使用Android的Bluetooth Chat示例应用程序作为指南http://developer.android.com/resources/samples/BluetoothChat/index.html，我尝试为正在开发的应用程序创建自己的蓝牙功能。
昨晚，我用两部Android手机对其进行了测试，但出现了一些问题，但没有强制关闭。当我要求连接时，它根本没有连接我的设备。我进入并添加了一些日志行，以确保程序遵循正确的过程。今天当我在手机上重新安装并启动该应用程序时，尝试打开蓝牙时遇到强制关闭错误，这是我昨晚根本没有遇到的问题。我更改的唯一代码是添加了2-3个日志命令。我检查了logcat并得到以下信息：
如您所见，问题是由NPE @第185行引起的。这是我的BluetoothService类代码，我将在其下面重新发布特定的问题区域。
package com.tagapp.android;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.UUID;
import android.bluetooth.BluetoothAdapter;
import android.bluetooth.BluetoothDevice;
import android.bluetooth.BluetoothServerSocket;
import android.bluetooth.BluetoothSocket;
import android.content.Context;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
public class BluetoothService {
private static final String TAG = "BluetoothService";
private static final boolean D = true;
private static final String NAME = "BluetoothTag";
private static final UUID MY_UUID =
UUID.fromString("93845760-234e-11e0-ac64-0800200c9a66");
private final BluetoothAdapter mAdapter;
private final Handler mHandler;
private AcceptThread mAcceptThread;
private ConnectThread mConnectThread;
private ConnectedThread mConnectedThread;
private int mState;
public static final int STATE_NONE = 0;
public static final int STATE_LISTEN = 1;
public static final int STATE_CONNECTING = 2;
public static final int STATE_CONNECTED = 3;
public BluetoothService(Context context, Handler handler) {
mAdapter = BluetoothAdapter.getDefaultAdapter();
mState = STATE_NONE;
mHandler = handler;
}
private synchronized void setState(int state) {
if (D) Log.d(TAG, "setState() " + mState + " -> " + state);
mState = state;
mHandler.obtainMessage(BluetoothTransfer.MESSAGE_STATE_CHANGE, state, -1).sendToTarget();
}
public synchronized int getState() {
return mState;
}
public synchronized void start() {
if (D) Log.d(TAG, "start");
if(mConnectThread != null) {
mConnectThread.cancel();
mConnectThread = null;
}
if(mConnectedThread != null) {
mConnectedThread.cancel();
mConnectedThread = null;
}
if(mAcceptThread == null) {
mAcceptThread = new AcceptThread();
mAcceptThread.start();
}
setState(STATE_LISTEN);
}
public synchronized void connect(BluetoothDevice device) {
if (D) Log.d(TAG, "connect to: " + device);
if(mState == STATE_CONNECTING) {
if(mConnectThread != null) {
mConnectThread.cancel();
mConnectThread = null;
}
}
if(mConnectedThread != null) {
mConnectedThread.cancel();
mConnectedThread = null;
}
mConnectThread = new ConnectThread(device);
mConnectThread.start();
setState(STATE_CONNECTING);
}
public synchronized void connected(BluetoothSocket socket, BluetoothDevice device) {
if (D) Log.d(TAG, "connected");
if(mConnectThread != null) {
mConnectThread.cancel();
mConnectThread = null;
}
if(mConnectedThread != null) {
mConnectedThread.cancel();
mConnectedThread = null;
}
if(mAcceptThread != null) {
mAcceptThread.cancel();
mAcceptThread = null;
}
mConnectedThread = new ConnectedThread(socket);
mConnectedThread.start();
Message msg = mHandler.obtainMessage(BluetoothTransfer.MESSAGE_DEVICE_NAME);
Bundle bundle = new Bundle();
bundle.putString(BluetoothTransfer.DEVICE_NAME, device.getName());
msg.setData(bundle);
mHandler.sendMessage(msg);
setState(STATE_CONNECTED);
}
public synchronized void stop() {
if (D) Log.d(TAG, "stop");
if(mConnectThread != null) {
mConnectThread.cancel();
mConnectThread = null;
}
if(mConnectedThread != null) {
mConnectedThread.cancel();
mConnectedThread = null;
}
if(mAcceptThread != null) {
mAcceptThread.cancel();
mAcceptThread = null;
}
setState(STATE_NONE);
}
public void write(byte[] out) {
ConnectedThread ct;
synchronized(this) {
if(mState != STATE_CONNECTED) return;
ct = mConnectedThread;
}
ct.write(out);
}
private void connectionFailed() {
setState(STATE_LISTEN);
Message msg = mHandler.obtainMessage(BluetoothTransfer.MESSAGE_TOAST);
Bundle bundle = new Bundle();
bundle.putString(BluetoothTransfer.TOAST, "Unable to connect device");
msg.setData(bundle);
mHandler.sendMessage(msg);
}
private void connectionLost() {
setState(STATE_LISTEN);
Message msg = mHandler.obtainMessage(BluetoothTransfer.MESSAGE_TOAST);
Bundle bundle = new Bundle();
bundle.putString(BluetoothTransfer.TOAST, "Device connection was lost");
msg.setData(bundle);
mHandler.sendMessage(msg);
}
private class AcceptThread extends Thread {
private final BluetoothServerSocket mmServerSocket;
public AcceptThread() {
BluetoothServerSocket tmp = null;
try {
tmp = mAdapter.listenUsingRfcommWithServiceRecord(NAME, MY_UUID);
}
catch (IOException e) {
Log.e(TAG, "listen() failed", e);
}
mmServerSocket = tmp;
}
public void run() {
if (D) Log.d(TAG, "BEGIN mAcceptThread" + this);
setName("AcceptThread");
BluetoothSocket socket = null;
while (mState != STATE_CONNECTED) {
try {
socket = mmServerSocket.accept();
}
catch (IOException e) {
Log.e(TAG, "accept() failed", e);
break;
}
if(socket != null) {
synchronized (BluetoothService.this) {
switch(mState) {
case STATE_LISTEN :
case STATE_CONNECTING :
connected(socket, socket.getRemoteDevice());
break;
case STATE_NONE :
case STATE_CONNECTED :
try {
socket.close();
}
catch (IOException e) {
Log.e(TAG, "Could not close unwanted socket", e);
}
break;
}
}
}
}
if (D) Log.i(TAG, "END mAcceptThread");
}
public void cancel() {
if (D) Log.d(TAG, "cancel " + this);
try {
mmServerSocket.close();
}
catch (IOException e) {
Log.e(TAG, "close() of server failed", e);
}
}
}
private class ConnectThread extends Thread {
private final BluetoothSocket mmSocket;
private final BluetoothDevice mmDevice;
public ConnectThread(BluetoothDevice device) {
mmDevice = device;
BluetoothSocket tmp = null;
try {
tmp = device.createRfcommSocketToServiceRecord(MY_UUID);
}
catch (IOException e) {
Log.e(TAG, "create() failed", e);
}
mmSocket = tmp;
}
public void run() {
Log.i(TAG, "BEGIN mConnectThread");
setName("ConnectThread");
mAdapter.cancelDiscovery();
try {
mmSocket.connect();
}
catch (IOException e) {
connectionFailed();
try {
mmSocket.close();
}
catch (IOException e2) {
Log.e(TAG, "unable to close() socket during connection failure", e2);
}
BluetoothService.this.start();
return;
}
synchronized (BluetoothService.this) {
mConnectThread = null;
}
connected(mmSocket, mmDevice);
}
public void cancel() {
try {
mmSocket.close();
}
catch (IOException e) {
Log.e(TAG, "close() of connect socket failed", e);
}
}
}
private class ConnectedThread extends Thread {
private final BluetoothSocket mmSocket;
private final InputStream mmInStream;
private final OutputStream mmOutStream;
public ConnectedThread(BluetoothSocket socket) {
Log.d(TAG, "create ConnectedThread");
mmSocket = socket;
InputStream tmpIn = null;
OutputStream tmpOut = null;
try {
tmpIn = socket.getInputStream();
tmpOut = socket.getOutputStream();
}
catch (IOException e) {
Log.e(TAG, "tempt sockets not created", e);
}
mmInStream = tmpIn;
mmOutStream = tmpOut;
}
public void run() {
Log.i(TAG, "BEGIN mConnectedThread");
byte[] buffer = new byte[1024];
int bytes;
while (true) {
try {
bytes = mmInStream.read(buffer);
mHandler.obtainMessage(BluetoothTransfer.CONTACT_RECEIVE, bytes, -1, buffer).sendToTarget();
}
catch (IOException e) {
Log.e(TAG, "disconnected", e);
connectionLost();
break;
}
}
}
public void write(byte[] buffer) {
try {
mmOutStream.write(buffer);
mHandler.obtainMessage(BluetoothTransfer.CONTACT_SEND, -1, -1, buffer).sendToTarget();
}
catch (IOException e) {
Log.e(TAG, "exception during write", e);
}
}
public void cancel() {
try {
mmSocket.close();
}
catch (IOException e) {
Log.e(TAG, "close() of connect socket failed", e);
}
}
}
}
引起NPE（AcceptThread）的行：
private class AcceptThread extends Thread {
private final BluetoothServerSocket mmServerSocket;
public AcceptThread() {
BluetoothServerSocket tmp = null;
try {
tmp = mAdapter.listenUsingRfcommWithServiceRecord(NAME, MY_UUID);
}
catch (IOException e) {
Log.e(TAG, "listen() failed", e);
}
mmServerSocket = tmp;
}
public void run() {
if (D) Log.d(TAG, "BEGIN mAcceptThread" + this);
setName("AcceptThread");
BluetoothSocket socket = null;
while (mState != STATE_CONNECTED) {
try {
socket = mmServerSocket.accept();//THIS LINE CAUSES NPE
}
catch (IOException e) {
Log.e(TAG, "accept() failed", e);
break;
}
对象套接字应该通过初始化@ null进行初始化，但是应该尝试使用socket = mmServerSocket.accept（）;。
这直接来自Android开发人员网站上提供的Google示例应用程序。我有两个问题：1，为什么这行不通，以及为什么几个小时前就行得通？
谢谢你的帮助。
最佳答案
关于“奇怪的NPE”-一点都不奇怪。您在此行获得NPE的原因
socket = mmServerSocket.accept(); //THIS LINE CAUSES NPE
很明显-变量mmServerSocket等于NULL，这是在此代码行获得NPE的唯一可能原因。 mmServerSocket等于NULL的原因也很简单-查看类的构造函数：
public AcceptThread() {
BluetoothServerSocket tmp = null;
try {
tmp = mAdapter.listenUsingRfcommWithServiceRecord(NAME, MY_UUID);
}
catch (IOException e) {
Log.e(TAG, "listen() failed", e);
}
mmServerSocket = tmp;
}
您正在调用抛出IOException的方法
listenUsingRfcommWithServiceRecord，捕获了此异常，正如我们在logcat中可以清楚看到的那样。接下来，tmp变量保持初始化为NULL，与mmServerSocket相同，这导致上述NPE。正如您在问题中提到的，
listenUsingRfcommWithServiceRecord引发IOException的原因-因为蓝牙已关闭。 Android文档并未说明您的假设，即如果此方法已关闭，则该方法应自动打开蓝牙。我认为您应该在调用listenUsingRfcommWithServiceRecord之前手动检查蓝牙是否已关闭并手动将其打开。您可以找到如何检查蓝牙是否已打开/关闭并将其打开here和here
关于android - 在我的一个蓝牙类(class)中获得了一个奇怪的NPE，以前从未发生过，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/4869891/