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add support for dynamically loaded JNI
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‎src/djni.c

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#include <dlfcn.h>
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#include "djni.h"
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static JNI_GetDefaultJavaVMInitArgs_fnptr JNI_GetDefaultJavaVMInitArgs_fn;
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static JNI_CreateJavaVM_fnptr JNI_CreateJavaVM_fn;
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static JNI_GetCreatedJavaVMs_fnptr JNI_GetCreatedJavaVMs_fn;
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static void *jni_dl = 0;
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static const char *last_error = 0;
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static int load_sym(const char *sym, void **ptr) {
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void *v = dlsym(jni_dl, sym);
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if (!v) {
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last_error = dlerror();
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dlclose(jni_dl);
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jni_dl = 0;
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return -1;
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}
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*ptr = v;
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return 0;
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}
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/* path: path to libjvm
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returns: 0 = success, -1 = already loaded, -2 = dlopen error, -3 symbols not found */
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int djni_load(const char *path) {
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last_error = 0;
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if (jni_dl) return -1;
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jni_dl = dlopen(path, RTLD_LOCAL | RTLD_NOW);
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if (!jni_dl) {
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last_error = dlerror();
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return -2;
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}
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if (load_sym("JNI_GetDefaultJavaVMInitArgs", (void**) &JNI_GetDefaultJavaVMInitArgs_fn) ||
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load_sym("JNI_CreateJavaVM", (void**) &JNI_CreateJavaVM_fn) ||
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load_sym("JNI_GetCreatedJavaVMs", (void**) &JNI_GetCreatedJavaVMs_fn)) {
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JNI_GetDefaultJavaVMInitArgs_fn = 0;
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JNI_CreateJavaVM_fn = 0;
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JNI_GetCreatedJavaVMs_fn = 0;
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return -3;
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}
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return 0;
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}
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/* returns: 0 = success, -1 = noting loaded, -2 = unload failed */
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int djni_unload(void) {
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if (jni_dl) {
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if (dlclose(jni_dl))
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return -2;
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jni_dl = 0;
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return 0;
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}
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return -1;
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}
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/* The following are the "normal" JNI API calls which are routed to libjvm JNI API.
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If no JNI was loaded, they return -99 to distinguish it from JNI error codes. */
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jint JNI_GetDefaultJavaVMInitArgs(void *args) {
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return JNI_GetDefaultJavaVMInitArgs_fn ? JNI_GetDefaultJavaVMInitArgs_fn(args) : -99;
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}
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jint JNI_CreateJavaVM(JavaVM **pvm, void **penv, void *args) {
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return JNI_CreateJavaVM_fn ? JNI_CreateJavaVM_fn(pvm, penv, args) : -99;
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}
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jint JNI_GetCreatedJavaVMs(JavaVM **vmBuf, jsize bufLen, jsize *nVMs) {
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return JNI_GetCreatedJavaVMs_fn ? JNI_GetCreatedJavaVMs_fn(vmBuf, bufLen, nVMs) : -99;
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}
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/* define DJNI_MAIN_TEST to test the load + create + print version + destroy + unload cycle
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it will create an executable - just compile djni.c alone for this (not used in rJava) */
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#ifdef DJNI_MAIN_TEST
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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/* calls java.lang.System.getProperty("java.version") and returns the result or NULL
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if something went wrong. Any non-NULL returns must be free()d */
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static char *java_version(JNIEnv *eenv) {
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char *jver = 0;
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jclass sys = (*eenv)->FindClass(eenv, "java/lang/System");
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if (sys) {
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jmethodID mid = (*eenv)->GetStaticMethodID(eenv, sys, "getProperty", "(Ljava/lang/String;)Ljava/lang/String;");
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if (mid) {
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jstring jvs = (*eenv)->NewStringUTF(eenv, "java.version");
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if (jvs) {
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jobject jres = (*eenv)->CallStaticObjectMethod(eenv, sys, mid, jvs);
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(*eenv)->DeleteLocalRef(eenv, jvs);
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if (jres) {
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const char *str = (*eenv)->GetStringUTFChars(eenv, (jstring) jres, NULL);
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if (str) {
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jver = strdup(str);
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(*eenv)->ReleaseStringUTFChars(eenv, (jstring) jres, str);
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}
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(*eenv)->DeleteLocalRef(eenv, jres);
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}
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}
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}
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(*eenv)->DeleteLocalRef(eenv, sys);
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}
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return jver;
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}
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/* create a JVM, call java_version(), destroy the JVM */
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static int run_jvm_test(void) {
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JavaVM *jvm;
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JNIEnv *eenv;
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JavaVMOption vm_options[4];
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JavaVMInitArgs vm_args;
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vm_args.version = JNI_VERSION_1_2;
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if(JNI_GetDefaultJavaVMInitArgs(&vm_args) != JNI_OK) {
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fprintf(stderr, "JNI 1.2 or higher is required\n");
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return -1;
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}
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vm_args.ignoreUnrecognized = JNI_TRUE;
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vm_args.options = vm_options;
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vm_args.nOptions = 0;
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jint res = JNI_CreateJavaVM(&jvm,(void **)&eenv, &vm_args);
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if (res != 0) {
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fprintf(stderr, "Cannot create Java virtual machine (JNI_CreateJavaVM returned %d)\n", (int) res);
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return -1;
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}
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if (!eenv) {
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fprintf(stderr, "Cannot obtain JVM environment");
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return -1;
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}
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printf("JVM initialized, JNI version: %x\n", (*eenv)->GetVersion(eenv));
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{
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char *jver = java_version(eenv);
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if (jver) {
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printf("Java version: %s\n", jver);
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free(jver);
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}
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}
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/* this shouldn't be even needed, but was last-ditch effort - makes no difference */
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if ((*jvm)->DetachCurrentThread(jvm) != JNI_OK)
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fprintf(stderr, "FWIW: cannot detach thread.\n");
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/* destroy the VM */
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if ((*jvm)->DestroyJavaVM(jvm) != JNI_OK) {
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fprintf(stderr, "ERROR: cannot destroy VM\n");
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return -1;
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}
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printf("JVM destroyed.\n");
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return 0;
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}
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int test_jvm(const char *path) {
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int e = djni_load(path);
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if (e) {
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printf("Cannot load: %d (%s)\n", e, last_error ? last_error : "not a dl error");
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return 1;
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}
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printf("JVM Loaded!\n");
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if (!run_jvm_test()) {
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if (djni_unload())
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fprintf(stderr, "ERROR: cannot unload JNI (%s)\n", last_error ? last_error : "no information");
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else
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printf("JNI unloaded.\n");
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}
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return 0;
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}
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/* for debugging on macOS - prints all dynamic loads */
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#ifdef __APPLE__
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#include <mach-o/dyld.h>
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static void add_image(const struct mach_header* mh, intptr_t vmaddr_slide) {
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Dl_info DlInfo;
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dladdr(mh, &DlInfo);
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printf("LOADED: %s\n", DlInfo.dli_fname);
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}
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static void rm_image(const struct mach_header* mh, intptr_t vmaddr_slide) {
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Dl_info DlInfo;
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dladdr(mh, &DlInfo);
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printf("REMOVE: %s\n", DlInfo.dli_fname);
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}
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#endif
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int main(int ac, char **av) {
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#ifdef __APPLE__
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_dyld_register_func_for_add_image(add_image);
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_dyld_register_func_for_remove_image(rm_image);
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#endif
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test_jvm("/Library/Java/JavaVirtualMachines/temurin-17.jdk/Contents/Home/lib/server/libjvm.dylib");
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#ifdef SAME_VM /* option 1: try the same VM twice (will load but fail to create VM) */
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test_jvm("/Library/Java/JavaVirtualMachines/temurin-17.jdk/Contents/Home/lib/server/libjvm.dylib");
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#else /* option 2: will even fail to load */
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test_jvm("/Library/Java/JavaVirtualMachines/temurin-11.jdk/Contents/Home/lib/server/libjvm.dylib");
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#endif
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return 0;
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}
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#endif

‎src/djni.h

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/* Dynamic JNI
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Allows to load JVM dynamically into a process.
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(C)2024 Simon Urbanek <[email protected]>
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License: LGPL-2.1 or MIT
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*/
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#ifndef DJNI_H__
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#define DJNI_H__
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#include "jni.h"
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int djni_load(const char *path);
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int djni_unload(void);
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jint JNI_GetDefaultJavaVMInitArgs(void *args);
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jint JNI_CreateJavaVM(JavaVM **pvm, void **penv, void *args);
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jint JNI_GetCreatedJavaVMs(JavaVM **vmBuf, jsize bufLen, jsize *nVMs);
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#endif

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