深入探索NDK与JNI技术的实践应用与学习路径

工程的目录结构如下&＃xff1a;

public class MainActivity extends Activity {/** Called when the activity is first created. */&＃64;Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.main);String[] argv&＃61;new String[]{"mylib","Gao","GaoMatrix"};Natives.libMain(argv);}
}

/*** &＃64;author GaoMatrix E-mail:gcquan08&＃64;gmail.com* &＃64;version Time&＃xff1a;2011-8-20*/
public class Natives {static{System.loadLibrary("mylib");}public static native int libMain(String[] argv);private static void onMessage(String text,int level){// System.out.println("Message:"&＃43;text&＃43;" Level:"&＃43;level);Log.d("JNI", "Message:"&＃43;text&＃43;" Level:"&＃43;level);}
}

Android.mk:

LOCAL_PATH:&＃61;$(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE:&＃61;mylib
LOCAL_SRC_FILES:&＃61;mylib.c
LOCAL_LDLIBS :&＃61; -llog
LOCAL_LDLIBS &＃43;&＃61; -L$(SYSROOT)/usr/lib -lloginclude $(BUILD_SHARED_LIBRARY)
include/Natives.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include
/* Header for class Natives */#ifndef _Included_Natives
#define _Included_Natives
#ifdef __cplusplus
extern "C" {
#endif
/** Class: Natives* Method: libMain* Signature: ([Ljava/lang/String;)I*/
JNIEXPORT jint JNICALL Java_com_gao_Natives_libMain(JNIEnv *, jclass, jobjectArray);#ifdef __cplusplus
}
#endif
#endifmylib.c:

#include
#include
#include
#include "include/Natives.h"#include
#define LOG_TAG "JNI"
#define LOGD(a) __android_log_write(ANDROID_LOG_DEBUG,LOG_TAG,a)#define CB_CLASS "com/gao/Natives"/**
* onMessage callback
*/
#define CB_CLASS_MSG_CB "onMessage"
#define CB_CLASS_MSG_SIG "(Ljava/lang/String;I)V"//lib main sub
int lib_main(int argc,char **argv);//Used to get the len of a java Array
const int getArrayLen(JNIEnv* env,jobjectArray jarray);//print str message back to java
void jni_printf(char* format,...);//Global env ref(for callback)
static JavaVM* g_VM;//Global Reference to the native java calss com.gao.Natives.java
static jclass jNativesCls;JNIEXPORT jint JNICALL Java_com_gao_Natives_libMain(JNIEnv *env, jclass cls, jobjectArray jarray)
{//obtain a global ref to the caller class(*env)->GetJavaVM(env,&g_VM);//Extract char **args from java arrayjsize len&＃61;getArrayLen(env,jarray);char* args[(int)len];int i;jstring jrow;for(i&＃61;0;iGetObjectArrayElement(env,jarray,i);const char* row&＃61;(*env)->GetStringUTFChars(env,jrow,0);args[i]&＃61;malloc(strlen(row)&＃43;1);strcpy(args[i],row);//print argsjni_printf("Main argv[%d]&＃61;%s",i,args[i]);//LOGD("From C "&＃43;*args[i]);LOGD("From C ");//free java string jrow(*env)->ReleaseStringUTFChars(env,jrow,row);}//Load the com.gao.Natives classjNativesCls&＃61;(*env)->FindClass(env,CB_CLASS);if(jNativesCls&＃61;&＃61;0){jni_printf("Unable to find class:%s",CB_CLASS);return -1;}//Invoke the lib main sub.This will loop forener//Program agrs com from Javalib_main(len,args);return 0;
}/**
*Send a string back to Java
*/
jmethodID mSendStr;
static void jni_send_str(const char* text,int level)
{JNIEnv* env;if(!g_VM){printf("I_JNI_NOVM:%s\n",text);return;}(*g_VM)->AttachCurrentThread(g_VM,(void **)&env,NULL);//Load jni.Natives if missingif(!jNativesCls){jNativesCls&＃61;(*env)->FindClass(env,CB_CLASS);if(jNativesCls&＃61;&＃61;0){printf("Unable to find class:%s",CB_CLASS);return;}}//Call com.gao.Natives.onMessage(String,int)if(!mSendStr){//Get a ref to the static method:com.gao.Natives.onMessagemSendStr&＃61;(*env)->GetStaticMethodID(env,jNativesCls,CB_CLASS_MSG_CB,CB_CLASS_MSG_SIG);}if(mSendStr){(*env)->CallStaticVoidMethod(env,jNativesCls,mSendStr,(*env)->NewStringUTF(env,text),(jint)(level));}else{printf("Unable to find method: %s, signature: %s\n", CB_CLASS_MSG_CB, CB_CLASS_MSG_SIG );}}/**
*Get java array length
*/const int getArrayLen(JNIEnv* env,jobjectArray jarray)
{return (*env)->GetArrayLength(env,jarray);
}/*** Printf into the java layer* does a varargs printf into a temp buffer* and calls jni_sebd_str*/
void jni_printf(char* format,...)
{va_list argptr;static char string[1024];va_start(argptr,format);vsprintf(string,format,argptr);va_end(argptr);jni_send_str(string,0);
}/**
*Library main sub
*/
int lib_main(int argc,char** argv)
{int i;jni_printf("Entering LIB MAIN");for(i&＃61;0;i}运行结果&＃xff1a;

08-20 21:50:50.350: DEBUG/JNI(7574): Message:Main argv[0]&＃61;mylib Level:0
08-20 21:50:50.350: DEBUG/JNI(7574): From C
08-20 21:50:50.350: DEBUG/JNI(7574): Message:Main argv[1]&＃61;Gao Level:0
08-20 21:50:50.350: DEBUG/JNI(7574): From C
08-20 21:50:50.350: DEBUG/JNI(7574): Message:Main argv[2]&＃61;GaoMatrix Level:0
08-20 21:50:50.350: DEBUG/JNI(7574): From C
08-20 21:50:50.350: DEBUG/JNI(7574): Message:Entering LIB MAIN Level:0
08-20 21:50:50.350: DEBUG/JNI(7574): Message:Lib Main argv[0]&＃61;mylib Level:0
08-20 21:50:50.350: DEBUG/JNI(7574): Message:Lib Main argv[1]&＃61;Gao Level:0
08-20 21:50:50.350: DEBUG/JNI(7574): Message:Lib Main argv[2]&＃61;GaoMatrix Level:0