哪种方法最终调用JNI_OnLoad

Question

我试图弄清楚JNI_OnLoad在内部是如何被调用的。我最终理解了下面的教程，但是它并没有说明什么代码部分实际上将JNI_OnLoad称为内部函数调用。请帮助我找到明确调用JNI_OnLoad的链接函数。我观察到System.loadLibrary调用运行时，运行时再次调用Classloader。但仍然找不到本机链接。

我对(android/platform_frameworks_base/blob/master/services/jni/onload.cpp)的OnLoad.cpp特别感兴趣

JNI_OnLoad

jint JNI_OnLoad(JavaVM *vm, void *reserved);

The VM calls JNI_OnLoad when the native library is loaded (for example, through 
System.loadLibrary). JNI_OnLoad must return the JNI version needed by the native library.
In order to use any of the new JNI functions, a native library must export a JNI_OnLoad function that returns JNI_VERSION_1_2. If the native library does not export a JNI_OnLoad function, the VM assumes that the library only requires JNI version JNI_VERSION_1_1. If the VM does not recognize the version number returned by JNI_OnLoad, the native library cannot be loaded.

编辑：基于@Code响应的文件跟踪如下：

       System.loadLibrary("android_servers");
            |
            |The call System.loadLibrary(name) is effectively equivalent
            |  to the call
            |
            V
        Runtime.getRuntime().loadLibrary(name)
            |
            |public static Runtime getRuntime() {
            |        return currentRuntime;}
            |
            | // Here, also,Classloader.loadlibrary is called, 
            | but this is over-ridden (?)
            | by the Native function of Runtime.java below
            V
        /dalvik/vm/native/java_lang_Runtime.cpp (The jni native
        implementation of Runtime.java):
        /*
         * static String nativeLoad(String filename, ClassLoader loader)
         *
         * Load the specified full path as a dynamic library filled with
         * JNI-compatible methods. Returns null on success, or a failure
         * message on failure.
         */
        static void Dalvik_java_lang_Runtime_nativeLoad{
        //
        success = dvmLoadNativeCode(fileName, classLoader, &reason);
        }

我现在了解到Runtime.loadlibrary重载了Dalvik_java_lang_Runtime_nativeLoad本机函数，而Classloader.loadlibrary没有被调用。如果我错了，请纠正我。

Answer 1

对于Android，您应该查看dalvik/vm/Native.c，它定义了JNI接口。

最相关的职能是：

bool dvmLoadNativeCode(const char* pathName, Object* classLoader);

这里是图书馆dlopen()-ed的地方。其中最有趣的部分是：

    vonLoad = dlsym(handle, "JNI_OnLoad");
    if (vonLoad == NULL) {
        LOGD("No JNI_OnLoad found in %s %p\n", pathName, classLoader);
    } else {
        /*
         * Call JNI_OnLoad.  We have to override the current class
         * loader, which will always be "null" since the stuff at the
         * top of the stack is around Runtime.loadLibrary().  (See
         * the comments in the JNI FindClass function.)
         */
        OnLoadFunc func = vonLoad;
        Object* prevOverride = self->classLoaderOverride;

        self->classLoaderOverride = classLoader;
        oldStatus = dvmChangeStatus(self, THREAD_NATIVE);
        LOGV("+++ calling JNI_OnLoad(%s)\n", pathName);
        version = (*func)(gDvm.vmList, NULL);
        dvmChangeStatus(self, oldStatus);
        self->classLoaderOverride = prevOverride;

如您所见，JNI_OnLoad只需使用dlsym()进行解析，并使用返回的指针进行调用。这个代码部分的其余部分是检查JNI_OnLoad返回的值，没有什么是真正令人兴奋的。

我认为其他VM的外观应该差不多--所以只有dlopen()和dlsym()的grep --毕竟，它只是简单的共享库加载和符号解析。

编辑:说到您提到的确切文件，同一个目录中的Android.mk编译并将该文件链接到libandroid_servers共享库中。浏览这个库名会发现services/java/com/android/server/SystemServer.java。

什么是相关的：

public static void main(String[] args) {
    // ...
    System.loadLibrary("android_servers");
    // ...
}

因此，加载库(以及在onload.cpp中调用onload.cpp)是在Android的系统服务启动上下文中执行的。如果您想更多地了解如何/何时加载系统服务，我建议使用这份报告。