Re: Synchronization of the constructor
markspace wrote:
Eric Sosman wrote:
MaciekL wrote:
I have a doubt because Java disables synchronization of the constructor
by default.
The constructor can be synchronized,
It really can't. Recall that the compiler will insert a call to a super=
constructor if the first statement doesn't have a such a call or a call=
to another class constructor. Consider this:
public class SomeClass {
public SomeClass() {
synchronized( SomeClass.class ) {
...
}
}
...
What you get is this:
public class SomeClass {
public SomeClass() {
super();
synchronized( SomeClass.class ) {
...
}
}
...
And thus you see that some writes in the construction occur outside of
the synchronized block, a classic case of incorrectly written
synchronization.
The closest Java gets to a synchronized constructor is immutable objects=
made with final fields.
public class Immutable {
private final SomeObject o;
public Immutable() {
o = new SomeObject();
}
...
This is thread safe, and immutable, because the fields written are
declared "final." Java takes special processing at the end of the
constructor to synchronize all final fields, and any writes made to
objects accessible via those final fields, with all other threads in the=
system. So now this Immutable class can be used safely by any thread in=
the system.
Others have explained your error quite well, OP, so I will merely add that =
you should read the Java Language Specification on threads, synchronization=
and the /happens-before/ relationship. Also, read and study /Effective Ja=
va/ by Joshua Bloch, /Java Concurrency in Practice/ by Brian Goetz, et al.,=
the book by Doug Lea whose title escapes me just now (lmgtfy?) and any art=
icle in the IBM Developerworks for Java site's series, especially those by =
Goetz, Bloch and others on the topic of threaded Java code.
Bottom line: constructors are wholly for _construction_, only construction=
and nothing but construction, not operations on the object. Do your logic=
outside the constructor, after the constructor's promises have a chance to=
be kept.
--
Lew