哪种方法最终调用JNI_OnLoad

Question

我试图弄清楚如何在内部调用JNI_OnLoad.我最终想出了下面的教程,但它没有说明什么代码部分实际上调用JNI_OnLoad作为内部函数调用.请帮我找到明确调用JNI_OnLoad的链接函数.我观察到System.loadLibrary调用Runtime,它再次调用Classloader.但仍无法找到原生链接.

我对OnLoad.cpp中的那个特别感兴趣(android/platform_frameworks_base/blob/master/services/jni/onload.cpp)

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 Painters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);
        }

我现在明白Runtime.loadlibrary是用Dalvik_java_lang_Runtime_nativeLoad本机函数重载的,并且不会引发Classloader.loadlibrary.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.

最相关的功能是这一个:

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

这是图书馆所在的地方dlopen().最有趣的部分:

    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;

正如您所看到的,JNI_OnLoad只需dlsym()使用返回的指针进行解析和调用.这段代码的其余部分是检查返回的值JNI_OnLoad,没有什么真正令人兴奋的.

我相信对于其他虚拟机看起来应该看起来几乎相同 - 所以只需要grep dlopen()和dlsym()- 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,Android.mk在同一目录中编译并将此文件链接到libandroid_servers共享库.这个库名称的左侧显示了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,在Android的系统服务启动的上下文中执行库的加载(以及对JNI_OnLoad()in 的调用onload.cpp).如果您想了解有关系统服务加载方式/时间的更多信息,我建议您使用此演示文稿.