android异步加载图片实战

android异步加载图片实战 研究了android从网络上异步加载图像，现总结如下: (1)由于android UI更新支持单一线程，所以从网络上取数据并更新到界面上，为了不阻塞主线程首先可能会想到以下。 在主线程中new 一个Handler对象，加载图像方法如下所示 private void loadImage(final String url, final int id) { handler.post(new Runnable() { public void run() { Drawable drawable = null; try { drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png"); } catch (IOException e) { } ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable); } }); } 上面这个方法缺点很显然，经测试，如果要加载多个图片，这并不能实现异步加载，而是等到所有的图片都加载完才一起显示，因为它们都运行在一个线程中。 然后，我们可以简单改进下，将Handler+Runnable模式改为Handler+Thread+Message模式不就能实现同时开启多个线程吗, (2)在主线程中new 一个Handler对象，代码如下: final Handler handler2=new Handler(){ @Override public void handleMessage(Message msg) { ((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)ms g.obj); } }; 对应加载图像代码如下: //采用handler+Thread模式实现多线程异步加载 private void loadImage2(final String url, final int id) { Thread thread = new Thread(){ @Override public void run() { Drawable drawable = null; try { drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png"); } catch (IOException e) { } Message message= handler2.obtainMessage() ; message.arg1 = id; message.obj = drawable; handler2.sendMessage(message); } }; thread.start(); thread = null; } 这样就简单实现了异步加载了。细想一下，还可以优化的，比如引入线程池、引入缓存等， 我们先介绍线程池。 (3)引入ExecutorService接口，于是代码可以优化如下: 在主线程中加入:private ExecutorService executorService = Executors.newFixedThreadPool(5); 对应加载图像方法更改如下: // 引入线程池来管理多线程 private void loadImage3(final String url, final int id) { executorService.submit(new Runnable() { public void run() { try { final Drawable drawable = Drawable.createFromStream(ne handler.post(new Runnable() { public void run() { ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable); } }); } catch (Exception e) { throw new RuntimeException(e); } } }); } (4)为了更方便使用我们可以将异步加载图像方法封装一个类，对外界只暴露一个方法即可， 考虑到效率问我们可以引入内存缓存机制，做法是 建立一个HashMap，其键(key)为加载图像url，其值(value)是图像对象Drawable。先 看一下我们封装的类 public class AsyncImageLoader3 { //为了加快速度，在内存中开启缓存(主要应用于重复图片较多时，或者同一个图片要多 次被访问，比如在ListView时来回滚动) public Map> imageCache = new HashMap>(); private ExecutorService executorService = Executors.newFixedThreadPool(5); //固定五个线程来执行任务 private final Handler handler=new Handler(); public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) { //如果缓存过就从缓存中取出数据 if (imageCache.containsKey(imageUrl)) { SoftReference softReference = imageCache.get(imageUrl); if (softReference.get() != null) { return softReference.get(); } } //缓存中没有图像，则从网络上取出数据，并将取出的数据缓存到内存中 executorService.submit(new Runnable() { public void run() { try { final Drawable drawable = Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png"); imageCache.put(imageUrl, new SoftReference(drawable)); handler.post(new Runnable() { public void run() { callback.imageLoaded(drawable); } }); } catch (Exception e) { throw new RuntimeException(e); } } }); return null; } //从网络上取数据方法 protected Drawable loadImageFromUrl(String imageUrl) { try { return Drawable.createFromStream(new URL(imageUrl).openStream(), "image.png"); } catch (Exception e) { throw new RuntimeException(e); } } //对外界开放的回调接口 public interface ImageCallback { //注意 此方法是用来设置目标对象的图像资源 public void imageLoaded(Drawable imageDrawable); } }这样封装好后使用 在主线程调用代码: 先实例化对象 private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3(); 调用异步加载方法: //引入线程池，并引入内存缓存功能,并对外部调用封装了接口，简化调用过程 private void loadImage4(final String url, final int id) { //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行 Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() { //请参见实现:如果第一次加载url时下面方法会执行 public void imageLoaded(Drawable imageDrawable) { ((ImageView) findViewById(id)).setImageDrawable(imageDrawable); } }); if(cacheImage!=null){ ((ImageView) findViewById(id)).setImageDrawable(cacheImage); } } (5)同理，下面也给出采用Thread+Handler+MessageQueue+内存缓存代码，原则同(4)， 只是把线程池换成了Thread+Handler+MessageQueue模式而已。代码如下: public class AsyncImageLoader { //为了加快速度，加入了缓存(主要应用于重复图片较多时，或者同一个图片要多次被访 问，比如在ListView时来回滚动) private Map> imageCache = new HashMap>(); public Drawable loadDrawable(final String imageUrl, final ImageCallback callback) { //如果缓存过就从缓存中取出数据 if (imageCache.containsKey(imageUrl)) { SoftReference softReference = imageCache.get(imageUrl); if (softReference.get() != null) { return softReference.get(); } } final Handler handler = new Handler() { @Override public void handleMessage(Message msg) { callback.imageLoaded((Drawable) msg.obj); } }; new Thread() { public void run() { Drawable drawable = loadImageFromUrl(imageUrl); imageCache.put(imageUrl, new SoftReference(drawable)); handler.sendMessage(handler.obtainMessage(0, drawable)); } }.start(); // new AsyncTask() { // @Override // protected Drawable doInBackground(Object... objects) { // Drawable drawable = loadImageFromUrl(imageUrl); // imageCache.put(imageUrl, new SoftReference(drawable)); // return drawable; // } // // @Override // // callback.imageLoaded((Drawable) o); // } // }.execute(); return null; } protected Drawable loadImageFromUrl(String imageUrl) { try { return Drawable.createFromStream(new URL(imageUrl).openStream(), "src"); } catch (Exception e) { throw new RuntimeException(e); } } //对外界开放的回调接口 public interface ImageCallback { public void imageLoaded(Drawable imageDrawable); } } 至此，异步加载就介绍完了，下面给出的代码为测试用的完整代码: package com.bshark.supertelphone.activity; import android.app.Activity; import android.graphics.drawable.Drawable; import android.os.Bundle; import android.os.Handler; import android.os.Message; import android.widget.ImageView; import com.bshark.supertelphone.R; import com.bshark.supertelphone.ui.adapter.util.AsyncImageLoader; import com.bshark.supertelphone.ui.adapter.util.AsyncImageLoader3; import java.io.IOException; import java.net.URL; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class LazyLoadImageActivity extends Activity { final Handler handler=new Handler(); final Handler handler2=new Handler(){ @Override public void handleMessage(Message msg) { ((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.obj); } }; private ExecutorService executorService = Executors.newFixedThreadPool(5); //固定五个线程来执行任务 private AsyncImageLoader asyncImageLoader = new AsyncImageLoader(); private AsyncImageLoader3 asyncImageLoader3 = new AsyncImageLoader3(); @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); // loadImage(";, R.id.image1); // loadImage(";, R.id.image2); // loadImage(";, R.id.image3); // loadImage(";, R.id.image4); // loadImage(";, R.id.image5); loadImage2(";, R.id.image1); loadImage2(";, R.id.image2); loadImage2(";, R.id.image3); loadImage2(";, R.id.image4); loadImage2(";, R.id.image5); // loadImage3(";, R.id.image1); // loadImage3(";, R.id.image2); // loadImage3(";, R.id.image3); // // loadImage3(";, R.id.image5); // loadImage4(";, R.id.image1); // loadImage4(";, R.id.image2); // loadImage4(";, R.id.image3); // loadImage4(";, R.id.image4); // loadImage4(";, R.id.image5); // loadImage5(";, R.id.image1); // //为了测试缓存而模拟的网络延时 // SystemClock.sleep(2000); // loadImage5(";, R.id.image2); // SystemClock.sleep(2000); // loadImage5(";, R.id.image3); // SystemClock.sleep(2000); // loadImage5(";, R.id.image4); // SystemClock.sleep(2000); // loadImage5(";, R.id.image5); // SystemClock.sleep(2000); // loadImage5(";, R.id.image4); } @Override protected void onDestroy() { executorService.shutdown(); super.onDestroy(); } //线程加载图像基本原理 private void loadImage(final String url, final int id) { handler.post(new Runnable() { public void run() { Drawable drawable = null; try { drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png"); } catch (IOException e) { } ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable); } }); } //采用handler+Thread模式实现多线程异步加载 private void loadImage2(final String url, final int id) { Thread thread = new Thread(){ @Override public void run() { Drawable drawable = null; try { drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png"); } catch (IOException e) { } Message message= handler2.obtainMessage() ; message.arg1 = id; message.obj = drawable; handler2.sendMessage(message); } }; thread.start(); thread = null; } // 引入线程池来管理多线程 private void loadImage3(final String url, final int id) { executorService.submit(new Runnable() { public void run() { try { final Drawable drawable = Drawable.createFromStream(new URL(url).openStream(), "image.png"); public void run() { ((ImageView) LazyLoadImageActivity.this.findViewById(id)).setImageDrawable(drawable); } }); } catch (Exception e) { throw new RuntimeException(e); } } }); } //引入线程池，并引入内存缓存功能,并对外部调用封装了接口，简化调用过程 private void loadImage4(final String url, final int id) { //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行 Drawable cacheImage = asyncImageLoader.loadDrawable(url,new AsyncImageLoader.ImageCallback() { //请参见实现:如果第一次加载url时下面方法会执行 public void imageLoaded(Drawable imageDrawable) { ((ImageView) findViewById(id)).setImageDrawable(imageDrawable); } }); if(cacheImage!=null){ ((ImageView) findViewById(id)).setImageDrawable(cacheImage); } } //采用Handler+Thread+封装外部接口 private void loadImage5(final String url, final int id) { //如果缓存过就会从缓存中取出图像，ImageCallback接口中方法也不会被执行 Drawable cacheImage = asyncImageLoader3.loadDrawable(url,new AsyncImageLoader3.ImageCallback() { //请参见实现:如果第一次加载url时下面方法会执行 public void imageLoaded(Drawable imageDrawable) { ((ImageView) findViewById(id)).setImageDrawable(imageDrawable); } }); if(cacheImage!=null){ ((ImageView) findViewById(id)).setImageDrawable(cacheImage); } } } xml文件大致如下: