Re: "Lock" on object - Synchronized keyword

Lew <>
Tue, 20 May 2008 23:02:14 -0400
Mark Space wrote:

If you want thread B to block, you have to do this:

  List newList = Collections.synchronizedList( lockedObject );

By this Mark Space shows how the instance 'A' acquires 'newList' as an already
synchronized object - all calls through List interface methods will be
synchronized on 'newList' for you, at least when made singly.

  synchronized( newList ) {
    ... // Thread A

It is actually sometimes necessary to grab the entire object in an outer lock
to allow multiple method calls on the object to work in a single critical
section. It is harmless that the individual method calls lock within an outer
lock against the same object, since the thread is allowed to reacquire the
same monitor it already has. However, for single method calls one can rely on
the synchronization provided by the SynchronizedList [sic].

One need not provide an explicit 'synchronized( newList )' in newList's
clients for a single method call at a time - rely on the synchronization
already there. One must provide it to lock several such calls together.

The rest of the explanation remains unaffected, modulo the "raw type" warning.


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