How to synchronize blocks instead of entire methods

[Java Tip]

This Java tip shows how to synchronize blocks instead of entire methods.

import java.util.ArrayList;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;

public class CriticalSection {
  public static void main(String[] args) {
    // Test the two different approaches:
    final PairManipulator pm1 = new PairManipulator(new PairManager1()), pm2 = new PairManipulator(
        new PairManager2());
    new Timer(true).schedule(new TimerTask() {
      public void run() {
        System.out.println("pm1: " + pm1);
        System.out.println("pm2: " + pm2);
        System.exit(0);
      }
    }, 500); // run() after 500 milliseconds
  }
} ///:~



class Pair { // Not thread-safe
  private int x, y;

  public Pair(int x, int y) {
    this.x = x;
    this.y = y;
  }

  public Pair() {
    this(0, 0);
  }

  public int getX() {
    return x;
  }

  public int getY() {
    return y;
  }

  public void incrementX() {
    x++;
  }

  public void incrementY() {
    y++;
  }

  public String toString() {
    return "x: " + x + ", y: " + y;
  }

  public class PairValuesNotEqualException extends RuntimeException {
    public PairValuesNotEqualException() {
      super("Pair values not equal: " + Pair.this);
    }
  }

  // Arbitrary invariant -- both variables must be equal:
  public void checkState() {
    if (x != y)
      throw new PairValuesNotEqualException();
  }
}

// Protect a Pair inside a thread-safe class:

abstract class PairManager {
  protected Pair p = new Pair();

  private List storage = new ArrayList();

  public synchronized Pair getPair() {
    // Make a copy to keep the original safe:
    return new Pair(p.getX(), p.getY());
  }

  protected void store() {
    storage.add(getPair());
  }

  // A "template method":
  public abstract void doTask();
}

// Synchronize the entire method:

class PairManager1 extends PairManager {
  public synchronized void doTask() {
    p.incrementX();
    p.incrementY();
    store();
  }
}

// Use a critical section:

class PairManager2 extends PairManager {
  public void doTask() {
    synchronized (this) {
      p.incrementX();
      p.incrementY();
    }
    store();
  }
}

class PairManipulator extends Thread {
  private PairManager pm;

  private int checkCounter = 0;

  private class PairChecker extends Thread {
    PairChecker() {
      start();
    }

    public void run() {
      while (true) {
        checkCounter++;
        pm.getPair().checkState();
      }
    }
  }

  public PairManipulator(PairManager pm) {
    this.pm = pm;
    start();
    new PairChecker();
  }

  public void run() {
    while (true) {
      pm.doTask();
    }
  }

  public String toString() {
    return "Pair: " + pm.getPair() + " checkCounter = " + checkCounter;
  }
}