如何从java中的线程返回一个值？

Question

In android i am creating thread for url connection.Inside the thread i store the response message in a string which is globally declared.When I access the method method it returns null.

    public class Rate_fetch {
       String total="";
          public String  rate(String dt)
       {
    new Thread(new Runnable(){ 

         public void run(){


            try  {

             URL url = new URL(tally_ipaddr+"/prorate.jsp?plist="+sss.toString().trim());

             HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
             InputStream in = new BufferedInputStream(urlConnection.getInputStream());
             BufferedReader r = new BufferedReader(new InputStreamReader(in));
             String x = "";
             String total = "";
             x = r.readLine();
             int i=0;

             while(x.length()>1)
             {
                 total=total+x.toString().trim();
                 i++;
                 x = r.readLine();
             }
             }
            catch(Exception e){
             return e.toString();
            }   
        }    

    }).start();

    return total;



}

When i call the method it returns null.

 Rate_fetch rf=new Rate_fetch();
      String amt= rf.rate(prodList);

Answer 1

It you really want to use Thread only, try this

    public class Rate_fetch {
    String total = "";
    boolean b = true;

    public String rate(String dt) {
        StringBuilder sb = new StringBuilder();
        new Thread(new Runnable() {

            public void run() {

                try {

                    URL url = new URL(tally_ipaddr + "/prorate.jsp?plist="
                            + sss.toString().trim());

                    HttpURLConnection urlConnection = (HttpURLConnection) url
                            .openConnection();
                    InputStream in = new BufferedInputStream(urlConnection
                            .getInputStream());
                    BufferedReader r = new BufferedReader(
                            new InputStreamReader(in));
                    StringBuilder sb = new StringBuilder();
                    String s;
                    while (true) {
                        s = r.readLine();
                        if (s == null || s.length() == 0)
                            break;
                        sb.append(s);
                    }
                    b = true;
                } catch (Exception e) {
                    b = true;
                }
            }

        }).start();
        while (b) {

        }
        total = sb.toString();
        return sb.toString();

    }
}

Answer 2

You are getting the value of result as null because the method rate is running a thread so the return is not waiting to finish the thread.

You can use one of the following technique to resolve the issue

• Handlers
• Async. Task

Also you can look at the link for the reference.

http://mobileorchard.com/android-app-developmentthreading-part-1-handlers/

Answer 3

You are missing the whole point of using a thread, because we use them to do expensive or potentially long operation in a separate thread in order to not block the UI thread. Therefore, you can not start a thread and expect to get the result instantly, as Premsuraj, it returns before the thread has finished.

As others suggested, a cleaner way to do this would be to use an AsyncTask, it encapsulates the process in three main stages: OnPreExecute, doInBackground and onPostExecute, they names are self-explanatory, the first do any initialization you might need, the second actually do the async work and the third is used to deal with the result of the entire operation.

Here's one quick example, of course it can be improved but should help you to understand it:

Rate_fetch

Your same class but without threading:

public class Rate_fetch {

       public String rate(String dt)
       {
            String total="";
            try  {

                URL url = new URL(tally_ipaddr+"/prorate.jsp?plist="+sss.toString().trim());

                HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
                InputStream in = new BufferedInputStream(urlConnection.getInputStream());
                BufferedReader r = new BufferedReader(new InputStreamReader(in));
                String x = "";
                String total = "";
                x = r.readLine();
                int i=0;

                while(x.length()>1)
                {
                    total=total+x.toString().trim();
                    i++;
                    x = r.readLine();
                }
            }
            catch(Exception e){
                return e.toString();
            };

            return total;
        }

    }

An example activity:

public class YourActivity extends Activity {

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.layout);

        GetRate asyncRate = new GetRate();
        asyncRate.execute(); // starting the task, can be done wherever you need, for example a Button click event
    }

    private class GetRate extends AsyncTask<Void, Integer, String> {

        @Override
        protected String doInBackground(Void... params) {

            Rate_fetch fetch = new Rate_fetch();
            string rate = fetch.rate();

            return rate;
        }

        @Override
        protected void onPostExecute(String rate) {                     
            // Do whatever you need with the string, you can update your UI from here
        }
    }

}

Hope it helps.

Answer 4

possibly you can pass reference of main/current thread/class to the thread. and thread will store its result in that referenced object.

Below is a simple example.

class CallerTest{
 private ResultData result; // any data that you want from thread

 public void caller() throws Exception{
   ThreadClass thread = new ThreadClass(this);
   thread.start();

   //wait for thread to finish
   thread.join();

   //now you have data in >> this.result
 }

 public void setResult(ResultData rd){
   this.result = rd; 
 }
}

class ThreadClass extends Thread{
    CallerTest cl;
    public ThreadClass(CallerTest cl){
       this.cl = cl;
    }
    public void run() {
     ResultData rd;
     rd = //do code to retrive result
     cl.setResult(rd);
    }

}

Answer 5

只需声明一个类变量并将该对象传递给您的线程

Answer 6

You can't return a value from a thread. Your function Rate_fetch would return before the thread has even started executing. A solution is to use AsyncTask and use the value in its onPostExecute.

Answer 7

If you choose to use a separate thread to execute your logic is implicit that the execution of the code inside of your thread will be asynchronous, so you cannot access the result immediatly ignoring the asynchronous thread execution. You should consider to use something like http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/FutureTask.html

Answer 8

看一下Thread Local ，它会让你携带一个参数，其中thread是它自己的局部变量。