Android JNI和NDK学习(2)--编程入门-优快云博客

本文详细介绍如何使用Android NDK进行开发，从Java代码到C/C++代码的编写过程，包括JNI的使用，生成和调用本地库的方法，以及如何在Android应用中集成本地代码。

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NDK代码编写

1. 首先是Java代码的编写

Android NDK Sample里面的Hello-jni工程.
Hellojni.java


    /*
     * 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.example.hellojni;

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


    public class HelloJni 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");
     }
    }

这段代码很简单，注释也很清晰，这里只提两点：:

    static{ 
    System.loadLibrary("hello-jni"); 
    }

表明程序开始运行的时候会加载hello-jni, static区声明的代码会先于onCreate方法执行,因为这是属于类的静态方法。如果你的程序中有多个类，而且如果HelloJni这个类不是你应用程序的入口，那么hello-jni（完整的名字是libhello-jni.so）这个库会在第一次使用HelloJni这个类的时候加载。

public native String stringFromJNI(); 
public native String unimplementedStringFromJNI();

可以看到这两个方法的声明中有 native 关键字，这个关键字表示这两个方法是本地方法，也就是说这两个方法是通过本地代码（C/C++）实现的，在java代码中仅仅是声明。

用eclipse编译该工程，生成相应的.class文件，这步必须在下一步之前完成，因为生成.h文件需要用到相应的.class文件。

2.编写C/C++代码

利用javah这个工具生成相应的.h文件，然后根据这个.h文件编写相应的C/C++代码。

2.1 生成相应.h文件：

就拿我这的环境来说，首先在终端下进入刚刚建立的HelloJni工程的目录：

~$ cd workspace/android/NDK/hello-jni/

ls查看工程文件

~/workspace/android/NDK/hello-jni$ ls 
AndroidManifest.xml  assets  bin  default.properties  gen  res  src

可以看到目前仅仅有几个标准的android应用程序的文件（夹）。

首先我们在工程目录下建立一个jni文件夹：

~/workspace/android/NDK/hello-jni$ mkdir jni 
~/workspace/android/NDK/hello-jni$ ls 
AndroidManifest.xml  assets  bin  default.properties  gen  jni  res  src

下面就可以生成相应的.h文件了：

~/workspace/android/NDK/hello-jni$ javah -classpath bin -d jni com.example.hellojni.HelloJni

-classpath bin：表示类的路劲

-d jni: 表示生成的头文件存放的目录

com.example.hellojni.HelloJni 则是完整类名

执行这个命令的时候可能会出错,因为需要加载android.jar关联类库.所以需要加上这个参数

-bootclasspath .../android.jar

这一步的成功要建立在已经在 bin/com/example/hellojni/ 目录下生成了 HelloJni.class的基础之上。

现在可以看到jni目录下多了个.h文件：

~/workspace/android/NDK/hello-jni$ cd jni/ 
~/workspace/android/NDK/hello-jni/jni$ ls 
com_example_hellojni_HelloJni.h

我们来看看com_example_hellojni_HelloJni.h的内容：

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

#ifndef _Included_com_example_hellojni_HelloJni
#define _Included_com_example_hellojni_HelloJni
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     com_example_hellojni_HelloJni
 * Method:    stringFromJNI
* Signature: ()Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_stringFromJNI
  (JNIEnv *, jobject);

/*
 * Class:     com_example_hellojni_HelloJni
 * Method:    unimplementedStringFromJNI
* Signature: ()Ljava/lang/String;
*/
JNIEXPORT jstring JNICALL Java_com_example_hellojni_HelloJni_unimplementedStringFromJNI
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

上面代码中的JNIEXPORT 和 JNICALL 是jni的宏，在android的jni中不需要

按照：java_pacakege_class_mathod 形式来命名。

也就是说：

Hello.java中 stringFromJNI() 方法对应于 C/C++中的Java_com_example_hellojni_HelloJni_stringFromJNI() 方法

2.2 编写Ｃ文件

    
    /*
     * 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.
     *
     */
    #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_example_hellojni_HelloJni_stringFromJNI( JNIEnv* env,
                                                      jobject thiz )
    {
        return (*env)->NewStringUTF(env, "Hello Wolrd JNI");
    }

在这里我们先不对Android.mk文件做改动.android.mk在jni目录下，Android.mk 文件是Android 的 makefile文件，内容如下：


    # 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
    LOCAL_SRC_FILES := hello-jni.c
    
    include $(BUILD_SHARED_LIBRARY)

在eclipse下进行重新编译.将生成的so库打包在apk内运行,就可以看到结果了.