Android 异步加载图片分析

研究了android从网络上异步加载图像，现总结如下：

（1）由于android UI更新支持单一线程原则，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方法。

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(){@Overridepublic void handleMessage(Message msg) {((ImageView) LazyLoadImageActivity.this.findViewById(msg.arg1)).setImageDrawable((Drawable)msg.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(){@Overridepublic 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(new URL(url).openStream(), "image.png");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，考虑到效率问题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(); /**** @param imageUrl 图像url地址* @param callback 回调接口* @return 返回内存中缓存的图像，第一次加载返回null*/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);}}

这样封装好后使用起来就方便多了。在主线程中首先要引入AsyncImageLoader3 对象，然后直接调用其loadDrawable方法即可，需要注意的是ImageCallback接口的imageLoaded方法是唯一可以把加载的图像设置到目标ImageView或其相关的组件上。

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：
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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>(); /**** @param imageUrl 图像url地址* @param callback 回调接口* @return 返回内存中缓存的图像，第一次加载返回null*/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() {@Overridepublic 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();/*下面注释的这段代码是Handler的一种代替方法*/// new AsyncTask() {// @Override// protected Drawable doInBackground(Object... objects) {// Drawable drawable = loadImageFromUrl(imageUrl);// imageCache.put(imageUrl, new SoftReference(drawable));// return drawable;// }//// @Override// protected void onPostExecute(Object o) {// 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(){@Overridepublic 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(); @Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.main); // loadImage("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);// loadImage("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);// loadImage("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);// loadImage("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);// loadImage("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5); loadImage2("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);loadImage2("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);loadImage2("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);loadImage2("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);loadImage2("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);// loadImage3("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);// loadImage3("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);// loadImage3("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);// loadImage3("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);// loadImage3("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5); // loadImage4("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);// loadImage4("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);// loadImage4("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);// loadImage4("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);// loadImage4("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5); // loadImage5("http://www.chinatelecom.com.cn/images/logo_new.gif", R.id.image1);// //为了测试缓存而模拟的网络延时// SystemClock.sleep(2000);// loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image2);// SystemClock.sleep(2000);// loadImage5("http://cache.soso.com/30d/img/web/logo.gif", R.id.image3);// SystemClock.sleep(2000);// loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);// SystemClock.sleep(2000);// loadImage5("http://cache.soso.com/30d/img/web/logo.gif", R.id.image5);// SystemClock.sleep(2000);// loadImage5("http://www.baidu.com/img/baidu_logo.gif", R.id.image4);} @Overrideprotected 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(){@Overridepublic 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");handler.post(new Runnable() { 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：

< ?xml version="1.0" encoding="utf-8"?> < LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:layout_width="fill_parent"android:orientation="vertical"android:layout_height="fill_parent" >< /LinearLayout>

这篇文章出自EOE：http://www.eoeandroid.com/forum.php?mod=viewthread&tid=236245
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