Android6.0 消息机制原理解析

2024-03-12 19:15:06 30

消息都是存放在一个消息队列中去，而消息循环线程就是围绕这个消息队列进入一个无限循环的，直到线程退出。如果队列中有消息，消息循环线程就会把它取出来，并分发给相应的Handler进行处理；如果队列中没有消息，消息循环线程就会进入空闲等待状态，等待下一个消息的到来。在编写Android应用程序时，当程序执行的任务比较繁重时，为了不阻塞UI主线程而导致ANR的发生，我们通常的做法的创建一个子线程来完成特定的任务。在创建子线程时，有两种选择，一种通过创建Thread对象来创建一个无消息循环的子线程；还有一种就是创建一个带有消息循环的子线程，而创建带有消息循环的子线程由于两种实现方法，一种是直接利用Android给我们封装好的HandlerThread类来直接生成一个带有消息循环的线程对象，另一种方法是在实现线程的run()方法内使用以下方式启动一个消息循环：

一、消息机制使用

通常消息都是有一个消息线程和一个Handler组成，下面我们看PowerManagerService中的一个消息Handler：

mHandlerThread=newServiceThread(TAG,
Process.THREAD_PRIORITY_DISPLAY,false/*allowIo*/);
mHandlerThread.start();
mHandler=newPowerManagerHandler(mHandlerThread.getLooper());

这里的ServiceThread就是一个HandlerThread，创建Handler的时候，必须把HandlerThread的looper传进去，否则就是默认当前线程的looper。

而每个handler，大致如下：

privatefinalclassPowerManagerHandlerextendsHandler{
publicPowerManagerHandler(Looperlooper){
super(looper,null,true/*async*/);
}

@Override
publicvoidhandleMessage(Messagemsg){
switch(msg.what){
caseMSG_USER_ACTIVITY_TIMEOUT:
handleUserActivityTimeout();
break;
caseMSG_SANDMAN:
handleSandman();
break;
caseMSG_SCREEN_BRIGHTNESS_BOOST_TIMEOUT:
handleScreenBrightnessBoostTimeout();
break;
caseMSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT:
checkWakeLockAquireTooLong();
Messagem=mHandler.obtainMessage(MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT);
m.setAsynchronous(true);
mHandler.sendMessageDelayed(m,WAKE_LOCK_ACQUIRE_TOO_LONG_TIMEOUT);
break;
}
}
}

二、消息机制原理
那我们先来看下HandlerThread的主函数run函数：

publicvoidrun(){
mTid=Process.myTid();
Looper.prepare();
synchronized(this){
mLooper=Looper.myLooper();//赋值后notifyall，主要是getLooper函数返回的是mLooper
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid=-1;
}

再来看看Lopper的prepare函数，最后新建了一个Looper对象，并且放在线程的局部变量中。

publicstaticvoidprepare(){
prepare(true);
}

privatestaticvoidprepare(booleanquitAllowed){
if(sThreadLocal.get()!=null){
thrownewRuntimeException("OnlyoneLoopermaybecreatedperthread");
}
sThreadLocal.set(newLooper(quitAllowed));
}

Looper的构造函数中创建了MessageQueue

privateLooper(booleanquitAllowed){
mQueue=newMessageQueue(quitAllowed);
mThread=Thread.currentThread();
}

我们再来看下MessageQueue的构造函数，其中nativeInit是一个native方法，并且把返回值保存在mPtr显然是用long型变量保存的指针

MessageQueue(booleanquitAllowed){
mQuitAllowed=quitAllowed;
mPtr=nativeInit();
}

native函数中主要创建了NativeMessageQueue对象，并且把指针变量返回了。

staticjlongandroid_os_MessageQueue_nativeInit(JNIEnv*env,jclassclazz){
NativeMessageQueue*nativeMessageQueue=newNativeMessageQueue();
if(!nativeMessageQueue){
jniThrowRuntimeException(env,"Unabletoallocatenativequeue");
return0;
}

nativeMessageQueue->incStrong(env);
returnreinterpret_cast<jlong>(nativeMessageQueue);
}

NativeMessageQueue构造函数就是获取mLooper，如果没有就是新建一个Looper

NativeMessageQueue::NativeMessageQueue():
mPollEnv(NULL),mPollObj(NULL),mExceptionObj(NULL){
mLooper=Looper::getForThread();
if(mLooper==NULL){
mLooper=newLooper(false);
Looper::setForThread(mLooper);
}
}

然后我们再看下Looper的构造函数，显示调用了eventfd创建了一个fd，eventfd它的主要是用于进程或者线程间的通信，我们可以看下这篇博客eventfd介绍

Looper::Looper(boolallowNonCallbacks):
mAllowNonCallbacks(allowNonCallbacks),mSendingMessage(false),
mPolling(false),mEpollFd(-1),mEpollRebuildRequired(false),
mNextRequestSeq(0),mResponseIndex(0),mNextMessageUptime(LLONG_MAX){
mWakeEventFd=eventfd(0,EFD_NONBLOCK);
LOG_ALWAYS_FATAL_IF(mWakeEventFd<0,"Couldnotmakewakeeventfd.errno=%d",errno);

AutoMutex_l(mLock);
rebuildEpollLocked();
}

2.1c层创建epoll

我们再来看下rebuildEpollLocked函数，创建了epoll，并且把mWakeEventFd加入epoll，而且把mRequests的fd也加入epoll

voidLooper::rebuildEpollLocked(){
//Closeoldepollinstanceifwehaveone.
if(mEpollFd>=0){
#ifDEBUG_CALLBACKS
ALOGD("%p~rebuildEpollLocked-rebuildingepollset",this);
#endif
close(mEpollFd);
}

//Allocatethenewepollinstanceandregisterthewakepipe.
mEpollFd=epoll_create(EPOLL_SIZE_HINT);
LOG_ALWAYS_FATAL_IF(mEpollFd<0,"Couldnotcreateepollinstance.errno=%d",errno);

structepoll_eventeventItem;
memset(&eventItem,0,sizeof(epoll_event));//zerooutunusedmembersofdatafieldunion
eventItem.events=EPOLLIN;
eventItem.data.fd=mWakeEventFd;
intresult=epoll_ctl(mEpollFd,EPOLL_CTL_ADD,mWakeEventFd,&eventItem);
LOG_ALWAYS_FATAL_IF(result!=0,"Couldnotaddwakeeventfdtoepollinstance.errno=%d",
errno);

for(size_ti=0;i<mRequests.size();i++){
constRequest&request=mRequests.valueAt(i);
structepoll_eventeventItem;
request.initEventItem(&eventItem);

intepollResult=epoll_ctl(mEpollFd,EPOLL_CTL_ADD,request.fd,&eventItem);
if(epollResult<0){
ALOGE("Erroraddingepolleventsforfd%dwhilerebuildingepollset,errno=%d",
request.fd,errno);
}
}
}

继续回到HandlerThread的run函数，我们继续分析Looper的loop函数

publicvoidrun(){
mTid=Process.myTid();
Looper.prepare();
synchronized(this){
mLooper=Looper.myLooper();
notifyAll();
}
Process.setThreadPriority(mPriority);
onLooperPrepared();
Looper.loop();
mTid=-1;
}

我们看看Looper的loop函数：

publicstaticvoidloop(){
finalLooperme=myLooper();
if(me==null){
thrownewRuntimeException("NoLooper;Looper.prepare()wasn'tcalledonthisthread.");
}
finalMessageQueuequeue=me.mQueue;//得到Looper的mQueue

//Makesuretheidentityofthisthreadisthatofthelocalprocess,
//andkeeptrackofwhatthatidentitytokenactuallyis.
Binder.clearCallingIdentity();
finallongident=Binder.clearCallingIdentity();

for(;;){
Messagemsg=queue.next();//mightblock这个函数会阻塞，阻塞主要是epoll_wait
if(msg==null){
//Nomessageindicatesthatthemessagequeueisquitting.
return;
}

//Thismustbeinalocalvariable,incaseaUIeventsetsthelogger
Printerlogging=me.mLogging;//自己打的打印
if(logging!=null){
logging.println(">>>>>Dispatchingto"+msg.target+""+
msg.callback+":"+msg.what);
}

msg.target.dispatchMessage(msg);

if(logging!=null){
logging.println("<<<<<Finishedto"+msg.target+""+msg.callback);
}

//Makesurethatduringthecourseofdispatchingthe
//identityofthethreadwasn'tcorrupted.
finallongnewIdent=Binder.clearCallingIdentity();
if(ident!=newIdent){
Log.wtf(TAG,"Threadidentitychangedfrom0x"
+Long.toHexString(ident)+"to0x"
+Long.toHexString(newIdent)+"whiledispatchingto"
+msg.target.getClass().getName()+""
+msg.callback+"what="+msg.what);
}

msg.recycleUnchecked();
}
}

MessageQueue类的next函数主要是调用了nativePollOnce函数，后面就是从消息队列中取出一个Message

Messagenext(){
//Returnhereifthemessageloophasalreadyquitandbeendisposed.
//Thiscanhappeniftheapplicationtriestorestartalooperafterquit
//whichisnotsupported.
finallongptr=mPtr;//之前保留的指针
if(ptr==0){
returnnull;
}

intpendingIdleHandlerCount=-1;//-1onlyduringfirstiteration
intnextPollTimeoutMillis=0;
for(;;){
if(nextPollTimeoutMillis!=0){
Binder.flushPendingCommands();
}

nativePollOnce(ptr,nextPollTimeoutMillis);

下面我们主要看下nativePollOnce这个native函数，把之前的指针强制转换成NativeMessageQueue，然后调用其pollOnce函数

staticvoidandroid_os_MessageQueue_nativePollOnce(JNIEnv*env,jobjectobj,
jlongptr,jinttimeoutMillis){
NativeMessageQueue*nativeMessageQueue=reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->pollOnce(env,obj,timeoutMillis);
}

2.2c层epoll_wait阻塞

pollOnce函数，这个函数前面的while一般都没有只是处理了indent大于0的情况，这种情况一般没有，所以我们可以直接看pollInner函数

intLooper::pollOnce(inttimeoutMillis,int*outFd,int*outEvents,void**outData){
intresult=0;
for(;;){
while(mResponseIndex<mResponses.size()){
constResponse&response=mResponses.itemAt(mResponseIndex++);
intident=response.request.ident;
if(ident>=0){
intfd=response.request.fd;
intevents=response.events;
void*data=response.request.data;
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~pollOnce-returningsignalledidentifier%d:"
"fd=%d,events=0x%x,data=%p",
this,ident,fd,events,data);
#endif
if(outFd!=NULL)*outFd=fd;
if(outEvents!=NULL)*outEvents=events;
if(outData!=NULL)*outData=data;
returnident;
}
}

if(result!=0){
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~pollOnce-returningresult%d",this,result);
#endif
if(outFd!=NULL)*outFd=0;
if(outEvents!=NULL)*outEvents=0;
if(outData!=NULL)*outData=NULL;
returnresult;
}

result=pollInner(timeoutMillis);
}
}

pollInner函数主要就是调用epoll_wait阻塞，并且java层会计算每次阻塞的时间传到c层，等待有mWakeEventFd或者之前addFd的fd有事件过来，才会epoll_wait返回。

intLooper::pollInner(inttimeoutMillis){
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~pollOnce-waiting:timeoutMillis=%d",this,timeoutMillis);
#endif

//Adjustthetimeoutbasedonwhenthenextmessageisdue.
if(timeoutMillis!=0&&mNextMessageUptime!=LLONG_MAX){
nsecs_tnow=systemTime(SYSTEM_TIME_MONOTONIC);
intmessageTimeoutMillis=toMillisecondTimeoutDelay(now,mNextMessageUptime);
if(messageTimeoutMillis>=0
&&(timeoutMillis<0||messageTimeoutMillis<timeoutMillis)){
timeoutMillis=messageTimeoutMillis;
}
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~pollOnce-nextmessagein%"PRId64"ns,adjustedtimeout:timeoutMillis=%d",
this,mNextMessageUptime-now,timeoutMillis);
#endif
}

//Poll.
intresult=POLL_WAKE;
mResponses.clear();//清空mResponses
mResponseIndex=0;

//Weareabouttoidle.
mPolling=true;

structepoll_eventeventItems[EPOLL_MAX_EVENTS];
inteventCount=epoll_wait(mEpollFd,eventItems,EPOLL_MAX_EVENTS,timeoutMillis);//epoll_wait主要线程阻塞在这，这个阻塞的时间也是有java层传过来的

//Nolongeridling.
mPolling=false;

//Acquirelock.
mLock.lock();

//Rebuildepollsetifneeded.
if(mEpollRebuildRequired){
mEpollRebuildRequired=false;
rebuildEpollLocked();
gotoDone;
}

//Checkforpollerror.
if(eventCount<0){
if(errno==EINTR){
gotoDone;
}
ALOGW("Pollfailedwithanunexpectederror,errno=%d",errno);
result=POLL_ERROR;
gotoDone;
}

//Checkforpolltimeout.
if(eventCount==0){
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~pollOnce-timeout",this);
#endif
result=POLL_TIMEOUT;
gotoDone;
}

//Handleallevents.
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~pollOnce-handlingeventsfrom%dfds",this,eventCount);
#endif

for(inti=0;i<eventCount;i++){
intfd=eventItems[i].data.fd;
uint32_tepollEvents=eventItems[i].events;
if(fd==mWakeEventFd){//通知唤醒线程的事件
if(epollEvents&EPOLLIN){
awoken();
}else{
ALOGW("Ignoringunexpectedepollevents0x%xonwakeeventfd.",epollEvents);
}
}else{
ssize_trequestIndex=mRequests.indexOfKey(fd);//之前addFd的事件
if(requestIndex>=0){
intevents=0;
if(epollEvents&EPOLLIN)events|=EVENT_INPUT;
if(epollEvents&EPOLLOUT)events|=EVENT_OUTPUT;
if(epollEvents&EPOLLERR)events|=EVENT_ERROR;
if(epollEvents&EPOLLHUP)events|=EVENT_HANGUP;
pushResponse(events,mRequests.valueAt(requestIndex));//放在mResponses中
}else{
ALOGW("Ignoringunexpectedepollevents0x%xonfd%dthatis"
"nolongerregistered.",epollEvents,fd);
}
}
}
Done:;

//Invokependingmessagecallbacks.
mNextMessageUptime=LLONG_MAX;
while(mMessageEnvelopes.size()!=0){//这块主要是c层的消息，java层的消息是自己管理的
nsecs_tnow=systemTime(SYSTEM_TIME_MONOTONIC);
constMessageEnvelope&messageEnvelope=mMessageEnvelopes.itemAt(0);
if(messageEnvelope.uptime<=now){
//Removetheenvelopefromthelist.
//WekeepastrongreferencetothehandleruntilthecalltohandleMessage
//finishes.Thenwedropitsothatthehandlercanbedeleted*before*
//wereacquireourlock.
{//obtainhandler
sp<MessageHandler>handler=messageEnvelope.handler;
Messagemessage=messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage=true;
mLock.unlock();

#ifDEBUG_POLL_AND_WAKE||DEBUG_CALLBACKS
ALOGD("%p~pollOnce-sendingmessage:handler=%p,what=%d",
this,handler.get(),message.what);
#endif
handler->handleMessage(message);
}//releasehandler

mLock.lock();
mSendingMessage=false;
result=POLL_CALLBACK;
}else{
//Thelastmessageleftattheheadofthequeuedeterminesthenextwakeuptime.
mNextMessageUptime=messageEnvelope.uptime;
break;
}
}

//Releaselock.
mLock.unlock();

//Invokeallresponsecallbacks.
for(size_ti=0;i<mResponses.size();i++){//这是之前addFd的事件的处理，主要是遍历mResponses，然后调用其回调
Response&response=mResponses.editItemAt(i);
if(response.request.ident==POLL_CALLBACK){
intfd=response.request.fd;
intevents=response.events;
void*data=response.request.data;
#ifDEBUG_POLL_AND_WAKE||DEBUG_CALLBACKS
ALOGD("%p~pollOnce-invokingfdeventcallback%p:fd=%d,events=0x%x,data=%p",
this,response.request.callback.get(),fd,events,data);
#endif
//Invokethecallback.Notethatthefiledescriptormaybeclosedby
//thecallback(andpotentiallyevenreused)beforethefunctionreturnsso
//weneedtobealittlecarefulwhenremovingthefiledescriptorafterwards.
intcallbackResult=response.request.callback->handleEvent(fd,events,data);
if(callbackResult==0){
removeFd(fd,response.request.seq);
}

//Clearthecallbackreferenceintheresponsestructurepromptlybecausewe
//willnotcleartheresponsevectoritselfuntilthenextpoll.
response.request.callback.clear();
result=POLL_CALLBACK;
}
}
returnresult;
}

继续分析Looper的loop函数，可以增加自己的打印来调试代码，之前调用Message的target的dispatchMessage来分配消息

for(;;){
Messagemsg=queue.next();//mightblock
if(msg==null){
//Nomessageindicatesthatthemessagequeueisquitting.
return;
}

//Thismustbeinalocalvariable,incaseaUIeventsetsthelogger
Printerlogging=me.mLogging;//自己的打印
if(logging!=null){
logging.println(">>>>>Dispatchingto"+msg.target+""+
msg.callback+":"+msg.what);
}

msg.target.dispatchMessage(msg);

if(logging!=null){
logging.println("<<<<<Finishedto"+msg.target+""+msg.callback);
}

//Makesurethatduringthecourseofdispatchingthe
//identityofthethreadwasn'tcorrupted.
finallongnewIdent=Binder.clearCallingIdentity();
if(ident!=newIdent){
Log.wtf(TAG,"Threadidentitychangedfrom0x"
+Long.toHexString(ident)+"to0x"
+Long.toHexString(newIdent)+"whiledispatchingto"
+msg.target.getClass().getName()+""
+msg.callback+"what="+msg.what);
}

msg.recycleUnchecked();
}
}

2.3增加调试打印

我们先来看自己添加打印，可以通过Lopper的setMessageLogging函数来打印

publicvoidsetMessageLogging(@NullablePrinterprinter){
mLogging=printer;
}
Printer就是一个interface

publicinterfacePrinter{
/**
*Writealineoftexttotheoutput.Thereisnoneedtoterminate
*thegivenstringwithanewline.
*/
voidprintln(Stringx);
}

2.4java层消息分发处理

再来看消息的分发，先是调用Handler的obtainMessage函数

Messagemsg=mHandler.obtainMessage(MSG_CHECK_WAKE_LOCK_ACQUIRE_TIMEOUT);
msg.setAsynchronous(true);
mHandler.sendMessageDelayed(msg,WAKE_LOCK_ACQUIRE_TOO_LONG_TIMEOUT);

先看obtainMessage调用了Message的obtain函数

publicfinalMessageobtainMessage(intwhat)
{
returnMessage.obtain(this,what);
}

Message的obtain函数就是新建一个Message，然后其target就是设置成其Handler

publicstaticMessageobtain(Handlerh,intwhat){
Messagem=obtain();//就是新建一个Message
m.target=h;
m.what=what;

returnm;
}

我们再联系之前分发消息

msg.target.dispatchMessage(msg);最后就是调用Handler的dispatchMessage函数，最后在Handler中，最后会根据不同的情况对消息进行处理。

publicvoiddispatchMessage(Messagemsg){
if(msg.callback!=null){
handleCallback(msg);//这种就是用post形式发送，带Runnable的
}else{
if(mCallback!=null){//这种是handler传参的时候就是传入了mCallback回调了
if(mCallback.handleMessage(msg)){
return;
}
}
handleMessage(msg);//最后就是在自己实现的handleMessage处理
}
}

2.3java层消息发送

我们再看下java层的消息发送，主要也是调用Handler的sendMessagepost之类函数，最终都会调用下面这个函数

publicbooleansendMessageAtTime(Messagemsg,longuptimeMillis){
MessageQueuequeue=mQueue;
if(queue==null){
RuntimeExceptione=newRuntimeException(
this+"sendMessageAtTime()calledwithnomQueue");
Log.w("Looper",e.getMessage(),e);
returnfalse;
}
returnenqueueMessage(queue,msg,uptimeMillis);
}

我们再来看java层发送消息最终都会调用enqueueMessage函数

privatebooleanenqueueMessage(MessageQueuequeue,Messagemsg,longuptimeMillis){
msg.target=this;
if(mAsynchronous){
msg.setAsynchronous(true);
}
returnqueue.enqueueMessage(msg,uptimeMillis);
}

最终在enqueueMessage中，把消息加入消息队列，然后需要的话就调用c层的nativeWake函数

booleanenqueueMessage(Messagemsg,longwhen){
if(msg.target==null){
thrownewIllegalArgumentException("Messagemusthaveatarget.");
}
if(msg.isInUse()){
thrownewIllegalStateException(msg+"Thismessageisalreadyinuse.");
}

synchronized(this){
if(mQuitting){
IllegalStateExceptione=newIllegalStateException(
msg.target+"sendingmessagetoaHandleronadeadthread");
Log.w(TAG,e.getMessage(),e);
msg.recycle();
returnfalse;
}

msg.markInUse();
msg.when=when;
Messagep=mMessages;
booleanneedWake;
if(p==null||when==0||when<p.when){
//Newhead,wakeuptheeventqueueifblocked.
msg.next=p;
mMessages=msg;
needWake=mBlocked;
}else{
//Insertedwithinthemiddleofthequeue.Usuallywedon'thavetowake
//uptheeventqueueunlessthereisabarrierattheheadofthequeue
//andthemessageistheearliestasynchronousmessageinthequeue.
needWake=mBlocked&&p.target==null&&msg.isAsynchronous();
Messageprev;
for(;;){
prev=p;
p=p.next;
if(p==null||when<p.when){
break;
}
if(needWake&&p.isAsynchronous()){
needWake=false;
}
}
msg.next=p;//invariant:p==prev.next
prev.next=msg;
}

//WecanassumemPtr!=0becausemQuittingisfalse.
if(needWake){
nativeWake(mPtr);
}
}
returntrue;
}

我们看下这个native方法，最后也是调用了Looper的wake函数

staticvoidandroid_os_MessageQueue_nativeWake(JNIEnv*env,jclassclazz,jlongptr){
NativeMessageQueue*nativeMessageQueue=reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->wake();
}
voidNativeMessageQueue::wake(){
mLooper->wake();
}

Looper类的wake，函数只是往mWakeEventfd中写了一些内容，这个fd只是通知而已，类似pipe，最后会把epoll_wait唤醒，线程就不阻塞了继续先发送c层消息，然后处理之前addFd的事件，然后处理java层的消息。

voidLooper::wake(){
#ifDEBUG_POLL_AND_WAKE
ALOGD("%p~wake",this);
#endif

uint64_tinc=1;
ssize_tnWrite=TEMP_FAILURE_RETRY(write(mWakeEventFd,&inc,sizeof(uint64_t)));
if(nWrite!=sizeof(uint64_t)){
if(errno!=EAGAIN){
ALOGW("Couldnotwritewakesignal,errno=%d",errno);
}
}
}

2.4c层发送消息

在c层也是可以发送消息的，主要是调用Looper的sendMessageAtTime函数，参数有有一个handler是一个回调，我们把消息放在mMessageEnvelopes中。

voidLooper::sendMessageAtTime(nsecs_tuptime,constsp<MessageHandler>&handler,
constMessage&message){
#ifDEBUG_CALLBACKS
ALOGD("%p~sendMessageAtTime-uptime=%"PRId64",handler=%p,what=%d",
this,uptime,handler.get(),message.what);
#endif

size_ti=0;
{//acquirelock
AutoMutex_l(mLock);

size_tmessageCount=mMessageEnvelopes.size();
while(i<messageCount&&uptime>=mMessageEnvelopes.itemAt(i).uptime){
i+=1;
}

MessageEnvelopemessageEnvelope(uptime,handler,message);
mMessageEnvelopes.insertAt(messageEnvelope,i,1);

//Optimization:IftheLooperiscurrentlysendingamessage,thenwecanskip
//thecalltowake()becausethenextthingtheLooperwilldoafterprocessing
//messagesistodecidewhenthenextwakeuptimeshouldbe.Infact,itdoes
//notevenmatterwhetherthiscodeisrunningontheLooperthread.
if(mSendingMessage){
return;
}
}//releaselock

//Wakethepolllooponlywhenweenqueueanewmessageatthehead.
if(i==0){
wake();
}
}

当在pollOnce中，在epoll_wait之后，会遍历mMessageEnvelopes中的消息，然后调用其handler的handleMessage函数

while(mMessageEnvelopes.size()!=0){
nsecs_tnow=systemTime(SYSTEM_TIME_MONOTONIC);
constMessageEnvelope&messageEnvelope=mMessageEnvelopes.itemAt(0);
if(messageEnvelope.uptime<=now){
//Removetheenvelopefromthelist.
//WekeepastrongreferencetothehandleruntilthecalltohandleMessage
//finishes.Thenwedropitsothatthehandlercanbedeleted*before*
//wereacquireourlock.
{//obtainhandler
sp<MessageHandler>handler=messageEnvelope.handler;
Messagemessage=messageEnvelope.message;
mMessageEnvelopes.removeAt(0);
mSendingMessage=true;
mLock.unlock();

#ifDEBUG_POLL_AND_WAKE||DEBUG_CALLBACKS
ALOGD("%p~pollOnce-sendingmessage:handler=%p,what=%d",
this,handler.get(),message.what);
#endif
handler->handleMessage(message);
}//releasehandler

mLock.lock();
mSendingMessage=false;
result=POLL_CALLBACK;
}else{
//Thelastmessageleftattheheadofthequeuedeterminesthenextwakeuptime.
mNextMessageUptime=messageEnvelope.uptime;
break;
}
}

有一个Looper_test.cpp文件，里面介绍了很多Looper的使用方法，我们来看下

sp<StubMessageHandler>handler=newStubMessageHandler();
mLooper->sendMessageAtTime(now+ms2ns(100),handler,Message(MSG_TEST1));
StubMessageHandler继承MessageHandler就必须实现handleMessage方法

classStubMessageHandler:publicMessageHandler{
public:
Vector<Message>messages;

virtualvoidhandleMessage(constMessage&message){
messages.push(message);
}
};

我们再顺便看下Message和MessageHandler类

structMessage{
Message():what(0){}
Message(intwhat):what(what){}

/*Themessagetype.(interpretationisleftuptothehandler)*/
intwhat;
};


/**
*InterfaceforaLoopermessagehandler.
*
*TheLooperholdsastrongreferencetothemessagehandlerwheneverithas
*amessagetodelivertoit.MakesuretocallLooper::removeMessages
*toremoveanypendingmessagesdestinedforthehandlersothatthehandler
*canbedestroyed.
*/
classMessageHandler:publicvirtualRefBase{
protected:
virtual~MessageHandler(){}

public:
/**
*Handlesamessage.
*/
virtualvoidhandleMessage(constMessage&message)=0;
};

2.5c层addFd

我们也可以在Looper.cpp的addFd中增加fd放入线程epoll中，当fd有数据来我们也可以处理相应的数据，下面我们先来看下addFd函数，我们注意其中有一个callBack回调

intLooper::addFd(intfd,intident,intevents,Looper_callbackFunccallback,void*data){
returnaddFd(fd,ident,events,callback?newSimpleLooperCallback(callback):NULL,data);
}

intLooper::addFd(intfd,intident,intevents,constsp<LooperCallback>&callback,void*data){
#ifDEBUG_CALLBACKS
ALOGD("%p~addFd-fd=%d,ident=%d,events=0x%x,callback=%p,data=%p",this,fd,ident,
events,callback.get(),data);
#endif

if(!callback.get()){
if(!mAllowNonCallbacks){
ALOGE("InvalidattempttosetNULLcallbackbutnotallowedforthislooper.");
return-1;
}

if(ident<0){
ALOGE("InvalidattempttosetNULLcallbackwithident<0.");
return-1;
}
}else{
ident=POLL_CALLBACK;
}

{//acquirelock
AutoMutex_l(mLock);

Requestrequest;
request.fd=fd;
request.ident=ident;
request.events=events;
request.seq=mNextRequestSeq++;
request.callback=callback;
request.data=data;
if(mNextRequestSeq==-1)mNextRequestSeq=0;//reservesequencenumber-1

structepoll_eventeventItem;
request.initEventItem(&eventItem);

ssize_trequestIndex=mRequests.indexOfKey(fd);
if(requestIndex<0){
intepollResult=epoll_ctl(mEpollFd,EPOLL_CTL_ADD,fd,&eventItem);//加入epoll
if(epollResult<0){
ALOGE("Erroraddingepolleventsforfd%d,errno=%d",fd,errno);
return-1;
}
mRequests.add(fd,request);//放入mRequests中
}else{
intepollResult=epoll_ctl(mEpollFd,EPOLL_CTL_MOD,fd,&eventItem);//更新
if(epollResult<0){
if(errno==ENOENT){
//TolerateENOENTbecauseitmeansthatanolderfiledescriptorwas
//closedbeforeitscallbackwasunregisteredandmeanwhileanew
//filedescriptorwiththesamenumberhasbeencreatedandisnow
//beingregisteredforthefirsttime.Thiserrormayoccurnaturally
//whenacallbackhastheside-effectofclosingthefiledescriptor
//beforereturningandunregisteringitself.Callbacksequencenumber
//checksfurtherensurethattheraceisbenign.
//
//Unfortunatelyduetokernellimitationsweneedtorebuildtheepoll
//setfromscratchbecauseitmaycontainanoldfilehandlethatweare
//nowunabletoremovesinceitsfiledescriptorisnolongervalid.
//Nosuchproblemwouldhaveoccurredifwewereusingthepollsystem
//callinstead,butthatapproachcarriesothersdisadvantages.
#ifDEBUG_CALLBACKS
ALOGD("%p~addFd-EPOLL_CTL_MODfailedduetofiledescriptor"
"beingrecycled,fallingbackonEPOLL_CTL_ADD,errno=%d",
this,errno);
#endif
epollResult=epoll_ctl(mEpollFd,EPOLL_CTL_ADD,fd,&eventItem);
if(epollResult<0){
ALOGE("Errormodifyingoraddingepolleventsforfd%d,errno=%d",
fd,errno);
return-1;
}
scheduleEpollRebuildLocked();
}else{
ALOGE("Errormodifyingepolleventsforfd%d,errno=%d",fd,errno);
return-1;
}
}
mRequests.replaceValueAt(requestIndex,request);
}
}//releaselock
return1;
}

在pollOnce函数中，我们先寻找mRequests中匹配的fd，然后在pushResponse中新建一个Response，然后把Response和Request匹配起来。

}else{
ssize_trequestIndex=mRequests.indexOfKey(fd);
if(requestIndex>=0){
intevents=0;
if(epollEvents&EPOLLIN)events|=EVENT_INPUT;
if(epollEvents&EPOLLOUT)events|=EVENT_OUTPUT;
if(epollEvents&EPOLLERR)events|=EVENT_ERROR;
if(epollEvents&EPOLLHUP)events|=EVENT_HANGUP;
pushResponse(events,mRequests.valueAt(requestIndex));
}else{
ALOGW("Ignoringunexpectedepollevents0x%xonfd%dthatis"
"nolongerregistered.",epollEvents,fd);
}
}

下面我们就会遍历mResponses中的Response，然后调用其request中的回调

for(size_ti=0;i<mResponses.size();i++){
Response&response=mResponses.editItemAt(i);
if(response.request.ident==POLL_CALLBACK){
intfd=response.request.fd;
intevents=response.events;
void*data=response.request.data;
#ifDEBUG_POLL_AND_WAKE||DEBUG_CALLBACKS
ALOGD("%p~pollOnce-invokingfdeventcallback%p:fd=%d,events=0x%x,data=%p",
this,response.request.callback.get(),fd,events,data);
#endif
//Invokethecallback.Notethatthefiledescriptormaybeclosedby
//thecallback(andpotentiallyevenreused)beforethefunctionreturnsso
//weneedtobealittlecarefulwhenremovingthefiledescriptorafterwards.
intcallbackResult=response.request.callback->handleEvent(fd,events,data);
if(callbackResult==0){
removeFd(fd,response.request.seq);
}

//Clearthecallbackreferenceintheresponsestructurepromptlybecausewe
//willnotcleartheresponsevectoritselfuntilthenextpoll.
response.request.callback.clear();
result=POLL_CALLBACK;
}
}

同样我们再来看看Looper_test.cpp是如何使用的？

Pipepipe;
StubCallbackHandlerhandler(true);

handler.setCallback(mLooper,pipe.receiveFd,Looper::EVENT_INPUT);

我们看下handler的setCallback函数

classCallbackHandler{
public:
voidsetCallback(constsp<Looper>&looper,intfd,intevents){
looper->addFd(fd,0,events,staticHandler,this);//就是调用了looper的addFd函数，并且回调
}

protected:
virtual~CallbackHandler(){}

virtualinthandler(intfd,intevents)=0;

private:
staticintstaticHandler(intfd,intevents,void*data){//这个就是回调函数
returnstatic_cast<CallbackHandler*>(data)->handler(fd,events);
}
};

classStubCallbackHandler:publicCallbackHandler{
public:
intnextResult;
intcallbackCount;

intfd;
intevents;

StubCallbackHandler(intnextResult):nextResult(nextResult),
callbackCount(0),fd(-1),events(-1){
}

protected:
virtualinthandler(intfd,intevents){//这个是通过回调函数再调到这里的
callbackCount+=1;
this->fd=fd;
this->events=events;
returnnextResult;
}
};

我们结合Looper的addFd一起来看，当callback是有的，我们新建一个SimpleLooperCallback

intLooper::addFd(intfd,intident,intevents,Looper_callbackFunccallback,void*data){
returnaddFd(fd,ident,events,callback?newSimpleLooperCallback(callback):NULL,data);
}

这里的Looper_callbackFunc是一个typedef
typedefint(*Looper_callbackFunc)(intfd,intevents,void*data);

我们再来看SimpleLooperCallback

classSimpleLooperCallback:publicLooperCallback{
protected:
virtual~SimpleLooperCallback();

public:
SimpleLooperCallback(Looper_callbackFunccallback);
virtualinthandleEvent(intfd,intevents,void*data);

private:
Looper_callbackFuncmCallback;
};SimpleLooperCallback::SimpleLooperCallback(Looper_callbackFunccallback):
mCallback(callback){
}

SimpleLooperCallback::~SimpleLooperCallback(){
}

intSimpleLooperCallback::handleEvent(intfd,intevents,void*data){
returnmCallback(fd,events,data);
}

最后我们是调用callback->handleEvent(fd,events,data)，而callback就是SimpleLooperCallback，这里的data，之前传进来的就是CallbackHandler的this指针
因此最后就是调用了staticHandler，而data->handler，就是this->handler，最后是虚函数就调用到了StubCallbackHandler的handler函数中了。

当然我们也可以不用这么复杂，直接使用第二个addFd函数，当然callBack我们需要自己定义一个类来实现LooperCallBack类就行了，这样就简单多了。
intaddFd(intfd,intident,intevents,constsp<LooperCallback>&callback,void*data);

2.6java层addFd

一直以为只能在c层的Looper中才能addFd，原来在java层也通过jni做了这个功能。

我们可以在MessageQueue中的addOnFileDescriptorEventListener来实现这个功能

publicvoidaddOnFileDescriptorEventListener(@NonNullFileDescriptorfd,
@OnFileDescriptorEventListener.Eventsintevents,
@NonNullOnFileDescriptorEventListenerlistener){
if(fd==null){
thrownewIllegalArgumentException("fdmustnotbenull");
}
if(listener==null){
thrownewIllegalArgumentException("listenermustnotbenull");
}

synchronized(this){
updateOnFileDescriptorEventListenerLocked(fd,events,listener);
}
}

我们再来看看OnFileDescriptorEventListener这个回调

publicinterfaceOnFileDescriptorEventListener{
publicstaticfinalintEVENT_INPUT=1<<0;
publicstaticfinalintEVENT_OUTPUT=1<<1;
publicstaticfinalintEVENT_ERROR=1<<2;

/**@hide*/
@Retention(RetentionPolicy.SOURCE)
@IntDef(flag=true,value={EVENT_INPUT,EVENT_OUTPUT,EVENT_ERROR})
public@interfaceEvents{}


@EventsintonFileDescriptorEvents(@NonNullFileDescriptorfd,@Eventsintevents);
}

接着调用了updateOnFileDescriptorEventListenerLocked函数

privatevoidupdateOnFileDescriptorEventListenerLocked(FileDescriptorfd,intevents,
OnFileDescriptorEventListenerlistener){
finalintfdNum=fd.getInt$();

intindex=-1;
FileDescriptorRecordrecord=null;
if(mFileDescriptorRecords!=null){
index=mFileDescriptorRecords.indexOfKey(fdNum);
if(index>=0){
record=mFileDescriptorRecords.valueAt(index);
if(record!=null&&record.mEvents==events){
return;
}
}
}

if(events!=0){
events|=OnFileDescriptorEventListener.EVENT_ERROR;
if(record==null){
if(mFileDescriptorRecords==null){
mFileDescriptorRecords=newSparseArray<FileDescriptorRecord>();
}
record=newFileDescriptorRecord(fd,events,listener);//fd保存在FileDescriptorRecord对象
mFileDescriptorRecords.put(fdNum,record);//mFileDescriptorRecords然后保存在
}else{
record.mListener=listener;
record.mEvents=events;
record.mSeq+=1;
}
nativeSetFileDescriptorEvents(mPtr,fdNum,events);//调用native函数
}elseif(record!=null){
record.mEvents=0;
mFileDescriptorRecords.removeAt(index);
}
}

native最后调用了NativeMessageQueue的setFileDescriptorEvents函数

staticvoidandroid_os_MessageQueue_nativeSetFileDescriptorEvents(JNIEnv*env,jclassclazz,
jlongptr,jintfd,jintevents){
NativeMessageQueue*nativeMessageQueue=reinterpret_cast<NativeMessageQueue*>(ptr);
nativeMessageQueue->setFileDescriptorEvents(fd,events);
}

setFileDescriptorEvents函数，这个addFd就是调用的第二个addFd，因此我们可以肯定NativeMessageQueue继承了LooperCallback

voidNativeMessageQueue::setFileDescriptorEvents(intfd,intevents){
if(events){
intlooperEvents=0;
if(events&CALLBACK_EVENT_INPUT){
looperEvents|=Looper::EVENT_INPUT;
}
if(events&CALLBACK_EVENT_OUTPUT){
looperEvents|=Looper::EVENT_OUTPUT;
}
mLooper->addFd(fd,Looper::POLL_CALLBACK,looperEvents,this,
reinterpret_cast<void*>(events));
}else{
mLooper->removeFd(fd);
}
}

果然是，需要实现handleEvent函数

classNativeMessageQueue:publicMessageQueue,publicLooperCallback{
public:
NativeMessageQueue();
virtual~NativeMessageQueue();

virtualvoidraiseException(JNIEnv*env,constchar*msg,jthrowableexceptionObj);

voidpollOnce(JNIEnv*env,jobjectobj,inttimeoutMillis);
voidwake();
voidsetFileDescriptorEvents(intfd,intevents);

virtualinthandleEvent(intfd,intevents,void*data);

handleEvent就是在looper中epoll_wait之后，当我们增加的fd有数据就会调用这个函数

intNativeMessageQueue::handleEvent(intfd,intlooperEvents,void*data){
intevents=0;
if(looperEvents&Looper::EVENT_INPUT){
events|=CALLBACK_EVENT_INPUT;
}
if(looperEvents&Looper::EVENT_OUTPUT){
events|=CALLBACK_EVENT_OUTPUT;
}
if(looperEvents&(Looper::EVENT_ERROR|Looper::EVENT_HANGUP|Looper::EVENT_INVALID)){
events|=CALLBACK_EVENT_ERROR;
}
intoldWatchedEvents=reinterpret_cast<intptr_t>(data);
intnewWatchedEvents=mPollEnv->CallIntMethod(mPollObj,
gMessageQueueClassInfo.dispatchEvents,fd,events);//调用回调
if(!newWatchedEvents){
return0;//unregisterthefd
}
if(newWatchedEvents!=oldWatchedEvents){
setFileDescriptorEvents(fd,newWatchedEvents);
}
return1;
}

最后在java的MessageQueue中的dispatchEvents就是在jni层反调过来的，然后调用之前注册的回调函数

//Calledfromnativecode.
privateintdispatchEvents(intfd,intevents){
//Getthefiledescriptorrecordandanystatethatmightchange.
finalFileDescriptorRecordrecord;
finalintoldWatchedEvents;
finalOnFileDescriptorEventListenerlistener;
finalintseq;
synchronized(this){
record=mFileDescriptorRecords.get(fd);//通过fd得到FileDescriptorRecord
if(record==null){
return0;//spurious,nolistenerregistered
}

oldWatchedEvents=record.mEvents;
events&=oldWatchedEvents;//filtereventsbasedoncurrentwatchedset
if(events==0){
returnoldWatchedEvents;//spurious,watchedeventschanged
}

listener=record.mListener;
seq=record.mSeq;
}

//Invokethelisteneroutsideofthelock.
intnewWatchedEvents=listener.onFileDescriptorEvents(//listener回调
record.mDescriptor,events);
if(newWatchedEvents!=0){
newWatchedEvents|=OnFileDescriptorEventListener.EVENT_ERROR;
}

//Updatethefiledescriptorrecordifthelistenerchangedthesetof
//eventstowatchandthelisteneritselfhasn'tbeenupdatedsince.
if(newWatchedEvents!=oldWatchedEvents){
synchronized(this){
intindex=mFileDescriptorRecords.indexOfKey(fd);
if(index>=0&&mFileDescriptorRecords.valueAt(index)==record
&&record.mSeq==seq){
record.mEvents=newWatchedEvents;
if(newWatchedEvents==0){
mFileDescriptorRecords.removeAt(index);
}
}
}
}

//Returnthenewsetofeventstowatchfornativecodetotakecareof.
returnnewWatchedEvents;
}

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持毛票票。

Android6.0 消息机制原理解析

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