Second Lombok Synchronized Example

Here is the second example of a program that deadlocks, with an example that I refactored using the @Synchronized annotation from Project Lombok.

This example was posted on one of the LinkedIn groups in which I first started asking about concurrency using the synchronized keyword. The basic idea to prevent deadlock is to mark the methods with the @Synchronized annotation. For one of them you should create a private, final object, and use the name of that object as the element for that annotation. For the other method that deadlocks I just use the annotation without an element.

Here is the original that was posted:

package fromlinkedin;

// posted on a LinkedIn group discussion

import java.util.UUID;


class A {

    private String uuid = UUID.randomUUID().toString();
    
    synchronized void doA() {
        System.out.println( "In doA for " + uuid );
    }
    
    synchronized void doB( A another ) {
        doA();
        another.doA();
    }

    public static void main( String[] args ) {
        System.out.println( "starting main" );
        final A a1 = new A();
        final A a2 = new A();
        Thread second = new Thread() {
            public void run() {
                a1.doB( a2 );
            }
        };
        second.start();
        a2.doB( a1 );
        System.out.println( "At end of main" );
    } // end method main
} // end class

Here is the version with the Lombok refactoring:

package fromlinkedin;

// Lombok-ed version of a class posted in a LinkedIn group discussion

import java.util.UUID;
import lombok.Synchronized;

class LombokA {
    private final Object objID2 = new Object();
    private String uuid = UUID.randomUUID().toString();
    
    // synchronized void doA(){
    @Synchronized void doA() {
        System.out.println( "In doA for " + uuid );
    }
    
    // synchronized void doB( LombokA another ) {
    @Synchronized( "objID2" ) void doB( LombokA another ) {
        System.out.println( "objID2.hashCode(): " + objID2.hashCode() );
        doA();
        another.doA();
    }

    public static void main( String[] args ) {
        System.out.println( "starting main" );
        final LombokA a1 = new LombokA();
        final LombokA a2 = new LombokA();
        Thread second = new Thread(){
            public void run() {
                a1.doB( a2 );
            }
        };
        second.start();
        a2.doB( a1 );
        System.out.println( "At end of main" );
    } // end method main
} // end class

I have a couple more examples before I get to my notes from No Fluff Just Stuff.

Image from the Wikipedia page for the Indonesian province of West Java, assumed allowed under Fair Use.