<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  xmlns:tools="http://schemas.android.com/tools"  android:layout_width="match_parent"  android:layout_height="match_parent"  android:orientation="vertical" >  <ImageView    android:id="@+id/imageview"    android:layout_width="wrap_content"    android:layout_height="wrap_content" />     <Button    android:id="@+id/bt_download"    android:layout_width="wrap_content"    android:layout_height="wrap_content"    android:text="圖片下載"    android:onClick="Image_Download" /></LinearLayout>

//操作SDCARD的文件工具類public class FileUtil { private static final String CACHE_DIR = Environment.getExternalStorageDirectory() + "/my_caches/images"; private static int COMP_JPG = 0; private static int COMP_PNG = 1;//判斷sdcard 是否掛載(是否有sdcard) public static boolean isMounted(){ String state = Environment.getExternalStorageState(); return state.equals(Environment.MEDIA_MOUNTED); } //獲取sdcard文件 根路徑的絕對路徑 public static String getSDCARD(){ return Environment.getExternalStorageDirectory().getAbsolutePath(); } //獲取文件名  public static String getFilename(String url){ return url.substring(url.lastIndexOf('/') + 1); } //保存文件 方法1  public static void sava1(String url,byte[] data){ //判斷是否有sdcard if(!isMounted()){ return ;  } //有sdcard //判斷是否有緩存文件夾 File dir = new File(CACHE_DIR); if(!dir.exists()){ //不存在緩存文件夾 創建文件夾用來保存文件 dir.mkdirs(); } //把文件 數據存到sdcard File file = new File(dir,getFilename(url)); try { FileOutputStream fos = new FileOutputStream(file); fos.write(data);  fos.close(); } catch (Exception e) { e.printStackTrace(); } }  //保存文件 方法2 public static void sava2(String url,Bitmap bitmap,int format){ //判斷 是否有sdcard if(!isMounted()){ return ; } File dir = new File(CACHE_DIR); if(!dir.exists()){ dir.mkdirs(); } //把 文件數據 寫到 sdcard File file = new File(dir,getFilename(url)); try { FileOutputStream fos = new FileOutputStream(file); //把圖片文件寫入緩存 bitmap.compress((format == COMP_JPG?CompressFormat.JPEG:CompressFormat.PNG), 100, fos);  fos.close(); } catch (Exception e) { e.printStackTrace(); }  } //讀取圖片 public static Bitmap readImage(String url){ if(!isMounted()){ return null; } File file = new File(CACHE_DIR,getFilename(url)); if(file.exists()){ return BitmapFactory.decodeFile(file.getAbsolutePath()); } return null; }  //清空 緩存目錄 public void clearCaches(){ File dir = new File(CACHE_DIR); File[] file_datas = dir.listFiles(); for(File file : file_datas){ file.delete(); } } }

public class MainActivity extends Activity { private ImageView imageview;  private String url = "http://b.hiphotos.baidu.com/image/pic/item/d1160924ab18972bf5f68cc8e0cd7b899f510ae7.jpg"; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main);  this.imageview = (ImageView) this.findViewById(R.id.imageview); //設置默認圖片 imageview.setImageResource(R.drawable.ic_launcher);  } //圖片下載 按鈕 點擊事件 監聽 public void Image_Download(View view){ //判斷本地是否有圖片 Bitmap bitmap = FileUtil.readImage(url); if(bitmap != null){ imageview.setImageBitmap(bitmap); }else{ //沒有圖片 就發送聯網請求 new MyAsyncTask().execute(url); } }  //聯網工具類 public class MyAsyncTask extends AsyncTask<String, Void, byte[]>{ @Override protected byte[] doInBackground(String... params) {  HttpGet get = new HttpGet(params[0]); HttpClient client = new DefaultHttpClient(); HttpResponse response; try { response = client.execute(get); if(response.getStatusLine().getStatusCode() == 200){ byte[] data = EntityUtils.toByteArray(response.getEntity()); FileUtil.sava1(params[0], data); return data; } } catch (Exception e) { e.printStackTrace(); } return null; } @Override protected void onPostExecute(byte[] result) { super.onPostExecute(result); if(result != null){ Bitmap bitmap = BitmapFactory.decodeByteArray(result, 0, result.length); imageview.setImageBitmap(bitmap); } }  }}