這一節的主要內容是講如何在native程序中嵌入一個java VM。一個java的實現通常是作爲一個native庫運行。native程序可以和這個庫鏈接,並且用調用接口來加載java VM。事實上,JDK或者java2 sdk的標準啓動命令,就是一個簡單鏈接了java VM的c程序。該啓動器,解析命令行參數、加載VM、並且通過調用接口運行java程序。
創建java虛擬機
先看一C程序,它加載一個java VM並且調用Prog.main方法:下面是C的代碼:public class Prog { public static void main(String[] args) { System.out.println("Hello World " + args[0]); } }
#include <jni.h> #define PATH_SEPARATOR ';' /* define it to be ':' on Solaris */ #define USER_CLASSPATH "." /* where Prog.class is */ main() { JNIEnv *env; JavaVM *jvm; jint res; jclass cls; jmethodID mid; jstring jstr; jclass stringClass; jobjectArray args; #ifdef JNI_VERSION_1_2 JavaVMInitArgs vm_args; JavaVMOption options[1]; options[0].optionString = "-Djava.class.path=" USER_CLASSPATH; vm_args.version = 0x00010002; vm_args.options = options; vm_args.nOptions = 1; vm_args.ignoreUnrecognized = JNI_TRUE; /* Create the Java VM */ res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args); #else JDK1_1InitArgs vm_args; char classpath[1024]; vm_args.version = 0x00010001; JNI_GetDefaultJavaVMInitArgs(&vm_args); /* Append USER_CLASSPATH to the default system class path */ sprintf(classpath, "%s%c%s", vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH); vm_args.classpath = classpath; /* Create the Java VM */ res = JNI_CreateJavaVM(&jvm, &env, &vm_args); #endif /* JNI_VERSION_1_2 */ if (res < 0) { fprintf(stderr, "Can't create Java VM\n"); exit(1); } cls = (*env)->FindClass(env, "Prog"); if (cls == NULL) { goto destroy; } mid = (*env)->GetStaticMethodID(env, cls, "main", "([Ljava/lang/String;)V"); if (mid == NULL) { goto destroy; } jstr = (*env)->NewStringUTF(env, " from C!"); if (jstr == NULL) { goto destroy; } stringClass = (*env)->FindClass(env, "java/lang/String"); args = (*env)->NewObjectArray(env, 1, stringClass, jstr); if (args == NULL) { goto destroy; } (*env)->CallStaticVoidMethod(env, cls, mid, args); destroy: if ((*env)->ExceptionOccurred(env)) { (*env)->ExceptionDescribe(env); } (*jvm)->DestroyJavaVM(jvm); }
和java VM一起鏈接native程序
如何和java VM鏈接,依賴於native程序對準備知部署於一個特定的java VM還是和各種VM實現一起工作。和一個已知的java VM鏈接
例如和上訴代碼invoke.c一起鏈接:1)在linux:cc -I<jni.h dir> -L<libjava.so dir> -lthread -ljava invoke.c-lthread選項表示需要native線程支持。-java選項指定了libjava.so是linux中java VM實現的動態庫。2)在win32:cl -I<jni.h dir> -MD invoke.c -link <javai.lib dir>\javai.lib-MD選項保證了native程序時和win32多線程C庫一起鏈接的。和未知java VM 鏈接
可以以動態的方式和未知的java VM鏈接在一起。這樣一套程序,就可以在不同的vm實現上執行了。這個方案的核心是,在運行時加載動態鏈接庫。(和前面提到的鏈接一個已知的java vm,這兩種方式,其實就是如何使用動態庫的兩種方式)。例如爲一個創建java VM的 函數尋找一個入口點:/* Win32 version */ void *JNU_FindCreateJavaVM(char *vmlibpath) { /* load the dynamical libraries */ HINSTANCE hVM = LoadLibrary(vmlibpath); if (hVM == NULL) { return NULL; } /* get the address of function "JNI_CreateJavaVM" */ return GetProcAddress(hVM, "JNI_CreateJavaVM"); } /* linux version */ void *JNU_FindCreateJavaVM(char *vmlibpath) { /* load the dynamical libraries */ void *libVM = dlopen(vmlibpath, RTLD_LAZY); if (libVM == NULL) { return NULL; } /* get the address of function "JNI_CreateJavaVM" */ return dlsym(libVM, "JNI_CreateJavaVM"); }
關聯native 線程
假設你有一個用native C實現的多線程web server程序。當HTTP請求進來時, 服務端,創建多個native線程來併發地處理請求。我們可能希望在服務中嵌入一個java虛擬機,這樣,同一時刻可以有多個線程在這個虛擬機上進行操作。例如:服務器產生的native方法的生命週期,可能比java VM要短,於是我們要有一種能夠將線程附加到已經存在的java VM上的方法,使得VM能夠執行附加了的native線程的JNI調用。並且在完成後分離該線程,而不中斷其他附加的線程。下面例子展示了,如何將一個native線程通過調用接口來附加到java VM上:/* Note: This program only works on Win32 */ #include <windows.h> #include <jni.h> JavaVM *jvm; /* The virtual machine instance */ #define PATH_SEPARATOR ';' #define USER_CLASSPATH "." /* where Prog.class is */ void thread_fun(void *arg) { jint res; jclass cls; jmethodID mid; jstring jstr; jclass stringClass; jobjectArray args; JNIEnv *env; char buf[100]; int threadNum = (int)arg; /* Pass NULL as the third argument */ #ifdef JNI_VERSION_1_2 res = (*jvm)->AttachCurrentThread(jvm, (void**)&env, NULL); #else res = (*jvm)->AttachCurrentThread(jvm, &env, NULL); #endif if (res < 0) { fprintf(stderr, "Attach failed\n"); return; } cls = (*env)->FindClass(env, "Prog"); if (cls == NULL) { goto detach; } mid = (*env)->GetStaticMethodID(env, cls, "main", "([Ljava/lang/String;)V"); if (mid == NULL) { goto detach; } sprintf(buf, " from Thread %d", threadNum); jstr = (*env)->NewStringUTF(env, buf); if (jstr == NULL) { goto detach; } stringClass = (*env)->FindClass(env, "java/lang/String"); args = (*env)->NewObjectArray(env, 1, stringClass, jstr); if (args == NULL) { goto detach; } (*env)->CallStaticVoidMethod(env, cls, mid, args); detach: if ((*env)->ExceptionOccurred(env)) { (*env)->ExceptionDescribe(env); } (*jvm)->DetachCurrentThread(jvm); } main() { JNIEnv *env; int i; jint res; #ifdef JNI_VERSION_1_2 JavaVMInitArgs vm_args; JavaVMOption options[1]; options[0].optionString = "-Djava.class.path=" USER_CLASSPATH; vm_args.version = 0x00010002; vm_args.options = options; vm_args.nOptions = 1; vm_args.ignoreUnrecognized = TRUE; /* Create the Java VM */ res = JNI_CreateJavaVM(&jvm, (void**)&env, &vm_args); #else JDK1_1InitArgs vm_args; char classpath[1024]; vm_args.version = 0x00010001; JNI_GetDefaultJavaVMInitArgs(&vm_args); /* Append USER_CLASSPATH to the default system class path */ sprintf(classpath, "%s%c%s",vm_args.classpath, PATH_SEPARATOR, USER_CLASSPATH); vm_args.classpath = classpath; /* Create the Java VM */ res = JNI_CreateJavaVM(&jvm, &env, &vm_args); #endif /* JNI_VERSION_1_2 */ if (res < 0) { fprintf(stderr, "Can't create Java VM\n"); exit(1); } for (i = 0; i < 5; i++) /* We pass the thread number to every thread */ _beginthread(thread_fun, 0, (void *)i); Sleep(1000); /* wait for threads to start */ (*jvm)->DestroyJavaVM(jvm); }
執行結果:Hello World from thread 1 Hello World from thread 0 Hello World from thread 4 Hello World from thread 2 Hello World from thread 3
JNI學習筆記(八)—— 調用接口
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