小弟我的Android进阶之旅->android中service的onStartCommand()方法中intent为null的有关问题

       今天在维护公司的一个APP的时候，突然爆了空指针异常，

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.content.Intent.getBooleanExtra(java.lang.String, boolean)' on a null object reference

       下面是报错的log。

D/AndroidRuntime( 4721): Shutting down VME/AndroidRuntime( 4721): FATAL EXCEPTION: mainE/AndroidRuntime( 4721): Process: com.tintele.sos, PID: 4721E/AndroidRuntime( 4721): java.lang.RuntimeException: Unable to start service [email protected] with null: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.content.Intent.getBooleanExtra(java.lang.String, boolean)' on a null object referenceE/AndroidRuntime( 4721):        at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:3152)E/AndroidRuntime( 4721):        at android.app.ActivityThread.access$2100(ActivityThread.java:178)E/AndroidRuntime( 4721):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1568)E/AndroidRuntime( 4721):        at android.os.Handler.dispatchMessage(Handler.java:111)E/AndroidRuntime( 4721):        at android.os.Looper.loop(Looper.java:194)E/AndroidRuntime( 4721):        at android.app.ActivityThread.main(ActivityThread.java:5637)E/AndroidRuntime( 4721):        at java.lang.reflect.Method.invoke(Native Method)E/AndroidRuntime( 4721):        at java.lang.reflect.Method.invoke(Method.java:372)E/AndroidRuntime( 4721):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:959)E/AndroidRuntime( 4721):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)E/AndroidRuntime( 4721): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.content.Intent.getBooleanExtra(java.lang.String, boolean)' on a null object referenceE/AndroidRuntime( 4721):        at com.tintele.sos.SosService.onStartCommand(SosService.java:309)E/AndroidRuntime( 4721):        at android.app.ActivityThread.handleServiceArgs(ActivityThread.java:3135)E/AndroidRuntime( 4721):        ... 9 more

     打印log后发现原来是在onStartCommand(Intent intent, int flags, int startId)方法中intent为null。这个错误是对onStartCommand()方法的flags参数没有深入理解造成的，报错的代码：

boolean groundAlarmFlag=intent.getBooleanExtra("flag", false);

      而这个flag参数是通过别处传过来的，代码如下：

Intent intent = new Intent(getBaseContext(), SosService.class);intent.putExtra("flag", true);startService(intent);

【报错原因】

       因此报错 intent的参数是null的原因是这个intent参数是通过startService(Intent)方法所传递过来的，但是如果Service在你的进程退出后有可能被系统自动重启，这个时候intent就会是null.
【解决方法】
        【方法一】在使用intent前需要判断一下是否为空。即：

boolean groundAlarmFlag = false;  if (intent!=null) {    groundAlarmFlag=intent.getBooleanExtra("flag", false);}

        【方法二】

      将onStartCommand()方法的返回值从

return super.onStartCommand(intent, flags, startId);

      改成

return super.onStartCommand(intent, Service.START_REDELIVER_INTENT, startId);

因为onStartCommand方法的默认值START_STICKY_COMPATIBILITY或者START_STICKY不能保证intent不为null.

/**     * Called by the system every time a client explicitly starts the service by calling      * [email protected] android.content.Context#startService}, providing the arguments it supplied and a      * unique integer token representing the start request.  Do not call this method directly.     *      * <p>For backwards compatibility, the default implementation calls     * [email protected] #onStart} and returns either [email protected] #START_STICKY}     * or [email protected] #START_STICKY_COMPATIBILITY}.     *      * <p>If you need your application to run on platform versions prior to API     * level 5, you can use the following model to handle the older [email protected] #onStart}     * callback in that case.  The <code>handleCommand</code> method is implemented by     * you as appropriate:     *      * [email protected] [email protected]egroundService.java     *   start_compatibility}     *     * <p class="caution">Note that the system calls this on your     * service's main thread.  A service's main thread is the same     * thread where UI operations take place for Activities running in the     * same process.  You should always avoid stalling the main     * thread's event loop.  When doing long-running operations,     * network calls, or heavy disk I/O, you should kick off a new     * thread, or use [email protected] android.os.AsyncTask}.</p>     *     * @param intent The Intent supplied to [email protected] android.content.Context#startService},      * as given.  This may be null if the service is being restarted after     * its process has gone away, and it had previously returned anything     * except [email protected] #START_STICKY_COMPATIBILITY}.     * @param flags Additional data about this start request.  Currently either     * 0, [email protected] #START_FLAG_REDELIVERY}, or [email protected] #START_FLAG_RETRY}.     * @param startId A unique integer representing this specific request to      * start.  Use with [email protected] #stopSelfResult(int)}.     *      * @return The return value indicates what semantics the system should     * use for the service's current started state.  It may be one of the     * constants associated with the [email protected] #START_CONTINUATION_MASK} bits.     *      * @see #stopSelfResult(int)     */    public int onStartCommand(Intent intent, int flags, int startId) {        onStart(intent, startId);        return mStartCompatibility ? START_STICKY_COMPATIBILITY : START_STICKY;    }

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下面具体讲解一下这几个flags的参数，首先参数有START_STICKY_COMPATIBILITY、

START_STICKY、START_NOT_STICKY、START_REDELIVER_INTENT四个。下面分别介绍一下，如下面表格所示：

   源代码如下：

/**     * Constant to return from [email protected] #onStartCommand}: compatibility     * version of [email protected] #START_STICKY} that does not guarantee that     * [email protected] #onStartCommand} will be called again after being killed.     */    public static final int START_STICKY_COMPATIBILITY = 0;        /**     * Constant to return from [email protected] #onStartCommand}: if this service's     * process is killed while it is started (after returning from     * [email protected] #onStartCommand}), then leave it in the started state but     * don't retain this delivered intent.  Later the system will try to     * re-create the service.  Because it is in the started state, it will     * guarantee to call [email protected] #onStartCommand} after creating the new     * service instance; if there are not any pending start commands to be     * delivered to the service, it will be called with a null intent     * object, so you must take care to check for this.     *      * <p>This mode makes sense for things that will be explicitly started     * and stopped to run for arbitrary periods of time, such as a service     * performing background music playback.     */    public static final int START_STICKY = 1;        /**     * Constant to return from [email protected] #onStartCommand}: if this service's     * process is killed while it is started (after returning from     * [email protected] #onStartCommand}), and there are no new start intents to     * deliver to it, then take the service out of the started state and     * don't recreate until a future explicit call to     * [email protected] Context#startService Context.startService(Intent)}.  The     * service will not receive a [email protected] #onStartCommand(Intent, int, int)}     * call with a null Intent because it will not be re-started if there     * are no pending Intents to deliver.     *      * <p>This mode makes sense for things that want to do some work as a     * result of being started, but can be stopped when under memory pressure     * and will explicit start themselves again later to do more work.  An     * example of such a service would be one that polls for data from     * a server: it could schedule an alarm to poll every N minutes by having     * the alarm start its service.  When its [email protected] #onStartCommand} is     * called from the alarm, it schedules a new alarm for N minutes later,     * and spawns a thread to do its networking.  If its process is killed     * while doing that check, the service will not be restarted until the     * alarm goes off.     */    public static final int START_NOT_STICKY = 2;        /**     * Constant to return from [email protected] #onStartCommand}: if this service's     * process is killed while it is started (after returning from     * [email protected] #onStartCommand}), then it will be scheduled for a restart     * and the last delivered Intent re-delivered to it again via     * [email protected] #onStartCommand}.  This Intent will remain scheduled for     * redelivery until the service calls [email protected] #stopSelf(int)} with the     * start ID provided to [email protected] #onStartCommand}.  The     * service will not receive a [email protected] #onStartCommand(Intent, int, int)}     * call with a null Intent because it will will only be re-started if     * it is not finished processing all Intents sent to it (and any such     * pending events will be delivered at the point of restart).     */    public static final int START_REDELIVER_INTENT = 3;

 官方文档上的解释如下：

public static final int START_STICKY_COMPATIBILITY

public static final int START_STICKY

public static final int START_REDELIVER_INTENT

public int onStartCommand (Intent intent, int flags, int startId)

// This is the old onStart method that will be called on the pre-2.0// platform.  On 2.0 or later we override onStartCommand() so this// method will not be called.@Overridepublic void onStart(Intent intent, int startId) {    handleCommand(intent);}@Overridepublic int onStartCommand(Intent intent, int flags, int startId) {    handleCommand(intent);    // We want this service to continue running until it is explicitly    // stopped, so return sticky.    return START_STICKY;}

  作者：欧阳鹏  欢迎转载，与人分享是进步的源泉！

  转载请保留原文地址：http://blog.csdn.net/ouyang_peng

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