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5 locksupport and thread interruption

2022-06-13 11:58:00 【It rains when the wind blows】

5 LockSupport With thread interrupts

Catalog

Thread interrupt mechanism

Ali ant financial interview questions

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voidinterrupt() Interrupt this thread
static booleaninterrupted() Tests whether the current thread has been interrupted
booleanisInterrupted() Test whether this thread has been interrupted

  • Have you understood the three methods ？ Where to use it ？

  • How to stop a running thread ？

  • How to interrupt a running thread ？？

What is interrupt mechanism ?

  • First

A thread should not be forced to interrupt or stop by other threads , It should be stopped by the thread itself .

therefore ,Thread.stop, Thread.suspend, Thread.resume It's been abandoned .

  • secondly

stay Java There is no way to stop a thread immediately , However, stopping threads is particularly important , For example, cancel a time-consuming operation .

therefore ,Java Provides a method for stopping threads Consultation mechanism —— interrupt .

Interruption is just a collaborative negotiation mechanism ,Java No syntax added to interrupt , The interruption process needs to be implemented by the programmer himself .

To interrupt a thread , You need to manually call the thread's interrupt Method , This method only sets the interrupt ID of the thread object to true;

Then you need to write your own code to constantly detect the identification bit of the current thread , If true, Indicates that another thread requests this thread to interrupt ,

What to do at this time requires you to write your own code to realize .

There is an identity in each thread object , Used to indicate whether a thread is interrupted ; The identification bit is true To interrupt , by false Means not interrupted ;

By calling the interrupt Method to set the identification bit of the thread to true; Can be invoked in other threads , You can also call it in your own thread .

  • eg. Customers smoke in smoke-free restaurants , The waiter wanted him to stop smoking , Not to force him to stop smoking , Instead, he marked his sign as true, Specifically, customers should stop smoking by themselves .（ It embodies the negotiation mechanism ）

Interrupt correlation API Description of the three methods

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public void interrupt() Example method , Example method interrupt() Just set the interrupt status of the thread to true, Initiate a negotiation without immediately stopping the thread
public static boolean interrupted() Static methods ,Thread.interrupted(); Determine whether the thread is interrupted , And clear the current interrupt state Two things ：1 Returns the interrupt status of the current thread 2 Set the interrupt status of the current thread to false（ This method is a little difficult to understand , Because the results of two consecutive calls may be different .）
public boolean isInterrupted() Example method , Determine whether the current thread is interrupted （ Check the interrupt flag bit ）

Interview questions of Dachang ： How to use interrupt ID to stop thread ？

1 How to stop interrupting a running thread ？

① Through one volatile Variable implementation

  • volatile Guaranteed visibility ,t2 After modifying the flag bit, it can be t1 notice 
public class interruptDemo {
    
    static volatile boolean isStop = false;

    public static void main(String[] args) {
    
        new Thread(()->{
    
            while(true){
    
                if(isStop){
    // If this flag bit is changed by another thread to true 了 
                    System.out.println(Thread.currentThread().getName()+"\t isStop Changed to true, Termination of procedure ");
                    break;
                }
                System.out.println("t1 ------hello volatile");//---------------------- If it doesn't stop , Then keep printing 
            }
        },"t1").start();

        try {
    TimeUnit.MILLISECONDS.sleep(20);} catch (InterruptedException e) {
    e.printStackTrace();}

        new Thread(()->{
    
            isStop = true;
        },"t2").start();
    }
}
//--
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 isStop Changed to true, Termination of procedure

② adopt AtomicBoolean（ Atomic Boolean ）

public class interruptDemo {
    

    static AtomicBoolean atomicBoolean = new AtomicBoolean(false);

    public static void main(String[] args) {
    
        m1_volatile();
    }

    public static void m1_volatile() {
    
        new Thread(()->{
    
            while(true){
    
                if(atomicBoolean.get()){
    // If this flag bit is changed by another thread to true 了 
                    System.out.println(Thread.currentThread().getName()+"\t isStop Changed to true, Termination of procedure ");
                    break;
                }
                System.out.println("t1 ------hello volatile");//---------------------- If it doesn't stop , Then keep printing 
            }
        },"t1").start();

        try {
    TimeUnit.MILLISECONDS.sleep(20);} catch (InterruptedException e) {
    e.printStackTrace();}

        new Thread(()->{
    
            atomicBoolean.set(true);
        },"t2").start();
    }
}
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 ------hello volatile
//t1 isStop Changed to true, Termination of procedure

③ adopt Thread Class has its own interrupt api Method realization 

    // The default interrupt flag bit is false, Then it was changed to true
    public static void main(String[] args) {
    
        m1_volatile();
    }

    public static void m1_volatile() {
    
        Thread t1 = new Thread(() -> {
    
            while (true) {
    
                if (Thread.currentThread().isInterrupted()) {
    // Once the interrupt flag bit is found to be modified 
                    System.out.println(Thread.currentThread().getName() + "\t isInterrupted() Changed to true, Termination of procedure ");
                    break;
                }
                System.out.println("t1 ------hello interrupt ");//---------------------- If it doesn't stop , Then keep printing 
            }
        }, "t1");
        t1.start();

        try {
    TimeUnit.MILLISECONDS.sleep(20);} catch (InterruptedException e) {
    e.printStackTrace();}

        new Thread(()->{
    
            t1.interrupt();// hold t1 interrupt 
        },"t2").start();
    }
}

//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 ------hello interrupt 
//t1 isInterrupted() Changed to true, Termination of procedure

—API Source code analysis

Example method interrupt(), no return value 

//Thread.java
public void interrupt() {
    
        if (this != Thread.currentThread())
            checkAccess();

        synchronized (blockerLock) {
    
            Interruptible b = blocker;
            if (b != null) {
    
                interrupt0();           // Just to set the interrupt flag---- Called interrupt0（） Method 
                b.interrupt(this);
                return;
            }
        }
        interrupt0();
    }

//Thread.java
    /* Some private helper methods */
    private native void setPriority0(int newPriority);
    private native void stop0(Object o);
    private native void suspend0();
    private native void resume0();
    private native void interrupt0();  //--------------------------- Called c Bottom 
    private native void setNativeName(String name);

Example method isInterrupted, Returns a Boolean value 

    //Thread.java
    public boolean isInterrupted() {
    
        return isInterrupted(false);
    }

    //Thread.java
    private native boolean isInterrupted(boolean ClearInterrupted);// Also called c Bottom

explain

  • say concretely , When it comes to a thread , call interrupt() when ：

    1. If the thread is in Normal active state , The thread's interrupt flag will be set to true, That's it . The thread with interrupt flag will continue to run normally , Unaffected . therefore , interrupt() It doesn't really interrupt threads , You need to cooperate with the called thread .

    2. ② If the thread is blocked （ For example, in the sleep, wait, join Equal state ）, To call the current thread object in another thread interrupt Method , So the thread will Immediately exit the blocked state （ The interrupt state will be cleared ）, And throw a InterruptedException abnormal .

    3. （ Interrupt inactivity The thread of Can't Any impact , Take a look at the following case ）

2 The interrupt ID of the current thread is true, Does the thread stop immediately ？

  • no

    • Only one interrupt state is set

  • See if the interrupt will stop this immediately 300 The thread of

    • no , Although the interrupt flag bit has changed . however i It's been circulating 
public class InterruptDemo02 {
    
    public static void main(String[] args) {
    
        Thread t1 = new Thread(()->{
    
            for(int i = 0;i < 300;i ++){
    
                System.out.println("---------" + i);
            }
            System.out.println("after t1.interrupt()--- The first 2 Time ----"+Thread.currentThread().isInterrupted());
        },"t1");
        t1.start();
        System.out.println("before t1.interrupt()----"+t1.isInterrupted());
        t1.interrupt();
        try {
    TimeUnit.MILLISECONDS.sleep(3);} catch (InterruptedException e) {
    e.printStackTrace();}
        System.out.println("after t1.interrupt()--- The first 1 Time ---"+t1.isInterrupted());
        try {
    TimeUnit.MILLISECONDS.sleep(3000);} catch (InterruptedException e) {
    e.printStackTrace();}
        System.out.println("after t1.interrupt()--- The first 3 Time ---"+t1.isInterrupted());
    }
}
//before t1.interrupt()----false
//---------0
//---------1
//---------2
//---------3
//....
//---------136
//after t1.interrupt()--- The first 1 Time ---true ------ The interrupt flag bit here is set to true, however t1 Still running 
//---------137
//---------298
//---------299
//after t1.interrupt()--- The first 2 Time ----true
//after t1.interrupt()--- The first 3 Time ---false// Interrupting an inactive thread does not have any effect , After the thread ends, it should automatically change to false

Follow up cases - thorough

  • On the skeleton of our basic interrupt program + One sleep Blocking

  • Interrupt exception And Will cause the program to loop indefinitely .

public class InterruptDemo03 {
    
    public static void main(String[] args) {
    
        Thread t1 =  new Thread(()->{
    
            while(true){
    
                if(Thread.currentThread().isInterrupted()){
    
                    System.out.println(Thread.currentThread().getName()+"\t"+
                            " Interrupt flag bit ："+Thread.currentThread().isInterrupted()+" Termination of procedure ");
                    break;
                }
                try {
    
                    Thread.sleep(200);
                } catch (InterruptedException e) {
    
                    e.printStackTrace();
                    // Thread.currentThread().interrupt();  If you add this , The program can be terminated , It will only explode abnormally 
                }
                System.out.println("-----hello InterruptDemo03");
            }
        },"t1");
        t1.start();
        try {
    TimeUnit.MILLISECONDS.sleep(1);} catch (InterruptedException e) {
    e.printStackTrace();}
        new Thread(() -> t1.interrupt()).start();
    }
}
// The explosion is abnormal , And the program has been running 
//java.lang.InterruptedException: sleep interrupted
// at java.lang.Thread.sleep(Native Method)
//-----hello InterruptDemo03
//-----hello InterruptDemo03
//-----hello InterruptDemo03
//......
//----------------------------
//--------- added Thread.currentThread().interrupt();
//java.lang.InterruptedException: sleep interrupted
// at java.lang.Thread.sleep(Native Method)
// at com.zhang.admin.controller.InterruptDemo03.lambda$main$0(InterruptDemo03.java:15)
// at java.lang.Thread.run(Thread.java:748)
//-----hello InterruptDemo03
//t1  Interrupt flag bit ：true Termination of procedure

  • above

    1. ② If the thread is blocked （ For example, in the sleep, wait, join Equal state ）, To call the current thread object in another thread interrupt Method , So the thread will Immediately exit the blocked state （ The interrupt state will be cleared ）, And throw a InterruptedException abnormal .
/** * 1  Interrupt flag bit   The default is false * 2 t2 ----->t1 An interrupt negotiation was issued ,t2 call t1.interrupt(), Interrupt flag bit true * 3  Interrupt flag bit true, Under normal circumstances , The program to stop ,^-^ * 4  Interrupt flag bit true, Under abnormal conditions ,InterruptedException, Will clear the interrupt state , And will receive InterruptedException. Interrupt flag bit false Cause infinite loop . * * 5  stay catch In block , The interrupt flag bit needs to be set to again true,2 Call stop  */

sleep Method throw InterruptedException after , The interrupt flag is also cleared to false, We are catch Don't pass th.interrupt() Method again sets the interrupt flag to true, This leads to infinite loops

A small summary

  • Interruption is just a collaborative mechanism , Modifying the interrupt flag bit is nothing more , Not immediately stop interrupt

3 Static methods Thread.interrupted(), Talk about your understanding

  • api The second one in

    • public static boolean interrupted()
    • Static methods ,Thread.interrupted(); Determine whether the thread is interrupted , And clear the current interrupt state Two things ：1 Returns the interrupt status of the current thread 2 Set the interrupt status of the current thread to false（ This method is a little difficult to understand , Because the results of two consecutive calls may be different .） |
public class InterruptDemo04 {
    
    public static void main(String[] args) {
    
        System.out.println(Thread.currentThread().getName()+"\t"+Thread.interrupted());
        System.out.println(Thread.currentThread().getName()+"\t"+Thread.interrupted());
        System.out.println("-----1");
        Thread.currentThread().interrupt();// The interrupt flag bit is set to true
        System.out.println("-----2");
        System.out.println(Thread.currentThread().getName()+"\t"+Thread.interrupted());
        System.out.println(Thread.currentThread().getName()+"\t"+Thread.interrupted());
    }
}
//main false
//main false
//-----1
//-----2
//main true
//main false

  • Look at the source ,interrupted() contrast isInterrupted()

    public static boolean interrupted() {
      
        return currentThread().isInterrupted(true);
    }
    
private native boolean isInterrupted(boolean ClearInterrupted);

    public boolean isInterrupted() {
      
        return isInterrupted(false);
    }

private native boolean isInterrupted(boolean ClearInterrupted);

    • They all call at the bottom native Method isInterrupted.

    • Just pass in the parameters ClearInterrupted A pass on parameter passed on true, One by one false.

      • Static methods interrupted() in true Indicates that the current interrupt state is cleared .

      • Example method isInterrupted Will not be .

LockSupport What is it?

  • Official explanation ： Basic thread blocking primitives for creating locks and other synchronization classes .

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  • The core Namely park() and unpark() Method

    • park() The method is Blocking threads

    • unpark() The method is Unblock threads
      Please add a picture description

Thread waiting wake-up mechanism

3 Ways to let threads wait and wake up

  1. Use Object Medium wait() Method to make the thread wait , Use Object Medium notify() Method wake up thread

  2. Use JUC In bag Condition Of await() Method to make the thread wait , Use signal() Method wake up thread

  3. LockSupport Class can block the current thread and wake up the specified blocked thread
    Insert picture description here

①Object Class wait and notify Method to realize thread waiting and wake-up

  • normal 

    public class LockSupportDemo
{
      
    public static void main(String[] args)
    {
      
        Object objectLock = new Object();

        new Thread(() -> {
      
            synchronized (objectLock) {
      
                System.out.println(Thread.currentThread().getName()+"\t ---- come in");
                try {
      
                    objectLock.wait();//---------------------- Let him wait here first 
                } catch (InterruptedException e) {
      
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName()+"\t"+"--- Wake up ");
        },"t1").start();

        // Pause the thread for a few seconds 
        try {
       TimeUnit.SECONDS.sleep(3L); } catch (InterruptedException e) {
       e.printStackTrace(); }

        new Thread(() -> {
      
            synchronized (objectLock) {
      
                objectLock.notify();//------------------------- Wake it up again 
                System.out.println(Thread.currentThread().getName()+"\t --- A notice ");
            }
        },"t2").start();
    }
}
//t1 ---- come in
//t2 --- A notice 
//t1 --- Wake up

  • abnormal 1— Get rid of synchronized

    • Explain to use wait and notify Must be added synchronized
    public class LockSupportDemo
{
      
    public static void main(String[] args)
    {
      
        Object objectLock = new Object();

        new Thread(() -> {
      
        // synchronized (objectLock) {
      
                System.out.println(Thread.currentThread().getName()+"\t ---- come in");
                try {
      
                    objectLock.wait();
                } catch (InterruptedException e) {
      
                    e.printStackTrace();
                }
      // }
            System.out.println(Thread.currentThread().getName()+"\t"+"--- Wake up ");
        },"t1").start();

        // Pause the thread for a few seconds 
        try {
       TimeUnit.SECONDS.sleep(3L); } catch (InterruptedException e) {
       e.printStackTrace(); }

        new Thread(() -> {
      
       // synchronized (objectLock) {
      
                objectLock.notify();
                System.out.println(Thread.currentThread().getName()+"\t --- A notice ");
       // }
        },"t2").start();
    }
}
//t1 ---- come in
//Exception in thread "t1" java.lang.IllegalMonitorStateException
// at java.lang.Object.wait(Native Method)
// at java.lang.Object.wait(Object.java:502)
// at com.zhang.admin.controller.LockSupportDemo.lambda$main$0(LockSupportDemo.java:15)
// at java.lang.Thread.run(Thread.java:748)

  • abnormal 2— hold notify and wait The execution sequence of is reversed

    • The order of instructions cannot be reversed 
    public class LockSupportDemo
{
      
    public static void main(String[] args)
    {
      
        Object objectLock = new Object();

        new Thread(() -> {
      
            try {
       TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) {
       e.printStackTrace(); }
            synchronized (objectLock) {
      
                System.out.println(Thread.currentThread().getName()+"\t ---- come in");
                try {
      
                    objectLock.wait();
                } catch (InterruptedException e) {
      
                    e.printStackTrace();
                }
            }
            System.out.println(Thread.currentThread().getName()+"\t"+"--- Wake up ");
        },"t1").start();



        new Thread(() -> {
      
            synchronized (objectLock) {
      
                objectLock.notify();// This is done first 
                System.out.println(Thread.currentThread().getName()+"\t --- A notice ");
            }
        },"t2").start();
    }
}
// Has been in a cycle

A small summary

  • wait and notify The method must be in Sync In a block or method , And Pair Appear to use

  • First wait after notify only OK, The order

②Condition Interface await after signal Method to realize thread waiting and wake-up

  • normal 

    public class LockSupportDemo
{
      
    public static void main(String[] args)
    {
      
        Lock lock = new ReentrantLock();
        Condition condition = lock.newCondition();

        new Thread(() -> {
      
            lock.lock();
            try
            {
      
                System.out.println(Thread.currentThread().getName()+"\t-----come in");
                condition.await();
                System.out.println(Thread.currentThread().getName()+"\t ----- Awakened ");
            } catch (InterruptedException e) {
      
                e.printStackTrace();
            } finally {
      
                lock.unlock();
            }
        },"t1").start();

        // Pause the thread for a few seconds 
        try {
       TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) {
       e.printStackTrace(); }

        new Thread(() -> {
      
            lock.lock();
            try
            {
      
                condition.signal();
            } catch (Exception e) {
      
                e.printStackTrace();
            } finally {
      
                lock.unlock();
            }
            System.out.println(Thread.currentThread().getName()+"\t"+" I want to wake up ");
        },"t2").start();

    }
}
//t1 -----come in
//t2  I want to wake up 
//t1 ----- Awakened

  • abnormal 1 The principle of same

    • Still back IllegalMonitorStateException

  • abnormal 2 The principle of same

    • It's still going round and round

A small summary

  • await and notify Similar to the above wait and notify

    • Condition Thread waiting and wakeup methods in , You need to acquire the lock first

    • Be sure to await after signal, It can't be reversed

Object and Condition Restrictions on use

  • summary

    • The thread must first acquire and hold the lock , Must be in the lock block （synchronized or lock） in

    • You have to wait before you wake up , Thread can be awakened

③LockSupport Class park Waiting and unpark Wake up the

What is it?

  • adopt park() and unpark(thread) Method to block and wake up threads

  • Website to explain

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    • LockSupport Is the basic thread blocking primitive used to create locks and other synchronization classes .

    • LockSupport Class uses a method called Permit（ The license ） To block and wake up threads , Each thread has a license (permit),

    • permit（ The license ） There are only two values 1 and 0, The default is 0.0 It's blocking ,1 It's wake up

    • You can think of licensing as a kind of (0,1) Semaphore （Semaphore）, But with Semaphore The difference is , The cumulative upper limit of the license is 1.

The license （Permit）

The main method

API

Insert picture description here

Blocking

  • park()/park(Object blocker)

  • call LockSupport.park() when , Found that it called unsafe class , And by default, it passes a 0

    public static void park() {
      
        UNSAFE.park(false, 0L);
    }

Insert picture description here

  • permit The default is zero , So at the beginning of the call park() Method , The current thread will block , Until another thread will permit Set to 1 when ,park Method will be awakened ,
    Then it will permit Set to zero again and return .

Wake up the

  • call LockSupport.unpark(); when , Also called unsafe class 

    public static void unpark(Thread thread) {
      
        if (thread != null)
            UNSAFE.unpark(thread);
    }

Insert picture description here

  • call unpark(thread) After the method , Will be thread Permission for threads permit Set to 1( Note multiple calls unpark Method , Not cumulative ,permit Value or 1) Will wake up automatically thread Threads , That is, in the previous blocking LockSupport.park() Method will immediately return .

Code

  • normal + No lock block requirements 

    public class LockSupportDemo
{
      
    public static void main(String[] args) {
      
        Thread t1 = new Thread(()->{
      
            System.out.println(Thread.currentThread().getName()+"\t----------come in");
            LockSupport.park();
            System.out.println(Thread.currentThread().getName()+"\t---------- Wake up ");
        },"t1");
        t1.start();

        new Thread(()->{
      
            LockSupport.unpark(t1);
            System.out.println(Thread.currentThread().getName()+"\t----- A notice , To wake up t1");
        },"t2").start();
    }
}
//t1 ----------come in
//t2 ----- A notice , To wake up t1
//t1 ---------- Wake up

  • The previous error wakes up first and then waits ,LockSupport Still support 

    public class LockSupportDemo
{
      
    public static void main(String[] args) {
      
        Thread t1 = new Thread(()->{
      
            try {
      TimeUnit.SECONDS.sleep(3);} catch (InterruptedException e) {
      e.printStackTrace();}
            System.out.println(Thread.currentThread().getName()+"\t----------come in"+"\t"+System.currentTimeMillis());
            LockSupport.park();
            System.out.println(Thread.currentThread().getName()+"\t---------- Wake up "+"\t"+System.currentTimeMillis());
        },"t1");
        t1.start();

        new Thread(()->{
      
            LockSupport.unpark(t1);
            System.out.println(Thread.currentThread().getName()+"\t----- A notice , To wake up t1");
        },"t2").start();
    }
}
//t2 ----- A notice , To wake up t1
//t1 ----------come in 1654750785663
//t1 ---------- Wake up  1654750785663

    sleep Method 3 Wake up in seconds , perform park Invalid , No blocking effect , Explain the following . First executed. unpark(t1) Cause the above park The method is null and void , Time is the same
    - Similar to the highway ETC, Bought the pass in advance unpark, Lift the handrail directly at the gate and let it go , No, park Intercepted .

  • In pairs, remember

Last

  • There is only one license 

    public class LockSupportDemo
{
      
    public static void main(String[] args) {
      
        Thread t1 = new Thread(()->{
      
            try {
      TimeUnit.SECONDS.sleep(3);} catch (InterruptedException e) {
      e.printStackTrace();}
            System.out.println(Thread.currentThread().getName()+"\t----------come in"+"\t"+System.currentTimeMillis());
            LockSupport.park();
            LockSupport.park();
            System.out.println(Thread.currentThread().getName()+"\t---------- Wake up "+"\t"+System.currentTimeMillis());
        },"t1");
        t1.start();

        new Thread(()->{
      
            LockSupport.unpark(t1);
            LockSupport.unpark(t1);
            System.out.println(Thread.currentThread().getName()+"\t----- A notice , To wake up t1");
        },"t2").start();
    }
}
//t2 ----- A notice , To wake up t1
//t1 ----------come in 1654750970677-------------------- It's stuck here

  • A small summary

    Lock Support Is the basic thread blocking primitive used to create locks and other synchronization classes .
    Lock Support Is a thread blocking tool class , All methods are static , The thread can be blocked at any location , There is a corresponding wake-up method after blocking . Root knot
    At the end of , Lock Support Called Unsafe Medium native Code .

    Lock Support Provide park() and unpark() Method realization Blocking threads and Unblock threads The process of
    Lock Support And every thread that uses it has a license (permit) relation .
    Each thread has an associated permit, permit At most one , Repeated calls to un park And they don't accumulate credentials .

    The understanding of image
    Thread blocking requires consuming credentials (permit) , This certificate is at most 1 individual .
    When calling When the method is used
    * If there is a certificate , It will consume the voucher directly and exit normally ;
    * If there is no certificate , You have to block and wait for credentials to be available ;
    and **** On the contrary , It will add a voucher , But the certificate can only have at most     1     individual , Accumulation is invalid .

Interview questions

  • Why can we break through wait/notify The original calling sequence of ?
    because un park Got a certificate , Call later park Method , You can have the right voucher consumption , So it won't block .
    After the vouchers are issued first, they can be unblocked later .

  • Why wake up twice and block twice , But the end result will also block the thread ?
    Because the maximum number of vouchers is 1, Two consecutive calls un park And call once un park The effect is the same , Only one voucher will be added ;
    And call twice park But you need to consume two vouchers , Not enough proof , No release .

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