Constructor synchronization in Java

Question

Someone somewhere told me that Java constructors are synchronized so that it can't be accessed concurrently during construction, and I was wondering: 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?

Let me demonstrate with some code:

public class Test {
    private static final Map<Integer, Test> testsById =
            Collections.synchronizedMap(new HashMap<>());
    private static final AtomicInteger atomicIdGenerator = new AtomicInteger();
    private final int id;

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

    public static Test getTestById(int id) {
        return testsById.get(id);
    }
}

Assume that put/get are the only operations on the map, so I won't get CME's via something like iteration, and try to ignore other obvious flaws here.

What I want to know is if another thread (that's not the one constructing the object, obviously) tries to access the object using getTestById and calling something on it, will it block? In other words:

Test test = getTestById(someId);
test.doSomething(); // Does this line block until the constructor is done?

I'm just trying to clarify how far the constructor synchronization goes in Java and if code like this would be problematic. I've seen code like this recently that did this instead of using a static factory method, and I was wondering just how dangerous (or safe) this is in a multi-threaded system.

Answer 1

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.

Answer 2

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.

Answer 3

You've been misinformed. What you describe is actually referred to as improper publication and discussed at length in the Java Concurrency In Practice book.

So yes, it will be possible for another thread to obtain a reference to your object and begin trying to use it before it is finished initializing. But wait, it gets worse consider this answer: https://stackoverflow.com/a/2624784/122207 ... basically there can be a reordering of reference assignment and constructor completion. In the example referenced, one thread can assign h = new Holder(i) and another thread call h.assertSanity() on the new instance with timing just right to get two different values for the n member that is assigned in Holder's constructor.

Answer 4

constructors are just like other methods, there's no additional synchronization (except for handling final fields).

the code would work if this is published later

public Test() 
{
    // Some lengthy operation to fully initialize this object

    this.id = atomicIdGenerator.getAndIncrement();
    testsById.put(this.id, this);
}

Answer 5

Although this question is answered but the code pasted in question doesn't follow safe construction techniques as it allows this reference to escape from constructor , I would like to share a beautiful explanation presented by Brian Goetz in the article: "Java theory and practice: Safe construction techniques" at the IBM developerWorks website.

Answer 6

It's unsafe. There are no additional synchronization in JVM. You can do something like this:

public class Test {
    private final Object lock = new Object();
    public Test() {
        synchronized (lock) {
            // your improper object reference publication
            // long initialization
        }
    }

    public void doSomething() {
        synchronized (lock) {
            // do something
        }
    }
}