Android JNI的学习经历

理解JNI
Java测MediaScanner的分析
- 静态注册：静态方法是根据函数名字建立java 和jni之间的关联
- 动态注册的原理：java native 函数直接调用jni函数的函数指针：JNINativeMethod

理解JNI

jni是java 和c/c++ 代码调用的桥梁
Java（MediaScanner）
|
jni (libmedia_jni.so)
|
Native(libmedia.so)

Java测MediaScanner的分析

public class MediaScanner implements AutoCloseable {
static {
System.loadLibrary(“media_jni”);//加载对应的jni so
native_init();
}

静态注册：静态方法是根据函数名字建立java 和jni之间的关联

// 关键字 native  申明一个个Native_init  函数体是在jni完成
private static native final void native_init();

// 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)
{
ALOGV(“native_init”);
jclass clazz = env->FindClass(kClassMediaScanner);
if (clazz == NULL) {
return;
}

fields.context = env->GetFieldID(clazz, "mNativeContext", "J");
if (fields.context == NULL) {return;
}

}

缺点：
1 每个包含 native class 都会生成一个对应的头文件
2 javah 生成的文件文件名很长
3 第一次启动，速率比较慢
动态注册

静态注册的原理：javah 生成.c文件，从java函数调用jni函数首先是根据名称来对应jni函数建立关联，然后 java native 函数通过函数指针来和jni函数建立关系

动态注册的原理：java native 函数直接调用jni函数的函数指针：JNINativeMethod

static const JNINativeMethod gMethods[] = {
{
“processDirectory”,
“(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
},

};

JNIEnv 功能是，保存了java测的函数指针，和java 对象
对象由成员函数和成员变量组成，
这就对应了两个函数
jmethodID GetMethodID(jclass clazz, const char* name, const char* sig)
jfieldID GetFieldID(jclass clazz, const char* name, const char* sig)
取出 jmethodID：
jclass mediaScannerClientInterface = env->FindClass(kClassMediaScannerClient);
mScanFileMethodID = env->GetMethodID(
mediaScannerClientInterface,
“scanFile”,
“(Ljava/lang/String;JJZZ)V”);

mHandleStringTagMethodID = env->GetMethodID(mediaScannerClientInterface,"handleStringTag","(Ljava/lang/String;Ljava/lang/String;)V");mSetMimeTypeMethodID = env->GetMethodID(mediaScannerClientInterface,"setMimeType","(Ljava/lang/String;)V");

使用 jmethodID （CallVoidMethod）

virtual status_t scanFile(const char* path, long long lastModified,
long long fileSize, bool isDirectory, bool noMedia)
{
ALOGV(“scanFile: path(%s), time(%lld), size(%lld) and isDir(%d)”,
path, lastModified, fileSize, isDirectory);

   jstring pathStr;if ((pathStr = mEnv->NewStringUTF(path)) == NULL) {mEnv->ExceptionClear();return NO_MEMORY;}mEnv->CallVoidMethod(mClient, mScanFileMethodID, pathStr, lastModified,fileSize, isDirectory, noMedia);mEnv->DeleteLocalRef(pathStr);return checkAndClearExceptionFromCallback(mEnv, "scanFile");

}
NativeType CallMethod(JNIEnv *env, jObject obj. jmethodID methodID,…)
NativeType CallStaticMethod(JNIEnv *env, jObject obj. jmethodID methodID,…)

以下是基于动态注册总结：
1 java 测调用native 和native调用java，都要经过jni
2 java 调用native，直接通过JMethodNative
3 native调用java测，可以使用jmethodID和jfieldID 组合使用。