Threading II

Deadlock Example


The following class is example of a java program which leads to a deadlock. Basically a person a is bowing to his friend b. and b in turn bows back to friend a. The problem is Person a and Person b both have obtained lock on their objects. When person b tries to bowback to a and at the same time, a tries to bowback to person b. In this case, both will wait endlessly to obtain a lock to bow to each other. This situation is refered as deadlock.

class Person extends Thread {

protected String myName;
Person friend;
public Person(String name)
{
 super(name);
 myName=name;
 //do nothing
}

public synchronized void bow()
{
 System.out.println(myName+" is bowing to "+friend.toStr());
 friend.bowBack();
}

public synchronized void bowBack()
{
 System.out.println(myName+" is bowing back to " +friend.toStr());
}

public void setFriend(Person p)
{
 this.friend=p;
}
public String toStr()
{
 return myName;
}
 public void run() {
     bow();
 }
}
public class Deadlock {

public static void main(String[] args) {
    Person a=new Person("A");
    Person b=new Person("B");
    a.setFriend(b);
    b.setFriend(a);
    a.start();
    b.start();

}
}

There are various techniques by which you can synchronize sequence of events.

In the above example, instead of synchronizing the entire method, just synchronize a block, so that the deadlock doesn't happen.

E.g,

public void bow()
{
   Synchronized(this) { 
     System.out.println(myName+" is bowing to "+friend.toStr());
   }
   friend.bowBack();
}

public synchronized void bowBack()
{
   Synchronized(this) {
     System.out.println(myName+" is bowing back to " +friend.toStr());
   }
}