Android-jni Summary (1)

1>>JNI structure (Java code >jni Code >c/c++ code)

2>> a Mediascanner invocation example

1.java layer: (Load function library name is configured by. mk file)

public class mediascanner{    static {        system.loadlibrary ("Media_jni");//Load Class Library        native_init ();//Call JNI layer Code    }       private static native final void Native_init ();}

2. JNI Layer: Static registration (function name to package name _ Class Name _ Method name)

Static Voidandroid_media_mediascanner_native_init (jnienv *env)//jni code Implementation {     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;}    }

Static registration ***********************************

Java files:

/** * @author Lean  @date: 2014-11-29   */public class Getstr {static{system.loadlibrary ("Jnisample");} public static native String testget ();}

>> get jni. h Code files

Generate the. h file, call Javah-jni Packagename.classname

/* Don't EDIT this file-it are machine generated */#include <jni.h>/* Header for class com_example_jnisample_getst R */#ifndef _included_com_example_jnisample_getstr#define _included_com_example_jnisample_getstr#ifdef __ Cplusplusextern "C" {#endif */* Class:     com_example_jnisample_getstr * Method:    testget * Signature: () Ljava/lang /string; */jniexport jstring jnicall java_com_example_jnisample_getstr_testget  (jnienv *, jclass); #ifdef __cplusplus}# Endif#endif

>> Generate. So correspondence

To write the Jni file, copy the Com_example_jnisample_getstr.h file function name to JNISample.cpp as follows:

#include <jni.h> #include "com_example_jnisample_getstr.h" extern "C" {#endif */* Class:     com_example_ JNISAMPLE_GETSTR * Method:    testget * Signature: () ljava/lang/string; */jniexport jstring Jnicall Java_com_example_ Jnisample_getstr_testget  (jnienv *env, Jclass) {return Env->newstringutf ("Hello");} #ifdef __cplusplus}

At this point, you can call into the JNI layer by calling the Java file and invoke the specific C + + code at the JNI layer.

*************************************** Dynamic Registration ***********************************

1. Disclaimer Gmethods (Jninativemethod is a struct that corresponds to a Java function and a JNI function one by one)

Static Jninativemethod gmethods[] = {{"Processdirectory", "(ljava/lang/string; ljava/lang/string; landroid/media/mediascannerclient;) V ", (void *) Android_media_me Diascanner_processdirectory}, {"Processfile", "(ljava/lang/string; ljava/lang/string; landroid/media/mediascannerclient;) V ", (void *) Android_media_me Diascanner_processfile}, {"SetLocale", "(ljava/lang/string;) V", (void *) Android_media_mediascanner_setloca    Le}, {"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},};

1-1.2nd parameter Method simplification of type signature

2. Actual invocation of the registration code:

This function was only registers the native methods, and was called from//Jni_onload in Android_media_mediaplayer.cppint re Gister_android_media_mediascanner (jnienv *env) {    return androidruntime::registernativemethods (env,                " Android/media/mediascanner ", Gmethods, Nelem (gmethods));

3. Declare the method in Android_media_mediaplayer.cpp and bind in Jni_onload, call Jni_onload () automatically after calling in System.loadlibrary ()

>>android_media_mediaplayer.cpp Statement

extern int Register_android_media_mediascanner (jnienv *env);

>>jni_onload () call

Jint jni_onload (javavm* vm, void* reserved) {jnienv* env = NULL;    Jint result =-1;    if (vm->getenv (void**) &env, jni_version_1_4)! = JNI_OK) {        LOGE ("error:getenv failed\n");        Goto bail;    }    ASSERT (env! = NULL);    if (Register_android_media_mediascanner (env) < 0) {        LOGE ("Error:mediascanner native registration failed\n");        Goto bail;    }    /* Success--Return valid version number */    result = Jni_version_1_4;bail:    return result;

3>> corresponding type conversions

4>>jnienv (a thread-related JNI environment struct)

1) jnienv operation Jobject equivalent to Operation Jfieldid and Jmethodid

Get Methodid

Call menv operation (Call<type>method) methodid

Get FieldID

5>> garbage collection mechanism Variable life cycle (global reference Local reference weak global Reference)

Global Reference:newglobalref declaration mclient (env->newglobalref (client))

deleteglobalref Destruction Menv->deleteglobalref (mclient);

Weak Global References: Special global references call Jnienv->issameoject before using to determine whether they are recycled

6>>jstring Mapping of String objects in Jni in Java

string of local strings to Java

Unicode:jnienv->newstring ();

Utf-8:jnienv->newstringutf ();

String -to-local strings for JAVA

Unicode:jnienv->getstringcharas ();

Utf-8:jnienv->Getstringutfcharas();

Release resources:

Unicode:jnienv->releasestringcharas ();

utf-8:jnienv->ReleaseStringutfcharas ();

Android-jni Summary (1)