register_android_os_Binder

register_jni_procs(gRegJNI, NELEM(gRegJNI), env)开始注册jni方法。下面重点查看一下gRegJNI数组的register_android_os_Binder类型。

int register_android_os_Binder(JNIEnv* env) {
    if (int_register_android_os_Binder(env) < 0)
        return -1;
    if (int_register_android_os_BinderInternal(env) < 0)
        return -1;
    if (int_register_android_os_BinderProxy(env) < 0)
        return -1;

   ......
    return 0;
}

首先查看int_register_android_os_Binder

static int int_register_android_os_Binder(JNIEnv* env) {
    jclass clazz;
    // 查找android/os/Binder
    clazz = env->FindClass(kBinderPathName);
    // 将Java层Binder类保存到mClass变量
    gBinderOffsets.mClass = (jclass) env->NewGlobalRef(clazz);
    // 将Binder类的execTransact保存到mExecTransact变量
    gBinderOffsets.mExecTransact = env->GetMethodID(clazz, "execTransact", "(IIII)Z");
    assert(gBinderOffsets.mExecTransact);
    // 将Binder类的mObject保存到mObject变量
    gBinderOffsets.mObject = env->GetFieldID(clazz, "mObject", "I");
    assert(gBinderOffsets.mObject);
    return AndroidRuntime::registerNativeMethods(env, kBinderPathName, gBinderMethods, NELEM(gBinderMethods));
}

const char* const kBinderPathName = "android/os/Binder";

上述方法首先将Java层的Binder类的相关信息保存到gBinderOffsets结构体中，然后对jni方法进行注册。

static struct bindernative_offsets_t {
    jclass mClass;
    jmethodID mExecTransact;
    jfieldID mObject;

} gBinderOffsets;

gBinderOffsets保存好Binder类的信息后为jni层访问Java层提供通道，而且因为gBinderOffsets是全局静态结构体，提高了Binder的查询效率，属于空间换时间的做法。

static const JNINativeMethod gBinderMethods[] = {
     /* name, signature, funcPtr */
    { "getCallingPid", "()I", (void*)android_os_Binder_getCallingPid },
    { "getCallingUid", "()I", (void*)android_os_Binder_getCallingUid },
    { "clearCallingIdentity", "()J", (void*)android_os_Binder_clearCallingIdentity },
    { "restoreCallingIdentity", "(J)V", (void*)android_os_Binder_restoreCallingIdentity },
    { "setThreadStrictModePolicy", "(I)V", (void*)android_os_Binder_setThreadStrictModePolicy },
    { "getThreadStrictModePolicy", "()I", (void*)android_os_Binder_getThreadStrictModePolicy },
    { "flushPendingCommands", "()V", (void*)android_os_Binder_flushPendingCommands },
    { "init", "()V", (void*)android_os_Binder_init },
    { "destroy", "()V", (void*)android_os_Binder_destroy }
};

AndroidRuntime::registerNativeMethods将为gBinderMethods数组中的方法建立了一一映射关系，为Java层访问JNI层提供通道。

下面继续查看int_register_android_os_BinderInternal -------> android_util_Binder.cpp

static int int_register_android_os_BinderInternal(JNIEnv* env) {
    jclass clazz;
    // 查找com/android/internal/os/BinderInternal类
    clazz = env->FindClass(kBinderInternalPathName);
    // 将BinderInternal类保存到mClass对象中
    gBinderInternalOffsets.mClass = (jclass) env->NewGlobalRef(clazz);
    // 将BinderInternal类中的forceBinderGc方法保存在mForceGc变量
    gBinderInternalOffsets.mForceGc = env->GetStaticMethodID(clazz, "forceBinderGc", "()V");
    // 注册jni方法
    return AndroidRuntime::registerNativeMethods(env, kBinderInternalPathName,
        gBinderInternalMethods, NELEM(gBinderInternalMethods));
}

const char* const kBinderInternalPathName = "com/android/internal/os/BinderInternal";

上述方法将Java层的BinderInternal类保存在gBinderInternalOffsets结构体中，然后进行jni方法注册。

static struct binderinternal_offsets_t {
    jclass mClass;
    jmethodID mForceGc;
} gBinderInternalOffsets;

static const JNINativeMethod gBinderInternalMethods[] = {
     /* name, signature, funcPtr */
    { "getContextObject", "()Landroid/os/IBinder;", (void*)android_os_BinderInternal_getContextObject },
    { "joinThreadPool", "()V", (void*)android_os_BinderInternal_joinThreadPool },
    { "disableBackgroundScheduling", "(Z)V", (void*)android_os_BinderInternal_disableBackgroundScheduling },
    { "handleGc", "()V", (void*)android_os_BinderInternal_handleGc }
};

AndroidRuntime::registerNativeMethods将为gBinderInternalMethods数组中的方法建立了一一映射关系，为Java层访问JNI层提供通道。

下面继续查看int_register_android_os_BinderProxy

static int int_register_android_os_BinderProxy(JNIEnv* env)
{
    ......
    clazz = env->FindClass(kBinderProxyPathName);
    gBinderProxyOffsets.mClass = (jclass) env->NewGlobalRef(clazz);
    gBinderProxyOffsets.mConstructor = env->GetMethodID(clazz, "<init>", "()V");

    gBinderProxyOffsets.mSendDeathNotice
        = env->GetStaticMethodID(clazz, "sendDeathNotice", "(Landroid/os/IBinder$DeathRecipient;)V");
    gBinderProxyOffsets.mObject = env->GetFieldID(clazz, "mObject", "I");
    gBinderProxyOffsets.mSelf = env->GetFieldID(clazz, "mSelf", "Ljava/lang/ref/WeakReference;");
    gBinderProxyOffsets.mOrgue = env->GetFieldID(clazz, "mOrgue", "I");

    clazz = env->FindClass("java/lang/Class");
    gClassOffsets.mGetName = env->GetMethodID(clazz, "getName", "()Ljava/lang/String;");

    return AndroidRuntime::registerNativeMethods(env, kBinderProxyPathName,
        gBinderProxyMethods, NELEM(gBinderProxyMethods));
}

上述方法与前面分析的方法类似，就不在赘述啦。

static const JNINativeMethod gBinderProxyMethods[] = {
     /* name, signature, funcPtr */
    {"pingBinder",          "()Z", (void*)android_os_BinderProxy_pingBinder},
    {"isBinderAlive",       "()Z", (void*)android_os_BinderProxy_isBinderAlive},
    {"getInterfaceDescriptor", "()Ljava/lang/String;", (void*)android_os_BinderProxy_getInterfaceDescriptor},
    {"transactNative",       "(ILandroid/os/Parcel;Landroid/os/Parcel;I)Z", (void*)android_os_BinderProxy_transact},
    {"linkToDeath",         "(Landroid/os/IBinder$DeathRecipient;I)V", (void*)android_os_BinderProxy_linkToDeath},
    {"unlinkToDeath",       "(Landroid/os/IBinder$DeathRecipient;I)Z", (void*)android_os_BinderProxy_unlinkToDeath},
    {"destroy",             "()V", (void*)android_os_BinderProxy_destroy},
};

jni注册的BinderProxy中的方法如上数组所示。