Question

我有一个基本的应用程序，它使用后台线程将值发布到我们的服务器。代码如下。当我运行代码时，在调试或正常情况下，以下行抛出NullException（尽管在调试时，设置了backgroundThread变量的值）。每一次后续运行都很好。我是Android开发的新手，我不确定导致这种差异的原因。我已经包含了活动功能和服务器发布功能。

backgroundThread.start();

这一行抛出了NullException。关于导致问题的原因是什么？感谢您提供的任何帮助。

- UPDATE @ jonas452＆amp; @Rudi

我更改了代码以使用asynctask并且正在努力使用返回值。 postResult总是为null，并且postTo函数中的断点似乎永远不会运行。可能是一个愚蠢的错误，但我不确定什么是错。

- UPDATE 我解决了这个问题。 AsyncTask设置正确，但由于它是异步的，因此postResult不能立即使用。我将支票移到了AsyncTask的onPostExecute函数中，一切正常。

public void onActivityResult(int requestCode, int resultCode, Intent intent) {
    try {
        //retrieve scan result
        IntentResult scanningResult = IntentIntegrator.parseActivityResult(requestCode, resultCode, intent);
        if (scanningResult != null) {
            //we have a result
            final String scanContent = scanningResult.getContents();
            String scanFormat = scanningResult.getFormatName();
            scan_format.setText("FORMAT: " + scanFormat);
            scan_content.setText("CONTENT: " + scanContent);
            error.setText(null);
            output.setText(null);
            neonetpost post = new neonetpost();
            post.execute(scanContent);
            /*
            Thread backgroundThread = new Thread(new Runnable() {
                   @Override
                   public void run() {
                    postResult = postTo(scanContent);
                   }
               });
            backgroundThread.start();
            */
            if(postResult.indexOf("ERROR: ")>-1)
                error.setText(postResult);
            else
                output.setText(postResult);
        }
        else{
            Toast toast = Toast.makeText(getApplicationContext(),
                "No scan data received!", Toast.LENGTH_SHORT);
            toast.show();
        }   
    } catch (Exception e) {
        // TODO Auto-generated catch block
        error.setText("ERROR: " + e.getMessage());
    } 
}


public class neonetpost extends AsyncTask<String,String,String>
{
    @Override
    protected void onPreExecute() {  
    }
    protected void onPostExecute(String result) {
    }
    protected String doInBackground(String... params) {
        return postTo(params[0], params[1]);
    }

    public String postTo(String operation) {
        return postTo(operation, "E:1");
    }
    public String postTo(String operation, String employee)
    {
        try {
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost("http://domain.com/page.php");

            // Add your data
            List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
            nameValuePairs.add(new BasicNameValuePair("operation", operation));
            nameValuePairs.add(new BasicNameValuePair("employee", employee));
            httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

            // Execute HTTP Post Request
            HttpResponse response = httpclient.execute(httppost);
            // According to the JAVA API, InputStream constructor do nothing. 
            //So we can't initialize InputStream although it is not an interface
            InputStream inputStream = response.getEntity().getContent();

            InputStreamReader inputStreamReader = new InputStreamReader(inputStream);

            BufferedReader bufferedReader = new BufferedReader(inputStreamReader);

            StringBuilder stringBuilder = new StringBuilder();

            String bufferedStrChunk = null;

            while((bufferedStrChunk = bufferedReader.readLine()) != null){
                stringBuilder.append(bufferedStrChunk);
            }

            postResult = stringBuilder.toString();
        } catch (Exception e) {
            // TODO Auto-generated catch block
            postResult =  "ERROR: " + e.getMessage();
        } 
        return postResult;
    }
}

Answer 1

不要使用Java Threads，请使用AsyncTask类。这是一个很好的教程： http://www.compiletimeerror.com/2013/01/why-and-how-to-use-asynctask.html

Answer 2

给你NPE的行是if(postResult.indexOf("ERROR: ")>-1)。你假设调用

backgroundThread.start();

使UI线程等待，直到backgroundThread完成执行并将值赋给postResult，但Thread是异步执行的。和backgroundThread.start(); 使系统产生一个新线程。执行此操作时，取决于调度程序

Android应用后台线程行为

2 个答案: