Constructor synchronization in Java

Someone somewhere told me that Java constructors are synchronized so that it can't be accessed concurrently during construction

This is certainly not the case. There is no implied synchronization with constructors. Not only can multiple constructors happen at the same time but you can get concurrency issues by, for example, forking a thread inside of a constructor with a reference to the this being constructed.

if I have a constructor that stores the object in a map, and another thread retrieves it from that map before its construction is finished, will that thread block until the constructor completes?

No it won't.

The big problem with constructors in threaded applications is that the compiler has the permission, under the Java memory model, to reorder the operations inside of the constructor so they take place after (of all things) the object reference is created and the constructor finishes. final fields will be guaranteed to be fully initialized by the time the constructor finishes but not other "normal" fields.

In your case, since you are putting your Test into the synchronized-map and then continuing to do initialization, as @Tim mentioned, this will allow other threads to get ahold of the object in a possibly semi-initialized state. One solution would be to use a static method to create your object:

private Test() {
    this.id = atomicIdGenerator.getAndIncrement();
    // Some lengthy operation to fully initialize this object
}

public static Test createTest() {
    Test test = new Test();
    // this put to a synchronized map forces a happens-before of Test constructor
    testsById.put(test.id, test);
    return test;
}

My example code works since you are dealing with a synchronized-map, which makes a call to synchronized which ensures that the Test constructor has completed and has been memory synchronized.

The big problems in your example is both the "happens before" guarantee (the constructor may not finish before Test is put into the map) and memory synchronization (the constructing thread and the get-ing thread may see different memory for the Test instance). If you move the put outside of the constructor then both are handled by the synchronized-map. It doesn't matter what object it is synchronized on to guarantee that the constructor has finished before it was put into the map and the memory has been synchronized.

I believe that if you called testsById.put(this.id, this); at the very end of your constructor, you may in practice be okay however this is not good form and at the least would need careful commenting/documentation. This would not solve the problem if the class was subclassed and initialization was done in the subclass after the super(). The static solution I showed is a better pattern.

Someone somewhere told me that Java constructors are synchronized

'Somebody somewhere' is seriously misinformed. Constructors are not synchronized. Proof:

public class A
{
    public A() throws InterruptedException
    {
        wait();
    }

    public static void main(String[] args) throws Exception
    {
        A a = new A();
    }
}

This code throws java.lang.IllegalMonitorStateException at the wait() call. If there was synchronization in effect, it wouldn't.

It doesn't even make sense. There is no need for them to be synchronized. A constructor can only be invoked after a new(), and by definition each invocation of new() returns a different value. So there is zero possibility of a constructor being invoked by two threads simultaneously with the same value of this. So there is no need for synchronization of constructors.

if I have a constructor that stores the object in a map, and another thread retrieves it from that map before its construction is finished, will that thread block until the constructor completes?

No. Why would it do that? Who's going to block it? Letting 'this' escape from a constructor like that is poor practice: it allows other threads to access an object that is still under construction.