在Android开发过程中，为了防止阻塞UI，图片加载时经常采用异步的方法来加载，异步加载图片的主要流程是进行判断缓存中是否存在图片，如果存在则直接返回，如果不存在则进行下载并进行缓存。

以下是建立一个异步下载类：

/**?
*?User:?Tom?
*?Date:?13-5-13?
*?Time:?下午8:07?
*/?
public?class?AsnycImageLoader?{?

//定义一个HashMap进行存放缓存的Image?key为String?Value为一个弱引用的一个资源文件?
//?图片?为了方便JAVA的回收?
private?Map<String,?SoftReference<Drawable>>?imageCache?=?null;?
public?AsnycImageLoader()?{?
imageCache?=?new?HashMap<String,?SoftReference<Drawable>>();?
}?

/**?
*?加载图片?
*?<p>imageurl为下载资源的URL，?
*?ImageCallback当缓存中不存在相关图片时时行网络下载?
*?在多线程下要使用Handler进行操作UI?利用回调接口的方式进行更新UI?
*?</p>?
*?@param?imageUrl?
*?@param?callback?
*?@return?
*/?
public?Drawable?loadDrawable(final?String?imageUrl,?final?ImageCallback?callback)?{?
//进行判断ImageCache中是否存在缓存图片?
if?(imageCache.containsKey(imageUrl))?{?
SoftReference<Drawable>?softReference?=?imageCache.get(imageUrl);?
if?(softReference.get()?!=?null)?{?
return?softReference.get();?
}?
}?
//定义操作UI的Handler?
final?Handler?handler?=?new?Handler()?{?
@Override?
public?void?handleMessage(Message?msg)?{?
callback.imageLoaded((Drawable)?msg.obj);?
}?
};?

new?Thread(new?Runnable()?{?
@Override?
public?void?run()?{?
Drawable?drawable?=?loadImageFromUrl(imageUrl);?
imageCache.put(imageUrl,?new?SoftReference<Drawable>(drawable));?
Message?message?=?handler.obtainMessage(0,?drawable);?
handler.sendMessage(message);?
}?
}).start();?
return?null;?
}?

//根据URL地址进行获取资源?
protected?Drawable?loadImageFromUrl(String?imageUrl)?{?
try?{?
return?Drawable.createFromStream(new?URL(imageUrl).openStream(),?"src");?
}?catch?(Exception?e)?{?
throw?new?RuntimeException();?
}?
}?

//回调接口?
public?interface?ImageCallback?{?
public?abstract?void?imageLoaded(Drawable?drawable);?
}?
}

主Activity：

/**?
*?User:?Tom?
*?Date:?13-5-13?
*?Time:?下午8:33?
*/?
public?class?LoadImage?extends?Activity?{?
private?AsnycImageLoader?loader?=?null;?

public?void?onCreate(Bundle?savedInstanceState)?{?
super.onCreate(savedInstanceState);?
setContentView(R.layout.loadimages);?

loader?=?new?AsnycImageLoader();?

loadImage("http://www.jb51.net/images/icon-partners.png",?R.id.image1);?
loadImage("http://www.jb51.net/images/icon-dev.png",?R.id.image2);?
loadImage("http://pic28.jb51.net/20130421/12302174_231210305323_2.jpg",?R.id.image3);?

}?

public?void?loadImage(String?url,?int?id)?{?
final?ImageView?imageView?=?(ImageView)?findViewById(id);?
Drawable?cacheImage?=?loader.loadDrawable(url,?new?AsnycImageLoader.ImageCallback()?{?
@Override?
public?void?imageLoaded(Drawable?drawable)?{?
imageView.setImageDrawable(drawable);?
}?
});?
if?(cacheImage?!=?null)?{?
imageView.setImageDrawable(cacheImage);?
}?
}?
}