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我正在开发两个蓝牙应用程序，它们都基于 bluetoothChat 示例，但其中一个也能够使用 USB 附件模式从微控制器发送/接收数据。这些应用程序在两个不同的设备上运行，一个 Nexus 4 和一个 V301（中国手机）。一般来说，一切正常，问题只是在第一次尝试时这两个设备无法建立蓝牙连接。bluetoothChat 应用程序根本无法建立连接，如果我尝试建立连接，另一个应用程序 (BT+USB) 会停止并崩溃。但问题是，在这次崩溃之后，一切正常，我最终能够从这两个应用程序建立连接。这是两个android manifest和BT+USB应用崩溃时出现的错误。

Eclipse 控制台显示的错误：

06-24 12:40:10.090: W/dalvikvm(5886): threadid=1: thread exiting with uncaught exception (group=0x40a98390)
06-24 12:40:10.110: E/AndroidRuntime(5886): FATAL EXCEPTION: main
06-24 12:40:10.110: E/AndroidRuntime(5886): java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=1, result=-1, data=Intent { (has extras) }} to activity {com.example.bluetoothx10y/com.example.bluetoothx10y.BluetoothChat}: java.lang.NullPointerException
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.ActivityThread.deliverResults(ActivityThread.java:2988)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.ActivityThread.handleSendResult(ActivityThread.java:3031)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.ActivityThread.access$1100(ActivityThread.java:126)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1185)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.os.Handler.dispatchMessage(Handler.java:99)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.os.Looper.loop(Looper.java:137)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.ActivityThread.main(ActivityThread.java:4482)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at java.lang.reflect.Method.invokeNative(Native Method)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at java.lang.reflect.Method.invoke(Method.java:511)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:787)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:554)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at dalvik.system.NativeStart.main(Native Method)
06-24 12:40:10.110: E/AndroidRuntime(5886): Caused by: java.lang.NullPointerException
06-24 12:40:10.110: E/AndroidRuntime(5886):     at com.example.bluetoothx10y.BluetoothChat.onActivityResult(BluetoothChat.java:537)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.Activity.dispatchActivityResult(Activity.java:4649)
06-24 12:40:10.110: E/AndroidRuntime(5886):     at android.app.ActivityThread.deliverResults(ActivityThread.java:2984)
06-24 12:40:10.110: E/AndroidRuntime(5886):     ... 11 more

BT+USB 应用（称为 bluetoothx10y）的 android manifest：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.bluetoothx10y"
    android:versionCode="1"
    android:versionName="1.0" >

<uses-sdk
    android:minSdkVersion="9"
    android:targetSdkVersion="15" />

<uses-feature android:name="android.hardware.usb.accessory"/>
<uses-permission android:name="android.permission.WRITE_INTERNAL_STORAGE" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission android:name="android.permission.BLUETOOTH"/>
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN"/>

<application
    android:allowBackup="true"
    android:icon="@drawable/ic_launcher"
    android:label="@string/app_name" >
    <activity
        android:name="com.example.bluetoothx10y.BluetoothChat"
        android:label="@string/app_name"
        android:screenOrientation="landscape" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>

        <intent-filter>
           <action android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"/>
        </intent-filter>

        <meta-data 
            android:name="android.hardware.usb.action.USB_ACCESSORY_ATTACHED"
            android:resource="@xml/accessory_filter">
        </meta-data>

    </activity>

    <activity android:name=".DeviceListActivity"
          android:label="@string/app_name"
          android:theme="@android:style/Theme.Dialog"
          android:screenOrientation="landscape" />

</application>
</manifest>

BT 应用程序的 android 清单（称为 BluetoothChat）：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="com.example.android.BluetoothChat"
     android:versionCode="1"
     android:versionName="1.0">

<uses-sdk
    android:minSdkVersion="6"
    android:targetSdkVersion="9" />

<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.BLUETOOTH" />

<application android:label="@string/app_name"
             android:icon="@drawable/app_icon" >
    <activity android:name=".BluetoothChat"
              android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
    <activity android:name=".DeviceListActivity"
              android:label="@string/select_device"
              android:theme="@android:style/Theme.Dialog" />
</application>
</manifest>

在第 536 和 537 行，我有：

mBluetoothAdapter.getRemoteDevice(address); // Attempt to connect to the device      
mChatService.connect(device);

mChatService 是负责建立和管理蓝牙连接的类的对象

public synchronized void connect(BluetoothDevice device) {
    if (D) Log.d(TAG, "connect to: " + device);

    // Cancel any thread attempting to make a connection
    if (mState == STATE_CONNECTING) {
        if (mConnectThread != null) {mConnectThread.cancel(); mConnectThread = null;}
    }

    // Cancel any thread currently running a connection
    if (mConnectedThread != null) {mConnectedThread.cancel(); mConnectedThread = null;}

    // Start the thread to connect with the given device
    mConnectThread = new ConnectThread(device);
    mConnectThread.start();
    setState(STATE_CONNECTING);
}
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1 回答 1

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根据您的代码，mChatService == null这就是导致 NullPointerException 的原因。您应该注意在使用前对其进行初始化。如果不出意外，请对该字段使用延迟初始化。但是，我建议您尝试将代码设计为具有尽可能多的final字段。在对象的构造函数中初始化你需要的一切，不要试图改变它们的值。使用可变（可设置）字段引入可变状态会增加代码的复杂性并使其非常难以调试。特别是在多线程应用程序中。

注意：对于未来的问题，学习阅读堆栈跟踪非常有用。这可能是一篇不错的文章，从http://marxsoftware.blogspot.hu/2010/10/reading-java-stack-traces-few-tips.html开始

于 2013-06-24T19:44:21.197 回答