实现了一个IntentService子类,但是运行的时候报空指针异常。
异常日志
04-03 18:19:53.849: W/dalvikvm(12857): threadid=1: thread exiting with uncaught exception (group=0x40015568)
04-03 18:19:53.849: E/AndroidRuntime(12857): FATAL EXCEPTION: main
04-03 18:19:53.849: E/AndroidRuntime(12857): java.lang.RuntimeException: Unable to start service com.paad.services.MyIntentService@4050f420 with Intent { cmp=com.paad.services/.MyIntentService }: java.lang.NullPointerException
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2069)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.ActivityThread.access$2800(ActivityThread.java:117)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:994)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.os.Handler.dispatchMessage(Handler.java:99)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.os.Looper.loop(Looper.java:130)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.ActivityThread.main(ActivityThread.java:3703)
04-03 18:19:53.849: E/AndroidRuntime(12857): at java.lang.reflect.Method.invokeNative(Native Method)
04-03 18:19:53.849: E/AndroidRuntime(12857): at java.lang.reflect.Method.invoke(Method.java:507)
04-03 18:19:53.849: E/AndroidRuntime(12857): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
04-03 18:19:53.849: E/AndroidRuntime(12857): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:624)
04-03 18:19:53.849: E/AndroidRuntime(12857): at dalvik.system.NativeStart.main(Native Method)
04-03 18:19:53.849: E/AndroidRuntime(12857): Caused by: java.lang.NullPointerException
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.IntentService.onStart(IntentService.java:110)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.IntentService.onStartCommand(IntentService.java:118)
04-03 18:19:53.849: E/AndroidRuntime(12857): at com.paad.services.MyIntentService.onStartCommand(MyIntentService.java:30)
04-03 18:19:53.849: E/AndroidRuntime(12857): at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:2056)
04-03 18:19:53.849: E/AndroidRuntime(12857): ... 10 more
从日志中可见,在IntentService的onStart()方法中,存在一处空指针。
究其原因,是没有在IntentService的onCreate()回调中调用super.onCreate()。
跟踪IntentService的源码,代码如下所示:
@Override
public void onStart(Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();// 110行
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
第110行使用了一个mServiceHandler对象,它是在onCreate()里面初始化的。
@Override
public void onCreate() {
// TODO: It would be nice to have an option to hold a partial wakelock
// during processing, and to have a static startService(Context, Intent)
// method that would launch the service & hand off a wakelock.
super.onCreate();
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
参考资料
http://stackoverflow.com/questions/7165215/null-pointer-exception-starting-intentservice
附录
/*
* Copyright (C) 2008 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package android.app;
import android.content.Intent;
import android.os.Handler;
import android.os.HandlerThread;
import android.os.IBinder;
import android.os.Looper;
import android.os.Message;
/**
* IntentService is a base class for {@link Service}s that handle asynchronous
* requests (expressed as {@link Intent}s) on demand. Clients send requests
* through {@link android.content.Context#startService(Intent)} calls; the
* service is started as needed, handles each Intent in turn using a worker
* thread, and stops itself when it runs out of work.
*
* <p>This "work queue processor" pattern is commonly used to offload tasks
* from an application's main thread. The IntentService class exists to
* simplify this pattern and take care of the mechanics. To use it, extend
* IntentService and implement {@link #onHandleIntent(Intent)}. IntentService
* will receive the Intents, launch a worker thread, and stop the service as
* appropriate.
*
* <p>All requests are handled on a single worker thread -- they may take as
* long as necessary (and will not block the application's main loop), but
* only one request will be processed at a time.
*
* @see android.os.AsyncTask
*/
public abstract class IntentService extends Service {
private volatile Looper mServiceLooper;
private volatile ServiceHandler mServiceHandler;
private String mName;
private boolean mRedelivery;
private final class ServiceHandler extends Handler {
public ServiceHandler(Looper looper) {
super(looper);
}
@Override
public void handleMessage(Message msg) {
onHandleIntent((Intent)msg.obj);
stopSelf(msg.arg1);
}
}
/**
* Creates an IntentService. Invoked by your subclass's constructor.
*
* @param name Used to name the worker thread, important only for debugging.
*/
public IntentService(String name) {
super();
mName = name;
}
/**
* Sets intent redelivery preferences. Usually called from the constructor
* with your preferred semantics.
*
* <p>If enabled is true,
* {@link #onStartCommand(Intent, int, int)} will return
* {@link Service#START_REDELIVER_INTENT}, so if this process dies before
* {@link #onHandleIntent(Intent)} returns, the process will be restarted
* and the intent redelivered. If multiple Intents have been sent, only
* the most recent one is guaranteed to be redelivered.
*
* <p>If enabled is false (the default),
* {@link #onStartCommand(Intent, int, int)} will return
* {@link Service#START_NOT_STICKY}, and if the process dies, the Intent
* dies along with it.
*/
public void setIntentRedelivery(boolean enabled) {
mRedelivery = enabled;
}
@Override
public void onCreate() {
// TODO: It would be nice to have an option to hold a partial wakelock
// during processing, and to have a static startService(Context, Intent)
// method that would launch the service & hand off a wakelock.
super.onCreate();
HandlerThread thread = new HandlerThread("IntentService[" + mName + "]");
thread.start();
mServiceLooper = thread.getLooper();
mServiceHandler = new ServiceHandler(mServiceLooper);
}
@Override
public void onStart(Intent intent, int startId) {
Message msg = mServiceHandler.obtainMessage();
msg.arg1 = startId;
msg.obj = intent;
mServiceHandler.sendMessage(msg);
}
@Override
public int onStartCommand(Intent intent, int flags, int startId) {
onStart(intent, startId);
return mRedelivery ? START_REDELIVER_INTENT : START_NOT_STICKY;
}
@Override
public void onDestroy() {
mServiceLooper.quit();
}
@Override
public IBinder onBind(Intent intent) {
return null;
}
/**
* This method is invoked on the worker thread with a request to process.
* Only one Intent is processed at a time, but the processing happens on a
* worker thread that runs independently from other application logic.
* So, if this code takes a long time, it will hold up other requests to
* the same IntentService, but it will not hold up anything else.
*
* @param intent The value passed to {@link
* android.content.Context#startService(Intent)}.
*/
protected abstract void onHandleIntent(Intent intent);
}
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