我有点困惑处理器,AsyncTask和线程之间的区别在Android。我在StackOverflow上读过不少博客和问题。

Handler是后台线程,为您提供与UI通信。例如,更新进度条应该通过Handler来完成。使用Handlers可以获得MessagingQueues的优势,因此如果您想调度消息或更新多个UI元素或有重复任务。

AsyncTask是类似的,事实上,他们利用Handler,但不运行在UI线程,所以它很适合抓取数据,例如抓取web服务。稍后您可以与UI交互。

然而,线程不能与UI交互,提供更多的“基本”线程,你错过了AsyncTask的所有抽象。

但是,我希望在服务中运行套接字连接。这应该运行在一个处理程序或线程,甚至一个AsyncTask?UI交互根本不需要。它对我使用的性能有影响吗?

同时,文档也得到了很大的改进。


当前回答

public class RequestHandler {

    public String sendPostRequest(String requestURL,
                                  HashMap<String, String> postDataParams) {

        URL url;

        StringBuilder sb = new StringBuilder();
        try {
            url = new URL(requestURL);

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(15000);
            conn.setConnectTimeout(15000);
            conn.setRequestMethod("POST");
            conn.setDoInput(true);
            conn.setDoOutput(true);


            OutputStream os = conn.getOutputStream();
            BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(os, "UTF-8"));
            writer.write(getPostDataString(postDataParams));

            writer.flush();
            writer.close();
            os.close();
            int responseCode = conn.getResponseCode();

            if (responseCode == HttpsURLConnection.HTTP_OK) {
                BufferedReader br = new BufferedReader(new InputStreamReader(conn.getInputStream()));
                sb = new StringBuilder();
                String response;
                while ((response = br.readLine()) != null){
                    sb.append(response);
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
        return sb.toString();
    }

    private String getPostDataString(HashMap<String, String> params) throws UnsupportedEncodingException {
        StringBuilder result = new StringBuilder();
        boolean first = true;
        for (Map.Entry<String, String> entry : params.entrySet()) {
            if (first)
                first = false;
            else
                result.append("&");

            result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
            result.append("=");
            result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
        }

        return result.toString();
    }

}

其他回答

AsyncTask用于执行一些后台计算并将结果发布到UI线程(带有可选的进度更新)。既然你不关心UI,那么处理器或线程似乎更合适。

您可以生成一个后台线程,并通过使用处理程序的post方法将消息传递回主线程。

让我试着用一个例子来回答这个问题:)- MyImageSearch[请参考这里的主活动屏幕的图像-包含编辑文本/搜索按钮/网格视图]

MyImageSearch -一旦用户在编辑文本字段输入详细信息并单击搜索按钮,我们将通过flickr提供的web服务在互联网上搜索图像(您只需要在那里注册以获得密钥/秘密令牌)-对于搜索,我们发送一个HTTP请求和get JSON数据返回响应,其中包含个人图像的url,然后我们将使用它来加载网格视图。

My Implementation - In the main activity I will define a inner class which extends the AsyncTask to send the HTTP Request in doInBackGround Method and fetch the JSON Response and update my local ArrayList of FlickrItems which I am going to use to update my GridView via the FlickrAdapter (extends the BaseAdapter) and call the adapter.notifyDataSetChanged() in the onPostExecute() of AsyncTask to reload the grid view. Note that here the HTTP Request is a blocking call because of which I have done it via the AsyncTask. And, I can cache the items in adapter to increase the performance or store them on SDCard. The grid that I will be inflating in the FlickrAdapter contains in my implementation a progressbar and image view. Below you can find the code for mainActivity which I used.

现在回答问题- 因此,一旦我们获得了用于获取单个图像的JSON数据,我们就可以实现通过Handlers或线程或AsyncTask在后台获取图像的逻辑。我们应该注意到,由于我的图像一旦下载就必须显示在UI/主线程上,我们不能简单地使用线程,因为它们不能访问上下文。 在FlickrAdapter中,我能想到的选择:

Choice 1: Create a LooperThread [extends thread] - and keep on downloading images sequentially in one thread by keeping this thread open [looper.loop()] Choice 2: Make use of a Thread Pool and post the runnable via myHandler which contains reference to my ImageView, but since the views in Grid View are recycled, again the problem might arise where image at index 4 is displayed at index 9 [download may take more time] Choice 3 [I used this]: Make use of a Thread Pool and send a message to myHandler, which contains data related to ImageView's index and ImageView itself, so while doing handleMessage() we will update the ImageView only if currentIndex matches the index of the Image we tried to download. Choice 4: Make use of AsyncTask to download the images in background, but here I will not have access to the number of threads I want in the thread pool and it varies with different android version, but in Choice 3 I can make of conscious decision of the size of thread pool depending on device configuration being used.

以下是源代码:

public class MainActivity extends ActionBarActivity {

    GridView imageGridView;
    ArrayList<FlickrItem> items = new ArrayList<FlickrItem>();
    FlickrAdapter adapter;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        imageGridView = (GridView) findViewById(R.id.gridView1);
        adapter = new FlickrAdapter(this, items);
        imageGridView.setAdapter(adapter);
    }

    // To avoid a memory leak on configuration change making it a inner class
    class FlickrDownloader extends AsyncTask<Void, Void, Void> {



        @Override
        protected Void doInBackground(Void... params) {
            FlickrGetter getter = new FlickrGetter();

            ArrayList<FlickrItem> newItems = getter.fetchItems();

            // clear the existing array
            items.clear();

            // add the new items to the array
            items.addAll(newItems);

            // is this correct ? - Wrong rebuilding the list view and should not be done in background
            //adapter.notifyDataSetChanged();

            return null;
        }

        @Override
        protected void onPostExecute(Void result) {
            super.onPostExecute(result);

            adapter.notifyDataSetChanged();
        }

    }

    public void search(View view) {
        // get the flickr data
        FlickrDownloader downloader = new FlickrDownloader();
        downloader.execute();
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

我希望我的回答虽然很长,但将有助于理解一些更精细的细节。

public class RequestHandler {

    public String sendPostRequest(String requestURL,
                                  HashMap<String, String> postDataParams) {

        URL url;

        StringBuilder sb = new StringBuilder();
        try {
            url = new URL(requestURL);

            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(15000);
            conn.setConnectTimeout(15000);
            conn.setRequestMethod("POST");
            conn.setDoInput(true);
            conn.setDoOutput(true);


            OutputStream os = conn.getOutputStream();
            BufferedWriter writer = new BufferedWriter(
                    new OutputStreamWriter(os, "UTF-8"));
            writer.write(getPostDataString(postDataParams));

            writer.flush();
            writer.close();
            os.close();
            int responseCode = conn.getResponseCode();

            if (responseCode == HttpsURLConnection.HTTP_OK) {
                BufferedReader br = new BufferedReader(new InputStreamReader(conn.getInputStream()));
                sb = new StringBuilder();
                String response;
                while ((response = br.readLine()) != null){
                    sb.append(response);
                }
            }

        } catch (Exception e) {
            e.printStackTrace();
        }
        return sb.toString();
    }

    private String getPostDataString(HashMap<String, String> params) throws UnsupportedEncodingException {
        StringBuilder result = new StringBuilder();
        boolean first = true;
        for (Map.Entry<String, String> entry : params.entrySet()) {
            if (first)
                first = false;
            else
                result.append("&");

            result.append(URLEncoder.encode(entry.getKey(), "UTF-8"));
            result.append("=");
            result.append(URLEncoder.encode(entry.getValue(), "UTF-8"));
        }

        return result.toString();
    }

}

处理器——线程之间的通信媒介。在android中,它主要用于通过处理器创建和发送消息来与主线程通信

AsyncTask -用于在后台线程中执行长时间运行的应用程序。使用nAsyncTask,你可以在后台线程中执行操作,并在应用程序的主线程中获得结果。

Thread -是一个轻量级的进程,可以实现并发性和最大的cpu利用率。在android中,你可以使用线程来执行不触及应用程序UI的活动

线程:

你可以在不影响UI线程的情况下使用新的线程来执行长期运行的后台任务。从java线程,你不能更新UI线程。

由于普通线程在Android架构中用处不大,因此引入了线程的辅助类。

您可以在线程性能文档页面中找到查询的答案。

处理程序:

Handler允许您发送和处理与线程的MessageQueue关联的Message和Runnable对象。每个Handler实例都与单个线程和该线程的消息队列相关联。

Handler有两个主要用途:

将消息和可运行程序安排在未来的某个时间点执行; 将要在与自己的线程不同的线程上执行的操作排队。

AsyncTask:

AsyncTask允许正确和简单地使用UI线程。该类允许您执行后台操作并在UI线程上发布结果,而无需操作线程和/或处理程序。

缺点:

默认情况下,应用程序将它创建的所有AsyncTask对象推入一个线程。因此,它们以串行方式执行,与主线程一样,一个特别长的工作包会阻塞队列。由于这个原因,使用AsyncTask来处理持续时间短于5毫秒的工作项。 AsyncTask对象也是隐式引用问题的最常见的肇事者。AsyncTask对象也存在与显式引用相关的风险。

HandlerThread:

您可能需要一种更传统的方法来在长时间运行的线程上执行工作块(不像AsyncTask,它应该用于5ms的工作负载),并且需要一些手动管理工作流的能力。处理程序线程实际上是一个长时间运行的线程,它从队列中获取工作并对其进行操作。

ThreadPoolExecutor:

该类管理一组线程的创建,设置它们的优先级,并管理如何在这些线程之间分配工作。随着工作负载的增加或减少,该类将启动或销毁更多的线程以适应工作负载。

如果工作负载比较大,单个HandlerThread不够用,那么可以使用ThreadPoolExecutor

然而,我想有一个套接字连接运行在服务。这应该运行在一个处理程序或线程,甚至一个AsyncTask?UI交互根本不需要。它对我使用的性能有影响吗?

因为UI交互不是必需的,你可能不需要AsyncTask。普通线程用处不大,因此HandlerThread是最好的选择。因为你必须维护套接字连接,主线程上的Handler一点用都没有。创建一个HandlerThread并从HandlerThread的循环程序中获取一个Handler。

 HandlerThread handlerThread = new HandlerThread("SocketOperation");
 handlerThread.start();
 Handler requestHandler = new Handler(handlerThread.getLooper());
 requestHandler.post(myRunnable); // where myRunnable is your Runnable object. 

如果希望与UI线程通信,可以使用另外一个Handler来处理响应。

final Handler responseHandler = new Handler(Looper.getMainLooper()) {
        @Override
        public void handleMessage(Message msg) {
            //txtView.setText((String) msg.obj);
            Toast.makeText(MainActivity.this,
                    "Foreground task is completed:"+(String)msg.obj,
                    Toast.LENGTH_LONG)
                    .show();
        }
    };

在Runnable中,您可以添加

responseHandler.sendMessage(msg);

关于实现的更多细节可以在这里找到:

Android:在线程中吐司