Android 异步加载图片分析总结

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
代码如下:
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：
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：
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：
代码如下:
// 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
代码如下:
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();
 

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();

// 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(), "class="lazy" data-src");
} catch (Exception e) {
throw new Run timeException(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：
代码如下:
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("/file/upload/202206/06/jn55tbp3pzd.gif", R.id.image1);
// loadImage("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image2);
// loadImage("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image3);
// loadImage("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image4);
// loadImage("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image5);
 
loadImage2("/file/upload/202206/06/jn55tbp3pzd.gif", R.id.image1);
loadImage2("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image2);
loadImage2("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image3);
loadImage2("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image4);
loadImage2("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image5);
// loadImage3("/file/upload/202206/06/jn55tbp3pzd.gif", R.id.image1);
// loadImage3("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image2);
// loadImage3("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image3);
// loadImage3("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image4);
// loadImage3("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image5);
 
// loadImage4("/file/upload/202206/06/jn55tbp3pzd.gif", R.id.image1);
// loadImage4("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image2);
// loadImage4("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image3);
// loadImage4("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image4);
// loadImage4("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image5);
 
// loadImage5("/file/upload/202206/06/jn55tbp3pzd.gif", R.id.image1);
// //为了测试缓存而模拟的网络延时
// SystemClock.sleep(2000);
// loadImage5("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image2);
// SystemClock.sleep(2000);
// loadImage5("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image3);
// SystemClock.sleep(2000);
// loadImage5("/file/upload/202206/06/rrovmj44rv5.gif", R.id.image4);
// SystemClock.sleep(2000);
// loadImage5("/file/upload/202206/06/dl3ozktgykj.gif", R.id.image5);
// SystemClock.sleep(2000);
// loadImage5("/file/upload/202206/06/rrovmj44rv5.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：
代码如下:
<SPAN style="FONT-SIZE: 18px"><STRONG>< ?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></STRONG></SPAN>

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