About

This is a repository for deep into Android OS. While I read source code, I would write down some comment to clear the Android architecture and principle. Welcome to add pull requests to help me.

Android Source Code

This repository hold complete Android source code(Froyo). It can be compiled and run image file in emulator.

Reading Code Notes

JNI(Java Native Interface)

It makes Java methods and Native mehtods can call each other.

• Example

Java(MediaScanner) <---> JNI(libmeida_jni.so) <---> Native(libmedia.so)

JNI must implemented by dynamic library

/* 
* MediaScanner.java
*
*/

public class MediaScanner
{
    static {
        System.loadLibrary("media_jni");
        native_init();
    }

    ...


    private static native final void native_init();

    ...
}

This function is JNI implementation of native_init

/*
* android_media_Media_Scanner.cpp 
*
*/


// This function gets a field ID, which in turn causes class initialization.
// It is called from a static block in MediaScanner, which won't run until the
// first time an instance of this class is used.
static void
android_media_MediaScanner_native_init(JNIEnv *env)
{
     jclass clazz;

    clazz = env->FindClass("android/media/MediaScanner");
    if (clazz == NULL) {
        jniThrowException(env, "java/lang/RuntimeException", "Can't find android/media/MediaScanner");
        return;
    }

    fields.context = env->GetFieldID(clazz, "mNativeContext", "I");
    if (fields.context == NULL) {
        jniThrowException(env, "java/lang/RuntimeException", "Can't find MediaScanner.mNativeContext");
        return;
    }
}

But, how know Java method native_init's countpart is android_media_MediaScanner_native_init?

• Reigster JNI function

  • Static register

  • Dynamic register

JNINativeMethod

typedef struct { 
    const char* name; // native function name in Java 
    const char* signature; // Java method's signature, include parameters type and return value type
    void*       fnPtr; // function pointer in JNI layer
} JNINativeMethod;

This array store the relationship of Java method and native function

static JNINativeMethod gMethods[] = {
    {"processDirectory",  "(Ljava/lang/String;Ljava/lang/String;Landroid/media/MediaScannerClient;)V",    
                                                        (void *)android_media_MediaScanner_processDirectory},
    {"processFile",       "(Ljava/lang/String;Ljava/lang/String;Landroid/media/MediaScannerClient;)V",    
                                                        (void *)android_media_MediaScanner_processFile},
    {"setLocale",         "(Ljava/lang/String;)V",      (void *)android_media_MediaScanner_setLocale},
    {"extractAlbumArt",   "(Ljava/io/FileDescriptor;)[B",     (void *)android_media_MediaScanner_extractAlbumArt},
    {"native_init",        "()V",                      (void *)android_media_MediaScanner_native_init},
    {"native_setup",        "()V",                      (void *)android_media_MediaScanner_native_setup},
    {"native_finalize",     "()V",                      (void *)android_media_MediaScanner_native_finalize},
};

• JNI_OnLoad function

When java layer load JNI dynamic library finish, then call JNI_OnLoad function, the dynamic register complete here.

• JNIEnv - A structure stand for JNI environment relate to thread

  • Operate jobject through JNIEnv