Basic concepts of multithreading (concurrency and parallelism, threads and processes) and entry cases

Program without jump statement , They are carried out from top to bottom , Now I want to design a program , Playing games and listening to music , How to design ？
To solve the above problems , We have to use multiple processes or threads to solve .

Concurrency and parallelism

• Concurrent ： Two or more events in Within the same time period happen .
• parallel ： Two or more events in At the same time happen （ At the same time ）.
  
  In the operating system , Multiple programs installed , Concurrency means that there are multiple programs running at the same time in a period of time , This is on the list CPU In the system , Only one program can be executed at a time , That is to say, these programs run alternately in time-sharing , It just feels like it's running at the same time , That's because the time-sharing operation is very short .
  And in many CPU In the system , Then these programs that can be executed concurrently can be allocated to multiple processors （CPU）, Implement multitask parallel execution , That is, using each processor to process a program that can be executed concurrently , So that multiple programs can be executed at the same time . Multi-core in the current computer market CPU, It's a multi-core processor , nucleus The more , More programs to process in parallel , Can greatly improve the efficiency of computer operation .

Be careful ： A computer with a single core processor can't handle multiple tasks in parallel , It can only be multiple tasks in a single CPU Run concurrently on . Empathy , Threads are the same , Understand that threads run in parallel from a macro perspective , But from the micro point of view, it is a serial operation , That is to say, one thread and one thread run , When there is only one system CPU when , Threads execute multiple threads in a certain order , We call this situation thread scheduling .

Threads and processes

• process ： An application running in memory , Each process has a separate memory space , One application can run multiple processes at the same time ; A process is also an execution of a program , It is the basic unit of system operation program ; The system runs a program that is a process created from 、 Run to die .
• Threads ： A thread is an execution unit in a process , Responsible for the execution of programs in the current process , At least one thread in a process . There can be multiple threads in a process , This application can also be called a multithreaded program .
  In short ： At least one process after a program runs , A process can contain multiple threads

We can use the taskbar at the bottom of the computer , Right click -----> Open Task Manager , Can view the progress of the current task ：
process

Threads

Thread scheduling :

• Time sharing scheduling
  All threads rotate CPU Right to use , Average per thread CPU Time for .

• preemptive scheduling
  Give priority to high priority threads CPU, If the threads have the same priority , Then we will randomly choose one ( Thread randomness ),Java Preemptive scheduling is used .

  • Set the priority of the thread
    

• Details of preemptive scheduling
  Most operating systems support concurrent running of multiple processes , Today's operating systems almost support running multiple programs at the same time . such as ： Now let's use the editor in class , While using the recording software , At the same time, the drawing board is on ,dos Windows and other software . here , These programs are running at the same time ,” It feels like the software is running at the same time “.
  actually ,CPU( a central processor ) Using preemptive scheduling mode to switch between multiple threads at high speed . about CPU For a core of , Some time , Only one thread can be executed , and CPU The speed of switching between multiple threads is faster than our feeling , It seems to run at the same time .
  Actually , Multithreaded program can not improve the running speed of program , But it can improve the efficiency of program operation , Give Way CPU Higher usage of .
  

Create thread class

Java Use java.lang.Thread Class representative Threads , All thread objects must be Thread An instance of a class or its subclass . The function of each thread is to complete certain tasks , In fact, it is to execute a program flow, that is, a piece of sequential code .Java Use thread executor to represent this program flow .Java Through inheritance Thread Class to establish and Start multithreading The steps are as follows ：

1. Definition Thread Subclasses of classes , And override the run() Method , The run() The method body of the method represents the tasks that the thread needs to complete , So the run() Method is called thread executor .
2. establish Thread Instances of subclasses , The thread object is created
3. Calling the start() Method to start the thread

The code is as follows ：
Test class ：

public class Demo01 {
    
	public static void main(String[] args) {
    
		// Create a custom thread object 
		MyThread mt = new MyThread(" New Threads ！");
		// Start a new thread 
		mt.start();
		// Execute... In the main method for loop 
		for (int i = 0; i < 10; i++) {
    
			System.out.println("main Threads ！"+i);
		}
	}
}

Custom thread class ：

public class MyThread extends Thread {
    
	// Define the construction method of the specified thread name 
	public MyThread(String name) {
    
		// Calling the String Parameter construction method , Specify the name of the thread 
		super(name);
	}
	/** *  rewrite run Method , Complete the logic of the thread execution  */
	@Override
	public void run() {
    
		for (int i = 0; i < 10; i++) {
    
			System.out.println(getName()+"： Being implemented ！"+i);
		}
	}
}