LCK10-J. Do not use incorrect forms of the double-checked locking idiom

Lazy initialization defers the construction of a member object until an instance is actually required, rather than initializing the member object in the class's constructor. Lazy initialization also helps to breaking harmful circularities in class and instance initialization and in performing other optimizations [[Bloch 2005]].

Lazy initialization uses either a class or an instance method, depending on whether the member object is static. The method checks whether the instance has already been created and, if not, creates it. When the instance already exists, the method simply returns the instance:

// Correct single threaded version using lazy initialization
final class Foo {
  private Helper helper = null;

  public Helper getHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    return helper;
  }
  // ...
}

Lazy initialization must be synchronized in multithreaded applications, to prevent multiple threads from creating extraneous instances of the member object:

// Correct multithreaded version using synchronization
final class Foo {
  private Helper helper = null;

  public synchronized Helper getHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    return helper;
  }
  // ...
}

The double-checked locking idiom improves performance by limiting synchronization to the rare case of new instance creation, and by foregoing synchronization during the common case of retrieving an already-created instance.

Incorrect forms of the double-checked idiom include those that allow publication of an uninitialized or partially initialized object. Consequently, use of incorrect forms of the double-checked locking idiom is forbidden.

Noncompliant Code Example

The double-checked locking pattern uses block synchronization rather than method synchronization, and installs an additional null check before attempting synchronization. This noncompliant code example uses the incorrect form of the double-checked locking idiom.

// "Double-Checked Locking" idiom
final class Foo {
  private Helper helper = null;
  public Helper getHelper() {
    if (helper == null) {
      synchronized (this) {
        if (helper == null) {
          helper = new Helper();
        }
      }
    }
    return helper;
  }
  // Other methods and members...
}

According to Pugh [[Pugh 2004]]

... writes that initialize the Helper object and the write to the helper field can be done or perceived out of order. As a result, a thread which invokes getHelper() could see a non-null reference to a helper object, but see the default values for fields of the helper object, rather than the values set in the constructor.

Even if the compiler does not reorder those writes, on a multiprocessor, the processor or the memory system may reorder those writes, as perceived by a thread running on another processor.

Also see rule TSM03-J. Do not publish partially initialized objects.

Compliant Solution (Volatile)

This compliant solution declares the helper field volatile.

// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
final class Foo {
  private volatile Helper helper = null;

  public Helper getHelper() {
    if (helper == null) {
      synchronized (this) {
        if (helper == null) {
          helper = new Helper(); // If the helper is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}

When a thread initializes the Helper object, a [happens-before relationship] is established between this thread and any other thread that retrieves and returns the instance [[Pugh 2004], [Manson 2004]].

Compliant Solution (Static Initialization)

This compliant solution initializes the helper field in the declaration of the static variable [[Manson 2006]].

final class Foo {
  private static final Helper helper = new Helper();

  public static Helper getHelper() {
    return helper;
  }
}

Variables that are declared static and initialized at declaration, or from a static initializer, are guaranteed to be fully constructed before being made visible to other threads. However, this solution forgoes the benefits of lazy initialization.

Compliant Solution (Initialize-on-Demand, Holder Class Idiom)

This compliant solution uses the initialize-on-demand, holder class idiom that implicitly incorporates lazy initialization by declaring a static variable within a static Holder inner class.

final class Foo {
  // Lazy initialization
  private static class Holder {
    static Helper helper = new Helper();
  }

  public static Helper getInstance() {
    return Holder.helper;
  }
}

Initialization of the static helper field is deferred until the getInstance() method is called. This idiom is a better choice than the double-checked locking idiom for lazily initializing static fields [[Bloch 2008]]. However, this idiom cannot be used to lazily initialize instance fields [[Bloch 2001]].

Compliant Solution (ThreadLocal Storage)

This compliant solution (originally suggested by Alexander Terekhov [[Pugh 2004]]) uses a ThreadLocal object to lazily create a Helper instance.

final class Foo {
  private final ThreadLocal<Foo> perThreadInstance = new ThreadLocal<Foo>();
  private Helper helper = null;

  public Helper getHelper() {
    if (perThreadInstance.get() == null) {
      createHelper();
    }
    return helper;
  }

  private synchronized void createHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    // Any non-null value can be used as an argument to set()
    perThreadInstance.set(this);
  }
}

Compliant Solution (Immutable)

In this compliant solution, the Helper class is immutable and, consequently, is guaranteed to be fully constructed before becoming visible. In this case, lacks any further requirements to ensure that the double-checked locking idiom avoids the publication of an uninitialized or partially initialized field.

public final class Helper {
  private final int n;

  public Helper(int n) {
    this.n = n;
  }

  // Other fields and methods, all fields are final
}

final class Foo {
  private Helper helper = null;

  public Helper getHelper() {
    if (helper == null) {
      synchronized (this) {
        if (helper == null) {
          helper = new Helper(42); // If the helper is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}

Exceptions

LCK10-EX1: Use of the noncompliant form of the double-checked locking idiom is permitted for 32-bit primitive values (for example, int or float) [[Pugh 2004]], although this usage is discouraged. Note that the noncompliant form fails for long or double because unsynchronized reads/writes of 64-bit primitives are lack a guarantee of atomicity. (See rule VNA05-J. Ensure atomicity when reading and writing 64-bit values.)

Risk Assessment

Using incorrect forms of the double-checked, locking idiom can lead to synchronization problems.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography

      08. Locking (LCK)