Re: Synchronization of the constructor

Robert Klemme <>
Sat, 13 Aug 2011 12:17:33 +0200
On 13.08.2011 11:58, MaciekL wrote:

I have a doubt because Java disables synchronization of the constructor
by default.

Btw, it's "question" and not "doubt". Side note: I see this quite
often. Does anybody have an idea why non native speakers often use
"doubt" instead of "question"? I'm not a native speaker myself but I
can't remember having mixed up the two. So I guess it has to do with
the way English is taught or the mother tongue.

Following example shows that lack of synchronization makes
[Thread[main,5,main]] {TestApp(0)} Begin
[Thread[runner,5,main]] Begin
[Thread[runner,5,main]] Test
Exception in thread "runner" java.lang.NullPointerException
         at TestApp.test(
[Thread[main,5,main]] Test
[Thread[main,5,main]] OBJ = java.lang.Object@9304b1
[Thread[main,5,main]] {TestApp(0)} End

Where is line 61? From what I can see your example only has about 30 lines.

Synchronized 'test' method may be called by another thread in case
the Object is not created.

What do you mean by that?

This undefined behaviour is very tricky, note that following example
may works correctly, it depends on the Constructor duration time.

You see an error. That doesn't necessarily mean that behavior is undefined.

I have found a soultion, adding whole definition of the Constructor into
synchronized block.
synchronized (this)
   ... // ConstructorBody

I don't understand why Java forbids simple declaration:
public synchronized TestApp(int timeout)

Probably because usually "this" is not leaked (to other objects or
threads) during construction. This is generally considered dangerous
because you leak a reference to a potentially incomplete instance (the
constructor whose task it is to initialize the object to a consistent
state did not yet finish).

I also remember that they changed the language spec at one point with
regard to thread visibility of "this" during construction but I don't
recall the details right now.

I have to say that your design is questionable: you inherit Runnable
which is intended to decouple thread handling from what is executed in
the thread. Yet inside the constructor you create a thread and start it.

Rather you should first let the construction of the object finish
properly and then create a Thread with that instance and start the
thread. If you want to do that in the same class a static method could
do, e.g.

public static void runTest(final int timeout)
   throws InterruptedException {
   final TestApp ta = new TestApp();
   final Thread th = new Thread(ta);
   ta.setObj(new Object());

Because of that all calls to "add*Listener(this)" from constructor
should be carrefully implemented.

public class TestApp implements Runnable
   Object obj = null;
   public static void info(String s)
     System.err.println("[" + Thread.currentThread().toString() + "] " + s);
   public TestApp(int timeout)
     info(" {TestApp(" + timeout + ")} Begin");
     Thread runner = (new Thread(this));
     try { Thread.sleep(timeout); }
     catch (InterruptedException ie) { }

Empty catch block without any handling or at least a comment explaining
why you consciously ignore the exception is really bad.

     obj = new Object();
     info(" {TestApp(" + timeout + ")} End");
   public void run()
   public synchronized void test()
     info("OBJ = " + obj.toString());

You don't need toString() - it's done automatically.

   public static void main(String [] args)
     new TestApp(1000);

Now why do you want to start a thread in a constructor and have it
executed code of "this"?

Kind regards


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