一. 简要
在Android中,JNI主要用于实现一些性能较高的功能,如图像处理、音频处理、视频处理等。同时,JNI也可以用于实现一些特殊的功能,如与硬件交互、与系统服务交互等。
二. Java层
在某个对象中添加如下代码,例如我在/frameworks/base/services/core/java/com/android/server/keepalive/KeepAliveManagerService.java中去使用native方法
public class KeepAliveManagerService extends IKeepAliveManager.Stub {private static final String TAG = "KeepAliveManagerService leilei";private final Context mContext;private final ActivityManagerService mActivityManagerService;private WindowManagerService mWindowManagerService;private boolean mSystemReady = false;**public static native int resumeNative(int v);****public static native int pauseNative(int v);****public static native String stopNative(String v);**// 应用保活方法,简单的调用了三个native方法:resumeNative,pauseNative,stopNative@Overridepublic boolean keepAliveApplicationByPackage(String packageName) throws RemoteException {Log.d(TAG, "keepAliveApplicationByPackage.packageName:"+packageName);if (TextUtils.isEmpty(packageName) || mActivityManagerService == null|| mContext == null || !mSystemReady){return false;}int s = resumeNative(1);int s1 = pauseNative(2);String s2 = stopNative("leilei");Log.d(TAG, "leilei keepAliveApplicationByPackage: onResumeNative:"+s);Log.d(TAG, "leilei keepAliveApplicationByPackage: onPauseNative:"+s1);Log.d(TAG, "leilei keepAliveApplicationByPackage: stopNative:"+s2);int curCallingUid = Binder.getCallingUid();return keepAliveApplicationByPackage(packageName,curCallingUid);}
}
至此java层的代码就写好了
三. C/C++层
-
JNI文件创建
因为我写jni方法是需要在我的service对象里使用,所以我frameworks/base/services/core/jni/目录下创建为需要的文件,如下代码所示
#include <string> #include <dlfcn.h> #include <pthread.h> #include <chrono> #include <thread> #include <jni.h> #include <nativehelper/JNIHelp.h> #include <android/binder_manager.h> #include <android/binder_stability.h> #include <android/hidl/manager/1.2/IServiceManager.h> #include <binder/IServiceManager.h> #include <hidl/HidlTransportSupport.h> #include <incremental_service.h> #include <memtrackproxy/MemtrackProxy.h> #include <schedulerservice/SchedulingPolicyService.h> #include <sensorservicehidl/SensorManager.h> #include <stats/StatsAidl.h> #include <stats/StatsHal.h> #include <bionic/malloc.h> #include <bionic/reserved_signals.h> #include <android-base/properties.h> #include <utils/Log.h> #include <utils/misc.h> #include <utils/AndroidThreads.h> #ifdef LOG_TAG #undef LOG_TAG #define LOG_TAG "leilei" #endif// jni静态注册 /*extern "C" jstring Java_com_android_server_keepalive_KeepAliveManagerService_onResumeNative(JNIEnv *env, jclass thiz, jlong value) {// 进行本地处理,生成返回值std::string hello = "Hello from C++";jstring result = env->NewStringUTF(hello.c_str());return result; }*/// jni动态注册 namespace android { static jint pauseNative(JNIEnv* env, jobject thiz, jint value){ALOGD("The leilei message is onPauseNative %d:",value);return value; }static jint resumeNative(JNIEnv *env, jobject thiz, jint value){ALOGD("The leilei message is onResumeNative %d:",value);return value; }static jstring stopNative(JNIEnv *env, jobject thiz,jstring value){const char* ptr = env->GetStringUTFChars(value, NULL);ALOGD("The leilei message is stopNative %s:",ptr);return value; }// 对应native的方法注册表 static const JNINativeMethod gKeepAliveManagerMethods[] = {/* name, signature, funcPtr */{"pauseNative","(I)I",(void *)pauseNative},{"resumeNative","(I)I",(void *)resumeNative},{"stopNative","(Ljava/lang/String;)Ljava/lang/String;",(void*) stopNative}, };// 自己实现一个跟jniRegisterNativeMethods一样的功能 int registerNativeMethods(JNIEnv *env,std::string name,const JNINativeMethod *methods) {// 反射拿到java对象jclass klass = env->FindClass(name.c_str());if (klass == NULL) {return -1;}// 第一个参数:反射拿到的对象// 第二个参数:类中的native方法--注册表// 第三个参数:native方法对象的个数return env->RegisterNatives(klass, methods,3); }// onload.cpp中调用了JNI_OnLoad,然后调用了register_android_server_KeepAliveManager进行注册 // jniRegisterNativeMethods对RegisterNatives封装了,所以可以很方便的使用,我们手动来实现一下 // JNI_OnLoad是jni.h中的对象,只有调用JNI_OnLoad和RegisterNatives才是动态注册 int register_android_server_KeepAliveManager(JNIEnv* env) {// return jniRegisterNativeMethods(env, "com/android/server/keepalive/KeepAliveManagerService",gKeepAliveManagerMethods, NELEM(gKeepAliveManagerMethods));return registerNativeMethods(env,"com/android/server/keepalive/KeepAliveManagerService",gKeepAliveManagerMethods); } };
文件名称必须规范:com.android.server.keepalive.KeepAliveManagerService.cpp,由包名+类名组成,然后实现对应上层的native方法即可,方法名称最好相同(也可以不同,只要在注册函数的第一个参数中对应起来就行),例如下方法,三个参数中,前两个参数必须有,而且不能变—>JNIEnv和jobject:
static jint pauseNative(JNIEnv* env, jobject thiz, jint value){... }
然后就是开始动态注册jni方法,如下代码所示,由于aosp已经封装好了
jniRegisterNativeMethods
方法可以直接使用来注册jni方法,但是为了更深刻的理解,我们手动来实现registerNativeMethods
// native方法注册表 static const JNINativeMethod gKeepAliveManagerMethods[] = {/* name, signature, funcPtr */{"pauseNative","(I)I",(void *)pauseNative},{"resumeNative","(I)I",(void *)resumeNative},{"stopNative","(Ljava/lang/String;)Ljava/lang/String;",(void*) stopNative}, };// 自己实现一个跟register_android_server_KeepAliveManager一样的功能 int registerNativeMethods(JNIEnv *env,std::string name,const JNINativeMethod *methods) {jclass klass = env->FindClass(name.c_str());if (klass == NULL) {return -1;}// 第一个参数:反射拿到的对象// 第二个参数:类中的native方法--注册表// 第三个参数:native方法对象的个数return env->RegisterNatives(klass, methods,3); }int register_android_server_KeepAliveManager(JNIEnv* env) {// return jniRegisterNativeMethods(env, "com/android/server/keepalive/KeepAliveManagerService",gKeepAliveManagerMethods, NELEM(gKeepAliveManagerMethods));return registerNativeMethods(env,"com/android/server/keepalive/KeepAliveManagerService",gKeepAliveManagerMethods); }
先实现一个native方法注册表,代表需要对应java层native方法,返回的对象是
JNINativeMethod
,该对象属于jni.h里的,结构如下typedef struct {const char* name;const char* signature;void* fnPtr; } JNINativeMethod;
第一个参数对应了java native方法的名称,第二个参数代表native方法里面的参数和返回值,第三个参数代表jni方法
回到
registerNativeMethods
方法,主要就是通过env的FindClass反射获取Java对象,然后通过RegisterNatives(klass, methods,3);
进行注册即可,第二个参数就是native方法gKeepAliveManagerMethods
注册表**思考一下:**既然需要注册jni,那么调用
register_android_server_KeepAliveManager
函数的注册的入口又在哪里?下文分析 -
JNI文件引入和注册流程
-
android.bp引入编译
需要让我们的jni文件参与编译,需要在frameworks/base/services/core/jni/Android.bp中添加该文件,如下所示
cc_library_static {name: "libservices.core",defaults: ["libservices.core-libs"],cpp_std: "c++2a",cflags: ["-Wall","-Werror","-Wno-unused-parameter","-Wthread-safety","-DEGL_EGLEXT_PROTOTYPES","-DGL_GLEXT_PROTOTYPES",],srcs: ["BroadcastRadio/JavaRef.cpp","BroadcastRadio/NativeCallbackThread.cpp","BroadcastRadio/BroadcastRadioService.cpp","BroadcastRadio/Tuner.cpp","BroadcastRadio/TunerCallback.cpp","BroadcastRadio/convert.cpp","BroadcastRadio/regions.cpp","gnss/GnssConfiguration.cpp","gnss/GnssMeasurement.cpp","gnss/GnssMeasurementCallback.cpp","gnss/Utils.cpp","stats/SurfaceFlingerPuller.cpp",**"com.android.server.keepalive.KeepAliveManagerService.cpp",**include_dirs: ["frameworks/base/libs","frameworks/native/services","system/gatekeeper/include","system/memory/libmeminfo/include",],header_libs: ["bionic_libc_platform_headers",], }
在此模块添加**“com.android.server.keepalive.KeepAliveManagerService.cpp”**,即可,模块名为
libservices.core
,会生成对应的so库 -
JNI注册入口声明
在frameworks中,上文分析了如何调用jni注册native方法,但是调用注册的入口在哪里?就是通过frameworks/base/services/core/jni/onload.cpp文件进行调用的,需要在此文件中声明我们的注册入口,如下代码所示
namespace android { int register_android_server_BatteryStatsService(JNIEnv* env); int register_android_server_ConsumerIrService(JNIEnv *env); int register_android_server_InputManager(JNIEnv* env); **int register_android_server_KeepAliveManager(JNIEnv* env);** int register_android_server_LightsService(JNIEnv* env); int register_android_server_PowerManagerService(JNIEnv* env); int register_android_server_PowerStatsService(JNIEnv* env); int register_android_server_HintManagerService(JNIEnv* env); int register_android_server_storage_AppFuse(JNIEnv* env); } extern "C" jint JNI_OnLoad(JavaVM* vm, void* /* reserved */) {JNIEnv* env = NULL;jint result = -1;if (vm->GetEnv((void**) &env, JNI_VERSION_1_4) != JNI_OK) {ALOGE("GetEnv failed!");return result;}ALOG_ASSERT(env, "Could not retrieve the env!");register_android_server_broadcastradio_BroadcastRadioService(env);register_android_server_broadcastradio_Tuner(vm, env);register_android_server_PowerManagerService(env);register_android_server_PowerStatsService(env);register_android_server_HintManagerService(env);register_android_server_SerialService(env);register_android_server_InputManager(env);**register_android_server_KeepAliveManager(env);}** }
只需要在
namespace android
中声明注册入口函数**register_android_server_KeepAliveManager
**,此函数在我们创建的jni文件中会实现。然后在
JNI_OnLoad
函数中添加**register_android_server_KeepAliveManager(env);
**,目的是为了方法可以被正确调用,以及传递了env对象(jni里的东西),再来看一遍我创建的jni文件,frameworks/base/services/core/jni/com.android.server.keepalive.KeepAliveManagerService.cpp... int **register_android_server_KeepAliveManager**(JNIEnv* env) {// return jniRegisterNativeMethods(env, "com/android/server/keepalive/KeepAliveManagerService",gKeepAliveManagerMethods, NELEM(gKeepAliveManagerMethods));return registerNativeMethods(env,"com/android/server/keepalive/KeepAliveManagerService",gKeepAliveManagerMethods); } };
**思考一下:**为什么需要在
extern "C" jint JNI_OnLoad(JavaVM* vm, void* /* reserved */)
方法中调用jni注册逻辑呢?这涉及到jni动态注册原理了,后面再分析,jni分为静态注册和动态注册 -
JNI注册表分析
Andoird 中使用了一种不同传统Java JNI的方式来定义其native的函数。其中很重要区别是Andorid使用了一种Java 和 C 函数的映射表数组,并在其中描述了函数的参数和返回值。这个数组的类型是JNINativeMethod——该结构体位于
jni.h
中,结构如下typedef struct {const char* name;const char* signature;void* fnPtr; } JNINativeMethod;
三个参数代表着:native方法名称,签名—用字符串是描述了Java中函数的参数和返回值,jni函数对象-指向了java的native方法
具体用法如下
static const JNINativeMethod gKeepAliveManagerMethods[] = {/* name, signature, funcPtr */{"pauseNative","(I)I",(void *)pauseNative},{"resumeNative","(I)I",(void *)resumeNative},{"stopNative","(Ljava/lang/String;)Ljava/lang/String;",(void*) stopNative}, };
第三个参数前面必须带有(void *),这里主要分析第二个参数,()代表native方法的参数,()外面部分代表着返回值,I代表着java的int,jni的jint,具体如下
字符 c/c++类型 Java类型 V void void Z jboolean boolean I jint int J jlong long D jdouble double F jfloat float B jbyte byte C jchar char S jshort short 以上都是基本数据类,如果是数组,则用[代表,如整型数值
[I
来表示,具体如下名称 c/c++类型 Java类型 [I jintArray int[] [F jfloatArray float[] [B jbyteArray byte[] [C jcharArray char[] [S jshortArray short[] [D jdoubleArray double[] [J jlongArray long[] [Z jbooleanArray boolean[] 那如果native参数中是对象呢,需要用如下方法表示—参数解释:
// 参数解释 "()" 中的字符表示参数,小括号后面的则代表返回值。 "()V" 就表示native void Fun(); "(II)V" 表示native void Func(int a, int b);参数是俩个整型。 "(Ljava/lang/String;)Ljava/lang/String;" 就表示native Sting Func(String value);
所以如果要用对象作为参数或者返回值,需要在前面加个”L”,中间是用”/" 隔开,后面跟包名和类名,以及分号即可。如果是对象数组,则在前面加个
[
即可
-