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步入正題
其實(shí)我們從網(wǎng)絡(luò)上拉取(加載)一張圖片很簡(jiǎn)單，隨隨便便幾行代碼搞定涯冠，那我們的圖片拉取池的意義在什么地方呢？首先我們用于圖片拉取的線程是可控的逼庞，也就是說(shuō)可以隨時(shí)取消蛇更。其次我們內(nèi)部封裝了一層線程池，保證了我們拉取的圖片是有序的赛糟。對(duì)于如何得到拉取之后的圖片派任，我采用的是接口回調(diào)，每拉取一次都會(huì)返回給調(diào)用者裆针。

好罢绽，現(xiàn)在我們看代碼舔痪。

public class ThreadPool {
    
    private static ExecutorService exec;
    private static ThreadPool threadPool;
    
    public static ThreadPool getInstance(){
        if(threadPool != null){
            return threadPool;
        }
        synchronized (ThreadPool.class) {
            if(threadPool == null){
                threadPool = new ThreadPool();
            }
        }
        return threadPool;
    }
    
    private ThreadPool(){
        exec = Executors.newFixedThreadPool(1);
    }
    
    public ThreadListener exec(Runnable runnable){
        Future<?> future = exec.submit(runnable);
        ThreadListener listener = new ThreadListener(future);
        return listener;
    }
    
    public static class ThreadListener{
        
        private Future<?> f;
        
        public ThreadListener(Future<?> future){
            this.f = future;
        }
        
        //試圖取消線程，返回true表示取消成功
        public boolean cancle(){
            return f.cancel(true);
        }
        
        //判斷是否完成
        public boolean isDone(){
            return f.isDone();
        }
    }
    
}

這里使用單例模式對(duì)線程池進(jìn)行了一層封裝豆混，注意這里我們并沒(méi)有使用execute()方法，而是采用submit方法并返回一個(gè)Future對(duì)象动知，這樣做的好處就是皿伺，我們可以控制線程，可以隨時(shí)取消并且判斷是否完成等等盒粮。

public interface Callback {

    void getBitmap(Bitmap bitmap);

}

接口鸵鸥，用于返回bitmap。

public class LoadImage {

    private ThreadPool.ThreadListener threadListener;
    private String url;
    private final int readTimeout = 8*1000;
    private final int connectTimeout = 8*1000;
    private final String requestMethod = "GET";
    private InputStream inputStream;
    private Callback callback;

    public LoadImage(String url, Callback callback){
        this.callback = callback;
        ThreadPool threadPool = ThreadPool.getInstance();
        this.url = url;
        threadListener = threadPool.exec(new Runnable() {
            @Override
            public void run() {
                runnableWorker();
            }
        });
    }

    private void runnableWorker(){
        try {
            URL mURL = new URL(url);
            HttpURLConnection httpURLConnection = (HttpURLConnection) mURL.openConnection();
            httpURLConnection.setRequestMethod(requestMethod);
            httpURLConnection.setDoOutput(true);
            httpURLConnection.setDoInput(true);
            httpURLConnection.setReadTimeout(readTimeout);
            httpURLConnection.setConnectTimeout(connectTimeout);
            inputStream = httpURLConnection.getInputStream();
            Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
            callback.getBitmap(bitmap);
        } catch (IOException e) {
            callback.getBitmap(null);
        } finally {
            try {
                inputStream.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    public boolean cancle(){
        return threadListener.cancle();
    }

    public boolean isDone(){
        return threadListener.isDone();
    }
}

這里我們對(duì)接口拆讯、線程池以及網(wǎng)絡(luò)拉取進(jìn)行二次封裝≈校現(xiàn)在核心的方法我們已經(jīng)封裝完畢养叛，針對(duì)于不同的使用，我們可以再進(jìn)行一層簡(jiǎn)單的封裝調(diào)用宰翅。

使用就非常簡(jiǎn)單了：

new LoadImage("圖片網(wǎng)址", new Callback() {
            @Override
            public void getBitmap(Bitmap bitmap) {

            }
        });

筆者能力有限弃甥，不足之處歡迎指出！