The Design of Callback between Java and Native

Overall Module Stack

This article is about the design of callback between Java and native modules.  There is no doubt that JNI is the key man. The sample design is referenced from Multimedia framework.

Below figure is the event path from native modules to java. Let’s see the detailed process.

Init Media JNI

First step is to initialize media JNI. In android_media_MediaPlayer_native_init, JNI getspostEventFromNavtive defined in MediaPlayer (java) and assigns it topost_event of fields_t structure. JNI module will use post_event to dispatch media events to Java layer MediaPlayer in JNIMediaPlayerListener::notify.

Create JNI MediaPlayer Listener

Since MediaPlayer (java) requires to receive media events from native module, it is kind of a listener. Android registers MediaPlayer (java) as events listener in JNI duringnative_setup. A wrapper class JNIMediaPlayerListener was designed to wrap the weak reference of MediaPlayer (java) object. Actually,  JNIMediaPlayerListener is the instance registered in native MedaiPlayer (cpp). Native media events are sent to MediaPlayer (java) via JNIMediaPlayerListener. Let’s take a look of it as below UML and code snippet. The member variables ofmClass and mObjectare assigned as MediaPlayer (java) class andWeakReference of MediaPlayer (java) respectively.

  Below is the code snippet of native_setup implementation.

Notification flow of native media events

Finally, we can trace how a event was sent from MediaPlayer (cpp) to MediaPlayer (Java).