聲明

• 前階段在項目中使用了Android的JNI技術，在此文中做些技術知識總結。
• 本文參考了一些書籍的若干章節，比如《Android進階解密-第9章-JNI原理》、《深入理解Android虛擬機-第4章-分析JNI》、《深入理解Android系統-第2章-分析JNI》、《Android NDK Beginner_'s Guide》等
• 本文使用的代碼時LineageOS的cm-14.1，對應Android 7.1.2，可以參考我的另一篇博客：cm-14.1 Android系統啟動過程分析（〇）-如何下載Nexus5的LineageOS14.1(cm-14.1)系統源碼并編譯、刷機

1 Java層和JNI層的數據類型轉換

????進入到源碼路徑：
vim ~/LineageOS/frameworks/base/media/jni/android_
media_MediaRecorder.cpp，查看函數android_media_MediaRecorder_native_setup:

static void android_media_MediaRecorder_native_setup(JNIEnv *env, jobject thiz, jobject weak_this,jstring packageName, jstring opPackageName) {ALOGV("setup");ScopedUtfChars opPackageNameStr(env, opPackageName);sp<MediaRecorder> mr = new MediaRecorder(String16(opPackageNameStr.c_str()));if (mr == NULL) {jniThrowException(env, "java/lang/RuntimeException", "Out of memory");return;}if (mr->initCheck() != NO_ERROR) {jniThrowException(env, "java/lang/RuntimeException", "Unable to initialize media recorder");return;}// create new listener and give it to MediaRecordersp<JNIMediaRecorderListener> listener = new JNIMediaRecorderListener(env, thiz, weak_this);mr->setListener(listener);// Convert client name jstring to String16const char16_t *rawClientName = reinterpret_cast<const char16_t*>(env->GetStringChars(packageName, NULL));jsize rawClientNameLen = env->GetStringLength(packageName);String16 clientName(rawClientName, rawClientNameLen);env->ReleaseStringChars(packageName,reinterpret_cast<const jchar*>(rawClientName));// pass client package name for permissions trackingmr->setClientName(clientName);setMediaRecorder(env, thiz, mr); }

????其中jobject、jstring類型參數都是JNI層的數據類型，Java層數據類型到JNI層就要轉換為JNI層數據結構。包括基本數據類型和引用數據類型

1.1 Java層至JNI層基本數據類型的轉換

Java類型JNI類型C++類型關系描述簽名占內存大小

boolean	jboolean	unsigned char 或 unit8_t	布爾類型	Z	1
int	jint	int或long	整形	I	4
float	jfloat	float	單精度類型	F	4
double	jdouble	double	雙精度類型	D	8
long	jlong	long	長整形	J	8
short	jshort	short	短整型	S	2
char	jchar	unsigned short	字符	C	2
byte	jbyte	signed char	字節類型	B	1
void	void	void	空類型	V

????從轉換表中可以看出Java層基本數據類型轉換到JNI層只需將數據類型前加個“j”即可（除了void類型）。

1.2 Java層至JNI層引用數據類型的轉換

JavaNative簽名(以；結尾)

所有對象	jobject	L+classname +;
Class	jclass	Ljava/lang/Clas;
String	jstring	Ljava/lang/String;
Trowable	jthrowable	Ljava/lang/Throwable;
Object[]	jobjectArray	[L+classname +;
byte[]	jbyteArray	[B
char[]	jcharArray	[C
double[]	jdoubleArray	[D
float[]	jfloatArray	[F
int[]	jintArray	[I
short[]	jshortArray	[S
long[]	jlongArray	[J
boolean[]	jbooleanArray	[Z

????以~/LineageOS/frameworks/base/media/java/android/media/
MediaRecorder.java中的native_setup方法為例：

private native final void native_setup(Object mediarecorder_this,String clientName, String opPackageName) throws IllegalStateException;

????對應在~/LineageOS/frameworks/base/media/jni/android_
media_MediaRecorder.cpp，查看函數android_media_MediaRecorder_native_setup:

static void android_media_MediaRecorder_native_setup(JNIEnv *env, jobject thiz, jobject weak_this,jstring packageName, jstring opPackageName) { ...省略n行... }

????可以發現：Object類型轉換為jobject類型，String類型轉換為jstring類型。

2 方法簽名

????進入到源碼路徑： vim ~/LineageOS/frameworks/base/media/jni/
android_media_MediaRecorder.cpp，在數組gMethods[]中可看到簽名信息：

static const JNINativeMethod gMethods[] = {{"setCamera", "(Landroid/hardware/Camera;)V", (void *)android_media_MediaRecorder_setCamera},{"setVideoSource", "(I)V", (void *)android_media_MediaRecorder_setVideoSource},{"setAudioSource", "(I)V", (void *)android_media_MediaRecorder_setAudioSource},{"setOutputFormat", "(I)V", (void *)android_media_MediaRecorder_setOutputFormat},{"setVideoEncoder", "(I)V", (void *)android_media_MediaRecorder_setVideoEncoder},{"setAudioEncoder", "(I)V", (void *)android_media_MediaRecorder_setAudioEncoder},{"setParameter", "(Ljava/lang/String;)V", (void *)android_media_MediaRecorder_setParameter},{"_setOutputFile", "(Ljava/io/FileDescriptor;JJ)V", (void *)android_media_MediaRecorder_setOutputFileFD},{"setVideoSize", "(II)V", (void *)android_media_MediaRecorder_setVideoSize},{"setVideoFrameRate", "(I)V", (void *)android_media_MediaRecorder_setVideoFrameRate},{"setMaxDuration", "(I)V", (void *)android_media_MediaRecorder_setMaxDuration},{"setMaxFileSize", "(J)V", (void *)android_media_MediaRecorder_setMaxFileSize},{"_prepare", "()V", (void *)android_media_MediaRecorder_prepare},{"getSurface", "()Landroid/view/Surface;", (void *)android_media_MediaRecorder_getSurface},{"getMaxAmplitude", "()I", (void *)android_media_MediaRecorder_native_getMaxAmplitude},{"start", "()V", (void *)android_media_MediaRecorder_start},{"stop", "()V", (void *)android_media_MediaRecorder_stop},{"pause", "()V", (void *)android_media_MediaRecorder_pause},{"resume", "()V", (void *)android_media_MediaRecorder_resume},{"native_reset", "()V", (void *)android_media_MediaRecorder_native_reset},{"release", "()V", (void *)android_media_MediaRecorder_release},{"native_init", "()V", (void *)android_media_MediaRecorder_native_init},{"native_setup", "(Ljava/lang/Object;Ljava/lang/String;Ljava/lang/String;)V",(void *)android_media_MediaRecorder_native_setup},{"native_finalize", "()V", (void *)android_media_MediaRecorder_native_finalize},{"native_setInputSurface", "(Landroid/view/Surface;)V", (void *)android_media_MediaRecorder_setInputSurface }, };

????簡單地說，存在簽名的原因就是Java語言的方法是可以重載的，重載的方法名字相同而參數不同，所以JNI僅通過方法名無法確定對應的是重載的哪個方法，必須要參數簽名來輔助其關聯。

避免博客拖太長，后續參見下篇Android系統的JNI原理分析（三）- JNIEnv

總結

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