【Android 开发】：UI控件之 ImageView 从网络上获取图像

    在我们的手机应用开发中，经常要进行客户端与服务端之间的交互，我们可能会从网络上获取一张图像显示在我们的手机上，这样我们可以使用HTTP协议进行操作。这一讲我们将学习如何从服务端获取图片到手机客户端

    案例：点击按钮从网络上获取ImageView图像显示在本地

程序实现：

1. 布局文件

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical" >

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />
    <Button 
        android:id="@+id/button"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="下载网络图片"/>
        

</LinearLayout>

2. 程序主要代码

1)HttpUtils.java 作为辅助类

public class HttpUtils {

    private final static String URL_PATH = "http://www.android.com/images/about/about-nexus-family.png"; //访问网络图片的路径
    
    public HttpUtils() {
        // TODO Auto-generated constructor stub
    }
    
    /**
     * 从网络中获取图片信息，以流的形式返回
     * @return
     * @throws IOException 
     */
    public static InputStream getImageViewInputStream() throws IOException{
        
        InputStream inputStream = null;
            URL url = new URL(URL_PATH);
            if(url != null){
                HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
                httpURLConnection.setConnectTimeout(3000); //设置连接超时的时间
                httpURLConnection.setRequestMethod("GET");
                httpURLConnection.setDoInput(true);
                //设置响应的代码
                int response_code = httpURLConnection.getResponseCode();
                if(response_code == 200){
                    inputStream = httpURLConnection.getInputStream();
                }
            }
        
        return inputStream;
    }
    
    public static byte[] getImageViewArray(){
        byte[] data = null;
        InputStream inputStream = null;
        
        //不需要关闭输出流，直接 写入到内存中
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        try {
            URL url = new URL("URL_PATH");
            if(url != null){
                HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
                httpURLConnection.setConnectTimeout(3000); //设置连接超时的时间
                httpURLConnection.setRequestMethod("GET");
                httpURLConnection.setDoInput(true);
                //设置响应的代码
                int response_code = httpURLConnection.getResponseCode();
                
                int len = 0;
                byte[] b_data = new byte[1024];
                if(response_code == 200){
                    inputStream = httpURLConnection.getInputStream();
                    while((len = inputStream.read(b_data)) != -1) {
                        outputStream.write(b_data, 0, len);
                    }
                    data = outputStream.toByteArray();
                }
            }
        } catch (Exception e) {
            // TODO: handle exception
        }finally{
            if(inputStream != null){
                try {
                    inputStream.close();
                } catch (IOException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
        }
        
        return data;
        
    }

}

2) 主程序

public class ImageViewDemo extends Activity {
    private Button button;
    private ImageView imageView;
    
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        initComponent();
        button.setOnClickListener(new View.OnClickListener() {
            
            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                new Thread(){
                    
                    public void run() {
                        
                        try {
                            InputStream inputStream = HttpUtils.getImageViewInputStream();
                            Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
                            imageView.setImageBitmap(bitmap); //需要一个Bitmap对象，可以从 BitmapFactory工厂中获得
                            
                        } catch (IOException e) {
                            // TODO: handle exception
                        }
                    };
                }.start();

            }
        });
        
    }
    
    private void initComponent(){
        button = (Button)findViewById(R.id.button);
        imageView = (ImageView)findViewById(R.id.imageView);
    }
}

[注意编程规范]: 如果方法A里面含有了 try catch模块来捕获异常，在方法B中调用A的过程中也需要try catch 一次异常。则这种情况下一般要把A的异常抛出，让方法B去捕获这个异常。

   访问网络的主程序最好另起一个线程而不是写在UI主线程中。