Basic concepts of multithreading ： Getting started with multithreading （ One ）
Multi thread basic implementation ： Getting started with multithreading （ Two ）
Multithreading Management ： Multithreading Foundation （ 3、 ... and ）
State transitions between threads ： Multithreading Foundation （ Four ）
Communication between threads ： Multi thread advanced （ 5、 ... and ）

This article , Let's talk about java Multithreaded " lock ".

First of all, make it clear ：Java Multithreaded locks are object-based ,Java Every... In ⼀ Both objects can be used as ⼀ A lock

1、synchronized

synchronized Chinese means synchronous , Also known as ” Synchronization lock “. Role is Make sure that at the same time , The modified code block or method will only have one thread executing , In order to achieve the effect of ensuring concurrent security .

1.1 Scope

We usually make ⽤ synchronized Keywords to ⼀ A piece of code or ⼀ individual ⽅ Can't lock . It usually has the following three forms ：

//  Keyword in instance ⽅ Legally , Lock is the current instance 
public synchronized void instanceLock() {
    
 // code
}
//  Keywords in static ⽅ Legally , Lock is current Class object 
public static synchronized void classLock() {
    
 // code
}
//  Keyword on code block , The lock is in parentheses ⾥⾯ The object of 
public void blockLock() {
    
 Object o = new Object();
 synchronized (o) {
    
 // code
 }
}

1.2 Several locks

An object actually has four lock states , They have low to high , In turn, is ：

• No lock state
• Biased lock state
• Lightweight lock state
• Heavyweight lock state

Several locks , As the competition escalates , Lock upgrades can easily occur , But the conditions under which demotion occurs are very harsh .
The following describes several locks and lock upgrades between them ：

1.3 Biased locking

⼤ Most of the time Locks don't just have multithreaded competition ,⽽ And always by the same ⼀ The thread gets more than once , So it leads to ⼊ The deflection lock .
Biased locking biases the first thread to access the lock , If in the next run , There are no other threads competing for the lock , Then the thread holding the lock will never trigger synchronization , It improves the running performance of the program .

1.3.1 principle

principle ： When a thread first enters a synchronized code block , Lock records in the object header and stack frame ⾥ Store lock biased threads ID. The next time the thread enters the synchronized code block again , Will go to the object head mark word Check the tag in the , Whether there is a thread with its own thread ID.
If it is , Indicates that the thread has For the lock , Go to sync code block , No longer need to spend CAS Operation to lock and unlock .
If not , Represents another thread competing for biased locks , Now CAS Will try to replace mark word The thread of ID Thread for new thread ID.

• success , Indicates the old thread ID Does not exist. , Replacement successful , At this time, I still prefer to lock .
• Failure , Indicates the old thread ID There is still a , At this time, it will be upgraded to lightweight lock , Will compete for locks in the way of lightweight locks .

1.3.2 Upgrade process

When the bias lock is upgraded to a lightweight lock , Will pause the thread using the lock , Reset the deflection lock identification , The general process is as follows ：

• The system will first find a time point when there is no bytecode in execution ,, Then stop the thread that owns the lock ;
• Traverse the thread stack , Repair or modify mark word, Make it unlocked ;
• Wake up the stopped thread , Make it a lightweight lock ;

therefore , If we should ⽤ Program ⾥ All locks are usually in a competitive state , Then the bias lock will be ⼀ A burden , We can turn it off by default .

-XX:UseBiasedLocking=false.

1.4 Lightweight lock

Multiple threads acquire the same lock at different times , There is no lock competition .

1.4.1 principle

If a thread finds a lightweight lock when it obtains a lock , You'll lock it mark word Copied to the dispace mark word Inside . Then try to use CAS Lock it mark word Replace with a pointer to the lock record .

• success , Current thread gets lock ;
• Failure , Table name mark word Has been replaced with lock records of other threads , Next, I'll try The spin To get the lock , If you can't get , Upgrade to heavyweight lock .

A diagram illustrates the process of locking and releasing the lock ：

1.5 Heavyweight lock

Heavyweight locks depend on the mutex of the operating system （mutex） Realized ,⽽ State transitions between threads in the operating system are required ⽐ a ⻓ Time for , therefore The efficiency of heavyweight lock is very low .
When multiple threads request an object lock at the same time , Object locks are set ⼏ States ⽤ To distinguish the thread of the request ：

• Contention List： All threads that request locks will be locked ⾸ Put into the competition queue first
• Entry List：Contention List Which of them are eligible to be candidates ⼈ Thread moved to Entry List
• Wait Set： Those tunes ⽤wait⽅ Method blocked threads are placed in Wait Set
• OnDeck： At most at any time ⼀ Threads are competing for locks , This thread is called OnDeck
• Owner： The thread that obtains the lock is called Owner
• !Owner： The thread that releases the lock
  When the thread releases the lock , From Contention List or EntryList in choose ⼀ Thread wakeup , The selected thread is called Heir presumptive Assumed inheritance ⼈, Assumed inheritance ⼈ When you wake up, you try to get a lock , but synchronized yes ⾮ Fair , So assume inheritance ⼈ No ⼀ You can get the lock . This is because for heavyweight locks , Xian Chengxian ⾃ Spin attempt to obtain lock , Doing so ⽬ To reduce obsession ⾏ Overhead of operating system synchronization . If ⾃ If the rotation fails, advance again ⼊ Waiting in line . This is for threads that are already in the waiting queue , Slightly unfair , also ⼀ An unfair land ⽅ yes ⾃ The thread may be preempted Ready Thread lock .

1.6 Comparison of various locks

1.7 Summarize the upgrade process of lock

1. Every time ⼀ When a thread is ready to acquire a shared resource ： The first ⼀ Step , Check MarkWord⾥⾯ Did you put it ⾃⼰ Of ThreadId , If it is , Indicates that the current thread is in “ Biased locking ” .
2. The first ⼆ Step , If MarkWord No ⾃⼰ Of ThreadId, Lock escalation , Now ,⽤CAS To hold ⾏ Switch , The new thread is based on MarkWord⾥⾯ The existing ThreadId, Thread paused before notification , Before the thread take Markword The content of the is set to null .
3. The third step , Both threads lock the object's HashCode Copied to the ⾃⼰ New ⽤ The record for the storage lock is empty between , And then it started to go through CAS operation , Lock the object MarKword The content of is modified to ⾃⼰ New record The address of the space ⽅ Competitive MarkWord.
4. Step four , In the third step, successfully execute ⾏CAS Access to resources , If you fail, go to ⼊⾃ Spin .
5. Step five ,⾃ The spinning thread is ⾃ During spinning , Successful access to resources ( That is, the thread execution of the previously obtained resource ⾏ End And released shared resources ), Then the whole state is still in the state of lightweight lock , If ⾃ Spin failed .
6. Step six , Into the ⼊ State of heavyweight lock , This is the time ,⾃ Screw the thread into ⾏ Blocking , The thread executes before waiting ⾏ Complete and wake up ⾃⼰.