Does synchronized if apply to whole block

Question

This is not a question about method synchronisation, only to statement synchronisation. Please do not incorrectly mark as duplicate.

Is this code:

synchronized (this) if (something)
{
    somecode();
    somemorecode();
}

equivalent to this code:

if (something)
{
    synchronized (this)
    {
        somecode();
        somemorecode();
    }
}

My IDE tells me that your first form (once typos corrected) is not valid syntax - `synchronized` requires a block I think: a single statement is not allowed. `synchronized (this) { if (something) { ... } }` is valid (braces added) and it is now clear that the `if` is in the sync block, rather than attaching a condition to it - i.e. Java is not Ruby! — Paul, May 21 '15 at 10:47
So does mine, I had not noticed the tiny red squiggle. And yes, I was bringing the question with me from another languqage. — Steve Waring, May 21 '15 at 10:51
@Paul: Not just your IDE, the JLS requires it. Nice catch! I've updated my answer to say that. — T.J. Crowder, May 21 '15 at 10:52

T.J. Crowder · Accepted Answer · 2015-05-21T11:00:30.817

2

Is this code...equivalent to this code

No, it isn't. In your first example, the if test is might be inside the synchronized section. In your second example, the if is outside the synchronized section.

This is a syntax error:

synchronized (this) if (something)
{
    somecode();
    somemorecode();
}

synchronized requires a block, per JLS§14.19:

SynchronizedStatement:
synchronized ( Expression ) Block

If it were valid, it would probably be equivalent to:

synchronized (this)
{
    if (something)
    {
        somecode();
        somemorecode();
    }
}

...but that's pure speculation; if it's not defined by the JLS, who knows what it would be. :-)

Side note: It's synchronized, not syncronized or syncronised.

edited May 21 '15 at 11:00

answered May 21 '15 at 10:43

T.J. Crowder

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Ooops I typed in the question wrong. I was trying to ask if it was the same as your answer, or somehow the syncronization only applied to the code within the if clause. – Steve Waring May 21 '15 at 10:45
@SteveWaring: Ah! :-) – T.J. Crowder May 21 '15 at 10:47
The problem of being a UK English user, Every now and then you type in a word like colour or syncrinised. The IDE picks them up. – Steve Waring May 21 '15 at 10:49
If it would have been valid, I would probably read it the other way around. I.e synchronize on this if it's valid: `if(something) { synchronized(this) { ... } }` – Alexis C. May 21 '15 at 10:55
@AlexisC.: That would be unlike all other uses of single statements vs. blocks (`if`, `while`, ...) – T.J. Crowder May 21 '15 at 10:56
Sorry I'm not sure I'm following you on this :-) – Alexis C. May 21 '15 at 11:02
`while (condition) if (otherCondition) { doSomething(); }` puts the `if` **inside** the `while`, not outside it. – T.J. Crowder May 21 '15 at 11:16
Ah yes of course. But the first OP's example is really counter-intuitive if you don't know the rules (maybe that's why it's not allowed).. – Alexis C. May 21 '15 at 12:02
@AlexisC.: Yeah, that's entirely possible. :-) – T.J. Crowder May 21 '15 at 12:09

Does synchronized if apply to whole block

1 Answers1