android JNI简单的java调C

1、Android工程里创建一类（定义要用的native方法）：

public class DataProvider {public native int add(int x,int y);public native String sayHello(String str);public native int[] intMethod(int[] num);}

2、用Javah进行编译，生成cn_uc_DataProvider.h文件
3、将该文件放在jni文件夹里，并创建Android.mk（要添加log 库）：
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE    := hello
LOCAL_SRC_FILES := hello.c
#增加 log 函数对应的log 库  liblog.so
LOCAL_LDLIBS += -llog
include $(BUILD_SHARED_LIBRARY)
4、在该文件夹中创建hello.c文件（记得导入log头文件）：
#include "cn_uc_DataProvider.h"
#include <string.h>
#include<android/log.h>
#define LOG_TAG "XIAOYUAN"
#define LOGD(...)__android_log_print(ANDROID_LOG_DEBUG,LOG_TAG,__VA_ARGS__)
#define LOGI(...)__android_log_print(ANDROID_LOG_INFO,LOG_TAG,__VA_ARGS__)
JNIEXPORT jint JNICALL Java_cn_uc_DataProvider_add(JNIEnv * env, jobject obj,
jint x, jint y) {
LOGI("x=%ld",x);
LOGI("t=%ld",y);
return x + y;
}
JNIEXPORT jstring JNICALL Java_cn_uc_DataProvider_sayHello(JNIEnv * env,
jobject obj, jstring str) {
char* p = Jstring2CStr(env, str);
LOGI("%s",p);
char* newStr = "yuan";
//strcat(dest, sorce) 把sorce字符串添加到dest字符串的后面
LOGI("END");
return (*env)->NewStringUTF(env, strcat(p, newStr));
}
JNIEXPORT jintArray JNICALL Java_cn_uc_DataProvider_intMethod(JNIEnv * env,
jobject obj, jintArray arr) {
// 1.获取到 arr的大小
int len = (*env)->GetArrayLength(env, arr);
LOGI("len=%d", len);
 if(len==0){
return arr;
   }
   jint* p = (*env)-> GetIntArrayElements(env,arr,0);
   int i=0;
   for(;i<len;i++){
LOGI("len=%ld", *(p+i));
*(p+i) += 5;
   }
   return arr;
}

// java中的jstring, 转化为c的一个字符数组
char* Jstring2CStr(JNIEnv* env, jstring jstr) {
char* rtn = NULL;
jclass clsstring = (*env)->FindClass(env, "java/lang/String");
jstring strencode = (*env)->NewStringUTF(env, "GB2312");
jmethodID mid = (*env)->GetMethodID(env, clsstring, "getBytes","(Ljava/lang/String;)[B");
jbyteArray barr = (jbyteArray)(*env)->CallObjectMethod(env, jstr, mid,strencode); // String .getByte("GB2312");
jsize alen = (*env)->GetArrayLength(env, barr);
jbyte* ba = (*env)->GetByteArrayElements(env, barr, JNI_FALSE);
if (alen > 0) {
rtn = (char*) malloc(alen + 1); //new   char[alen+1]; "\0"
memcpy(rtn, ba, alen);
rtn[alen] = 0;
}
(*env)->ReleaseByteArrayElements(env, barr, ba, 0); //释放内存
return rtn;
}


注意：发现问题:
/**
$ ndk-build
Compile thumb  : hello <= hello.c
D:/Program/Demo/jni/hello.c: In function 'Java_cn_uc_DataProvider_sayHello':
D:/Program/Demo/jni/hello.c:18: warning: initialization makes pointer from integ
er without a cast
D:/Program/Demo/jni/hello.c: At top level:
D:/Program/Demo/jni/hello.c:46: error: conflicting types for 'Jstring2CStr'
D:/Program/Demo/jni/hello.c:18: note: previous implicit declaration of 'Jstring2
CStr' was here
make: *** [/cygdrive/d/Program/Demo/obj/local/armeabi/objs/hello/hello.o] Error
1
*/
5、用Cygwin进行编译......
6、在Activity中调用DataProvider中的方法