René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"[Pattern]","articleBody":"### Category\n\nlanguage\n\n### Slug\n\ncall-c-from-java\n\n### Title\n\nCalling out to C code from Java\n\n### Difficulty\n\nadvanced\n\n### Since JDK\n\n22\n\n### Summary\n\nSimplifies C/C++ code to be called from Java, allowing most existing libraries to be used without adapters or cumbersome C code modification.\n\n### Old code label\n\nJava 1.1+\n\n### Old code\n\n```java\npublic class CallCFromJava {\n\tstatic { System.loadLibrary(\"hello-jni\"); }\n\tpublic static native int greet(String name);\n\tpublic static void main(String[] args) {\n\t\tint ret = greet(\"Bambi\");\n\t\tSystem.out.println(\"Return value \" + ret);\n\t}\n}\n\nThen run javac -h, then view the generated .h file, then write your .c file:\n\n#include \"CallCFromJava.h\"\n\nJNIEXPORT int JNICALL Java_CallCFromJava_greet(JNIEnv *env, jclass this, jstring str) {\n\tconst char* name = (*env)-\u003eGetStringUTFChars(env, str, NULL);\n\tprintf(\"Hello %s\\n\", name);\n\treturn 0;\n}\n```\n\n### Modern code label\n\nJava 22+\n\n### Modern code\n\n```java\npublic class CallCFromJava {\n  void main() throws Throwable {\n\n    try (Arena arena = Arena.ofConfined()) {\n      SymbolLookup ourLib = \n        SymbolLookup.libraryLookup(\"libhello-ffi.so\", arena);\n      Optional\u003cMemorySegment\u003e segment = ourLib.find(\"greet\");\n      MemorySegment foreignFuncAddr = segment.get();\n      FunctionDescriptor greet_sig= // return and input parameter\n          FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS);\n      MethodHandle greetMethod =\n        Linker.nativeLinker().downcallHandle(foreignFuncAddr, greet_sig);\n      var ret = (long) greetMethod.invokeExact(arena.allocateFrom(\"Bambi\"));\n      System.out.println(\"Return value \" + ret);\n    }\n  }\n}\n\nC Code:\nint greet(char* name) {\n    printf(\"Hello %s\\n\", name);\n    return 0;\n}\n```\n\n### Explanation\n\nJava 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. \n\nFFM 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!\n\nMinimal runnable demo can be found at https://github.com/IanDarwin/javaevolved-java_callling_c \n\n### Why the modern way wins\n\n👁 Better readability — C code doesn't have to learn Java!\n⚡ More flexible - can directly call most existing C/C++ libraries\n⚡ Easier - don't have to stop, run javac -h, and implement the \"interface\" defined in .h file.","author":{"url":"https://github.com/IanDarwin","@type":"Person","name":"IanDarwin"},"datePublished":"2026-03-02T14:15:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/129/javaevolved.github.io/issues/129"}