Re: synchronized block question...
But what puzzles me is that --by experimentation-- if I touch the
semaphore object inside the synchronized-block it seems the lock on
the object gets released.
For example, if I put a stmt:
as the first stmt inside the syncronized block, it appears there is no
locking anymore and 2 different client-calls can now execute the block
Is this really how it's supposed to work?
Or what am I missing?
Let's analyze what the statement is doing in better detail. Since
semaphore is an actual Integer object, we need to box and unbox. So the
statement is the same as this:
semaphore = new Integer(semaphore.intValue() + 1);
Now we can see what's going on. Suppose semaphore begins at 0. When the
first call gets to there, it synchronizes on semaphore, an Integer with
a value of 0. It increments the value, which sets the variable to a new
Integer with a value of 1. The next call then comes in, sees that
nothing is holding a lock on the Integer with a value of 1, and then
executes the code.
In short, your code is locking on two different objects.
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth
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