套接字代碼拋出空指針異常

Question

嗨frends im工作在我基於sockets..im使用ssynctask來連接套接字etc..Everything工作正常，因爲我使用AsyncTask來創建一個Socket連接...我的android最後一年項目...套接字在doInBackground()方法中工作正常，但是當我嘗試從onSensorChanged()方法發送傳感器數據時，我得到空指針異常。我不知道哪裏出了問題。總之Socket在asyncTask Class之外返回NULL ...可以有一個幫助我嗎？

這裏是我的代碼：

package com.example.sensorsmart; 

import java.io.BufferedReader; 
import java.io.BufferedWriter; 
import java.io.File; 
import java.io.FileWriter; 
import java.io.IOException; 
import java.io.InputStreamReader; 
import java.io.PrintWriter; 
import java.net.ServerSocket; 
import java.net.Socket; 

import android.app.Activity; 
import android.content.Context; 
import android.hardware.Sensor; 
import android.hardware.SensorEvent; 
import android.hardware.SensorEventListener; 
import android.hardware.SensorManager; 
import android.os.AsyncTask; 
import android.os.Bundle; 
import android.util.Log; 
import android.view.Menu; 
import android.view.View; 
import android.widget.TextView; 
import android.widget.Toast; 

public class MainActivity extends Activity implements SensorEventListener{ 


private static final int SERVERPORT = 8222; 
private static final String SERVER_IP = "192.168.0.101"; 
private SensorManager mSensorManager; 
private Sensor mAccelerometer; 
private TextView tv ; 
public ServerSocket serverSocket=null; 
public Socket socket = null; 
public BufferedReader in ; 
public BufferedWriter out; 
public PrintWriter pw ; 
public FileWriter writer=null; 

@Override 
protected void onCreate(Bundle savedInstanceState) { 
super.onCreate(savedInstanceState); 
setContentView(R.layout.activity_main); 
new MyTask().execute(); 
mSensorManager = (SensorManager)getSystemService(Context.SENSOR_SERVICE); 
mAccelerometer mSensorManager.getDefaultSensor(Sensor.TYPE_ACCELEROMETER); 
mSensorManager.registerListener(this, mAccelerometer,SensorManager.SENSOR_DELAY_NORMAL); 

} 
public void onStopClick(View view) { 
mSensorManager.unregisterListener(this);} 

private class MyTask extends AsyncTask<Void, Void, Void> 
{ 
@Override 
protected Void doInBackground(Void... arg0) { 

try { 
serverSocket = new ServerSocket(8222); 
} catch (IOException e) { 
e.printStackTrace(); 
} 
try { 
socket=serverSocket.accept(); 
Log.i("TcpServer", "CONNECTED"); 
in = new BufferedReader(new InputStreamReader(socket.getInputStream())); 
pw = new PrintWriter(socket.getOutputStream(),true); 
if(!in.ready()) 
{ 
Log.i("TcpServer", "READER IS NOT READY"); 
} 

final String g; 
final String c; 
String b = null; 
Log.i("TcpServer", "GOING"); 
g=in.readLine(); 
runOnUiThread(new Runnable() { 
public void run() { 
Toast.makeText(getApplicationContext(),g, Toast.LENGTH_LONG).show(); 
} 
}); 

} catch (IOException e) { 
e.printStackTrace(); 
String response = e.getCause().toString(); 
Log.i("TCP",response); 
} 
return null; 
} 

} 


@Override 
public void onSensorChanged(SensorEvent event) { 
float x = event.values[0]; 
float y = event.values[1]; 
float z = event.values[2]; 
pw.write("g"); //NULL POINT EXCEPTION THROWS HERE 
} 
} 

這裏日誌貓：

02-06 11:31:54.250: E/AndroidRuntime(4108): FATAL EXCEPTION: main 
02-06 11:31:54.250: E/AndroidRuntime(4108): java.lang.NullPointerException 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at com.example.sensorsmart.MainActivity.onSensorChanged(MainActivity.java:196) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at  android.hardware.SystemSensorManager$ListenerDelegate$1.handleMessage(SystemSensorManager.java:204) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at android.os.Handler.dispatchMessage(Handler.java:99) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at android.os.Looper.loop(Looper.java:137) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at android.app.ActivityThread.main(ActivityThread.java:4759) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at java.lang.reflect.Method.invokeNative(Native Method) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at java.lang.reflect.Method.invoke(Method.java:511) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:558) 
02-06 11:31:54.250: E/AndroidRuntime(4108):  at dalvik.system.NativeStart.main(Native Method) 

Answer 1

這裏

pw.write("g"); //NULL POINT EXCEPTION THROWS HERE 

，因爲你用相同的名字裏面doInBackground創造PrintWriter不同的對象，而不是初始化爲pw。做爲：

.... 
Log.i("TcpServer", "CONNECTED"); 
in = new BufferedReader(new InputStreamReader(socket.getInputStream())); 
pw = new PrintWriter(socket.getOutputStream(),true); //initializing `pw` here 
if(!in.ready()) 
{ 
    Log.i("TcpServer", "READER IS NOT READY"); 
    } 
....