Is a synchronized static method legal in Java?

Yes, and it simplifies static factory methods like this:

class Foo {
    private Foo() {}

    public static synchronized Foo getInstance() {
        if (instance == null) {
            instance = new Foo();
        }
        return instance;
    }

    private static Foo instance = null;
}

      

        
        
        
      

    

This is what it would look like if the methods static

couldn't be synchronized

:

class Foo {
    private Foo() {}

    public static Foo getInstance() {
        synchronized (LOCK) {
            if (instance == null) {
                instance = new Foo();
            }
        }
        return instance;
    }

    private static Foo instance = null;

    private static final Object LOCK = Foo.class;
    // alternative: private static final Object LOCK = new Object();
}

      

        
        
        
      

    

Not that big a deal, it just saves 2 lines of code.