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Example of ManagedBlocker in Java

By Arvind Rai, December 27, 2013
ManagedBlocker is an interface within the class java.util.concurrent.ForkJoinPool in JDK 7. ManagedBlocker manages the block and release of ForkJoinWorkerThread. ManagedBlocker has two methods, block() and isReleasable(). block() blocks the current thread and isReleasable() method checks if blocking is required. ManagedBlocker can be implemented with the help of ReentrantLock or BlockingQueue and is used by the ForkJoinPool.managedBlock() method in java.

ManagedBlocker using ReentrantLock in Java

Find the code which is displaying how to implement ManagedBlocker using ReentrantLock.
Locker.java
package com.concretepage.util.concurrent;
import java.util.concurrent.ForkJoinPool.ManagedBlocker;
import java.util.concurrent.locks.ReentrantLock;
public class Locker implements ManagedBlocker {
	  final ReentrantLock rtlock;
	  boolean isLocked = false;
	  Locker(ReentrantLock rtlock) {
		this.rtlock = rtlock;
	  }
	  public boolean block() {
		if (!isLocked){
		   rtlock.lock();
		}
		return true;
	  }
	  public boolean isReleasable() {
		return isLocked || (isLocked = rtlock.tryLock());
	  }
}
 
Within block() method , ReentrantLock.lock() locks the current thread if already not locked. If already locked by another thread then current thread becomes disabled till it does not acquire lock.

ManagedBlocker using BlockingQueue in Java

Now look at the code how to implement ManagedBlocker using BlockingQueue.
QueueManagedBlocker.java
package com.concretepage.util.concurrent;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ForkJoinPool.ManagedBlocker;
public class QueueManagedBlocker<T> implements ManagedBlocker {
	  final BlockingQueue<T> queue;
	  volatile T value = null;
	  QueueManagedBlocker(BlockingQueue<T> queue) {
		this.queue = queue;
	  }
	  public boolean block() throws InterruptedException {
		if (value == null)
		   value = queue.take();
		return true;
	  }
	  public boolean isReleasable() {
		return value != null || (value = queue.poll()) != null;
	  }
	  public T getValue() {
		return value;
	  }
}
 
To get the value from BlockingQueue there are two methods take() and poll(). take() waits to get the value whereas poll not and returns null. getValue() method can be used to get the value.

ForkJoinPool.managedBlock() in Java

ForkJoinPool .ManagedBlocker is used by method ForkJoinPool.managedBlock(). managedBlock() internally calls block() and isReleasable() method to manage block for ForkJoinWorkerThread. Find a sample example of ForkJoinPool.managedBlock() with BlockingQueue.
ManagedBlockerDemo.java
package com.concretepage.util.concurrent;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ForkJoinPool;
public class ManagedBlockerDemo {
       public static void main(String[] args) throws InterruptedException {
           BlockingQueue<String> bq = new ArrayBlockingQueue<String>(2);
           bq.put("A");
           bq.put("B");
           QueueManagedBlocker<String> blocker  =  new QueueManagedBlocker<String>(bq);
           ForkJoinPool.managedBlock(blocker);
           System.out.println(blocker.getValue());
       }
}
 
In case current thread is ForkJoinWorkerThread and is blocked, then to ensure sufficient parallelism, managedBlock() method arranges a thread to be activated.
POSTED BY
ARVIND RAI
ARVIND RAI
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