[concurrent programming] thread foundation and sharing between threads

 2022.02.08

Thread basis

CPU The relationship between the number of cores and the number of threads

The core number : Number of threads =1:1  ; After using hyper threading technology ---> 1:2

CPU Time slice rotation mechanism

also called RR Dispatch , Will cause context switching

What are processes and threads

process ： The smallest unit of program running resource allocation , There are multiple threads inside the process , Will share the resources of this process

Threads ：CPU Minimum unit of scheduling , Must exist depending on the process .

Clarify parallelism and concurrency

parallel ： At the same time , The ability to handle things at the same time

Concurrent ： Related to unit time , The ability to handle things per unit of time

The significance of high concurrency programming 、 Benefits and precautions

benefits ： make the best of cpu Resources for 、 Speed up user response time , Program modularization , Asynchronism

problem ：

Threads share resources , There is a conflict ;

It is easy to cause deadlock ;

Enable too many threads , It's possible to break the machine

know Java Thread in

Threads

Thread vs Runnable

A new way to start threads

Three

How can we make Java The threads in stop working safely

Thread natural termination ： Execute naturally or throw unhandled exception

stop(),resume(),suspend() It is no longer recommended to use ,stop() It will cause the thread not to release resources correctly ,suspend() It is easy to cause deadlock .

java Threads are collaborative , Not preemptive

Call a thread's interrupt() Method to interrupt a thread , It's not forcing the thread to close , Just say hello to this thread , Set the thread interrupt flag position to true, Whether the thread is interrupted , It's up to the thread itself to decide .

isInterrupted() Determines whether the current thread is in an interrupted state .

static Method interrupted() Determines whether the current thread is in an interrupted state , At the same time, the interrupt flag bit is changed to false.

If the method throws InterruptedException, The thread's interrupt flag bit will be reset to false, If you really need to interrupt the thread , Ask ourselves to catch Call... Again in the statement block interrupt().

Thread common methods and thread status

Thread only 5 States . The whole life cycle is the switching of these States .

run() and start() ：run Methods are ordinary methods of ordinary objects , Only called start() after ,Java Will map the thread object to the actual thread in the operating system , We'll do it again run Method .

yield() ： Give up cpu The enforcement of , Turn the thread from running to runnable , But the next time slice , The thread may still be selected to run again .

Thread priority

The value is 1~10, Default is 5, But thread priority is unreliable , Not recommended as a means of thread development

The guardian thread

Die with the main thread ,finally There is no guarantee that

Sharing between threads

synchronized Built in lock

Object lock , The lock is an object instance of a class .

Kind of lock , The lock is for each class Class object , Of each class Class There is only one object in a virtual machine , So there is only one class lock .

volatile keyword

The lightest synchronization mechanism , Suitable for only one thread to write , Multi thread reading scenario , Because it can only ensure visibility .

ThreadLocal

Thread variable . It can be understood as a map, type Map<Thread,Integer>

Spring When dealing with transactions ThreadLocal, Make each thread keep its own connection .

Each thread has a copy of the variable （hash）   Thread isolation , Synchronization of variables is not guaranteed , Only ensure the isolation of variables

ThreadLocal Thread unsafe , Causes a memory leak （ Weak reference ）

virtual machine /JVM The underlying knowledge （ Strong citation 、 Soft citation 、 Weak reference 、 Virtual reference ）

Object o = new Object(); // Strong citation ： Stack frame ---》 Heap instance

SoftRefence  To overflow