AsyncHttpClient 源码分析

开源的AsyncHttp是基于Apache HTTP Client包装的一个库，比较简单，回调进行获取数据，不用自己处理线程和实例化Handler

上一篇写了个 AsyncHttpClient 获取图片和网页数据演示 简单用法

AsyncHttpClient.java 是暴露的接口，最主要的就是Post和Get 2种,还有其他的HTTP请求方式 比如还有Delete，Patch操作，不是很常用

线程池用的是 Executors.newCachedThreadPool() 这个源码里面显示 核心线程是0，创建的线程60秒后不用就销毁，感觉比较符合网络频繁操作一段时间，

比如ListView异步下载Bitmap图片

  public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
  }
    
RequestHandle 返回的请求运行对象，可以cancel一个请求 ,但不一定能成功。

创建异步请求用于任务执行

protected AsyncHttpRequest newAsyncHttpRequest(DefaultHttpClient client, HttpContext httpContext, HttpUriRequest uriRequest, String contentType, ResponseHandlerInterface responseHandler, Context context) {
     return new AsyncHttpRequest(client, httpContext, uriRequest, responseHandler);
}

//发送请求
    protected RequestHandle sendRequest(DefaultHttpClient client, HttpContext httpContext, HttpUriRequest uriRequest, String contentType, ResponseHandlerInterface responseHandler, Context context) {
        //判断代码省去。。。。。只看关键的

        if (contentType != null) {
            if (uriRequest instanceof HttpEntityEnclosingRequestBase && ((HttpEntityEnclosingRequestBase) uriRequest).getEntity() != null) {
                Log.w(LOG_TAG, "Passed contentType will be ignored because HttpEntity sets content type");
            } else {
                uriRequest.setHeader(HEADER_CONTENT_TYPE, contentType);
            }
        }

        responseHandler.setRequestHeaders(uriRequest.getAllHeaders());
        responseHandler.setRequestURI(uriRequest.getURI());

 	    //构造一个请求对象，线程运行
        AsyncHttpRequest request = newAsyncHttpRequest(client, httpContext, uriRequest, contentType, responseHandler, context);
        threadPool.submit(request);//提交任务
        
        RequestHandle requestHandle = new RequestHandle(request);
        
       


	    //这个地方用来判断之前是否有请求，回收不必要的请求对象,因为RequestHandle可以取消任务，用的是Weak对象
        if (context != null) {
            // Add request to request map
            List<RequestHandle> requestList = requestMap.get(context);
            synchronized (requestMap) {
                if (requestList == null) {
                    requestList = Collections.synchronizedList(new LinkedList<RequestHandle>());
                    requestMap.put(context, requestList);
                }
            }

            requestList.add(requestHandle);

            Iterator<RequestHandle> iterator = requestList.iterator();
            while (iterator.hasNext()) {
                if (iterator.next().shouldBeGarbageCollected()) { //清理不用的Weak对象
                    iterator.remove();
                }
            }
        }

        return requestHandle;
    }

构造请求返回对象,可以取消请求
RequestHandle requestHandle = new RequestHandle(request);


ResponseHandlerInterface 回调接口定义了 任务的开始，过程，结束还有错误消息
UI消息在哪里发送？ 在抽象类AsyncHttpResponseHandler构造函数里面

  //UI线程里面的Handler实例化是在构造函数里面,默认就是myLooper，默认构造函数是传null的
  public AsyncHttpResponseHandler(Looper looper) {
        this.looper = looper == null ? Looper.myLooper() : looper;

        // Use asynchronous mode by default.默认异步的
        setUseSynchronousMode(false);

        // Do not use the pool's thread to fire callbacks by default.
        setUsePoolThread(false);
    }

    
    @Override
    public void setUseSynchronousMode(boolean sync) {
        // A looper must be prepared before setting asynchronous mode.
        if (!sync && looper == null) {
            sync = true;
            Log.w(LOG_TAG, "Current thread has not called Looper.prepare(). Forcing synchronous mode.");
        }

        // If using asynchronous mode.
        if (!sync && handler == null) {
            // Create a handler on current thread to submit tasks
            handler = new ResponderHandler(this, looper);//初始化,看ResponderHandler
        } else if (sync && handler != null) {
            // TODO: Consider adding a flag to remove all queued messages.
            handler = null;
        }

        useSynchronousMode = sync;
    }
    
    
    
     private static class ResponderHandler extends Handler {
        private final AsyncHttpResponseHandler mResponder;

        ResponderHandler(AsyncHttpResponseHandler mResponder, Looper looper) {
            super(looper);
            this.mResponder = mResponder;
        }

        @Override
        public void handleMessage(Message msg) {
            mResponder.handleMessage(msg); //最终提交消息处理的地方
        }
    }

AsyncHttpResponseHandler 的子类有多个，包括Binary,Text,JSON 回调

UML 简单的类图