套接字代码抛出 NullPointerException

Question

嗨，我正在做我的基于套接字的安卓最后一年项目。IM 使用 SsyncTask 连接套接字等。一切正常，因为我正在使用AsyncTask来创建套接字连接......套接字在doInBackground()方法中工作正常，但是当我尝试从onSensorChanged()方法发送传感器数据时，出现空指针异常。我不知道出了什么问题。简而言之，套接字在异步任务类之外返回 NULL...一些 1 可以帮助我吗？

这是我的代码：

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
}
}

这是日志猫：

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

这里

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

因为您要在doInBackground中创建具有相同名称的不同PrintWriter对象，而不是初始化pw。 执行如下：

....
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");
}
....