Understanding threads

Recognize threads

What is a thread ？

A thread is the smallest unit of program execution , Depending on the process .

A process can contain multiple threads , Multithreading can also share the same memory space and system resources , Therefore, switching between threads saves more resources than switching between processes 、 More lightweight .

State of thread

• NEW, New state ： Threads are created , But the thread state when it is not started

• RUNNABLE, Ready state ： Indicates the state that the thread can run , It may be running , Or waiting in line for the operating system to allocate  CPU  resources ;

• BLOCKED, Blocked thread state waiting for lock ： Indicates a thread in a blocked state , Waiting for lock release , Like waiting for execution  synchronized  Code block or use  synchronized  Method of marking , But it has been executed by other threads , So I'm in a jam waiting .

• WAITING, Wait state ： One thread is waiting for another thread to perform a specific action , This thread is waiting . For example, a thread calls  Object.wait() , Then it is waiting for another thread to call  Object.notify()  or  Object.notifyAll()  Method , It is called waiting state .

• TIMED_WAITING, Timeout wait status ： When waiting , A timeout will be set , When the waiting time exceeds the specified time , The state of the thread is called timeout waiting state . such as  Object.wait(long timeout)  and  Thread.join(long timeout)  Other methods , It will make it possible for the thread to enter this state .

• TERMINATED, Termination status ： Indicates that the thread has completed execution .

Enumeration source code of thread state ：

public enum State {

 NEW,

 RUNNABLE,

 BLOCKED,

 WAITING,

 TIMED_WAITING,

 TERMINATED
}


Working mode of thread

• First create a thread , The thread is in NEW state .

• Specify the business methods that the thread needs to execute , The thread is in RUNNABLE state

• During thread execution , If you encounter a synchronized code block , And other threads execute one step first , Then the current thread will enter BLOCKED state . Wait until other threads finish executing synchronized code blocks , The current thread will continue to wait .

• If the code in the thread calls Object.wait() Methods or Thread.join() When the method is used , The thread will enter WAITING state , And when there is a set timeout , Then the thread will enter TIMED_WAITING Wait state . Until another thread executes notify()  or  notifyAll()  Method , The current thread will be awakened , Continue code execution .

• Finally, the thread method execution is completed , Threads become TERMINATED state .