Condition and AQS principle

Condition brief introduction

Any one of them java Objects are all inherited from Object class , Communication between online programs is often applied to Object Several methods of , such as wait(),wait(long timeout),wait(long timeout, int nanos) And notify(),notifyAll() Several ways to implement wait / A notification mechanism , alike , stay java Lock Under the system, there will still be the same way to achieve waiting / A notification mechanism . On the whole Object Of wait and notify/notify It is to cooperate with the object monitor to complete the inter thread waiting / A notification mechanism , and Condition And Lock Cooperate with the completion waiting notification mechanism , The former is java At the bottom level , The latter is at the language level , It has higher controllability and expansibility . The two are different in use , There are still many differences in functional features ：

• Condition Can support no response interrupt , But by using Object The way doesn't support ;
• Condition Can support multiple waiting queues （new Multiple Condition object ）, and Object Only one... Can be supported ;
• Condition It can support the setting of timeout time , and Object I won't support it

Condition analysis ：
（1）Condition The essence of waiting for notification is waiting queue and The interactive process of synchronizing queues , Follow object Of wait()/notify() The mechanism is the same ;Condition Based on synchronous lock state Realized , and objec Is based on monitor Pattern implementation .
（2） One lock(AQS) There can be multiple Condition, That is, multiple waiting queues , There is only one synchronization queue .
（3）Condition.await() Method execution , Will synchronize the... In the queue head Lock released , Encapsulate threads into new node Add to the waiting queue ;Condition.signal() Method execution , The first node in the waiting queue will be moved to the synchronization queue , Until the lock state It's only when you get it that you wake up .

AQS Principle Overview

It's a framework for building locks and synchronizers , Use AQS A large number of synchronizers can be constructed simply and efficiently

AQS The core idea is , If the requested shared resource is free , Set the thread of the current request resource to a valid worker thread , And set the shared resources to the locked state . If the requested shared resource is occupied , Then we need a set of mechanism for thread blocking waiting and lock allocation when it is awakened , This mechanism AQS Yes, it is CLH The implementation of queue lock , Join the queue with the thread temporarily unable to acquire the lock .

CLH(Craig,Landin,andHagersten) A queue is a virtual two-way queue （ Virtual two-way queue means there is no queue instance , There is only an association between nodes ）.AQS It is to encapsulate each thread requesting shared resources into a CLH A node in a lock queue （Node） To achieve lock allocation .

AQS Define two ways to share resources
1)Exclusive（ Monopoly ）
Only one thread can execute , Such as ReentrantLock. It can be divided into fair lock and unfair lock ,ReentrantLock Two locks are supported at the same time , Let's say ReentrantLock Introduce the definition of these two kinds of locks ：

• Fair lock ： In the order in which threads are queued in the queue , First come, first get the lock
• Not fair lock ： When a thread wants to acquire a lock , Pass it twice first CAS Operate to grab the lock , If you don't get , The current thread then joins the queue waiting for wakeup .

2)Share（ share ）