JNINativeMethod结构体及 系统 JNI 接口的注册流程

zygote进程在启动ART虚拟机后,会有一个注册系统JNI接口的调用:

AndroidRuntime::start()中: 

.....
 if (startVm(&mJavaVM, &env) != 0) {
        return;
    }
    onVmCreated(env);

    /*
     * Register android functions.
     */
    if (startReg(env) < 0) {
        ALOGE("Unable to register all android natives\n");
        return;
    }
....

startReg(env)即是进行系统JNI接口的注册.

    if (register_jni_procs(gRegJNI, NELEM(gRegJNI), env) < 0) {
        env->PopLocalFrame(NULL);
        return -1;
    }

gRegJNI的定义如下:

static const RegJNIRec gRegJNI[] = {
    REG_JNI(register_com_android_internal_os_RuntimeInit),
    ......
    REG_JNI(register_android_os_Process),
    ......
}

register_jni_procs()的实现:

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 0;
}

startReg(), 调用register_jni_procs(), 通过for循环,遍历调用gRegJNI数组中的函数(通过指向函数的指针实现).

 

关于gRegJNI:

可以看到, gRegJNI是一个成员为RegJNIRec类型的数组.RegJNIRec的定义如下:

#ifdef NDEBUG
    #define REG_JNI(name)      { name }
    struct RegJNIRec {
        int (*mProc)(JNIEnv*);
    };
#else
    #define REG_JNI(name)      { name, #name }
    struct RegJNIRec {
        int (*mProc)(JNIEnv*);
        const char* mName;
    };
#endif

根据代码推断,宏开关NDEBUG是打开的,即RegJNIRec的实际定义为:

    #define REG_JNI(name)      { name }
    struct RegJNIRec {
        int (*mProc)(JNIEnv*);
    };

即,RegJNIRec是一个结构体,该结构体有一个指向函数的指针成员,且该指针指向的函数的参数类型为:JNIEnv*.

再看 gRegJNI[] 数组的一个成员:REG_JNI(register_android_os_Process),REG_JNI为宏定义, 宏替换后的代码应该为:

{ register_android_os_Process}

上面已经分析, 注册的过程,就是通过遍历gRegJNI[] 数组,调用其成员中的函数指针指向的函数来实现. 而:

{ register_android_os_Process}

就是其中的结构体的成员,该结构体的唯一的成员为一个函数指针:register_android_os_Process,跟踪代码,该函数位于android_util_Process.cpp中,完整定义:

int register_android_os_Process(JNIEnv* env)
{
    return RegisterMethodsOrDie(env, "android/os/Process", methods, NELEM(methods));
}

static inline int RegisterMethodsOrDie(JNIEnv* env, const char* className,
                                       const JNINativeMethod* gMethods, int numMethods) {
    int res = AndroidRuntime::registerNativeMethods(env, className, gMethods, numMethods);
    LOG_ALWAYS_FATAL_IF(res < 0, "Unable to register native methods.");
    return res;
}

根据文件名(android_util_Process)可以推断,该函数注册的是Process相关的JNI接口, 具体哪些接口,由RegisterMethodsOrDie()的参数:methods指定.显然, methods是一个结构体数组,且对应的结构体,就是  文章标题中提到的结构体: JNINativeMethod. 

 

该结构体对应一个JNI方法的相关信息

typedef struct {
    const char* name;     // java层对应的方法名称
    const char* signature;// 该方法的返回值类型和参数类型
    void*       fnPtr;    // native中对应的函数指针
} JNINativeMethod;

 

android_util_Process指定的结构体数组 methods为:

static const JNINativeMethod methods[] = {
    {"getUidForName",       "(Ljava/lang/String;)I", (void*)android_os_Process_getUidForName},
    {"getGidForName",       "(Ljava/lang/String;)I", (void*)android_os_Process_getGidForName},
    {"setThreadPriority",   "(II)V", (void*)android_os_Process_setThreadPriority},
    {"setThreadScheduler",  "(III)V", (void*)android_os_Process_setThreadScheduler},
    {"setCanSelfBackground", "(Z)V", (void*)android_os_Process_setCanSelfBackground},
    {"setThreadPriority",   "(I)V", (void*)android_os_Process_setCallingThreadPriority},
    {"getThreadPriority",   "(I)I", (void*)android_os_Process_getThreadPriority},
    {"setThreadGroup",      "(II)V", (void*)android_os_Process_setThreadGroup},
    {"setProcessGroup",     "(II)V", (void*)android_os_Process_setProcessGroup},
    {"getProcessGroup",     "(I)I", (void*)android_os_Process_getProcessGroup},
    {"setSwappiness",   "(IZ)Z", (void*)android_os_Process_setSwappiness},
    {"setArgV0",    "(Ljava/lang/String;)V", (void*)android_os_Process_setArgV0},
    {"setUid", "(I)I", (void*)android_os_Process_setUid},
    {"setGid", "(I)I", (void*)android_os_Process_setGid},
    {"sendSignal", "(II)V", (void*)android_os_Process_sendSignal},
    {"sendSignalQuiet", "(II)V", (void*)android_os_Process_sendSignalQuiet},
    {"getFreeMemory", "()J", (void*)android_os_Process_getFreeMemory},
    {"getTotalMemory", "()J", (void*)android_os_Process_getTotalMemory},
    {"readProcLines", "(Ljava/lang/String;[Ljava/lang/String;[J)V", (void*)android_os_Process_readProcLines},
    {"getPids", "(Ljava/lang/String;[I)[I", (void*)android_os_Process_getPids},
    {"readProcFile", "(Ljava/lang/String;[I[Ljava/lang/String;[J[F)Z", (void*)android_os_Process_readProcFile},
    {"parseProcLine", "([BII[I[Ljava/lang/String;[J[F)Z", (void*)android_os_Process_parseProcLine},
    {"getElapsedCpuTime", "()J", (void*)android_os_Process_getElapsedCpuTime},
    {"getPss", "(I)J", (void*)android_os_Process_getPss},
    {"getPidsForCommands", "([Ljava/lang/String;)[I", (void*)android_os_Process_getPidsForCommands},
    //{"setApplicationObject", "(Landroid/os/IBinder;)V", (void*)android_os_Process_setApplicationObject},
    {"killProcessGroup", "(II)I", (void*)android_os_Process_killProcessGroup},
    {"removeAllProcessGroups", "()V", (void*)android_os_Process_removeAllProcessGroups},
};

以上便可以看到该接口注册了哪些jni接口,相关接口在Java层便可通过jni调用,有些还是很眼熟的.

最终的注册工作，是调用 AndroidRuntime.registerNativeMethods(env, className, gmethods,  numMethods)来实现。

系统JNI接口的注册流程大致就是上面所述.