尝试调用信使时后台线程中出现 NullPointerException

Question

我正在测试一个使用服务、线程和信使的简单 Android 应用程序。我注意到，当我快速启动和停止服务时，有时会出现 NullPointerException 并强制关闭。我对其进行了调试，发现我的 Messenger 实例有时为空，因此我的处理程序不会被调用。我做错了什么？我需要等待才能重新启动服务吗？

好吧，刚刚发生的事情没有快速停止/启动服务。所以我不确定如何重现这个问题。它可能随机发生。

我可以捕获异常并摆脱强制关闭，但这并不能解决当 Messenger 实例为空时处理程序不被调用的问题。我查看了有关管理服务生命周期，并且我没有看到任何提及在重新启动服务之前必须等待的内容。

这就是我启动服务的方式：

    Intent backgroundService = new Intent(PlayAudio.this, com.marie.mp3player.BackgroundService.class);
    backgroundService.putExtra(BackgroundService.EXTRA_MESSENGER, new Messenger(handler));
    backgroundService.putExtra(BackgroundService.EXTRA_SONG, song);
    startService(backgroundService);

这是我停止服务的方式：

Intent backgroundService = new Intent(MP3Player.this, BackgroundService.class);
stopService(backgroundService);

这是一个在底部附近包含两个 NullPointerException 的 logcat：

D/AndroidRuntime(17736): 

D/AndroidRuntime(17736): >>>>>> AndroidRuntime START com.android.internal.os.RuntimeInit <<<<<<

D/AndroidRuntime(17736): CheckJNI is OFF

D/dalvikvm(17736): creating instr width table

D/AndroidRuntime(17736): Calling main entry com.android.commands.pm.Pm

I/ActivityManager(   96): Start proc com.android.defcontainer for service com.android.defcontainer/.DefaultContainerService: pid=17744 uid=10011 gids={1015, 2001}

D/dalvikvm(17744): GC_EXPLICIT freed 46K, 50% free 2720K/5379K, external 1625K/2137K, paused 34ms

W/ActivityManager(   96): No content provider found for: 

D/PackageParser(   96): Scanning package: /data/app/vmdl-1782488429.tmp

W/ActivityManager(   96): No content provider found for: 

I/PackageManager(   96): Removing non-system package:com.marie.mp3player

I/ActivityManager(   96): Force stopping package com.marie.mp3player uid=10091

D/PackageManager(   96): Scanning package com.marie.mp3player

I/PackageManager(   96): Package com.marie.mp3player codePath changed from /data/app/com.marie.mp3player-2.apk to /data/app/com.marie.mp3player-1.apk; Retaining data and using new

I/PackageManager(   96): Unpacking native libraries for /data/app/com.marie.mp3player-1.apk

D/installd(   70): DexInv: --- BEGIN '/data/app/com.marie.mp3player-1.apk' ---

D/dalvikvm(17752): creating instr width table

D/dalvikvm(17752): DexOpt: load 5ms, verify+opt 18ms

D/installd(   70): DexInv: --- END '/data/app/com.marie.mp3player-1.apk' (success) ---

D/PackageManager(   96):   Services: com.marie.mp3player.BackgroundService

D/PackageManager(   96):   Activities: com.marie.mp3player.MP3Player com.marie.mp3player.PlayAudio

I/ActivityManager(   96): Force stopping package com.marie.mp3player uid=10091

W/PackageManager(   96): Code path for pkg : com.marie.mp3player changing from /data/app/com.marie.mp3player-2.apk to /data/app/com.marie.mp3player-1.apk

W/PackageManager(   96): Resource path for pkg : com.marie.mp3player changing from /data/app/com.marie.mp3player-2.apk to /data/app/com.marie.mp3player-1.apk

I/installd(   70): move /data/dalvik-cache/data@[email protected]@classes.dex -> /data/dalvik-cache/data@[email protected]@classes.dex

D/PackageManager(   96): New package installed in /data/app/com.marie.mp3player-1.apk

I/ActivityManager(   96): Force stopping package com.marie.mp3player uid=10091

D/dalvikvm(   96): GC_EXPLICIT freed 842K, 41% free 6421K/10759K, external 2424K/3027K, paused 106ms

D/dalvikvm(  192): GC_EXPLICIT freed 16K, 45% free 4617K/8327K, external 15977K/17649K, paused 67ms

D/dalvikvm(  158): GC_EXPLICIT freed 525K, 54% free 3151K/6791K, external 2480K/3071K, paused 49ms

I/ActivityManager(   96): Start proc com.appspot.swisscodemonkeys.apps for broadcast com.appspot.swisscodemonkeys.apps/.PackageReceiver: pid=17753 uid=10084 gids={3003}

I/ActivityThread(17753): Pub appbrain.SuggestionProvider: com.appspot.swisscodemonkeys.apps.search.SearchSuggestionSampleProvider

D/dalvikvm(11468): GC_EXPLICIT freed 2K, 49% free 2965K/5767K, external 1625K/2137K, paused 314ms

I/ActivityManager(   96): Start proc com.noshufou.android.su for broadcast com.noshufou.android.su/.UninstallReceiver: pid=17761 uid=10056 gids={1015, 3003}

D/dalvikvm(   96): GC_EXPLICIT freed 273K, 41% free 6367K/10759K, external 2424K/3027K, paused 101ms

I/installd(   70): unlink /data/dalvik-cache/data@[email protected]@classes.dex

D/AndroidRuntime(17736): Shutting down VM

D/dalvikvm(17736): GC_CONCURRENT freed 101K, 72% free 296K/1024K, external 0K/0K, paused 0ms+1ms

D/jdwp    (17736): Got wake-up signal, bailing out of select

D/dalvikvm(17736): Debugger has detached; object registry had 1 entries

I/ActivityManager(   96): Start proc com.android.voicedialer for broadcast com.android.voicedialer/.VoiceDialerReceiver: pid=17770 uid=10048 gids={3002}

D/VoiceDialerReceiver(17770): onReceive Intent { act=android.intent.action.PACKAGE_REMOVED dat=package:com.marie.mp3player flg=0x10000000 cmp=com.android.voicedialer/.VoiceDialerReceiver (has extras) }

V/RecognizerEngine(17770): deleteCachedGrammarFiles /data/data/com.android.voicedialer/files/openentries.txt

I/ActivityManager(   96): Start proc com.android.vending for broadcast com.android.vending/.PackageMonitorReceiver: pid=17779 uid=10046 gids={1015, 3003}

D/dalvikvm(   67): GC_EXPLICIT freed 11K, 50% free 2717K/5379K, external 1625K/2137K, paused 94ms

D/dalvikvm(   67): GC_EXPLICIT freed <1K, 50% free 2717K/5379K, external 1625K/2137K, paused 81ms

I/ActivityThread(17779): Pub com.android.vending.SuggestionsProvider: com.android.vending.SuggestionsProvider

D/vending (17779): [1] VendingApplication.onCreate(): Resetting ServiceLocator from app

D/dalvikvm(   67): GC_EXPLICIT freed <1K, 50% free 2717K/5379K, external 1625K/2137K, paused 93ms

D/InstallReceiver(17761): com.marie.mp3player

I/ActivityManager(   96): Start proc com.google.android.partnersetup for broadcast com.google.android.partnersetup/.AppInstalledReceiver: pid=17791 uid=10021 gids={}

D/AndroidRuntime(17783): 

D/AndroidRuntime(17783): >>>>>> AndroidRuntime START com.android.internal.os.RuntimeInit <<<<<<

D/AndroidRuntime(17783): CheckJNI is OFF

D/dalvikvm(17783): creating instr width table

D/GTalkService(  211): handlePackageInstalled: re-initialize providers

D/GTalkService(  211): [RawStanzaProvidersMgr] ##### searchProvidersFromIntent

D/GTalkService(  211): [RawStanzaProvidersMgr] no intent receivers found

D/VoiceDialerReceiver(17770): onReceive Intent { act=android.intent.action.PACKAGE_ADDED dat=package:com.marie.mp3player flg=0x10000000 cmp=com.android.voicedialer/.VoiceDialerReceiver (has extras) }

V/RecognizerEngine(17770): deleteCachedGrammarFiles /data/data/com.android.voicedialer/files/openentries.txt

I/ActivityManager(   96): Start proc com.google.android.googlequicksearchbox for broadcast com.google.android.googlequicksearchbox/.CorporaUpdateReceiver: pid=17798 uid=10022 gids={3003}

I/ActivityThread(17798): Pub com.google.android.googlequicksearchbox.shortcuts: com.google.android.googlequicksearchbox.ShortcutsProvider

I/ActivityThread(17798): Pub com.google.android.googlequicksearchbox.google: com.google.android.googlequicksearchbox.google.GoogleSuggestionProvider

I/ActivityManager(   96): Start proc com.google.android.apps.maps:FriendService for broadcast com.google.android.apps.maps/com.google.googlenav.friend.android.ServiceReceiver: pid=17812 uid=10033 gids={3003, 1015}

D/AndroidRuntime(17783): Calling main entry com.android.commands.am.Am

I/ActivityManager(   96): Starting: Intent { act=android.intent.action.MAIN cat=[android.intent.category.LAUNCHER] flg=0x10000000 cmp=com.marie.mp3player/.MP3Player } from pid 17783

D/AndroidRuntime(17783): Shutting down VM

D/dalvikvm(17783): GC_CONCURRENT freed 102K, 69% free 320K/1024K, external 0K/0K, paused 0ms+0ms

D/jdwp    (17783): Got wake-up signal, bailing out of select

D/dalvikvm(17783): Debugger has detached; object registry had 1 entries

I/ActivityManager(   96): Start proc com.marie.mp3player for activity com.marie.mp3player/.MP3Player: pid=17822 uid=10091 gids={}

I/AndroidRuntime(17783): NOTE: attach of thread 'Binder Thread #3' failed

I/ActivityManager(   96): Start proc com.google.android.apps.maps:NetworkLocationService for service com.google.android.apps.maps/com.google.android.location.internal.server.NetworkLocationService: pid=17830 uid=10033 gids={3003, 1015}

I/ActivityManager(   96): Displayed com.marie.mp3player/.MP3Player: +429ms

I/ActivityManager(   96): Start proc com.noinnion.android.greader.readerpro for broadcast com.noinnion.android.greader.readerpro/.OnUpgradeReceiver: pid=17831 uid=10070 gids={3003, 1015}

V/RenderScript_jni(  192): surfaceDestroyed

I/ActivityThread(17831): Pub com.noinnion.android.greader.readerpro: com.noinnion.android.greader.readerpro.ReaderProvider

D/InternalNlpService(17830): onCreate

D/NetworkLocationServiceThread(17830): start monitoring enabled

D/NetworkLocationServiceThread(17830): updateState

D/NetworkLocationServiceThread(17830): Creating RealOs

D/vending (17779): [11] LocalAssetCache.updateOnePackage(): No local info for com.marie.mp3player

D/szipinf (17830): Initializing inflate state

D/szipinf (17830): Initializing zlib to inflate

D/vending (17779): [12] LocalAssetCache.updateOnePackage(): No local info for com.marie.mp3player

D/dalvikvm(17830): GC_CONCURRENT freed 460K, 43% free 3851K/6727K, external 1625K/2137K, paused 2ms+3ms

D/WifiLocator(17830): Too many cache  misses. Need server request. hasLocation=0 noLocation=1 cacheMiss=2

I/System.out(17830): [INFO:1747]: f: Running flush

I/System.out(17830): [INFO:1761]: f: Sending payload [bytes=300]

I/System.out(17830): [INFO:6772]: f: Running flush

I/System.out(17830): [INFO:6761]: f: Running delayed flush

I/ActivityManager(   96): Start proc org.my_pod.mypod for service org.my_pod.mypod/net.robmunro.mypod.UpdateFeedsService: pid=17853 uid=10080 gids={3003, 1015}

I/ActivityThread(17853): Pub org.my_pod.mypod: net.robmunro.mypod.util.MyPodContentProvider

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +297ms

W/KeyCharacterMap(17822): No keyboard for id 131074

W/KeyCharacterMap(17822): Using default keymap: /system/usr/keychars/qwerty.kcm.bin

D/MP3Player(17822): Stop the background Service

D/WifiLocator(17830): Too many cache  misses. Need server request. hasLocation=0 noLocation=1 cacheMiss=2

E/GlsClient-query(17830): requestFailed

E/GlsClient-query(17830): ay.g

E/GlsClient-query(17830):   at ay.d.t(Unknown Source)

E/GlsClient-query(17830):   at ay.b.a(Unknown Source)

E/GlsClient-query(17830):   at Z.b.g(Unknown Source)

E/GlsClient-query(17830):   at Z.a.g(Unknown Source)

E/GlsClient-query(17830):   at Z.d.run(Unknown Source)

E/GlsClient-query(17830):   at aE.d.run(Unknown Source)

D/dalvikvm(13124): GC_EXPLICIT freed 2K, 50% free 2715K/5379K, external 1625K/2137K, paused 56ms

D/dalvikvm(  192): GC_EXPLICIT freed 88K, 44% free 4690K/8327K, external 16026K/17649K, paused 85ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +305ms

D/MP3Player(17822): Stop the background Service

D/dalvikvm(17830): GC_CONCURRENT freed 1201K, 51% free 3612K/7239K, external 1625K/2137K, paused 3ms+3ms

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/System.out(17830): [INFO:50519]: f: Response [http=200,length=147]

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +307ms

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +255ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +302ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +296ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +294ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

E/(Messenger)messenger: (17822): NullPointerException

W/System.err(17822): java.lang.NullPointerException

W/System.err(17822):    at com.marie.mp3player.BackgroundService$ServiceWorker.run(BackgroundService.java:65)

W/System.err(17822):    at java.lang.Thread.run(Thread.java:1019)

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +278ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

E/(Messenger)messenger: (17822): NullPointerException

W/System.err(17822): java.lang.NullPointerException

W/System.err(17822):    at com.marie.mp3player.BackgroundService$ServiceWorker.run(BackgroundService.java:65)

W/System.err(17822):    at java.lang.Thread.run(Thread.java:1019)

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +282ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +307ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +284ms

D/MP3Player(17822): Stop the background Service

I/ActivityManager(   96): Starting: Intent { cmp=com.marie.mp3player/.PlayAudio (has extras) } from pid 17822

I/handleMessge(17822): songNum: 10

I/ActivityManager(   96): Displayed com.marie.mp3player/.PlayAudio: +291ms

建议我在发生 NullPointerException 的位置添加代码。非常重要。事实上，这是发生异常的整个线程，用箭头和星号标记：

/*
 * This is the ServiceWorker thread that calls messenger to pass messages
 * and data to the PlayAudio activity.
 */
class ServiceWorker implements Runnable
{
    public void run() {
        // do background processing here... a simple message for now

        // send a message to the handler defined in the PlayAudio
        try {
            Message msg1 = Message.obtain();
            msg1.obj = "Song";
            msg1.arg1 = song;
            if (messenger == null) Log.e("(Messenger)messenger: ", "NullPointerException");
            messenger.send(msg1);   // <== *** EXCEPTION OCCURS HERE ***
        } catch (RemoteException e) {
            e.printStackTrace();
        } catch (NullPointerException e) {
            e.printStackTrace();
        }

        // stop the service when done...
        // BackgroundService.this.stopSelf();
        // Or use the unbindBtn in the MP3Player activity.
    }
}

如果有人在这里看到一个错误，有时会导致我的 Message 实例为空，我肯定会很感激一个答案。

Answer 1

玛丽，假设您的其余服务与您的其他问题相同。以下是发生的情况以及导致 NPE 的原因。

• 在 onCreate 中，您创建并启动线程，但在 onStart 方法中提取消息。如果您的工作方法在 onStart 之前执行，您将得到 NPE。它们的顺序是随机的，因此有时会得到 NPE，有时则不会。