Thread safety and processing caused by multithreading

1. Multithreading

(1) What is multithreading ？

         Multithreading means that multiple threads can execute tasks in an application . When multiple tasks need to be executed at the same time in the program , You need multithreading .

(2) The advantages and disadvantages of multithreading

advantage : Speed up the program ;
        Improved cpu Utilization ratio ;
        Improve program structure .

shortcoming : Multi wire programs consume more memory ;
         Multithread switch execution , Yes cpu The requirements of ( These two disadvantages can be solved by upgrading the hardware equipment );
         Multiple threads access the same shared resource , There will be thread safety issues appear Thread safety problem .

Single multithreading does not cause thread safety problems , Each thread is doing its own thing .

(3) Thread parallelism and concurrency

Thread parallel execution : Many people do different things at the same time , Mutual interference .
Threads execute concurrently : To perform something in a period of time .

2. Solving thread safety problems

To solve the thread safety problem, we must lock the program ,

(1)synchronized

synchronized keyword
synchronized Code blocks can be decorated
synchronized( Synchronization lock object ){ }

synchronized When modifying a method ：
If it's a non static method , So the lock object is this ; If it's a static method , So the lock object is a class Class object ( There is only one class Class object ).

synchronized It is realized by low-level instruction control .

synchronized Code blocks can be decorated , Modification methods , The object of the lock ( The lock object may change ).

synchronized Locking is implicit , When entering the synchronized code block , Automatically acquire locks , After the synchronization code block is executed , Automatic release lock .

(2) Lock Interface

        Lock The lock depends on java Code to control ,ReentrantLock Class implements the Lock Interface , Can control and synchronized Same function .. But the implementation details of the two are completely different ,Lock Locking can only lock a certain piece of code , And it is the lock and release lock shown .

3. Deadlock

Reasons for appearance ： Incorrect locking may lead to deadlock . That is, different threads occupy the synchronization resources required by each other without giving up .

consequence ： After deadlock , The program will not report an error , Just waiting ..

Solution ： Deadlock is easy to occur when locks are nested , When locking , Consider the sequence of locks , Minimize nested use .

4. Thread communication

(1) Thread communication

         Thread communication means that multiple threads are processing the same resource , And the tasks are different , Thread communication is needed to help solve the use or operation of the same variable between threads . When multiple threads operate on the same data , Avoid contention for the same shared variable . Multiple threads hold each other back , Restrict operation .

(2) Method

wait() Let thread wait , call wait() Method, the thread enters a blocking state , Must wake up through another thread
notify() Wake up waiting thread
notifyAll() Wake up all threads waiting
this 3 Methods must be executed in synchronized code blocks .

sleep() and  wait()
The same thing : Can make the thread enter the blocking state .
Difference : sleep() yes Thread Class , It won't release the lock , When the sleep time is up , Will automatically enter the ready state ;
             wait() yes Object Class ,wait() It can release the lock ,wait() Thread after , Need to use notify() perhaps notifyAll() Wake up the