下文转自:

http://blog.csdn.net/sgl870927/archive/2011/03/29/6285535.aspx

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

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(){
@Override
public 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(){
@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(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<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();
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<Drawable> 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>(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:

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<String, SoftReference<Drawable>> imageCache = new HashMap<String, SoftReference<Drawable>>();

/**
*
* @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<Drawable> 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>(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>(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(){
@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("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);
}

@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");
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" >
<ImageView android:id="@+id/image1" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
<ImageView android:id="@+id/image2" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
<ImageView android:id="@+id/image3" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
<ImageView android:id="@+id/image5" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
<ImageView android:id="@+id/image4" android:layout_height="wrap_content" android:layout_width="fill_parent"></ImageView>
</LinearLayout>

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