Java ReentrentLock class

The ReentrantLock class implements the Lock interface which is used to enhance the multithreading. It provides the capability to avoid the use of synchronization to maintain the database consistency so that the waiting time of the threads can be reduced. A Thread owns the ReentrantLock which is intended to perform the write operation on some particular data.

Java ReentrentLock class declaration

public class ReentrantLock
extends Object
implements Lock, Serializable

List of Array class Methods

NO	Method	Description
1.	public int getHoldCount()	The getHoldCount() method of ReentrantLock class Queries the number of holds on this lock by the current thread.
2.	protected Thread getOwner()	The getOwner() method of ReentrantLock class returns the thread that currently owns this lock, or null if not owned. When this method is called by a thread that is not the owner, the return value reflects a best-effort approximation of current lock status.
3.	protected Collection getQueuedThreads()	The getQueuedThreads() returns a collection containing threads that may be waiting to acquire this lock. Because the actual threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate.
4.	public final boolean hasQueuedThread(Thread thread)	The hasQueuedThread(Thread thread) method of ReentrantLock class Queries whether the given thread is waiting to acquire this lock. Note that because cancellations may occur at any time, a true return does not guarantee that this thread will ever acquire this lock. This method is designed primarily for use in monitoring of the system state.
5.	public final boolean hasQueuedThreads()	The hasQueuedThreads() Queries whether any threads are waiting to acquire this lock. Note that because cancellations may occur at any time, a true return does not guarantee that any other thread will ever acquire this lock.
6.	public final boolean isFair()	The isFair() method of ReentrantLock class returns true if this lock has fairness set true.
7.	public boolean isHeldByCurrentThread()	The isHeldByCurrentThread() method of Reentrant Class checks if the current thread occupies this lock.
8.	public boolean isLocked()	The isLocked()method of ReentrantLock class checks if any thread occupies this lock. This method is created for use in monitoring of the system state, not for synchronization control.
9.	public void lock()	The lock() method of Reentrant Class hold the lock if it is not held by another thread and returns immediately, and set the lock hold count to one.
10.	public boolean tryLock()	The tryLock() method of ReentrantLock class holds the lock only when any other thread does not hold it at the time of invocation. If the current thread already holds this lock, then the hold count is incremented by one, and the method returns true. Otherwise false.
11.	public final int getQueueLength()	The getQueueLength() method of Reentrant Class returns the number of threads waiting to get the lock. The value is approximate because the number of threads can change dynamically while traversing internal data structures. It is used to monitor the state.
12.	public void lockInterruptibly() throws InterruptedException	The lockInterruptibly() method of ReentrantLock class hold the lock until or unless the current thread is interrupted.
13.	public String toString()	The setByte() method of Array class sets the value of the indexed component of the specified array object to the specified byte value.

Java ReentrentLock class Example: hasQueuedThreads()

import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockhasQueuedThreadsExample1_1  {
    public static void main(String[] args) {
 
           ReentrantLock lock=new ReentrantLock();
           MyRunnableClass33 myRnbl=new MyRunnableClass33(lock);
           new Thread(myRnbl,"Thread-1").start();
                     
    }
}
  
class MyRunnableClass33 implements Runnable{
    
    ReentrantLock lock;
    public MyRunnableClass33(ReentrantLock lock) { 
           this.lock=lock;
    }
    
    public void run(){
           
           System.out.println(Thread.currentThread().getName()
                        +" is Waiting to get the lock");
           
           lock.lock();
 
           System.out.println(Thread.currentThread().getName()
                        +" has got the  lock.");
           
           try {
                  Thread.sleep(2000);
           } catch (InterruptedException e) {
                  e.printStackTrace();
           }
 
 
           System.out.println(" has queued Threads = "+lock.hasQueuedThreads());
           
           System.out.println(Thread.currentThread().getName()
                        +" has released the lock.");
           
           lock.unlock();    //read explanation for 5sec
    }
}

Test it Now

Output:

Thread-1 is Waiting to get the lock
Thread-1 has got the  lock.
 has queued Threads = false
Thread-1 has released the lock.

Java ReentrentLock Class Example: lock()

package com.app.Rentrant;

import java.text.SimpleDateFormat; 
import java.util.Date; 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
import java.util.concurrent.locks.ReentrantLock; 
  
class WorkerClassLL implements Runnable 
{ 
  String nm; 
  ReentrantLock relc; 
  public WorkerClassLL(ReentrantLock rl, String n) 
  { 
    relc = rl; 
    nm = n; 
  } 
  public void run() 
  { 
    boolean dn = false; 
    while (!dn) 
    { 
      //Outer Lock 
      boolean ans = relc.tryLock(); 
  
      //  True if lock is free 
      if(ans) 
      { 
        try
        { 
          int i=0;
          for(i=0;i<=6;i++) {
        	  
        	  if(i>=2)
        	  {
        		  relc.lock();
        		  Thread t = new Thread();
        		  System.out.println("Thread Created.....");
        		  if(i==3) {
        			  t.setName("Maint Thread2");
        			  System.out.println("Thread Created.....");
        		  }
        		
        		  
        	  }
        	  if(i==4)
        		  relc.unlock();
        		  break;
        	  
          }
          
          System.out.println(" after sleep(1500) Is locked - " + relc.isLocked()); 
          System.out.println("task name - " + nm + " work done"); 
  
          dn = true; 
        } 
        catch(Exception e) 
        { 
          e.printStackTrace(); 
        } 
        
      } 
      
      } 
    } 
  
  } 
   
public class ReentrantLockLockExample1_1
{ 
  static final int MAX_Time = 2; 
  public static void main(String[] args) 
  { 
    ReentrantLock rel = new ReentrantLock(); 
    ExecutorService pool = Executors.newFixedThreadPool(MAX_Time); 
    Runnable wrk1 = new WorkerClassLL(rel, "Job1"); 
    pool.execute(wrk1); 
    pool.shutdown(); 
  } 
} 

Test it Now

Output:

after sleep(1500) Is locked - true
task name - Job1 work done

Java ReentrentLock Class Example: isHeldByCurrentThread()

package com.app.Rentrant;

 import java.text.SimpleDateFormat; 
 import java.util.Date; 
 import java.util.concurrent.ExecutorService; 
 import java.util.concurrent.Executors; 
 import java.util.concurrent.locks.ReentrantLock; 
   
 class WorkerClass21 implements Runnable 
 { 
   String nm; 
   ReentrantLock relc; 
   public WorkerClass21(ReentrantLock rl, String n) 
   { 
     relc = rl; 
     nm = n; 
   } 
   public void run() 
   { 
     boolean dn = false; 
     while (!dn) 
     { 
       //Outer Lock 
       boolean ans = relc.tryLock(); 
   
       //  True if lock is free 
       if(ans) 
       { 
         try
        
         { 
        	 Thread t=new Thread();
        	 
        	 for(int i =0 ; i<=5 ; i++)
        	 {
        		if(i>3){        				
        			System.out.println("    "+i+  "  " +t.getPriority());	
        			t.sleep(1500);
        		}
        		if(i<2) {
        			System.out.println("    "+i+  "  " +t.getName());
        			t.sleep(1500);
        		}
        		
        	 }
           
           relc.lock(); 
         }
         
           catch(InterruptedException e) 
           { 
             e.printStackTrace(); 
           } 
           
           System.out.println(" after sleep(1500) Is held by Current Thread - " + relc.isHeldByCurrentThread()); 
           System.out.println("task name - " + nm + " work done"); 
   
           dn = true; 
         } 
          { 
          
         
       } 
   
         
       } 
     } 
   
   
   } 
  
   
 public class ReentrantLockisHeldByCurrentThreadExample1_1 
 { 
   static final int Time = 2; 
   public static void main(String[] args) 
   { 
     ReentrantLock rel = new ReentrantLock(); 
     ExecutorService pool = Executors.newFixedThreadPool(Time); 
     Runnable wrk1 = new WorkerClass21(rel, "Job1"); 
     pool.execute(wrk1); 
     pool.shutdown(); 
   } 
 } 

Test it Now

Output:

    0  Thread-0
    1  Thread-0
    4  5
    5  5
 after sleep(1500) Is held by Current Thread - true
task name - Job1 work done

Next TopicJava ReentrantLock

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ReentrantLock Class

Java ReentrentLock class

Java ReentrentLock class declaration

List of Array class Methods

Java ReentrentLock class Example: hasQueuedThreads()

Java ReentrentLock Class Example: lock()

Java ReentrentLock Class Example: isHeldByCurrentThread()

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