注意：

下載圖片、儲存圖片都需要非同步操作。

java程式碼:

package com.example.administrator.downloadimgdemo;

import android.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import
 
 android.os.Message;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

import java.io.BufferedOutputStream;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileOutputStream;
import
 
 java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

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();//獲取網路圖片的位元組陣列或者流，轉換成bitmap並顯示圖片

        // 下載圖片
        mBtnSave.setOnClickListener(new Button.OnClickListener() {
            public void onClick(View v) {
                mSaveDialog = ProgressDialog.show(IcsTestActivity.this, "儲存圖片", "圖片正在儲存中，請稍等...", true);
                new Thread(saveFileRunnable).start();
            }
        });
    }


    /**
     * 儲存檔案
     *
     * @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();//建立不存在的目錄
        }
        File myCaptureFile = new File(ALBUM_PATH + fileName);//將字串轉換成抽象路徑名

        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 = "圖片儲存成功！";
            } catch (IOException e) {
                mSaveMessage = "圖片儲存失敗！";
                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();
        }
    };

    /*
     * 連線網路
     * 由於在4.0中不允許在主執行緒中訪問網路，所以需要在子執行緒中訪問
     */
    private Runnable connectNet = new Runnable() {
        @Override
        public void run() {
            try {
                String filePath = "http://img.my.csdn.net/uploads/201402/24/1393242467_3999.jpg";
                mFileName = "test.jpg";

                //以下是取得圖片的兩種方法
                //////////////// 方法1：取得的是byte陣列, 從byte陣列生成bitmap
                byte[] data = getImage(filePath);
                if (data != null) {
                    mBitmap = BitmapFactory.decodeByteArray(data, 0, data.length);// bitmap
                } else {
                    Toast.makeText(IcsTestActivity.this, "Image error!",Toast.LENGTH_LONG).show();
                }
                ////////////////////////////////////////////////////////

                //******** 方法2：取得的是InputStream，直接從InputStream生成bitmap :推薦使用***********/
                mBitmap = BitmapFactory.decodeStream(getImageStream(filePath));
                //********************************************************************/

                // 傳送訊息，通知handler在主執行緒中更新UI
                connectHanlder.sendEmptyMessage(0);
                Log.d(TAG, "set image ...");
            } catch (Exception e) {
                Toast.makeText(IcsTestActivity.this, "無法連結網路！", Toast.LENGTH_SHORT).show();
                e.printStackTrace();
            }

        }

    };

    private Handler connectHanlder = new Handler() {
        @Override
        public void handleMessage(Message msg) {
            Log.d(TAG, "display image");
            // 更新UI，顯示圖片
            if (mBitmap != null) {
                mImageView.setImageBitmap(mBitmap);// display image
            }
        }
    };

      /*---------------------------------------------獲取網路圖片-----------------------------------------------*/
    /**
     * 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 byte[] 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();
    }

    /*---------------------------------------------------------------------------------------------------------*/

}

佈局：

<?xml version="1.0" encoding="utf-8"?>
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical">


        <Button
            android:id="@+id/btnSave"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="儲存圖片" />

        <ImageView
            android:id="@+id/imgSource"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:adjustViewBounds="true" />

    </LinearLayout>
</ScrollView>