Android中的NDK（二）

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JNI Reference

其实是对jobject的分类

    局部引用 LocalRef 本地方法栈内有效     全局引用 GlobalRef 虚拟机全局生效 不会被GC回收     若全局引用 WeakGlobalRef  虚拟机全局生效 GC时被回收

以下全都是局部引用：

这里写图片描述

局部引用最大数量为512个，可能会产生内部泄漏
建议采用下面这种方式

    (*env)->PushLocalFrame(env,3);    jobject localobj1 =            (*env)->NewLocalRef(env,message);    jobject localobj2 =            (*env)->NewLocalRef(env,message);    jobject localObj3 =            (*env)->NewLocalRef(env,message);    jobject localObj4 =            (*env)->NewLocalRef(env,message);    (*env)->PopLocalFrame(env,NULL);

GlobalReference

    //声明全局引用    jclass cls = (*env)->FindClass(env,"android/os/Message");    globalCls = (*env)->NewGlobalRef(env, cls);    //删除全局引用         (*jvm)->GetEnv(jvm,(void**)&env,JNI_VERSION_1_6);     if(globalCls!=NULL) {         (*env)->DeleteGlobalRef(env, globalCls);     }

    //全局化field和methodId    whatField = (*env)->GetFieldID(env,cls,"what","I");    obtainMethod =(*env)->GetStaticMethodID                (env,cls, "obtain","()Landroid/os/Message;");

初始化

jint JNI_OnLoad(JavaVM* jvm, void* reserved) {    JNIEnv* env = NULL;    if((*jvm)->GetEnv(jvm,(void**)&env,JNI_VERSION_1_6)!=JNI_OK){        return JNI_ERR;    }    //声明全局引用    jclass cls = (*env)->FindClass(env,"android/os/Message");    globalCls = (*env)->NewGlobalRef(env, cls);    //全局化field和methodId    whatField = (*env)->GetFieldID(env,cls,"what","I");    obtainMethod =(*env)->GetStaticMethodID                (env,cls, "obtain","()Landroid/os/Message;");    jclass jnDemoClass = (*env)->FindClass(env,classPathName);    //通过registerNatives调用    (*env)->RegisterNatives(env,jnDemoClass,methods,                            sizeof(methods)/sizeof(methods[0]));    return JNI_VERSION_1_6;}

 void  JNI_OnUnload(JavaVM* jvm, void* reserved){     JNIEnv* env = NULL;     (*jvm)->GetEnv(jvm,(void**)&env,JNI_VERSION_1_6);     if(globalCls!=NULL) {         (*env)->DeleteGlobalRef(env, globalCls);     }     whatField= NULL;     obtainMethod = NULL; }

registerNative

//创建一个jni与java的关联表static JNINativeMethod methods[] = {    {"changeMessageWhatInNative","(Landroid/os/Message;)V",            (void*)changeMessageWhatInNative },    {"getMessageFromNative","()Landroid/os/Message;",            (void*)getMessageFromNative },};    //通过registerNatives调用,一次registerNative只能注册一个class    (*env)->RegisterNatives(env,jnDemoClass,methods,                            sizeof(methods)/sizeof(methods[0]));

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