對於從事Android開發的人來說，遇到ANR(Application Not Responding)是比較常見的問題。一般情況下，如果有ANR發生，系統都會在/data/anr/目錄下生成trace文件，通過分析trace文件，可以定位產生ANR的原因。產生ANR的原因有很多，比如CPU使用過高、事件沒有得到及時的響應、死鎖等，下面將通過一次因為死鎖導致的ANR問題，來說明如何通過trace文件分析ANR問題。
對應的部分trace文件內容如下：
"PowerManagerService" prio=5 tid=24 MONITOR
| group="main" sCount=1 dsCount=0 obj=0x41dd0eb0 self=0x5241b218
| sysTid=567 nice=0 sched=0/0 cgrp=apps handle=1380038664
| state=S schedstat=( 6682116007 11324451214 33313 ) utm=450 stm=219 core=1
at com.android.server.am.ActivityManagerService.broadcastIntent(ActivityManagerService.java:~13045)
- waiting to lock <0x41a874a0> (a com.android.server.am.ActivityManagerService) held by tid=12 (android.server.ServerThread)
at android.app.ContextImpl.sendBroadcast(ContextImpl.java:1144)
at com.android.server.power.PowerManagerService$DisplayBlankerImpl.unblankAllDisplays(PowerManagerService.java:3442)
at com.android.server.power.DisplayPowerState$PhotonicModulator$1.run(DisplayPowerState.java:456)
at android.os.Handler.handleCallback(Handler.java:800)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:194)
at android.os.HandlerThread.run(HandlerThread.java:60)
"Binder_B" prio=5 tid=85 MONITOR
| group="main" sCount=1 dsCount=0 obj=0x42744770 self=0x58329e88
| sysTid=3700 nice=-20 sched=0/0 cgrp=apps handle=1471424616
| state=S schedstat=( 1663727513 2044643318 6806 ) utm=132 stm=34 core=1
at com.android.server.power.PowerManagerService$DisplayBlankerImpl.toString(PowerManagerService.java:~3449)
- waiting to lock <0x41a7e420> (a com.android.server.power.PowerManagerService$DisplayBlankerImpl) held by tid=24 (PowerManagerService)
at java.lang.StringBuilder.append(StringBuilder.java:202)
at com.android.server.power.PowerManagerService.dump(PowerManagerService.java:3052)
at android.os.Binder.dump(Binder.java:264)
at android.os.Binder.onTransact(Binder.java:236)
at android.os.IPowerManager$Stub.onTransact(IPowerManager.java:373)
at android.os.Binder.execTransact(Binder.java:351)
at dalvik.system.NativeStart.run(Native Method)
"android.server.ServerThread" prio=5 tid=12 MONITOR
| group="main" sCount=1 dsCount=0 obj=0x41a76178 self=0x507837a8
| sysTid=545 nice=-2 sched=0/0 cgrp=apps handle=1349936616
| state=S schedstat=( 15368096286 21707846934 69485 ) utm=1226 stm=310 core=0
at com.android.server.power.PowerManagerService.isScreenOnInternal(PowerManagerService.java:~2529)
- waiting to lock <0x41a7e2e8> (a java.lang.Object) held by tid=85 (Binder_B)
at com.android.server.power.PowerManagerService.isScreenOn(PowerManagerService.java:2522)
at com.android.server.wm.WindowManagerService.sendScreenStatusToClientsLocked(WindowManagerService.java:7749)
at com.android.server.wm.WindowManagerService.setEventDispatching(WindowManagerService.java:7628)
at com.android.server.am.ActivityManagerService.updateEventDispatchingLocked(ActivityManagerService.java:8083)
at com.android.server.am.ActivityManagerService.wakingUp(ActivityManagerService.java:8077)
at com.android.server.power.Notifier.sendWakeUpBroadcast(Notifier.java:474)
at com.android.server.power.Notifier.sendNextBroadcast(Notifier.java:455)
at com.android.server.power.Notifier.access$700(Notifier.java:62)
at com.android.server.power.Notifier$NotifierHandler.handleMessage(Notifier.java:600)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:194)
at com.android.server.ServerThread.run(SystemServer.java:1328)
從trace文件看，是因為TID為24的線程等待一個TID為12的線程持有的鎖，TID為12的線程等待一個TID為85的線程持有的鎖，而TID為85的線程確等待一個TID為24的線程持有的鎖，導致了循環等待的現象，對應的trace文件的語句如下：
TID 24：- waiting to lock <0x41a874a0> (a com.android.server.am.ActivityManagerService) held by tid=12 (android.server.ServerThread)
TID 12： - waiting to lock <0x41a7e2e8> (a java.lang.Object) held by tid=85 (Binder_B)
TID 85：- waiting to lock <0x41a7e420> (a com.android.server.power.PowerManagerService$DisplayBlankerImpl) held by tid=24 (PowerManagerService)
既然是死鎖，那麼先看各線程都有那些鎖。
先看TID=24的線程的棧頂，ActivityManagerService的broadcastIntent函數代碼如下:
public final int broadcastIntent(IApplicationThread caller,
Intent intent, String resolvedType, IIntentReceiver resultTo,
int resultCode, String resultData, Bundle map,
String requiredPermission, boolean serialized, boolean sticky, int userId) {
enforceNotIsolatedCaller("broadcastIntent");
synchronized(this) {
intent = verifyBroadcastLocked(intent);
final ProcessRecord callerApp = getRecordForAppLocked(caller);
final int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
int res = broadcastIntentLocked(callerApp,
callerApp != null ? callerApp.info.packageName : null,
intent, resolvedType, resultTo,
resultCode, resultData, map, requiredPermission, serialized, sticky,
callingPid, callingUid, userId);
Binder.restoreCallingIdentity(origId);
return res;
}