Log.java

源码目录：frameworks/base/core/java/android/util
编译目录：frameworks/base

我惊奇的发现，Log.java里没有定义println_native方法，也没有System.loadLibary任何的so库。带着这个疑问，我们来看c层代码。

android_util_Log.cpp

源码目录：frameworks/base/core/jni
编译目录：frameworks/base/core/jni

/*
 * JNI registration.
 */
static JNINativeMethod gMethods[] = {
    /* name, signature, funcPtr */
    { "isLoggable",      "(Ljava/lang/String;I)Z", (void*) android_util_Log_isLoggable },
    { "println_native",  "(IILjava/lang/String;Ljava/lang/String;)I", (void*) android_util_Log_println_native },
};

int register_android_util_Log(JNIEnv* env)
{
    jclass clazz = env->FindClass("android/util/Log");

    if (clazz == NULL) {
        ALOGE("Can't find android/util/Log");
        return -1;
    }

    levels.verbose = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "VERBOSE", "I"));
    levels.debug = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "DEBUG", "I"));
    levels.info = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "INFO", "I"));
    levels.warn = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "WARN", "I"));
    levels.error = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ERROR", "I"));
    levels.assert = env->GetStaticIntField(clazz, env->GetStaticFieldID(clazz, "ASSERT", "I"));

    return AndroidRuntime::registerNativeMethods(env, "android/util/Log", gMethods, NELEM(gMethods));
}

看了https://blog.csdn.net/flydream0/article/details/7371692这篇博客才知道，这种是显示注册native方法的一种方式。 android源码里基本都采用这种方式来注册native方法。