1.java代码，下载图片的主程序

先实现显示图片，然后点击下载图片按钮，执行下载功能。

从网络上取得的图片，生成Bitmap时有两种方法，一种是先转换为byte[]，再生成bitmap；一种是直接用InputStream生成bitmap。

（1）ICS4.0及更高版本中的实现

4.0中不允许在主线程，即UI线程中操作网络，所以必须新开一个线程，在子线程中执行网络连接；然后在主线程中显示图片。

public class IcsTestActivity extends Activity {    private final static String TAG = "IcsTestActivity";    private final static String ALBUM_PATH            = Environment.getExternalStorageDirectory() + "/download_test/";    private ImageView mImageView;    private Button mBtnSave;    private ProgressDialog mSaveDialog = null;    private Bitmap mBitmap;    private String mFileName;    private String mSaveMessage;    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);        mImageView = (ImageView)findViewById(R.id.imgSource);        mBtnSave = (Button)findViewById(R.id.btnSave);        new Thread(connectNet).start();        // 下载图片        mBtnSave.setOnClickListener(new Button.OnClickListener(){            public void onClick(View v) {                mSaveDialog = ProgressDialog.show(IcsTestActivity.this, "保存图片", "图片正在保存中，请稍等...", true);                new Thread(saveFileRunnable).start();        }        });    }    /**     * Get image from newwork     * @param path The path of image     * @return byte[]     * @throws Exception     */    public byte[] getImage(String path) throws Exception{        URL url = new URL(path);        HttpURLConnection conn = (HttpURLConnection) url.openConnection();        conn.setConnectTimeout(5 * 1000);        conn.setRequestMethod("GET");        InputStream inStream = conn.getInputStream();        if(conn.getResponseCode() == HttpURLConnection.HTTP_OK){            return readStream(inStream);        }        return null;    }    /**     * Get image from newwork     * @param path The path of image     * @return InputStream     * @throws Exception     */    public InputStream getImageStream(String path) throws Exception{        URL url = new URL(path);        HttpURLConnection conn = (HttpURLConnection) url.openConnection();        conn.setConnectTimeout(5 * 1000);        conn.setRequestMethod("GET");        if(conn.getResponseCode() == HttpURLConnection.HTTP_OK){            return conn.getInputStream();        }        return null;    }    /**     * Get data from stream     * @param inStream     * @return byte[]     * @throws Exception     */    public static byte[] readStream(InputStream inStream) throws Exception{        ByteArrayOutputStream outStream = new ByteArrayOutputStream();        byte[] buffer = new byte[1024];        int len = 0;        while( (len=inStream.read(buffer)) != -1){            outStream.write(buffer, 0, len);        }        outStream.close();        inStream.close();        return outStream.toByteArray();    }    /**     * 保存文件     * @param bm     * @param fileName     * @throws IOException     */    public void saveFile(Bitmap bm, String fileName) throws IOException {        File dirFile = new File(ALBUM_PATH);        if(!dirFile.exists()){            dirFile.mkdir();        }        File myCaptureFile = new File(ALBUM_PATH + fileName);        BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(myCaptureFile));        bm.compress(Bitmap.CompressFormat.JPEG, 80, bos);        bos.flush();        bos.close();    }    private Runnable saveFileRunnable = new Runnable(){        @Override        public void run() {            try {                saveFile(mBitmap, mFileName);                mSaveMessage = "图片保存成功！";            } catch (IOException e) {                mSaveMessage = "图片保存失败！";                e.printStackTrace();            }            messageHandler.sendMessage(messageHandler.obtainMessage());        }    };    private Handler messageHandler = new Handler() {        @Override        public void handleMessage(Message msg) {            mSaveDialog.dismiss();            Log.d(TAG, mSaveMessage);            Toast.makeText(IcsTestActivity.this, mSaveMessage, Toast.LENGTH_SHORT).show();        }    };    /*     * 连接网络     * 由于在4.0中不允许在主线程中访问网络，所以需要在子线程中访问     */    private Runnable connectNet = new Runnable(){        @Override        public void run() {            try {                String filePath = "https://img-my.csdn.net/uploads/201211/21/1353511891_4579.jpg";                mFileName = "test.jpg";                //以下是取得图片的两种方法                 方法1：取得的是byte数组, 从byte数组生成bitmap                byte[] data = getImage(filePath);                if(data!=null){                    mBitmap = BitmapFactory.decodeByteArray(data, 0, data.length);// bitmap                }else{                    Toast.makeText(IcsTestActivity.this, "Image error!", 1).show();                }                                //******** 方法2：取得的是InputStream，直接从InputStream生成bitmap ***********/                mBitmap = BitmapFactory.decodeStream(getImageStream(filePath));                //********************************************************************/                // 发送消息，通知handler在主线程中更新UI                connectHanlder.sendEmptyMessage(0);                Log.d(TAG, "set image ...");            } catch (Exception e) {                Toast.makeText(IcsTestActivity.this,"无法链接网络！", 1).show();                e.printStackTrace();            }        }    };    private Handler connectHanlder = new Handler() {        @Override        public void handleMessage(Message msg) {            Log.d(TAG, "display image");            // 更新UI，显示图片            if (mBitmap != null) {                mImageView.setImageBitmap(mBitmap);// display image            }        }    };}

（2）2.3以及以下版本可以在主线程中操作网络连接，但最好不要这样做，因为连接网络是阻塞的，如果5秒钟还没有连接上，就会引起ANR。

public class AndroidTest2_3_3 extends Activity {    private final static String TAG = "AndroidTest2_3_3";    private final static String ALBUM_PATH             = Environment.getExternalStorageDirectory() + "/download_test/";    private ImageView imageView;    private Button btnSave;    private ProgressDialog myDialog = null;    private Bitmap bitmap;    private String fileName;    private String message;            @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);                imageView = (ImageView)findViewById(R.id.imgSource);        btnSave = (Button)findViewById(R.id.btnSave);                String filePath = "http://hi.csdn.net/attachment/201105/21/134671_13059532779c5u.jpg";        fileName = "test.jpg";                try {             取得的是byte数组, 从byte数组生成bitmap            byte[] data = getImage(filePath);                  if(data!=null){                      bitmap = BitmapFactory.decodeByteArray(data, 0, data.length);// bitmap                      imageView.setImageBitmap(bitmap);// display image                  }else{                      Toast.makeText(AndroidTest2_3_3.this, "Image error!", 1).show();                  }                        //******** 取得的是InputStream，直接从InputStream生成bitmap ***********/            bitmap = BitmapFactory.decodeStream(getImageStream(filePath));            if (bitmap != null) {                imageView.setImageBitmap(bitmap);// display image            }            //********************************************************************/            Log.d(TAG, "set image ...");        } catch (Exception e) {               Toast.makeText(AndroidTest2_3_3.this,"Newwork error!", 1).show();               e.printStackTrace();           }                   // 下载图片        btnSave.setOnClickListener(new Button.OnClickListener(){            public void onClick(View v) {                myDialog = ProgressDialog.show(AndroidTest2_3_3.this, "保存图片", "图片正在保存中，请稍等...", true);                new Thread(saveFileRunnable).start();        }        });    }    /**       * Get image from newwork       * @param path The path of image       * @return byte[]     * @throws Exception       */      public byte[] getImage(String path) throws Exception{           URL url = new URL(path);           HttpURLConnection conn = (HttpURLConnection) url.openConnection();           conn.setConnectTimeout(5 * 1000);           conn.setRequestMethod("GET");           InputStream inStream = conn.getInputStream();           if(conn.getResponseCode() == HttpURLConnection.HTTP_OK){               return readStream(inStream);           }           return null;       }         /**       * Get image from newwork       * @param path The path of image       * @return InputStream     * @throws Exception       */    public InputStream getImageStream(String path) throws Exception{           URL url = new URL(path);           HttpURLConnection conn = (HttpURLConnection) url.openConnection();           conn.setConnectTimeout(5 * 1000);           conn.setRequestMethod("GET");        if(conn.getResponseCode() == HttpURLConnection.HTTP_OK){               return conn.getInputStream();              }           return null;     }    /**       * Get data from stream      * @param inStream       * @return byte[]     * @throws Exception       */      public static byte[] readStream(InputStream inStream) throws Exception{           ByteArrayOutputStream outStream = new ByteArrayOutputStream();           byte[] buffer = new byte[1024];           int len = 0;           while( (len=inStream.read(buffer)) != -1){               outStream.write(buffer, 0, len);           }           outStream.close();           inStream.close();           return outStream.toByteArray();       }     /**     * 保存文件     * @param bm     * @param fileName     * @throws IOException     */    public void saveFile(Bitmap bm, String fileName) throws IOException {        File dirFile = new File(ALBUM_PATH);        if(!dirFile.exists()){            dirFile.mkdir();        }        File myCaptureFile = new File(ALBUM_PATH + fileName);        BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream(myCaptureFile));        bm.compress(Bitmap.CompressFormat.JPEG, 80, bos);        bos.flush();        bos.close();    }        private Runnable saveFileRunnable = new Runnable(){        @Override        public void run() {            try {                saveFile(bitmap, fileName);                message = "图片保存成功！";            } catch (IOException e) {                message = "图片保存失败！";                e.printStackTrace();            }            messageHandler.sendMessage(messageHandler.obtainMessage());        }                };        private Handler messageHandler = new Handler() {        @Override        public void handleMessage(Message msg) {            myDialog.dismiss();            Log.d(TAG, message);            Toast.makeText(AndroidTest2_3_3.this, message, Toast.LENGTH_SHORT).show();        }    };}

下载进度条的可以参考我的另外一个帖子：

2.main.xml文件，只有一个button和一个ImageView

3.在mainfest文件中增加互联网权限和写sd卡的权限

4.预览图：

 

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