Communication between threads

itread01 2021-01-21 06:46:31
communication threads

# Communication between threads JVM During execution, the memory area managed by itself will be , Divided into different data areas , It's called the runtime data area . Each thread has its own private memory space , As shown below :![ Insert picture description here ](,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mzc2MDAyMA==,size_16,color_FFFFFF,t_70#pic_center)Java Threads follow the method code in their virtual machine stack step by step , In this process, it is inevitable to use the memory area, heap or method area shared by threads . To prevent multiple threads from accessing the same memory address at the same time , Need to tell each other their status to avoid resource contention . Threads can communicate in three ways :① Share memory ② Messaging ③ Piped flow shared memory : Through reading shared memory between threads - Write to achieve implicit communication .Java The specific implementation of is :volatile Share memory . Messaging : Threads communicate with each other through explicit message transmission .Java The specific implementation of is : wait for / Notification mechanism (wait/notify),join Method . Pipe flow : Pipeline input / Output stream .## 1、 wait for / The process of notification mechanism is : Thread A For some reason , Autonomously called the object o Of wait Method , Enter WAITING Status , Release the lock in possession and wait for notification . And the thread B Then call the object o Of notify Method or notifyall Method to notify , Thread A Will receive notice , And from wait Return in method , Continue to run the following code . It can be found that , Thread A And threads B It's through objects o Of wait Methods and notify Method to send a message , Communicate .wait Methods and notify The method is Object Method of class , and Object Class is the parent of all classes , So all objects are implemented Object Method of class . That is, all objects have wait Methods and notify Method .| Method | effect | Note ||--|--|--|| wait | The thread calls the wait() Method will enter WAITING Status , Release the held object lock and wait for other threads to notify or interrupt before returning from this method .| This method can pass the number ,wait(long n): To wait for n millisecond , Enter TIME-WAITING Status , If in n No notification or interruption in milliseconds , Then return to || notify | The thread calls the notify() Method will notify a call wait Method and the thread waiting here returns . But because there may be more than one thread waiting on a shared variable , So the specific notification of which thread is random .| notifyAll() Method and notify() Methods work in the same way , But notify Is to randomly notify a thread , and notifyAll Is to notify all threads waiting on the shared variable | Due to the thread waiting / The notification mechanism needs to use shared objects , So it's calling wait Before the method , The thread must first acquire the lock for the object , That is, only in the synchronization method or block (synchronized Code block ) Call me wait Method , Calling wait After the method , Thread releases lock . The same notify Method also needs to obtain the lock of the object before calling , That is, it can only be called in synchronization method or synchronization block notify Method . If there are multiple threads waiting , Then the thread scheduler randomly selects a thread to notify . It should be noted that , The notified thread does not immediately get notification from wait Method returns , You need to wait for the lock of the object before you can get it from wait Method returns . And called notify The thread of the method does not immediately release the lock on the object when called , It's after executing the synchronization method or block (synchronized Code block ) After , To release the lock of the object . therefore , The notified thread is waiting for the call notify After releasing the lock , From wait Return in method . To sum up , wait for / The classical paradigm of notification mechanism is as follows :```java/** * Wait for the thread ( call wait Method thread ) */synchronized( Sharing objects ){ // Synchronization block , The entry condition is to get the lock while( Judging conditions ){ // To carry out wait Enter when the condition of thread task is not satisfied Sharing objects .wait() } Thread task code }/** * Notify the thread ( call notify Method thread ) */synchronized( Sharing objects ){ // Synchronization block , The entry condition is to get the lock Thread task code Change wait Conditions for thread tasks Sharing objects .notify()}``` According to the above normalization , The code is as follows :```javapublic class WaitNotify { static boolean flag = true; // Conditions that wait for the thread to continue to run down static Object lock = new Object(); // Locked objects public static void main(String[] args) throws InterruptedException { Thread waitThread = new Thread(new WaitRunnable(),"waitThread"); // With WaitRunnable Is the thread of the task class Thread notifyThread = new Thread(new NotifyRunnable(),"notifyThread"); // With NotifyRunnable Is the thread of the task class waitThread.start(); //wait Thread start Thread.sleep(2000); // Main thread sleep 2s notifyThread.start(); //notify Thread start } /** * Runnable Waiting for the implementation class * synchronized Keywords : You can modify methods or use them as synchronous blocks */ static class WaitRunnable implements Runnable{ @Override public void run() { // Yes lock Lock synchronized(lock){ // Judge , if flag For true, Then continue to wait (wait) while(flag){ try { System.out.println( Thread.currentThread().getName()+ "---flag For true, wait for @"+ new SimpleDateFormat("hh:mm:ss").format(new Date()) ); lock.wait(); // wait for , And release the lock resource } catch (InterruptedException e) { e.printStackTrace(); } } // if flag For false, Then we will work System.out.println( Thread.currentThread().getName()+ "---flag For false, Execute @"+ new SimpleDateFormat("hh:mm:ss").format(new Date()) ); } } } /** * Runnable Notification implementation class */ static class NotifyRunnable implements Runnable{ @Override public void run(){ // Yes lock Lock synchronized(lock){ // With NotifyRunnable Release... For the thread of the task class lock Lock , And after notification , With Wait Only threads of task class can jump out of loop System.out.println( Thread.currentThread().getName()+ "--- Currently holding lock , Release @"+ new SimpleDateFormat("hh:mm:ss").format(new Date()) ); lock.notifyAll(); // Notifies all threads that are waiting from wait return flag = false; try { Thread.sleep(5000); //notifyThread Thread sleep 5s } catch (InterruptedException e) { e.printStackTrace(); } } // Again lock Lock , And sleep synchronized (lock){ System.out.println( Thread.currentThread().getName()+ "--- Hold the lock again , Sleep @"+ new SimpleDateFormat("hh:mm:ss").format(new Date()) ); try { Thread.sleep(2000); // Once again let notifyThread Thread sleep 2s } catch (InterruptedException e) { e.printStackTrace(); } } } }}// This code example comes from 《Java The art of concurrent programming 》``` The results are as follows :```javawaitThread---flag For true, wait for @01:53:51notifyThread--- Currently holding lock , Release @01:53:53waitThread---flag For false, Execute @01:53:58notifyThread--- Hold the lock again , Sleep @01:53:58``` The above code is based on wait / The classic paradigm of notification , A conditional variable that sets whether a thread continues to run flag, And a shared object lock, And use synchronized Keywords are right lock Lock up .waitThread Threads are waiting threads , Try to get lock on startup , Success leads to synchronized Code block . stay synchronized In the code block , If the conditions are not met ( namely flag For true), Then waitThread The thread will enter while Turn around , And call... In the loop wait Method , Enter WAITING State and release lock resources . Until another thread calls notify Method notification from wait Method returns .notifyThread A thread is a notification thread , It will also try to get a lock on startup , Success goes into synchronized Code block . stay synchronized In the code block ,notifyThread Threads change conditions , send waitThread The thread can continue to run down ( But even flag For false), At the same time notifyThread The thread also calls notyfiAll Method , Let waitThread Thread receives notification . But pay attention to ,notifyThread Threads don't call notyfiAll Release the lock immediately after the method , It's at the end of execution synchronized The lock is released after the contents of the code block . We're in notifyThread Thread call notyfiAll After , Hibernate the thread 5s. You can see from the print results that , stay notifyThread Thread dormant 5s in , Even if waitThread The thread is notified , And the conditions for continued execution have been met (flag For flase), but waitThread The thread is here 5s It's still not being implemented . stay notifyThread Thread 5s After the end of the sleep time of , And from synchronized Block exit ,waitThread The thread continues to run . therefore , Wait for the thread to be notified , You still need to wait for the thread to release the lock , And after trying to get the lock successfully, we can really get it from wait Return in method , And continue to carry out .## 2、 Shared memory has the following code ,```java/** * @Author Feng Jian * @Date 2021/1/20 13:18 * @Version 1.0 */public class JMMTest { private static boolean run = true; public static void main(String[] args) throws InterruptedException { Thread My_Thread = new Thread(new Runnable() { @Override public void run() { while(run){ //... } } }, "My_Thread"); My_Thread.start(); // Start My_Thread Thread System.out.println(Thread.currentThread().getName()+" Sleeping @"+new SimpleDateFormat("hh:mm:ss").format(new Date())+"--"+run); Thread.sleep(1000); // Main thread sleep 1s run = false; // Change My_Thread Thread execution conditions , but My_Thread The thread doesn't stop System.out.println(Thread.currentThread().getName()+" In the process of carrying out @"+new SimpleDateFormat("hh:mm:ss").format(new Date())+"--"+run); }}``` Defines a variable run, And use it as My_Thread In thread while Conditions for loop execution . Starting My_Thread Thread , And Hibernate the main thread 1s After , Change variables run Value . The results are as follows :![ Insert picture description here ](,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mzc2MDAyMA==,size_16,color_FFFFFF,t_70#pic_center) It can be seen that , Even if it's run The value of has changed , but My_Thread The thread will not stop . Why ? This requires understanding Java The memory model of (JMM). We know ,CPU To read data from memory for calculation , But actually CPU It's not always read directly from memory . Because of CPU And memory ( It's often called main memory ) It doesn't match the speed of (CPU It's much faster than main memory ), In order to make effective use of CPU, Use multilevel cache Mechanism of , As shown in the picture ![ Insert picture description here ](,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mzc2MDAyMA==,size_16,color_FFFFFF,t_70#pic_center) therefore ,CPU The order of reading data is : register - Cache memory - Main memory . Part of the data in main memory , I'll make a copy and put it in cache in , When CPU When calculating , Will come directly from cache Read data from , After the calculation, place the calculation results in cache in , Finally, rearrange the calculation results in main memory . So each CPU Will have a copy . These are just CPU Access memory , The basic way of doing calculations . Actually , Different hardware , There will be varying degrees of differences in the interview process . such as , Different computers ,CPU There may be three levels of cache between and main memory 、 Level 4 cache 、 Level 5 cache and so on . In order to mask the memory access differences between various hardware and operating systems , Realize let Java The program can achieve consistent memory access effect under various platforms , Defined Java The memory model of (Java Memory Model,JMM).JMM The main goal of is to define access rules for variables in a program , That is, in the virtual machine, the variables are stored in the main memory and the variables are removed from the main memory . Variables are variables that can be shared by multiple threads , It includes example fields 、 Static fields and elements that make up array objects , The culture variables in the method and the arguments of the method are private to the thread , Not subject to JMM Impact of .Java The memory model of is as follows ,![ Insert picture description here ](,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mzc2MDAyMA==,size_16,color_FFFFFF,t_70#pic_center)JMM Defines the relationship between threads and main memory : Shared variables between threads are stored in main memory , Each thread has a private local memory , The local memory stores a copy of the shared variable in the main memory .JMM Regulations : Put all shared variables in main memory , When a thread uses a variable , Will copy the variables to your local memory , When a thread reads or writes, it operates on a copy of a variable in local memory . One thread cannot access the local memory of other threads . Local memory is just an abstract concept , It doesn't really exist , It contains cache 、 Write buffer 、 Registers and other hardware and compiler optimizations . In a multithreaded environment , Since each thread has a copy of the shared variable in main memory , So when the thread is running , Read a copy of the shared variable in your local memory , This raises the issue of thread security : For example, shared variables in main memory i For 1, Thread A and B Fetching variables from main memory i, Put it into your own local memory to become a shared variable i Copy of . When the thread A On execution , Will read the replica variables directly from your local memory i Value , Add 1 Calculation , Update the copy in local memory when finished i Value , Write back to main memory , At this point in main memory i The value of is 2. And if the thread B You also need variables i Value , It doesn't go to the main memory to read i Value , Instead, it reads directly from its own local memory i Copy of , And the thread B Copy in local memory of i The value of is still 1, Not through threads A Revised , The value in main memory 2. This is also why in the above code ,main The thread has obviously changed the variable run Value , but My_Thread The thread is still running while The reason for the loop . As shown in the figure ,![ Insert picture description here ](,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80Mzc2MDAyMA==,size_16,color_FFFFFF,t_70#pic_center) This is also JMM The problem of multithreading visibility to deal with : When a shared variable has copies in the working memory of multiple threads , If a thread changes the copy value of the shared variable , Then other threads should be able to see the modified value . That is, how to ensure that the instruction will not be affected by cpu The impact of caching . Back to the code above , How to make My_Thread The thread can receive main Thread has been modified `run = false` Information about ? namely My_Thread Threads and main How threads can communicate . According to Java The memory model of , If the two threads need to communicate , You have to go through the following two steps :>①main The thread changes the shared variable in the local memory run To the main memory .>②My_Thread Thread to the main memory to read main Shared variables that have been previously updated by the thread run Value . This means , Communication between two threads must go through main memory .Java Provide volitale Keyword to achieve this requirement .volitale Keywords can be used to modify fields ( Member variable ), inform Java Any program access to this variable is from shared memory ( Main memory ) Get from , All changes to it have to be synchronized back to shared memory , so volitale Keyword can ensure the visibility of all threads to variable access . That is, the read and write of shared variables need to go through the main memory , Therefore, the purpose of thread communication through shared memory is achieved . We know how threads communicate through shared memory , Let's rewrite the above code , send main The thread is modified `run = false` After ,My_Thread In the thread while The loop will stop immediately . In fact, you just need to give shared variables run add volitale Key words :```javaprivate static volatile boolean run = true;``` The modified execution result is as follows :![ Insert picture description here ]( Visible , stay main Threads modify shared variables run After the value of , That is to rearrange the main memory . and My_Thread The thread reads... In main memory run The discovery value is false Then it stopped while Turn around . Actually , You can also use synchronized Keyword to ensure memory visibility , Realize thread communication . The mechanism is : stay synchronized In the modified synchronization block , If you operate on a shared variable , Will clear the value of this variable in the thread's local memory , And read the value of the shared variable in the main memory before using it . And at the end of the synchronization block , When the lock resource is released , The shared variable must be synchronized back to the main memory .## 3、 Pipe flow has not learned to use , Let's skip for a moment ...> The above is my notes in the learning process . Reference books 、 The article or blog is as follows :[1] Fang Tengfei , Wei Peng , Cheng Xiaoming . Java The art of concurrent programming [M]. Mechanical Industry Press .[2] Huo Luxu , Xue Bintian . Java The beauty of concurrent programming [M]. Electronic Industry Press .[3][Simen Lang . Please , Communication between threads is really simple . You know .]([4][ King of bliss .Java Thread memory model , Thread 、 Working memory 、 Main memory . You know .](

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