【问题汇总】IntentService报空指针异常的问题

实现了一个IntentService子类，但是运行的时候报空指针异常。

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

/*
 * 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);
}