In depth understanding of JUC concurrency (I) what is JUC

Basic knowledge of

What is? JUC

JUC Namely java.util.concurrent The following class package , Dedicated to multithreading development .

The advantages and disadvantages of concurrent programming

advantage

1. Make full use of multicore CPU Computing power

   Multicore CPU In the background of , The trend of concurrent programming , Through the form of concurrent programming, multi-core can be CPU To the extreme , Improved performance .

2. It is convenient for business splitting , Improve the concurrent ability and performance of the system

   The current system requires millions or even tens of millions of levels of concurrency , And multithreading concurrent programming is the foundation of developing high concurrent system , Using many thread mechanisms can greatly improve the overall concurrency and performance of the system .

shortcoming

1. Frequent context switching

   The task is a context switch from saving to reloading , And every time you switch , Need to save the current status , In order to restore the previous state , And this switch is very lossy , Too often it can't take advantage of multithreaded programming

2. Thread safety problem

   The most difficult problem in multithreading programming is thread safety in critical area , If you don't pay attention to it a little bit, you will get deadlock , Once a deadlock occurs, it will make the system function unavailable

Blocking and non blocking

The focus of blocking and non blocking is The behavior of a thread waiting for a message , While waiting for the news , The current thread is suspended , Or not suspended .

• Blocking ： Call after sending out , Before the message returns , The current thread will be suspended , Until a message returns , The current thread will be activated
• Non blocking ： Call after sending out , Does not block the current thread , And will return immediately

Synchronous and asynchronous

• Sync ： When a synchronous call is sent , caller Always wait for the call result to return after , To carry out subsequent operations
• asynchronous ： When an asynchronous call is sent , caller Regardless of whether the called method is completed , Will continue to execute the following code . Asynchronous call , To get results , There are generally two ways ：
  1. Results of active polling asynchronous calls
  2. Callee passed callback To notify the caller of the result of the call

Concurrency and parallelism

Concurrent

Threads take turns using cpu This is called concurrency

parallel

Multiple cpu Next , Each core can schedule running threads , At this time, the thread can be parallel

Processes and threads

process

A process can be considered an instance of a program . Most programs can run multiple instance processes at the same time （ For example, Notepad 、 drawing 、 The browser etc. ）, Some programs can only start one instance process （ For example, Netease cloud music 、360 Safety guard ） etc.

Threads

1. A process can be divided into one or more threads . A thread is a stream of instructions , Give instructions in a certain order to CPU perform
2. Java in , Thread as the minimum scheduling unit , Process is the smallest unit of resource allocation , stay windows The middle process is inactive , Just as a container for threads

State of thread

public enum State {
    
	// function 
    NEW,

	// function 
    RUNNABLE,

	// Blocking 
    BLOCKED,

	// wait for 
    WAITING,

	// Overtime waiting 
    TIMED_WAITING,

	// End 
    TERMINATED;
}

1. After the thread is created, it will be in the NEW（ newly build ） state
2. call start() Start running after method , And get the CPU Time slice （timeslice） After that RUNNING（ function ） state
3. When a thread calls Synchronization method when , Without obtaining the lock , The thread will enter the BLOCKED（ Blocking ） state
4. When the thread executes wait() After method , Thread entry WAITING（ wait for ） state
5. stay Wait state On the basis of the increase of the timeout limit , Such as through sleep（long millis） Method or wait（long millis） Method can put the thread into waiting timeout （TIMED WAITING） state
6. After the thread runs, it will enter TERMINATED（ End ） state