Android 软引用和弱引用详解及实例代码

package com.stevenhu.lit;
 
import java.lang.ref.SoftReference;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;
 
import android.graphics.drawable.Drawable;
import android.os.Handler;
import android.os.Message;
 
//实现图片异步加载的类
public class AsyncImageLoader
{
  //以Url为键，SoftReference为值，建立缓存HashMap键值对。
  private Map<String, SoftReference<Drawable>> mImageCache =
    new HashMap<String, SoftReference<Drawable>>();
   
  //实现图片异步加载
  public Drawable loadDrawable(final String imageUrl, final ImageCallback callback)
  {
    //查询缓存，查看当前需要下载的图片是否在缓存中
    if(mImageCache.containsKey(imageUrl))
    {
      SoftReference<Drawable> softReference = mImageCache.get(imageUrl);
      if (softReference.get() != null)
      {
        return softReference.get();
      }
    }
     
    final Handler handler = new Handler()
    {
      @Override
      public void dispatchMessage(Message msg)
      {
        //回调ImageCallbackImpl中的imageLoad方法，在主线(UI线程)中执行。
        callback.imageLoad((Drawable)msg.obj);
      }
    };
     
    /*若缓存中没有，新开辟一个线程，用于进行从网络上下载图片，
     * 然后将获取到的Drawable发送到Handler中处理，通过回调实现在UI线程中显示获取的图片
     */
    new Thread()
    {   
      public void run()
      {
        Drawable drawable = loadImageFromUrl(imageUrl);
        //将得到的图片存放到缓存中
        mImageCache.put(imageUrl, new SoftReference<Drawable>(drawable));
        Message message = handler.obtainMessage(0, drawable);
        handler.sendMessage(message);
      };
    }.start();
     
    //若缓存中不存在，将从网上下载显示完成后，此处返回null；
    return null;
     
  }
   
  //定义一个回调接口
  public interface ImageCallback
  {
    void imageLoad(Drawable drawable);
  }
   
  //通过Url从网上获取图片Drawable对象；
  protected Drawable loadImageFromUrl(String imageUrl)
  {
    try {
      return Drawable.createFromStream(new URL(imageUrl).openStream(),"debug");
    } catch (Exception e) {
      // TODO: handle exception
      throw new RuntimeException(e);
    }
  }
}
　　　　

package rxnet.zyj.com.myapplication;
 
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.support.v7.app.AppCompatActivity;
import java.lang.ref.WeakReference;
 
public class MainActivity extends AppCompatActivity {
 
  private Handler handler ;
 
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
 
    handler = new MyHandler( this ) ;
 
    new Thread(new Runnable() {
      @Override
      public void run() {
        handler.sendEmptyMessage( 0 ) ;
      }
    }).start() ;
 
  }
 
  private static class MyHandler extends Handler {
    WeakReference<MainActivity> weakReference ;
 
    public MyHandler(MainActivity activity ){
      weakReference = new WeakReference<MainActivity>( activity) ;
    }
 
    @Override
    public void handleMessage(Message msg) {
      super.handleMessage(msg);
      if ( weakReference.get() != null ){
        // update android ui
      }
    }
  }
 
}