Almost taken away by this wave of handler interview cannons~

Handler yes Android The most frequently used message mechanism in , During the interview process in large factories these years , More and more . Although there are many friends who can complete the solution, there are few , What's going on ？

In fact, the reason is self-evident , Dachang right Handler All the interview questions have a certain depth . And I like the details ;

For example, I mentioned in my previous article ,Handler Of runWithScissors() Can achieve A Thread to B The thread sends a message , send A Thread is blocking , wait for B After the thread processes the message, it continues to execute . So the extended 2 The problem is ;

• runWithScissors() What is the principle of ？

• It's marked as @hide Is there any problem in not allowing it to be used ？

Another example , We know MessageQueue, In fact, it is based on the trigger time when Priority message queue . So in what scenario ,MessageQueue The news at the back of the middle , Will precede the news ahead , Scheduled to execute ？

These problems , If you don't look at the source code , Just stay at the use level , It's basically hard to figure out .

1：Handler In mechanism , What roles exist ？ What functions do they undertake ？

• Handler： Message auxiliary class & External interface & towards MQ Deliver a message & The target handler of the message ;
• Message： The carrier of the message & By Handler The delivery & Bring their own Handler Handle & Built in message pool ;
• Looper： Circulator & hold MQ & Cycle from MQ Get message from & TLS Thread unique ;
• MessageQueue： Time based priority queue & Chain table structure & Java And C++ The ties of the layers ;

2：Handler Distribution event priority , Whether it can be intercepted ？ What is the priority of interception ？

Handler in , adopt dispatchMessage() Process the message , There is a priority policy ;

• priority 1：msg,callback,run() - Monopoly ;
• priority 2：mCallback.handleMessage(msg) - The return value determines whether to intercept the message ;
• priority 3：handle.handleMessage();

3： The main thread Looper When to run ？

App Startup time , Will be called to ActivityThread in ,Looper It's in it main() Method ;main() Will actively call Looper.prepareMainLooper() and Looper.loop();Tips：ActivityThread Do not inherit from Thread, It's just an object running on the main thread ;

4：Handler Of Message It can be divided into 3 class ？ What are the marks ？

• Sync Message： Ordinary Message;
• asynchronous Message：msg.setAsynchronous(true)
• Synchronization barrier ：msg.target == null

5： The same Message Whether the object can be repeated send？

The key is how to define the same Message.

• Angle one ：Java Object level , Reusable ;
  reason ：Message Maintained by the message pool , That is, the same object will be reused after being recycled ;｜ new Message & Message.obtain()
• Angle two ： Business level , It can't be reused ;
  reason ：Message adopt enqueueMessage() When you join the team , Will pass markInUse() Mark , Joining the team again can't pass isInUse() Check , Throw an exception ;

I sum up here BAT Dachang about Handler More than 100+ High frequency interview questions , Now it has been sorted into HD PDF Learning documents , If you need anything, you can Reply to me by private letter 666 Ready to pick up ！！！ It basically covers all angles , You can take a self-test . You can also brush it before the interview , After all Handler Although interview questions appear frequently , But don't panic when you meet .

6： scene ：MessageQueue Is based on the trigger time when Priority queue of , Under what circumstances , Messages later in the queue will be executed first ？ What is the principle ？

• scene ： The first message is the synchronization message , The next message is asynchronous , And the queue head of the message queue is the synchronization barrier ;
• principle ： The synchronization barrier will block MQ Synchronization messages in , Prioritize asynchronous messages ;

7：Message What's the use of synchronization barriers ？ What's the point ？ How to send a synchronization barrier ？

• purpose ： Blocking MQ Synchronization Message Distribution of , Prioritize asynchronous messages , When there is no asynchronous message, it enters sleep , Until the synchronization barrier is removed ;
• significance ： Allow asynchronous messages to take precedence over synchronous messages ;
• Synchronization barrier ： special Message,target == null, Unable to get Handler Join the team and leave the team , Direct operation is required MQ; The team ：postSyncBarrier()： Return to a barrier token; Out of the team ：removeSyncBarrier()

8： What is asynchronous message ？ How to send ？

• significance ： It needs to be used with synchronous barrier , No, it's no different from synchronous messages ;
• Asynchronous messaging ：setAsynchronous(true) → towards flags add to FLAG_ASYNCHRONOUS Mark
  Send by Through asynchrony Handler send out → structure Handler when ,async Pass on true Before sending a message , Active call setAsynchronous(true)
  Be on the safe side ,Android 9.0 Ordinary developers cannot use asynchronous messages , All sending methods are marked as @hide

9：Handler Of IdleHandler Mechanism , How to understand ？ What's the use ？

Interface , Need to implement queueIdle() Method & It's defined in MQ in & With MQ mIdleHandlers Maintain storage

• purpose ： Can be found in MQ When I'm about to be free , Processing tasks ;
• Logical point ：MQ.next() in , When there are currently no pending messages , perform mIdleHandlers;

basis queueIdle() The return value is divided into ： Continuous callback （true） & One time callback （false）,false Will result in after execution , from mIdleHandlers Remove

10：IdleHandler Will the execution time affect the normal message distribution ？Handler How to deal with it internally ？

Meeting ;IdleHandler The time is uncontrollable ; After execution, it will reset nextPollTimeoutMillis = 0, Redistribute recent messages

11： Remove the message removeMessage() Why do you need two cycles ？

Optimize efficiency ;

• while-1： Remove message & Find the next pending message , Deposit in mMessages in ;
• while-2： from mMessages Start , Remove the subsequent qualified messages ;

12：Handler Of runWithScissors() Can be realized A Thread blocking waiting B The function that the thread continues to execute after processing the message , Why is it marked hide？ What are the problems ？ What's the reason ？

• Realization ： take Runnable The packing is BlockingRunnable, It passes through synchronized + wait Has reached the awaited , stay r After the execution , call notifyAll() Wake up the child thread waiting for the queue , The child thread continues to execute ;
• problem ： In the child thread Looper Use in , May lead to A Thread entry wait wait for , And never get notify Wake up the ;
• reason ： Sub thread Looper Allow to exit , If packed BlockingRunnable Before being executed ,MessageQueue sign out , Then runnable It will never be carried out , Will lead to A Thread has been in wait wait for , Never be notify Wake up the ;

13：Looper.loop in , If there are no pending messages , Why not block UI？

The main thread is in MessageQueue When there is no message , Will be blocked. loop Of queue.next() Methods nativePollOnce() In the method .

The main thread is released CPU Resource goes to sleep , Until the next message arrives or a transaction occurs , Through to pipe How to write data at the writing end of the pipeline , To wake up the main thread . What we use here is epoll Mechanism .

epoll Mechanism is a kind of IO Multiplexing mechanism , Multiple descriptors can be monitored at the same time , When something happens , Immediately notify the corresponding program to read or write , Like a kind of callback The callback mechanism of . The main thread is dormant most of the time , It doesn't consume a lot of CPU resources . When a new message or transaction arrives , Will immediately wake up the main thread for processing , So it is imperceptible to users .

14： If Java layer MQ There is little news in , But the response time is very long , What is the reason ？

MQ In line , The Message Before Message Processing is time-consuming ;Native Too many layer messages ,Java layer MQ Message priority is the lowest , Finally deal with ;

15：Looper Of Printer Output log , What other uses ？ What principle does it depend on ？ What's the downside ？

• purpose ： Performance monitoring ;
• principle ： By filtering log memory , distinguish Message The point in time at which the execution of starts and ends , It can be judged and handled Message Time consuming , That is, the main route is stuck for a long time ;
• shortcoming ：Printer There are a lot of string concatenation , When the message volume is large , Will cause performance damage ;｜ Measured data ： There is Printer when , When the list slides quickly , The average frame rate is reduced 5 frame ;

16：Handler Sure IPC Correspondence ？

You can't ;Handler Can only be used for shared memory addresses 2 Threads communicating , That is, the same process 2 Threads communicating ;

17：Handler Why do I need to use the underlying epoll To sleep ？

Need to take into account Native Layer messages , Messages may come from the underlying hardware ; If only consider Java layer ,notify/wait That is to say ;

I sum up here BAT Dachang about Handler More than 100+ High frequency interview questions , Now it has been sorted into HD PDF Learning documents , If you need anything, you can Reply to me by private letter 666 Ready to pick up ！！！ It basically covers all angles , You can take a self-test . You can also brush it before the interview , After all Handler Although interview questions appear frequently , But don't panic when you meet .