Question

我希望能够将服务器上的文本文件内容显示为textview。请有人告诉我为什么当我尝试运行这个应用程序时，我一直收到NullPointerException。我甚至在manifest.xml文件中添加了Internet权限。

package com.example.httpclient_examples;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity implements Runnable {



     final String textSource = "http://www.website.com/text.txt";

       /** Called when the activity is first created. */
       @Override
       public void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.fragment_main);



           TextView textPrompt = (TextView)findViewById(R.id.textprompt);          


           textPrompt.setText("Wait...");

           //thread

           Runnable r = new MainActivity();
           Thread t = new Thread(r);
           t.start();





           textPrompt.setText("Finished!");

       }

    @Override
    public void run() {
          TextView textMsg = (TextView)findViewById(R.id.textmsg);

          try {
               URL   textUrl = new URL(textSource);
           BufferedReader bufferReader = new BufferedReader(new InputStreamReader(textUrl.openStream()));
                 String StringBuffer;
                 String stringText = "";
           while ((StringBuffer = bufferReader.readLine()) != null) {
            stringText += StringBuffer;
           }


                 bufferReader.close();
                 textMsg.setText(stringText);
          } 


          catch (MalformedURLException e) {

           e.printStackTrace();
           textMsg.setText(e.toString());
          } catch (IOException e) {

           e.printStackTrace();
           textMsg.setText(e.toString());
          }       

    }
}

07-28 12:06:29.477: I/Choreographer(7101): Skipped 121 frames!  The application may be doing too much work on its main thread.
07-28 12:06:31.087: I/Choreographer(7101): Skipped 271 frames!  The application may be doing too much work on its main thread.
07-28 12:06:32.387: D/gralloc_goldfish(7101): Emulator without GPU emulation detected.
07-28 12:06:34.817: I/Process(7101): Sending signal. PID: 7101 SIG: 9
07-28 17:25:58.658: D/AndroidRuntime(14851): Shutting down VM
07-28 17:25:58.658: W/dalvikvm(14851): threadid=1: thread exiting with uncaught exception (group=0xb3ab7ba8)
07-28 17:25:58.858: E/AndroidRuntime(14851): FATAL EXCEPTION: main
07-28 17:25:58.858: E/AndroidRuntime(14851): Process: com.example.httpclient_examples, PID: 14851
07-28 17:25:58.858: E/AndroidRuntime(14851): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.httpclient_examples/com.example.httpclient_examples.MainActivity}: java.lang.NullPointerException
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2121)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.os.Handler.dispatchMessage(Handler.java:102)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.os.Looper.loop(Looper.java:136)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.ActivityThread.main(ActivityThread.java:5017)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at java.lang.reflect.Method.invokeNative(Native Method)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at java.lang.reflect.Method.invoke(Method.java:515)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at dalvik.system.NativeStart.main(Native Method)
07-28 17:25:58.858: E/AndroidRuntime(14851): Caused by: java.lang.NullPointerException
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.Activity.findViewById(Activity.java:1884)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at com.example.httpclient_examples.MainActivity.<init>(MainActivity.java:16)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at java.lang.Class.newInstanceImpl(Native Method)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at java.lang.Class.newInstance(Class.java:1208)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.Instrumentation.newActivity(Instrumentation.java:1061)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2112)
07-28 17:25:58.858: E/AndroidRuntime(14851):    ... 11 more
07-28 17:26:06.898: I/Process(14851): Sending signal. PID: 14851 SIG: 9

Answer 1

07-28 17:25:58.858: E/AndroidRuntime(14851): Caused by: java.lang.NullPointerException
07-28 17:25:58.858: E/AndroidRuntime(14851):    at android.app.Activity.findViewById(Activity.java:1884)
07-28 17:25:58.858: E/AndroidRuntime(14851):    at com.example.httpclient_examples.MainActivity.<init>(MainActivity.java:16)

您发布的代码与堆栈跟踪不匹配。但是，此堆栈跟踪表示您在初始化findViewById()时过早地调用MainActivity，可能尝试在第16行初始化成员变量。这就是NPE。要解决此问题，请在findViewById()之后将onCreate()移至setContentView()。

还有其他问题。

这是完全错误的：
```
Runnable r = new MainActivity();
```
永远不要使用new实例化活动。在这种情况下，您可以使用this来获取对当前活动实例的引用。
在后台帖子run()中，您无法触及您的用户界面视图，例如setText()上的TextView来电。{/ p>

请阅读AsyncTask上的一些介绍性文字 - 它是在后台线程上工作的基本构建块，同时在UI中提供更新和结果。

Answer 2

它现在正在工作。我将textview声明移动到oncreate（）并更改了Runnable r = new MainActivity（）;到Runnable r = this;我还使用runOnUiThread（）将更新后的文本发布到屏幕上。

package com.example.httpclient_examples;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class MainActivity extends Activity implements Runnable {

     TextView textPrompt, textMsg;
     String stringText = "";

     final String textSource = "http://www.website.com/text.txt";

       /** Called when the activity is first created. */
       @Override
       public void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.fragment_main);



           textPrompt = (TextView)findViewById(R.id.textprompt);           
           textMsg = (TextView)findViewById(R.id.textmsg);

           textPrompt.setText("Wait...");

           //thread

           Runnable r = this;
           Thread t = new Thread(r);
           t.start();





           textPrompt.setText("Finished!");

       }

    @Override
    public void run() {


          try {
               URL   textUrl = new URL(textSource);
           BufferedReader bufferReader = new BufferedReader(new InputStreamReader(textUrl.openStream()));
                 String StringBuffer;

           while ((StringBuffer = bufferReader.readLine()) != null) {
            stringText += StringBuffer;
           }


                 bufferReader.close();

                 runOnUiThread(new Runnable(){

                     @Override
                     public void run(){
                         textMsg.setText(stringText);

                    }
                 });
          } 


          catch (MalformedURLException e) {

           e.printStackTrace();
           textMsg.setText(e.toString());
          } catch (IOException e) {

           e.printStackTrace();
           textMsg.setText(e.toString());
          }       

    }
}

如何使用android中的httpget从服务器上的文件中读取文本

2 个答案: