Thread lifecycle

One 、 summary

The life cycle of a thread includes 5 Stages , Include ： newly build 、 be ready 、 function 、 Blocking 、 Death .
When the thread enters the running state , The general operating system uses preemptive mode to let the thread get CPU. therefore CPU Need to switch between multiple threads , So the thread state will run many times 、 Blocking 、 Switch between ready .

Two 、 newly build

newly build ： Use new Method ,new Come out of the thread , Only by JAVA The virtual machine allocates memory for it , And initialize the value of the member variable . This is just an object .

3、 ... and 、 be ready

be ready ： It's the calling thread start() After the method , The thread is waiting CPU Resource allocation stage , Who robbed first CPU resources , Who is going to start ;
The thread is in ready state ,JAVA The virtual machine creates method call stacks and program counters for it . Thread execution is controlled by the underlying platform , It has a certain randomness .

Four 、 function

function ： When the ready thread is scheduled and gets CPU Resource time , It goes into operation ,run Method defines the operation and function of the thread ;（ When the thread in the ready state gets CPU, It will execute run() Method ）
For a single core cpu（ Or a kernel ） Come on , Only one instruction can be executed at the same time , and JVM By quickly switching threads to execute instructions to achieve multi-threaded , A real processor can process an instruction at the same time , But this switching speed is very fast , We don't even feel . In order to recover to the correct execution position after thread switching , Each thread has a separate program counter , Counters between threads do not affect each other , Independent storage .
When a thread starts running , It cannot always hold CPU（ Unless the thread execution is very short , The execution is over in an instant ）. therefore , The thread needs to be interrupted during execution , The purpose is to let other threads get execution CPU The opportunity of . The details of thread scheduling depend on the strategy adopted by the underlying platform .

5、 ... and 、 Blocking

Blocking ： In the running state , For some reason, the running thread may become blocked . Here's why ：

1. wait for I/O The input and output of the stream 
2. Waiting for network resources , Network speed 
3. call sleep（） Method , Need to wait sleep Time is over 
4. call wait（） Method , Need to call notify（） Wake up the thread 
5. Other threads execute join（） Method , The current thread will block , You need to wait for other threads to finish executing .

The state switching diagram is as follows ：

6、 ... and 、 Death

1、run()/call() Method execution complete , Thread ends normally ;
2、 Thread throws an uncaught Exception or Error;
3、 Directly calling the thread stop() Method ends the thread —— This method is easy to cause deadlock , It is generally not recommended to use .