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From: MaciekL <__nospam__maclab@o2.pl>
Newsgroups: comp.lang.java.programmer
Subject: Synchronization of the constructor
Date: Sat, 13 Aug 2011 11:58:15 +0200
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Hi,

I have a doubt because Java disables synchronization of the constructor
by default.
Following example shows that lack of synchronization makes
NullPointerException.
................
[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(TestApp.java:61)
        at TestApp.run(TestApp.java:55)
        at java.lang.Thread.run(Thread.java:662)
[Thread[main,5,main]] Test
[Thread[main,5,main]] OBJ = java.lang.Object@9304b1
[Thread[main,5,main]]  {TestApp(0)} End
................

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

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

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)
...............

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

/*--:BEG:--[TestApp.java]----------------------------------------------------*/
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));
    runner.setName("runner");
    runner.start();
    try { Thread.sleep(timeout); }
    catch (InterruptedException ie) { }
    obj = new Object();
    test();
    info(" {TestApp(" + timeout + ")} End");
  }
  public void run()
  {
    info("Begin");
    test();
    info("End");
  }
  public synchronized void test()
  {
    info("Test");
    info("OBJ = " + obj.toString());
  }
  public static void main(String [] args)
  {
    new TestApp(1000);
  }
}
/*--:EOF:--[TestApp.java]----------------------------------------------------*/

Regards

-- 
Maciek