Double-checked locking: Clever, but broken

Do you know what synchronized <em>really</em> means?

By Brian Goetz, JavaWorld.com, 02/09/01

What is DCL?

The DCL idiom was designed to support lazy initialization, which occurs when a class defers initialization of an owned object until it is actually needed:

class SomeClass {
  private Resource resource = null;
  public Resource getResource() {
    if (resource == null)
      resource = new Resource();
    return resource;
  }
}

Why would you want to defer initialization? Perhaps creating a Resource is an expensive operation, and users of SomeClass might not actually call getResource() in any given run. In that case, you can avoid creating the Resource entirely. Regardless, the SomeClass object can be created faster if it doesn't have to also create a Resource at construction time. Delaying some initialization operations until a user actually needs their results can help programs start up faster.

What if you try to use SomeClass in a multithreaded application? Then a race condition results: two threads could simultaneously execute the test to see if resource is null and, as a result, initialize resource twice. In a multithreaded environment, you should declare getResource() to be synchronized.

Unfortunately, synchronized methods run much slower -- as much as 100 times slower -- than ordinary unsynchronized methods. One of the motivations for lazy initialization is efficiency, but it appears that in order to achieve faster program startup, you have to accept slower execution time once the program starts. That doesn't sound like a great trade-off.

DCL purports to give us the best of both worlds. Using DCL, the getResource() method would look like this:

class SomeClass {
  private Resource resource = null;
  public Resource getResource() {
    if (resource == null) {
      synchronized {
        if (resource == null) 
          resource = new Resource();
      }
    }
    return resource;
  }
}

After the first call to getResource(), resource is already initialized, which avoids the synchronization hit in the most common code path. DCL also averts the race condition by checking resource a second time inside the synchronized block; that ensures that only one thread will try to initialize resource. DCL seems like a clever optimization -- but it doesn't work.

Meet the Java Memory Model

More accurately, DCL is not guaranteed to work. To understand why, we need to look at the relationship between the JVM and the computer environment on which it runs. In particular, we need to look at the Java Memory Model (JMM), defined in Chapter 17 of the Java Language Specification, by Bill Joy, Guy Steele, James Gosling, and Gilad Bracha (Addison-Wesley, 2000), which details how Java handles the interaction between threads and memory.