Testing and debugging multithreaded programs is extremely difficult, because concurrency hazards often do not manifest themselves uniformly or reliably. Most threading problems are unpredictable by their nature, and may not occur at all on certain platforms (like uniprocessor systems) or below a certain level of load. Because testing multithreaded programs for correctness is so difficult and bugs can take so long to appear, it becomes even more important to develop applications with thread safety in mind from the beginning. In this article, we're going to explore how a particular thread-safety problem -- allowing the this reference to escape during construction (which we'll call the escaped reference problem) -- can create some very undesirable results. We'll then establish some guidelines for writing thread-safe constructors.

Following "safe construction" techniques

Analyzing programs for thread-safety violations can be very difficult and requires specialized experience. Fortunately, and perhaps surprisingly, creating thread-safe classes from the outset is not as difficult, although it requires a different specialized skill: discipline. Most concurrency errors stem from programmers attempting to break the rules in the name of convenience, perceived performance benefits, or just plain laziness. Like many other concurrency problems, you can avoid the escaped reference problem by following a few simple rules when you write constructors.

Hazardous race conditions

Most concurrency hazards boil down to some sort of data race. A data race, or race condition, occurs when multiple threads or processes are reading and writing a shared data item, and the final result depends on the order in which the threads are scheduled. Listing 1 gives an example of a simple data race in which a program may print either 0 or 1, depending on the scheduling of the threads.

public class DataRace {
  static int a = 0;

  public static void main() {
    new MyThread().start();
    a = 1;
  }

  public static class MyThread extends Thread {
    public void run() { 
      System.out.println(a);
    }
  }
}

The second thread could be scheduled immediately, printing the initial value of 0 for a. Alternately, the second thread might not run immediately, resulting in the value 1 being printed instead. The output of this program may depend on the JDK you are using, the scheduler of the underlying operating system, or random timing artifacts. Running it multiple times could produce different results.

Visibility hazards

There is actually another data race in Listing 1, besides the obvious race of whether the second thread starts executing before or after the first thread sets a to 1. The second race is a visibility race: the two threads are not using synchronization, which would ensure visibility of data changes across threads. Because there's no synchronization, if the second thread runs after the assignment to a is completed by the first thread, changes made by the first thread may or may not be immediately visible to the second thread. It is possible that the second thread might still see a as having a value of 0 even though the first thread already assigned it a value of 1. This second class of data race, where two threads are accessing the same variable in the absence of proper synchronization, is a complicated subject, but fortunately you can avoid this class of data race by using synchronization whenever you are reading a variable that might have been last written by another thread, or writing a variable that might next be read by another thread. We won't be exploring this type of data race further here, but see the "Synching up with the Java Memory Model" sidebar and the Resources section for more information on this complicated issue.

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Don't publish the "this" reference during construction

One of the mistakes that can introduce a data race into your class is to expose the this reference to another thread before the constructor has completed. Sometimes the reference is explicit, such as directly storing this in a static field or collection, but other times it can be implicit, such as when you publish a reference to an instance of a non-static inner class in a constructor. Constructors are not ordinary methods -- they have special semantics for initialization safety. An object is assumed to be in a predictable, consistent state after the constructor has completed, and publishing a reference to an incompletely constructed object is dangerous. Listing 2 shows an example of introducing this sort of race condition into a constructor. It may look harmless, but it contains the seeds of serious concurrency problems.

public class EventListener { 

  public EventListener(EventSource eventSource) {
    // do our initialization
    ...

    // register ourselves with the event source
    eventSource.registerListener(this);
  }

  public onEvent(Event e) { 
    // handle the event
  }
}

On first inspection, the EventListener class looks harmless. The registration of the listener, which publishes a reference to the new object where other threads might be able to see it, is the last thing that the constructor does. But even ignoring all the Java Memory Model (JMM) issues such as differences in visibility across threads and memory access reordering, this code still is in danger of exposing an incompletely constructed EventListener object to other threads. Consider what happens when EventListener is subclassed, as in Listing 3:

public class RecordingEventListener extends EventListener {
  private final ArrayList list;

  public RecordingEventListener(EventSource eventSource) {
    super(eventSource);
    list = Collections.synchronizedList(new ArrayList());
  }

  public onEvent(Event e) { 
    list.add(e);
    super.onEvent(e);
  }

  public Event[] getEvents() {
    return (Event[]) list.toArray(new Event[0]);
  }
}

Because the Java language specification requires that a call to super() be the first statement in a subclass constructor, our not-yet-constructed event listener is already registered with the event source before we can finish the initialization of the subclass fields. Now we have a data race for the list field. If the event listener decides to send an event from within the registration call, or we just get unlucky and an event arrives at exactly the wrong moment, RecordingEventListener.onEvent() could get called while list still has the default value of null, and would then throw a NullPointerException exception. Class methods like onEvent() shouldn't have to code against final fields not being initialized.

The problem with Listing 2 is that EventListener published a reference to the object being constructed before construction was complete. While it might have looked like the object was almost fully constructed, and therefore passing this to the event source seemed safe, looks can be deceiving. Publishing the this reference from within the constructor, as in Listing 2, is a time bomb waiting to explode.

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Don't implicitly expose the "this" reference

It is possible to create the escaped reference problem without using the this reference at all. Non-static inner classes maintain an implicit copy of the this reference of their parent object, so creating an anonymous inner class instance and passing it to an object visible from outside the current thread has all the same risks as exposing the this reference itself. Consider Listing 4, which has the same basic problem as Listing 2, but without explicit use of the this reference:

public class EventListener2 {
  public EventListener2(EventSource eventSource) {

    eventSource.registerListener(
      new EventListener() {
        public void onEvent(Event e) { 
          eventReceived(e);
        }
      });
  }

  public void eventReceived(Event e) {
  }
}

The EventListener2 class has the same disease as its EventListener cousin in Listing 2: a reference to the object under construction is being published -- in this case indirectly -- where another thread can see it. If we were to subclass EventListener2, we would have the same problem where the subclass method could be called before the subclass constructor completes.

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Don't start threads from within constructors

A special case of the problem in Listing 4 is starting a thread from within a constructor, because often when an object owns a thread, either that thread is an inner class or we pass the this reference to its constructor (or the class itself extends the Thread class). If an object is going to own a thread, it is best if the object provides a start() method, just like Thread does, and starts the thread from the start() method instead of from the constructor. While this does expose some implementation details (such as the possible existence of an owned thread) of the class via the interface, which is often not desirable, in this case the risks of starting the thread from the constructor outweigh the benefit of implementation hiding.

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What do you mean by "publish"?

Not all references to the this reference during construction are harmful, only those that publish the reference where other threads can see it. Determining whether it is safe to share the this reference with another object requires detailed understanding of that object's visibility and what that object will do with the reference. Listing 5 contains some examples of safe and unsafe practices with respect to letting the this reference escape during construction:

public class Safe { 

  private Object me;
  private Set set = new HashSet();
  private Thread thread;

  public Safe() { 
    // Safe because "me" is not visible from any other thread
    me = this;

    // Safe because "set" is not visible from any other thread
    set.add(this);

    // Safe because MyThread won't start until construction is complete
    // and the constructor doesn't publish the reference
    thread = new MyThread(this);
  }

  public void start() {
    thread.start();
  }

  private class MyThread(Object o) {
    private Object theObject;

    public MyThread(Object o) { 
      this.theObject = o;
    }

    ...
  }
}

public class Unsafe {
  public static Unsafe anInstance;
  public static Set set = new HashSet();
  private Set mySet = new HashSet();

  public Unsafe() {
    // Unsafe because anInstance is globally visible
    anInstance = this;

    // Unsafe because SomeOtherClass.anInstance is globally visible
    SomeOtherClass.anInstance = this;

    // Unsafe because SomeOtherClass might save the "this" reference
    // where another thread could see it
    SomeOtherClass.registerObject(this);

    // Unsafe because set is globally visible 
    set.add(this);

    // Unsafe because we are publishing a reference to mySet
    mySet.add(this);
    SomeOtherClass.someMethod(mySet);

    // Unsafe because the "this" object will be visible from the new
    // thread before the constructor completes
    thread = new MyThread(this);
    thread.start();
  }

  public Unsafe(Collection c) {
    // Unsafe because "c" may be visible from other threads
    c.add(this);
  }
}

As you can see, many of the unsafe constructs in the Unsafe class bear a significant resemblance to the safe constructs in the Safe class. Determining whether the this reference can become visible to another thread can be tricky. The best strategy is to avoid using the this reference at all (directly or indirectly) in constructors. In reality, however, that's not always possible. Just remember to be very careful with the this reference and with creating instances of nonstatic inner classes in constructors.

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More reasons not to let references escape during construction

The practices detailed above for thread-safe construction take on even more importance when we consider the effects of synchronization. For example, when thread A starts thread B, the Java Language Specification (JLS) guarantees that all variables that were visible to thread A when it starts thread B are visible to thread B, which is effectively like having an implicit synchronization in Thread.start(). If we start a thread from within a constructor, the object under construction is not completely constructed, and so we lose these visibility guarantees.

Because of some of its more confusing aspects, the JMM is being revised under Java Community Process JSR 133, which will (among other things) change the semantics of volatile and final to bring them more in line with general intuition. For example, under the current JMM semantics, it is possible for a thread to see a final field have more than one value over its lifetime. The new memory model semantics will prevent this, but only if a constructor is defined properly -- which means not letting the this reference escape during construction.

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Conclusion

Making a reference to an incompletely constructed object visible to another thread is clearly undesirable. After all, how can we tell the properly constructed objects from the incomplete ones? But by publishing a reference to this from inside a constructor -- either directly or indirectly through inner classes -- we do just that, and invite unpredictable results. To prevent this hazard, try to avoid using this, creating instances of inner classes, or starting threads from constructors. If you cannot avoid using this either directly or indirectly in a constructor, be very sure that you are not making the this reference visible to other threads.

Resources

• Doug Lea's Concurrent Programming in Java, Second Edition (Addison-Wesley, 1999) is a masterful book on the subtle issues surrounding multithreaded programming in Java applications.
  
  
• Synchronization and the Java Memory Model is an excerpt from Doug Lea's book that focuses on the actual meaning of synchronized.
  
  
• "Double-checked locking: Clever, but broken" (JavaWorld, February 2001) and "Can double-checked locking be fixed?" (JavaWorld, May 2001) explore the JMM and the surprising consequences of failing to synchronize in certain situations.
  
  
• In "Double-checked locking and the Singleton pattern" (developerWorks, May 2002), Peter Haggar gives a step-by-step explanation of how strange things can happen when you fail to synchronize.
  
  
• Semantics of Multithreaded Java (PDF) details the proposed changes in the Java Memory Model as a result of JSR 133.
  
  
• In "Writing multithreaded Java applications" (developerWorks, February 2001), Alex Roetter gives a basic overview of threads, synchronization, and locking in Java classes.
  
  
• Find other Java technology content in the developerWorks Java technology zone.
  
  

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Table of contents

• Safe construction techniques
• Don't publish the "this" reference during construction
• Don't implicitly expose the "this" reference
• Don't start threads from within constructors
• What do you mean by "publish"?
• More reasons ...
• Conclusion
• Resources
• About the author
• Comments

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