在多个线程上同步var实例化

Question

I'm trying to sync a var instantiation like this one:

Object o = new Object();
String s = null;

void getS() {
  if (s != null) {
    return s;
  }
  // multiple threads stopping here
  // maybe using readwritelock? write.lock?
  syncronize(o) {
    // if previous thread stopped by sync block
    // completed before, bypass this
    if (s != null) {
      return s;
    }

    // no one before, instantiate s
    s = "abc";
  }
  return s; 
}

Is there a better way to handle a single time instantiation of var s? Maybe using locks?

Answer 1

Declare s volatile

volatile String s;

and we'll get a classic double-checked locking design pattern implementation. Patterns are formalized best practices so you dont need to try and further improve this code.

BTW the example with lazy String initialization makes no sense, it should be an expensive to create Object

Answer 2

The simplest to write:

private Foo foo;
public synchronized Foo getFoo() {
    if (foo == null) {
        foo = new Foo();
    }
    return foo;
}

The downside is that you have synchronization happen every time you access this property, even though synchronization is only needed the first time.

Google "double checked locking in java" for lots of information about ways that you can accomplish the same thing, but with less locking (and therefore potentially better performance).