public abstract class AsyncTask<Params, Progress, Result> {
 private static final String LOG_TAG = "AsyncTask";

 private static final int CORE_POOL_SIZE = 5;
 private static final int MAXIMUM_POOL_SIZE = 128;
 private static final int KEEP_ALIVE = 1;

 private static final ThreadFactory sThreadFactory = new ThreadFactory() {
  private final AtomicInteger mCount = new AtomicInteger(1);

  public Thread newThread(Runnable r) {
   return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
  }
 };

 private static final InternalHandler sHandler = new InternalHandler();

 mFuture = new FutureTask<Result>(mWorker) {
   @Override
   protected void More ...done() {
    Message message;
    Result result = null;
    try {
     result = get();
    } catch (InterruptedException e) {
     android.util.Log.w(LOG_TAG, e);
    } catch (ExecutionException e) {
     throw new RuntimeException("An error occured while executing doInBackground()",
       e.getCause());
    } catch (CancellationException e) {
     message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
       new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
     message.sendToTarget();
     return;
    } catch (Throwable t) {
     throw new RuntimeException("An error occured while executing "
       + "doInBackground()", t);
    }

    message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
      new AsyncTaskResult<Result>(AsyncTask.this, result));
    message.sendToTarget();
   }
  };
 private static class InternalHandler extends Handler {
  @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
  @Override
  public void More ...handleMessage(Message msg) {
   AsyncTaskResult result = (AsyncTaskResult) msg.obj;
   switch (msg.what) {
    case MESSAGE_POST_RESULT:
     // There is only one result
     result.mTask.finish(result.mData[0]);
     break;
    case MESSAGE_POST_PROGRESS:
     result.mTask.onProgressUpdate(result.mData);
     break;
    case MESSAGE_POST_CANCEL:
     result.mTask.onCancelled();
     break;
   }
  }
 }

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.widget.ImageView;

/**
 * 網(wǎng)絡(luò)緩存
 * 
 * @author Ace
 * @date 2016-02-18
 */
public class NetCacheUtils {

 private LocalCacheUtils mLocalUtils;
 private MemoryCacheUtils mMemoryUtils;

 public NetCacheUtils(LocalCacheUtils localUtils,
   MemoryCacheUtils memoryUtils) {
  mLocalUtils = localUtils;
  mMemoryUtils = memoryUtils;
 }

 public void getBitmapFromNet(ImageView imageView, String url) {
  BitmapTask task = new BitmapTask();
  task.execute(imageView, url);
 }

 /**
  * AsyncTask是線程池+handler的封裝 第一個泛型: 傳參的參數(shù)類型類型(和doInBackground一致) 第二個泛型:
  * 更新進(jìn)度的參數(shù)類型(和onProgressUpdate一致) 第三個泛型: 返回結(jié)果的參數(shù)類型(和onPostExecute一致,
  * 和doInBackground返回類型一致)
  */
 class BitmapTask extends AsyncTask<Object, Integer, Bitmap> {

  private ImageView mImageView;
  private String url;

  // 主線程運行, 預(yù)加載
  @Override
  protected void onPreExecute() {
   super.onPreExecute();
  }

  // 子線程運行, 異步加載邏輯在此方法中處理
  @Override
  protected Bitmap doInBackground(Object... params) {
   mImageView = (ImageView) params[0];
   url = (String) params[1];
   mImageView.setTag(url);// 將imageView和url綁定在一起
   // publishProgress(values)//通知進(jìn)度

   // 下載圖片
   return download(url);
  }

  // 主線程運行, 更新進(jìn)度
  @Override
  protected void onProgressUpdate(Integer... values) {
   super.onProgressUpdate(values);
  }

  // 主線程運行, 更新主界面
  @Override
  protected void onPostExecute(Bitmap result) {
   if (result != null) {
    // 判斷當(dāng)前圖片是否就是imageView要的圖片, 防止listview重用導(dǎo)致的圖片錯亂的情況出現(xiàn)
    String bindUrl = (String) mImageView.getTag();
    if (bindUrl.equals(url)) {
     // 給imageView設(shè)置圖片
     mImageView.setImageBitmap(result);
 // 將圖片保存在本地
     mLocalUtils.setBitmapToLocal(result, url);

     // 將圖片保存在內(nèi)存
     mMemoryUtils.setBitmapToMemory(url, result);
    }
   }
  }

 }

 /**
  * 下載圖片
  * 
  * @param url
  */
 public Bitmap download(String url) {
  HttpURLConnection conn = null;
  try {
   conn = (HttpURLConnection) (new URL(url).openConnection());

   conn.setConnectTimeout(5000);
   conn.setReadTimeout(5000);
   conn.setRequestMethod("GET");

   conn.connect();

   int responseCode = conn.getResponseCode();
   if (responseCode == 200) {
    InputStream in = conn.getInputStream();
    // 將流轉(zhuǎn)化為bitmap對象
    Bitmap bitmap = BitmapFactory.decodeStream(in);
    return bitmap;
   }

  } catch (Exception e) {
   e.printStackTrace();
  } finally {
   if (conn != null) {
    conn.disconnect();
   }
  }

  return null;
 }

}

* A cache that holds strong references to a limited number of values. Each time 

* a value is accessed, it is moved to the head of a queue. When a value is 

* added to a full cache, the value at the end of that queue is evicted and may 
* become eligible for garbage collection. 

* Cache保存一個強(qiáng)引用來限制內(nèi)容數(shù)量，每當(dāng)Item被訪問的時候，此Item就會移動到隊列的頭部。
* 當(dāng)cache已滿的時候加入新的item時，在隊列尾部的item會被回收。
* <p>If your cached values hold resources that need to be explicitly released, 
* override {@link #entryRemoved}. 
* 如果你cache的某個值需要明確釋放，重寫entryRemoved()

 

* <p>By default, the cache size is measured in the number of entries. Override 
* {@link #sizeOf} to size the cache in different units. For example, this cache 
* is limited to 4MiB of bitmaps: 默認(rèn)cache大小是測量的item的數(shù)量，重寫sizeof計算不同item的
* 大小。

 {@code 
 * int cacheSize = 4 * 1024 * 1024; // 4MiB 
 * LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) { 
 *  protected int sizeOf(String key, Bitmap value) { 
 *   return value.getByteCount(); 
 *  } 
 * }}
 

-------------------------------------------------------------------

<p>This class is thread-safe. Perform multiple cache operations atomically by 
* synchronizing on the cache: <pre> {@code 
* synchronized (cache) { 
* if (cache.get(key) == null) { 
* cache.put(key, value); 
* } 
* }}</pre> 
* 他是線程安全的,自動地執(zhí)行多個緩存操作并且加鎖

 

-------------------------

<p>This class does not allow null to be used as a key or value. A return 
* value of null from {@link #get}, {@link #put} or {@link #remove} is 
* unambiguous: the key was not in the cache.
* 不允許key或者value為null
* 當(dāng)get（），put（），remove（）返回值為null時，key相應(yīng)的項不在cache中

import android.graphics.Bitmap;
import android.support.v4.util.LruCache;

/**
 * 內(nèi)存緩存工具類
 * 
 * @author Ace
 * @date 2016-02-19
 */
public class MemoryCacheUtils {


 // Android 2.3 (API Level
 // 9)開始，垃圾回收器會更傾向于回收持有軟引用或弱引用的對象，這讓軟引用和弱引用變得不再可靠,建議用LruCache,它是強(qiáng)引用

 private LruCache<String, Bitmap> mCache;

 public MemoryCacheUtils() {
  int maxMemory = (int) Runtime.getRuntime().maxMemory();// 獲取虛擬機(jī)分配的最大內(nèi)存
                // 16M
  // LRU 最近最少使用, 通過控制內(nèi)存不要超過最大值(由開發(fā)者指定), 來解決內(nèi)存溢出,就像上面翻譯的所說 如果cache滿了會清理最近最少使用的緩存對象
  mCache = new LruCache<String, Bitmap>(maxMemory / 8) {
   @Override
   protected int sizeOf(String key, Bitmap value) {
    // 計算一個bitmap的大小
    int size = value.getRowBytes() * value.getHeight();// 每一行的字節(jié)數(shù)乘以高度
    return size;
   }
  };
 }

 public Bitmap getBitmapFromMemory(String url) {

  return mCache.get(url);
 }

 public void setBitmapToMemory(String url, Bitmap bitmap) {

  mCache.put(url, bitmap);
 }

}

/**
 * 本地緩存工具類
 * 
 * @author Ace
 * @date 2016-02-19
 */
public class LocalCacheUtils {

 // 圖片緩存的文件夾
 public static final String DIR_PATH = Environment
   .getExternalStorageDirectory().getAbsolutePath()
   + "/ace_bitmap_cache";

 public Bitmap getBitmapFromLocal(String url) {
  try {
   File file = new File(DIR_PATH, MD5Encoder.encode(url));

   if (file.exists()) {
    Bitmap bitmap = BitmapFactory.decodeStream(new FileInputStream(
      file));
    return bitmap;
   }

  } catch (Exception e) {
   e.printStackTrace();
  }

  return null;
 }

 public void setBitmapToLocal(Bitmap bitmap, String url) {
  File dirFile = new File(DIR_PATH);

  // 創(chuàng)建文件夾 文件夾不存在或者它不是文件夾 則創(chuàng)建一個文件夾.mkdirs,mkdir的區(qū)別在于假如文件夾有好幾層路徑的話,前者會創(chuàng)建缺失的父目錄 后者不會創(chuàng)建這些父目錄
  if (!dirFile.exists() || !dirFile.isDirectory()) {
   dirFile.mkdirs();
  }

  try {
   File file = new File(DIR_PATH, MD5Encoder.encode(url));
   // 將圖片壓縮保存在本地,參1:壓縮格式;參2:壓縮質(zhì)量(0-100);參3:輸出流
   bitmap.compress(CompressFormat.JPEG, 100,
     new FileOutputStream(file));
  } catch (Exception e) {
   e.printStackTrace();
  }
 }

}

import java.security.MessageDigest;

public class MD5Encoder {
 
 public static String encode(String string) throws Exception {
  byte[] hash = MessageDigest.getInstance("MD5").digest(string.getBytes("UTF-8"));
  StringBuilder hex = new StringBuilder(hash.length * 2);
  for (byte b : hash) {
   if ((b & 0xFF) < 0x10) {
    hex.append("0");
   }
   hex.append(Integer.toHexString(b & 0xFF));
  }
  return hex.toString();
 }
}

/**
 * 三級緩存工具類
 * 
 * @author Ace
 * @date 2016-02-19
 */
public class MyBitmapUtils {

 // 網(wǎng)絡(luò)緩存工具類
 private NetCacheUtils mNetUtils;
 // 本地緩存工具類
 private LocalCacheUtils mLocalUtils;
 // 內(nèi)存緩存工具類
 private MemoryCacheUtils mMemoryUtils;

 public MyBitmapUtils() {
  mMemoryUtils = new MemoryCacheUtils();
  mLocalUtils = new LocalCacheUtils();
  mNetUtils = new NetCacheUtils(mLocalUtils, mMemoryUtils);
 }

 public void display(ImageView imageView, String url) {
  // 設(shè)置默認(rèn)加載圖片
  imageView.setImageResource(R.drawable.news_pic_default);

  // 先從內(nèi)存緩存加載
  Bitmap bitmap = mMemoryUtils.getBitmapFromMemory(url);
  if (bitmap != null) {
   imageView.setImageBitmap(bitmap);
   System.out.println("從內(nèi)存讀取圖片啦...");
   return;
  }

  // 再從本地緩存加載
  bitmap = mLocalUtils.getBitmapFromLocal(url);
  if (bitmap != null) {
   imageView.setImageBitmap(bitmap);
   System.out.println("從本地讀取圖片啦...");
   // 給內(nèi)存設(shè)置圖片
   mMemoryUtils.setBitmapToMemory(url, bitmap);
   return;
  }

  // 從網(wǎng)絡(luò)緩存加載
  mNetUtils.getBitmapFromNet(imageView, url);
 }

}