Android开发之JNI调用本地C库专题（一）：JNI的使用

public class DataProvider {

	public native int add(int x, int y); //

	public native String sayHelloInC(String s);

	public native int[] intMethod(int[] iNum);

}

#include <stdio.h>
#include "com_example_ndkpassdata_DataProvider.h"
#include <android/log.h>
#include <string.h>
#define LOG_TAG "clog"
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)

//将java语言中的字符串格式转换为C语言中的字符串格式。
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);         //"\0"
		memcpy(rtn, ba, alen);
		rtn[alen] = 0;
	}
	(*env)->ReleaseByteArrayElements(env, barr, ba, 0);  //
	return rtn;
}

JNIEXPORT jint JNICALL Java_com_example_ndkpassdata_DataProvider_add(
		JNIEnv * env, jobject jobject, jint x, jint y) {
	// 想在logcat控制台上 打印日志
	LOGD("x=%d", x);
	LOGI("y=%d", y);
	// log.i(TAG,"sss");
	return x + y;

}

JNIEXPORT jstring JNICALL Java_com_example_ndkpassdata_DataProvider_sayHelloInC(
		JNIEnv * env, jobject jobject, jstring str) {

	char* c = "hello";
	// 在C语言中不能直接操作java中的字符串
	// 把java中的字符串转换成c语言中 char数组
	char* cstr = Jstring2CStr(env, str);

	strcat(cstr, c);
	LOGD("%s", cstr);
	return (*env)->NewStringUTF(env, cstr);
}

JNIEXPORT jintArray JNICALL Java_com_example_ndkpassdata_DataProvider_intMethod(
		JNIEnv * env, jobject jobject, jintArray jarray) {
	// jArray  遍历数组   jint*       (*GetIntArrayElements)(JNIEnv*, jintArray, jboolean*);
	// 数组的长度    jsize       (*GetArrayLength)(JNIEnv*, jarray);
	// 对数组中每个元素 +5
	int length = (*env)->GetArrayLength(env, jarray);
	//拿到指针初始位置
	int* array = (*env)->GetIntArrayElements(env, jarray, 0);
	int i = 0;
	for (; i < length; i++) {
		*(array + i) += 5;
	}
	return jarray;
}

5、编写android.mk文件

 LOCAL_PATH := $(call my-dir)

   include $(CLEAR_VARS)

   LOCAL_MODULE    := libhello
   LOCAL_SRC_FILES := Hello.c
	LOCAL_LDLIBS += -llog

   include $(BUILD_SHARED_LIBRARY)

public class MainActivity extends Activity {
	DataProvider provider;
	static {
		System.loadLibrary("hello");
	}

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		provider = new DataProvider();

	}

	public void click1(View view) {
		int result = provider.add(6, 8);
		System.out.println(result);
	}

	public void click2(View view) {
		String str = provider.sayHelloInC("freedom");
		Toast.makeText(getApplicationContext(), str, 0).show();

	}

	public void click3(View view) {
		int[] arr = new int[] { 5, 6, 7, 8, 9 };
		provider.intMethod(arr);
		for (int i : arr) {
			System.out.println(i);
		}

	}
}