Why can't Java constructors be synchronized?

Question

According to the Java Language Specification, constructors cannot be marked synchronized because other threads cannot see the object being created until the thread creating

Answer 1

Note that constructors cannot be synchronized — using the synchronizedkeyword with a constructor is a syntax error. Synchronizing constructors doesn't make sense, because only the thread that creates an object should have access to it while it is being constructed.

Answer 2

The question has been raised on a discussion list used by the writers of the Java concurrent API and the Java Memory Model. Several answers were given, in particular Hans Boehm replied:

Some of us (myself included IIRC) actually argued during the Java memory model deliberations that synchronized constructors should be allowed. Now I could go either way on it. Client code shouldn't use races to communicate the reference, so it shouldn't matter. But if you don't trust the clients of [your class], I think synchronized constructors could possibly be useful. And that was much of the reasoning behind final field semantics. [...] As David said, you can use synchronized blocks.

Answer 3

The following code can achieve the expected result for synchronized constructor.

public class SynchronisedConstructor implements Runnable {

    private int myInt;

    /*synchronized*/ static {
        System.out.println("Within static block");
    }

    public SynchronisedConstructor(){
        super();
        synchronized(this){
            System.out.println("Within sync block in constructor");
            myInt = 3;
        }
    }

    @Override
    public void run() {
        print();
    }

    public synchronized void print() {
        System.out.println(Thread.currentThread().getName());
        System.out.println(myInt);
    }

    public static void main(String[] args) {

        SynchronisedConstructor sc = new SynchronisedConstructor();

        Thread t1 = new Thread(sc);
        t1.setName("t1");
        Thread t2 = new Thread(sc);
        t2.setName("t2");

        t1.start();
        t2.start();
    }
}