Android JNI 原理分析

引言：分析Android源码6.0的过程，一定离不开Java与C/C++代码直接的来回跳转，那么就很有必要掌握JNI，这是链接Java层和Native层的桥梁，本文涉及相关源码：

frameworks/base/core/jni/AndroidRuntime.cpp  libcore/luni/src/main/java/java/lang/System.java libcore/luni/src/main/java/java/lang/Runtime.java libnativehelper/JNIHelp.cpp libnativehelper/include/nativehelper/jni.h  frameworks/base/core/java/android/os/MessageQueue.java frameworks/base/core/jni/android_os_MessageQueue.cpp  frameworks/base/core/java/android/os/Binder.java frameworks/base/core/jni/android_util_Binder.cpp  frameworks/base/media/java/android/media/MediaPlayer.java frameworks/base/media/jni/android_media_MediaPlayer.cpp 

int AndroidRuntime::startReg(JNIEnv* env) {     //设置线程创建方法为javaCreateThreadEtc     androidSetCreateThreadFunc((android_create_thread_fn) javaCreateThreadEtc);      env->PushLocalFrame(200);     //进程NI方法的注册     if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {         env->PopLocalFrame(NULL);         return -1;     }     env->PopLocalFrame(NULL);     return 0; } 

static int register_jni_procs(const RegJNIRec array[], size_t count, JNIEnv* env) {     for (size_t i = 0; i < count; i++) {         if (array[i].mProc(env) < 0) {             return -1;         }     }     return 0; } 

static const RegJNIRec gReggRegJNIJNI[] = {     REG_JNI(register_android_os_MessageQueue),     REG_JNI(register_android_os_Binder),     ... }; 

 private native void nativePollOnce(long ptr, int timeoutMillis); 

步骤1：

MessageQueue.java 的全限定名为android.os.MessageQueue.java，方法名：android.os.MessageQueue.nativePollOnce()，而相对应的native层方法名只是将点号替换为下划线，可得 android_os_MessageQueue_nativePollOnce() 。

Tips：nativePollOnce ==> android_os_MessageQueue_nativePollOnce()

步骤2：

有了native方法，那么接下来需要知道该native方法所在那个文件。前面已经介绍过Android系统启动时就已经注册了大量的JNI方法，见附录 gRegJNI 数组。这些注册方法命令方式：

register_[包名]_[类名] 

extern int register_android_os_MessageQueue(JNIEnv* env); 

[包名]_[类名].cpp [包名]_[类名].h 

static void android_os_MessageQueue_nativePollOnce(JNIEnv* env, jobject obj,         jlong ptr, jint timeoutMillis) {     NativeMessageQueue* nativeMessageQueue = reinterpret_cast<NativeMessageQueue*>(ptr);     nativeMessageQueue->pollOnce(env, obj, timeoutMillis); } 

public static final native int getCallingPid(); 

static jint android_os_Binder_getCallingPid(JNIEnv* env, jobject clazz) {     return IPCThreadState::self()->getCallingPid(); } 

public class MediaPlayer{     static {         System.loadLibrary("media_jni");         native_init();     }      private static native final void native_init();     ... } 

android_media_MediaPlayer.cpp android_media_MediaPlayer_native_init() 

  static void android_media_MediaPlayer_native_init(JNIEnv *env) {     jclass clazz;     clazz = env->FindClass("android/media/MediaPlayer");     fields.context = env->GetFieldID(clazz, "mNativeContext", "J");     ... } 

public static void loadLibrary(String libName) {     //接下来调用Runtime方法     Runtime.getRuntime().loadLibrary(libName, VMStack.getCallingClassLoader()); } 

void loadLibrary(String libraryName, ClassLoader loader) {     //loader不会空，则进入该分支     if (loader != null) {         //查找库所在路径         String filename = loader.findLibrary(libraryName);         if (filename == null) {             throw new UnsatisfiedLinkError(loader + " couldn't find /"" +                                            System.mapLibraryName(libraryName) + "/"");         }         //加载库         String error = doLoad(filename, loader);         if (error != null) {             throw new UnsatisfiedLinkError(error);         }         return;     }      //loader为空，则会进入该分支     String filename = System.mapLibraryName(libraryName);     List<String> candidates = new ArrayList<String>();     String lastError = null;     for (String directory : mLibPaths) {         String candidate = directory + filename;         candidates.add(candidate);         if (IoUtils.canOpenReadOnly(candidate)) {              //加载库             String error = doLoad(candidate, loader);             if (error == null) {                 return;//加载成功             }             lastError = error;         }     }     if (lastError != null) {         throw new UnsatisfiedLinkError(lastError);     }     throw new UnsatisfiedLinkError("Library " + libraryName + " not found; tried " + candidates); } 

private String doLoad(String name, ClassLoader loader) {     ...     synchronized (this) {         return nativeLoad(name, loader, ldLibraryPath);     } } 

jint JNI_OnLoad(JavaVM* vm, void* reserved) {     JNIEnv* env = NULL;     //【见3.3】 注册JNI方法      if (register_android_media_MediaPlayer(env) < 0) {         goto bail;     }     ... } 

static int register_android_media_MediaPlayer(JNIEnv *env) {     //【见3.4】     return AndroidRuntime::registerNativeMethods(env,                 "android/media/MediaPlayer", gMethods, NELEM(gMethods)); } 

static JNINativeMethod gMethods[] = {     {"prepare",      "()V",  (void *)android_media_MediaPlayer_prepare},     {"_start",       "()V",  (void *)android_media_MediaPlayer_start},     {"_stop",        "()V",  (void *)android_media_MediaPlayer_stop},     {"seekTo",       "(I)V", (void *)android_media_MediaPlayer_seekTo},     {"_release",     "()V",  (void *)android_media_MediaPlayer_release},     {"native_init",  "()V",  (void *)android_media_MediaPlayer_native_init},     ... }; 

typedef struct {     const char* name;  //Java层native函数名     const char* signature; //Java函数签名，记录参数类型和个数，以及返回值类型     void*       fnPtr; //Native层对应的函数指针 } JNINativeMethod; 

int AndroidRuntime::registerNativeMethods(JNIEnv* env,     const char* className, const JNINativeMethod* gMethods, int numMethods) {     //【见3.5】     return jniRegisterNativeMethods(env, className, gMethods, numMethods); } 

extern "C" int jniRegisterNativeMethods(C_JNIEnv* env, const char* className,     const JNINativeMethod* gMethods, int numMethods) {     JNIEnv* e = reinterpret_cast<JNIEnv*>(env);     scoped_local_ref<jclass> c(env, findClass(env, className));     if (c.get() == NULL) {         e->FatalError("");//无法查找native注册方法     }     //【见3.6】 调用JNIEnv结构体的成员变量     if ((*env)->RegisterNatives(e, c.get(), gMethods, numMethods) < 0) {         e->FatalError("");//native方法注册失败     }     return 0; } 

struct _JNIEnv {     const struct JNINativeInterface* functions;      jint RegisterNatives(jclass clazz, const JNINativeMethod* methods,             jint nMethods)     { return functions->RegisterNatives(this, clazz, methods, nMethods); }     ... } functions是指向`JNINativeInterface`结构体指针，也就是将调用下面方法：  struct JNINativeInterface {     jint (*RegisterNatives)(JNIEnv*, jclass, const JNINativeMethod*,jint);     ... }