Code after synchronized block in java

Question

I've a simple question but couldn't manage to find a proper answer. Imagine we have;

public void addName(String name) {
    synchronized(this) {
        lastName = name;
        nameCount++;
    }

    nameList.add(name);
}

What about the code after sync. block here? I mean sync. blocks are used to reduce scope of lock but here the code after it ( namelist.add(name) ) will be blocked, right ?

Assume thread A called this function above but it's gonna wait for 'this' lock to be released by thread B which had the lock before on some other method. Now, I wondered if the execution will resume from B's nameList.add(name) method while thread A is waiting on 'this' lock object - since nameList.add(name) is not in the sync block.

The question is not clear, what do you mean by "block" ? any code outside of synchronized block is not synchronized - if that's what you mean. — Nir Alfasi
– Nir Alfasi, Commented Jan 12, 2016 at 16:00
Please clarify what "blocked" means most specifically, as we are unable to answer your question without knowing what that actually means (it isn't a standard Java term and I assume it means something to you personally...). — Emily Mabrey
– Emily Mabrey, Commented Jan 12, 2016 at 16:05
Ok. Let me try again. Assume thread A called this function above but it's gonna wait for 'this' lock to be released by thread B which had the lock before on some other method. Now, I wondered if the execution will resume from B's nameList.add(name) method while thread A is waiting on 'this' lock object - since nameList.add(name) is not in the sync block. — stdout
– stdout, Commented Jan 13, 2016 at 7:59

Nathan Hughes · Accepted Answer · 2016-01-13 17:06:50Z

Now, I wondered if the execution will resume from B's nameList.add(name) method while thread A is waiting on 'this' lock object - since nameList.add(name) is not in the sync block.

No, the thread executing the method can't just skip over the block and execute the remaining part of the method. What it will do is block until it can acquire the monitor on this, then execute the synchronized block, then release the monitor on this, then add the string to the nameList.

If concurrent threads execute this, there's no guarantee of which threads will insert into the nameList first. It's possible that between the time that a thread releases the monitor on this and the time that it adds to the nameList one or more other threads might barge in and add to the list.

Also whatever nameList is implemented as needs to be a thread-safe collection, so that concurrent changes don't cause errors and so that changes are visible across threads. If nameList is an ArrayList or a HashSet, for instance, then this would not be safe.

Community · Accepted Answer · 2020-06-20 09:12:55Z

0

In principle, keyword synchronized does following two things:

Code guarded by the synchronized keyword cannot be executed simultaneously by more than one thread;
.. it controls the visibility of data (variables) between threads;

In your example, the code which is not in synchronized scope would be accessible by all other Threads which call this method;

Assuming the operations which is out of scope is writing operation, you'd (generally) want it to be synchronized to reduce any anomalies which may creep up.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jan 12, 2016 at 16:00

e.doroskevic

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Ctx · Accepted Answer · 2016-01-12 16:01:58Z

0

synchronized(this) will block if and while another thread is inside this block. If the thread leaves this block, one waiting thread has a chance (amongst other waiting threads) to enter the block. namelist.add() is executed outside the synchronized scope, so it may be executed in parallel with other threads.

answered Jan 12, 2016 at 16:01

Ctx

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