Synchronized method not blocking other threads

Question

I've a singleton class like this:

private static StringsHandler INSTANCE = null;
private int count = 0;
//I have 2 methods so I don't have to be sending/checking nulls on getInstance
//once it's created
public static void createInstance(List<String> strings){
    if(StringsHandler.INSTANCE == null)
        StringsHandler.INSTANCE = new StringsHandler(strings);
}

public static StringsHandler getInstance(){
    return StringsHandler.INSTANCE;
}

public synchronized File use(){
    count++;
    System.out.println("threads using .use " + count);
    while(true){} //Keep the thread here so count should stay as 1
}

Instance is created on the application main class, main method just like this:

if(stringList.isEmpty()){
        LOGGER.error("List was empty");
        System.exit(0);
    }
StringsHandler.createInstance(stringList);

And I call it using something like this:

list.parallelStream().forEach(element -> {
    SingletonClass.getInstance().use();
});

This should print threads using .use 1 but it prints threads using .use 5 why is the synchronized keyword allowing for more than 1 thread?

Post short but full example which will let us reproduce your problem [SSCCE](http://sscce.org) / [MCVE](http://stackoverflow.com/help/mcve). Don't make us guess *what* went wrong, even if we guess that there is lower chance to guess and properly explain *why* part which may prevent us from suggesting proper solution. — Pshemo, May 11 '16 at 18:50
I'm pretty sure that your bug is in SingletonClass.getInstance() so just show us this method and you will have your answer — Nicolas Filotto, May 11 '16 at 19:09
MCVE means we should be able to copy the code and paste it into a file, then compile and run it. voted to close due to this not being a MCVE. — Nathan Hughes, May 11 '16 at 20:09
@NathanHughes That makes sense, specially on a question like this. Unfortunately I wasn't able to reproduce this on a MCVE way, so I'll close the question. — Artemio Ramirez, May 12 '16 at 18:12

score 2 · Answer 1 · answered May 11 '16 at 18:43

2

The synchronized keyword only allows one thread to lock the particular object on which it is used. So if you have more than one instance of this object, and call use on more than one of them at a time, they will be able to execute at the same time.

It is your responsibility to decide on, and consistently enforce, a mapping of data elements to exclusion methods. If more than one exclusion method protects the same data element, then you will not get correct exclusion.

answered May 11 '16 at 18:43

David Schwartz

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Forgot to mention I was using a singleton class, edited question. – Artemio Ramirez May 11 '16 at 18:44
Log the object identifier on entry to `use` and you'll see they're not the same object. – David Schwartz May 11 '16 at 18:45
As long as `count` isn’t declared `static`, there still shouldn’t be any thread printing a value of `5`. Unless there is other code manipulating the variable… – Holger May 11 '16 at 18:53
It's actually a bit more complicated than that, it does print a number different from 1, but it only prints once. – Artemio Ramirez May 11 '16 at 18:56
@Holger there is not. – Artemio Ramirez May 11 '16 at 18:56
5

@ArtemioRamirez, sounds like the problem lies in code that you have not shown us. – Solomon Slow May 11 '16 at 19:05
@jameslarge I'm building a small class containing all the logic and flow to provide to you. – Artemio Ramirez May 11 '16 at 19:18

Synchronized method not blocking other threads

1 Answers1