NDK 学习 ndk基础

1.安装

1.下载NDK，注意下载的是r4版本

http://dl.google.com/android/ndk/android-ndk-r4b-linux-x86.zip

2.下载完成后解压到某一目录，我的是在

zhangweia@ubuntu:~/android/android-ndk-r4b$pwd
　　　　/home/zhangweia/android/android-ndk-r4b
　　zhangweia@ubuntu:~/android/android-ndk-r4b$echo $HOME
　　　　/home/zhangweia

3.配置NDK的环境变量

zhangweia@ubuntu:~/android/android-ndk-r4b$vi ~/.bashrc
在打开的文件末尾添加：

NDK=$HOME/android/android-ndk-r4b/
export NDK

执行下面的命令，保存环境便利，然后查看是否生效：

zhangweia@ubuntu:~/android/android-ndk-r4b$source ~/.bashrc

zhangweia@ubuntu:~/android/android-ndk-r4b$echo $NDK
/home/zhangweia/android/android-ndk-r4b/

4.编译例子。

进入NDK 的例子目录samples/hello-jni，然后在该目录下执行$NDK/ndk-build,编译成功后会在该目录多生成2个子目录libs,obj目录

然后打开eclipse ,添加存在的hello-jni工程 ，运行则

2.HelloNDk

bin/Hello.c

#include <string.h>
#include <jni.h>

/* This is a trivial JNI example where we use a native method
 * to return a new VM String. See the corresponding Java source
 * file located at:
 *
 *   apps/samples/hello-jni/project/src/com/example/hellojni/HelloJni.java
 */
jstring Java_com_su_testjni_HelloJniActivity_stringFromJNI(JNIEnv* env,
		jobject thiz) {
	return (*env)->NewStringUTF(env, "Hello from JNI !");
}

src/HelloJniActivity

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.su.testjni;

import android.app.Activity;
import android.widget.TextView;
import android.os.Bundle;

public class HelloJniActivity extends Activity {
	/** Called when the activity is first created. */
	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);

		/*
		 * Create a TextView and set its content. the text is retrieved by
		 * calling a native function.
		 */
		TextView tv = new TextView(this);
		tv.setText(stringFromJNI());
		setContentView(tv);
	}

	/*
	 * A native method that is implemented by the 'hello-jni' native library,
	 * which is packaged with this application.
	 */
	public native String stringFromJNI();

	/*
	 * This is another native method declaration that is *not* implemented by
	 * 'hello-jni'. This is simply to show that you can declare as many native
	 * methods in your Java code as you want, their implementation is searched
	 * in the currently loaded native libraries only the first time you call
	 * them.
	 * 
	 * Trying to call this function will result in a
	 * java.lang.UnsatisfiedLinkError exception !
	 */
	public native String unimplementedStringFromJNI();

	/*
	 * this is used to load the 'hello-jni' library on application startup. The
	 * library has already been unpacked into
	 * /data/data/com.example.hellojni/lib/libhello-jni.so at installation time
	 * by the package manager.
	 */
	static {
		System.loadLibrary("hello-jni");
	}
}

Android.mk

# Copyright (C) 2009 The Android Open Source Project
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#      http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
#
LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hello-jni 这里是jni库将来的名字 源码同名就可以
LOCAL_SRC_FILES := hello-jni.c c源码

include $(BUILD_SHARED_LIBRARY)

编译的方法:cd到项目目录 然后ndk-build 就ok

3.java和c之间 数据交换

android.mk:

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE :=Hello
LOCAL_SRC_FILES :=Hello.c
#liblog.so libGLESv2.so
LOCAL_LDLIBS += -llog //添加log
include $(BUILD_SHARED_LIBRARY)

制作头文件:

cd到bin目录然后javah com.test.testndkdata.DataProvider 含有nativte的方法

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class cn_itcast_ndk3_DataProvider */

#ifndef _Included_cn_itcast_ndk3_DataProvider
#define _Included_cn_itcast_ndk3_DataProvider
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     cn_itcast_ndk3_DataProvider
 * Method:    add
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_cn_itcast_ndk3_DataProvider_add__II
  (JNIEnv *, jobject, jint, jint);

/*
 * Class:     cn_itcast_ndk3_DataProvider
 * Method:    sub
 * Signature: (II)I
 */
JNIEXPORT jint JNICALL Java_cn_itcast_ndk3_DataProvider_sub
  (JNIEnv *, jclass, jint, jint);

/*
 * Class:     cn_itcast_ndk3_DataProvider
 * Method:    add
 * Signature: (CC)C
 */
JNIEXPORT jchar JNICALL Java_cn_itcast_ndk3_DataProvider_add__CC
  (JNIEnv *, jobject, jchar, jchar);

/*
 * Class:     cn_itcast_ndk3_DataProvider
 * Method:    sayHelloInC
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_cn_itcast_ndk3_DataProvider_sayHelloInC
  (JNIEnv *, jobject, jstring);

/*
 * Class:     cn_itcast_ndk3_DataProvider
 * Method:    intMethod
 * Signature: ([I)[I
 */
JNIEXPORT jintArray JNICALL Java_cn_itcast_ndk3_DataProvider_intMethod
  (JNIEnv *, jobject, jintArray);

/*
 * Class:     cn_itcast_ndk3_DataProvider
 * Method:    byteMethod
 * Signature: ([B)[B
 */
JNIEXPORT jbyteArray JNICALL Java_cn_itcast_ndk3_DataProvider_byteMethod
  (JNIEnv *, jobject, jbyteArray);

#ifdef __cplusplus
}
#endif
#endif

然后是c代码:

#include<stdio.h>
#include<jni.h>
#include "cn_itcast_ndk3_DataProvider.h";
#include <android/log.h>
#include<malloc.h>
#define LOG_TAG "System.out.c"
#define LOGD(...) __android_log_print(ANDROID_LOG_DEBUG, LOG_TAG, __VA_ARGS__)
#define LOGI(...) __android_log_print(ANDROID_LOG_INFO, LOG_TAG, __VA_ARGS__)


/**
 * 返回值 char* 这个代表char数组的首地址
 *  Jstring2CStr 把java中的jstring的类型转化成一个c语言中的char 字符串
 */
char*   Jstring2CStr(JNIEnv*   env,   jstring   jstr)
{
	 char*   rtn   =   NULL;
	 jclass   clsstring   =   (*env)->FindClass(env,"java/lang/String"); //String
	 jstring   strencode   =   (*env)->NewStringUTF(env,"GB2312");  // 得到一个java字符串 "GB2312"
	 jmethodID   mid   =   (*env)->GetMethodID(env,clsstring,   "getBytes",   "(Ljava/lang/String;)[B"); //[ String.getBytes("gb2312");
	 jbyteArray   barr=   (jbyteArray)(*env)->CallObjectMethod(env,jstr,mid,strencode); // String .getByte("GB2312");
	 jsize   alen   =   (*env)->GetArrayLength(env,barr); // byte数组的长度
	 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_cn_itcast_ndk3_DataProvider_add
  (JNIEnv * env, jobject obj , jint x, jint y){

	LOGD("x=%d",x);
	LOGD("y=%d",y);
	return x+y;


}
JNIEXPORT jstring JNICALL Java_cn_itcast_ndk3_DataProvider_sayHelloInC
  (JNIEnv * env, jobject obj, jstring jstr ){
	//在c语言中 是没有java的String
	char* cstr = Jstring2CStr(env, jstr);
	LOGD("cstr=%s",cstr);
	// c语言中的字符串 都是以'/0' 作为结尾
	char arr[7]= {' ','h','e','l','l','o','\0'};
	strcat(cstr,arr);
	LOGD("new cstr=%s",cstr);
	return (*env)->NewStringUTF(env,cstr);
}

/**env java 虚拟机 结构体c实现的指针 包含的有很多jni方法
 *jobject obj 代表的是调用这个c代码的java对象 代表的是DataProider的对象
 */

JNIEXPORT jintArray JNICALL Java_cn_itcast_ndk3_DataProvider_intMethod
  (JNIEnv * env , jobject obj , jintArray arr){
	//1.知道数组的长度
	//2.操作这个数组里面的每一个元素
	int len = (*env)->GetArrayLength(env,arr);
	LOGD("shuzu len =%d",len);
	//    jint*       (*GetIntArrayElements)(JNIEnv*, jintArray, jboolean*);
	jint* intarr = (*env)->GetIntArrayElements(env,arr,1);
	int i =0; //c99
	for(;i<len;i++){
		//*(intarr+i) += 10;

		LOGD("intarr[%d]=%d", i, intarr[i]);

		intarr[i]+= 10;
	}
//    void        (*ReleaseIntArrayElements)(JNIEnv*, jintArray,
//                        jint*, jint);
//

//	(*env)->ReleaseIntArrayElements(env,arr,intarr,0); // c语言释放掉 刚才申请的内存空间
	return arr;
}

/**
 * 代表的是调用c代码 的class类
 * jclass DataProvider  类
 */

JNIEXPORT jint JNICALL Java_cn_itcast_ndk3_DataProvider_sub
  (JNIEnv * env, jclass clazz, jint x, jint y){
	LOGD("x=%d",x);
	LOGD("y=%d",y);
	return x-y;


}

然后是dataprovider代码

package com.test.testndkdata;


import android.R.integer;


public class DataProvider {
	public native int add(int a, int b);


	public native String sayHello(String s);


	public native int[] ints(int[] nums);


}

所有代码

http://dl.vmall.com/c0hivk447p