value " + ret); } } Then run javac -h, then view the generated .h file, then write your .c file: #include "CallCFromJava.h" JNIEXPORT int JNICALL Java_CallCFromJava_greet(JNIEnv *env, jclass this, jstring str) { const char* name = (*env)->GetStringUTFChars(env, str, NULL); printf("Hello %s\n", name); return 0; } Modern code label Java 22+ Modern code public class CallCFromJava { void main() throws Throwable { try (Arena arena = Arena.ofConfined()) { SymbolLookup ourLib = SymbolLookup.libraryLookup("libhello-ffi.so", arena); Optional segment = ourLib.find("greet"); MemorySegment foreignFuncAddr = segment.get(); FunctionDescriptor greet_sig= // return and input parameter FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS); MethodHandle greetMethod = Linker.nativeLinker().downcallHandle(foreignFuncAddr, greet_sig); var ret = (long) greetMethod.invokeExact(arena.allocateFrom("Bambi")); System.out.println("Return value " + ret); } } } C Code: int greet(char* name) { printf("Hello %s\n", name); return 0; } Explanation Java has two techniques for interfacing between Java and C/C++ code, the traditional JNI and the newer FFM. With JNI you simply declare your method as native, but then you have to run javac -h to generate a C-language .h (header) file, then have your C code implement that, and deal with the slightly cumbersome JNI C-language API. FFM simplifies the C code to the point that it doesn't have to know anything about Java. This makes it much easier to interface with existing C/C++-language libraries. It does take a few more lines of Java code, but the net effect is more readable and more maintainable. Compare the C code in both versions and you'll see the difference! Minimal runnable demo can be found at https://github.com/IanDarwin/javaevolved-java_callling_c Why the modern way wins 👁 Better readability — C code doesn't have to learn Java! ⚡ More flexible - can directly call most existing C/C++ libraries ⚡ Easier - don't have to stop, run javac -h, and implement the "interface" defined in .h file. Comments on the Issue (you are @copilot in this section) @brunoborges @IanDarwin what do you think of this code snippet for the Modern side? void main() throws Throwable { try (var arena = Arena.ofConfined()) { // Use a system library to prove FFM compiles and links var stdlib = Linker.nativeLinker().defaultLookup(); var foreignFuncAddr = stdlib.find("strlen").orElseThrow(); var strlen_sig = FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS); var strlenMethod = Linker.nativeLinker().downcallHandle(foreignFuncAddr, strlen_sig); var ret = (long) strlenMethod.invokeExact(arena.allocateFrom("Bambi")); IO.println("Return value " + ret); } } Fixes #129 💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

Open Graph Description: Adds a new advanced language pattern contrasting the legacy JNI approach with the modern Foreign Function & Memory (FFM) API introduced in Java 22, highlighting how FFM eliminates the need for ...

X Description: Adds a new advanced language pattern contrasting the legacy JNI approach with the modern Foreign Function & Memory (FFM) API introduced in Java 22, highlighting how FFM eliminates the need ...

Opengraph URL: https://github.com/javaevolved/javaevolved.github.io/pull/130

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