Registration method of native method in JNI

JNI in native Methods registration

jni yes Java Native Interface Abbreviation ,Java Local interface , yes Java With other languages （native Language ） A bridge of communication
Native Method registration is divided into static registration （ be chiefly used in NDK Development ） And dynamic registration （ be chiefly used in framework Development ）

native Method static registration

1. AndroidStudio Create a new one in lib package , This example follows MediaRecorder.java give an example ,

public class MediaRecorder {
    static {
        System.loadLibrary("media_jni");
        //Native Method static registration , According to the method name Java Methods and JNI Function to establish association 
        native_init();
    }
    private static native final void native_init();
    public native void start() throws  IllegalStateException;
}

2. Enter the terminal to execute the command under the current package path ：

javac com/by/jnilib/MediaRecorder.java
javah com.by.jnilib.MediaRecorder

3. jni Method definitions in the document

/*
 * Native Methods are statically registered. According to the method name Java Methods and JNI Function to establish association 
 * Class:     com_by_jnilib_MediaRecorder
 * Method:    native_init
 * Signature: ()V  Java Method signature information   because Java Method overload , ( Parameter type ) return type  V representative void,
 *  Method signature reason ：  because Java Method overload , ( Parameter type ) return type  V representative void,
 * com_by_jnilib_MediaRecorder_native_1init  Package name + Class name + Method name ,
 * Java Of native_init The method contains _ Turn into jni The method will become _1
 * JNIEnv *  The pointer can be in native In the world of Java The code in , Only valid in the thread that created it , Cannot pass... Across threads 
 * jclass  yes JNI Data type of , Corresponding Java Medium class class 
 */
JNIEXPORT void JNICALL Java_com_by_jnilib_MediaRecorder_native_1init
  (JNIEnv *, jclass);

native Method dynamic registration

Reference system MediaRecorder Registration method
JNI There is a structure for recording Java in native Methods and JNI The connection of methods , Namely JNINativeMethod

1. Definition JNINativeMethod Type of gMethods Array , What is stored inside is MediaRecorder Of Native Methods and JNI The correspondence of layer functions .
   
2. gMethods Array in register_android_media_MediaRecoder Method is called , This register_android_media_MediaRecoder The method is in JNI_OnLoad Call in method , And back to registerNativeMethods Method ,JNI_OnLoad The method is described above System.loadLibrary(“media_jni”) After the method is executed , also JNI_OnLoad The registration of the entire multimedia framework is invoked in the function. JNINativeMethod Array functions , Finally by registerNativeMethods Method returns jniRegisterNativeMethods Function RegisterNatives Function completion JNI Dynamic registration of . So the order is ：
   System.loadLibrary ——> JNI_OnLoad ——> register_android_media_MediaRecoder——> return registerNativeMethods ——> return jniRegisterNativeMethods function ——> call RegisterNatives（…,gMethods,…） Function complete registration .
   
   

Data type conversion

Method signature ：

because Java Method overload , That is, the method name is the same , Parameters and returns are different ;JNI You can't find it just by the method name Java The corresponding method in

/*
 * Class:     com_by_jnilib_MediaRecorder
 * Method:    start
 * Signature: ()V Java Method signature information  ( Parameter type ) return type  V representative void
 */

Use javap Automatic generation of method signature

public class MediaRecorder {
    static {
        System.loadLibrary("media_jni");
        //Native Method static registration , According to the method name Java Methods and JNI Function to establish association 
        native_init();
    }
    private static native final void native_init();
    public native void start() throws  IllegalStateException;
    //javap Command test auto generate method signature 
    private native final void native_setup(Object media_recorder_this,
                                           String clientName,String opPackageName) throws IllegalStateException;
}

javac -encoding UTF-8 MediaRecorder.java
javap -s -p MediaRecorder.class

-s Represents the output internal type signature ,p Indicates that all methods and members are printed （ The default is public member ）

JNIEnv

JNIEnv yes Native In the world Java The representative of the environment , adopt JNIEnv * The pointer can be in native Visit... In the world Java The code in （ call Java Method , operation Java Variables and objects in ） To operate , Only valid in the thread that created it , Cannot pass... Across threads , Different JNIEnv It's independent of each other .
PS： There is only one virtual machine process JavaVM