android系统的jni程序的编写

jni的编写

java调用c/c++的框架
java代码-->jni代码--->c/c++代码
写一个简单的java app来调用c/c++代码打印“HelloWorld!”的过程。分为3步：
1、编写java代码，创建一个类（HelloWorld.java）声明native method
2、编写jni文件。
3、将jni实现文件编译成一个native lib
HelloWorld.java
public class HelloActivity extends Activity{
 public void onCreate(Bundle savedInstanceState){
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);
  this.fsSayHello();
 }
 static {
  System.loadLibrary("hello_jni");
 }
 private static native int fsSayHello();
}

构建jni函数，并且编译成本地库
jint JNI_OnLoad(JavaVM* vm, void* reserved){
 jint result = -1;
 JNIEnv* env = NULL;
 LOGI("JNI_OnLoad");
 if(vm->GetEnv( (void **)&env, JNI_VERSION_1_4) != JNI_OK){
  LOGE("ERROR: GetEnv failed");
  return result;
 }
 if (registerNatives(env) != JNI_TRUE){
  LOGE("ERROR:registerNatives failed");
  reture result;
 }
 result = JNI_VERSION_1_4;
}

static JNINativeMethod gMethods[] = {
 {"fsSayHello" , "()I" , (void *)hello_printf},
};

//声明fsSayHello()所在类
static const char *classPathName = "com/farsight/HelloActivity";
static int registerNatives(JNIEnv* env){
 jclass clazz;
 clazz = env->FindClass(class PathName);
 if (clazz == NULL){
  return JNI_FALSE;
 }
 //向VM(即AndroidRuntime)登记gMethods[]表格所含的本地函数
 if (env->RegisterNatives(clazz, gMethods, sizeof(gMethods)/sizeof(gMethods[0])) < 0){
  return JNI_FALSE;
 }
 return JNI_TRUE;
}

typedef struct{
 const char* name;
 const char* signature;
 void* fnPtr;
}JNINativeMethod;

编写c/c++代码
static jint hello_printf(JNIEnv* env, jobject thiz){
 LOGI("<<<<<<>>>>>>>>>");
 return 0;
}

Android.mk
LOCAL_PATH:=$(call my-dir)
include $(CLEAR_VARS)
LOCAL_SRC_FILES:= \
 com.farsight.hello.cpp
LOCAL_SHARED_LIBRARIES:= \
 libutils
LOCAL_C_INCLUDES+= \
 $(JNI_H_INCLUDE)
LOCAL_MODULE:= libhello_jni
LOCAL_PRELILNK_MODULE:= false
include $(BUILD_SHARED_LILBRARY)