Page History

...

// 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;
  }
  // ...
}

...

// Correct multithreaded version using synchronization
final class Foo { 
  private Helper helper = null;
  
  public synchronized Helper getHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    return helper;
  }
  // ...
}

...

Incorrect forms of the double-checked idiom include those that allow an uninitialized or partially initialized object to be published.

Noncompliant Code Example

The double-checked locking pattern uses block synchronization instead of method synchronization; installing 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...
}

...

See also CON28-J. Do not publish partially initialized objects.

Compliant Solution (Volatile)

This compliant solution declares the Helper object 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
  }
}

If a thread initializes the {{Helper}} object, a [happens-before relationship|BB. Definitions#happens-before order] is established between this thread and another that retrieves and returns the instance. \[[Pugh 04|AA. Java References#Pugh 04]\] and \[[Manson 04|AA. Java References#Manson 04]\] 

Compliant Solution (Static Initialization)

This compliant solution initializes the {{helper}} field in the declaration of the static variable \[[Manson 06|AA. Java References#Manson 06]\]. 

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.

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 inner class Holder.

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 \[[Bloch 08|AA. Java References#Bloch 08]\].  However, this idiom cannot be used to lazily initialize instance fields \[[Bloch 01|AA. Java References#Bloch 01]\]. 

Compliant Solution (ThreadLocal Storage)

This compliant solution (originally suggested by Alexander Terekhov \[[Pugh 04|AA. Java References#Pugh 04]\]) uses a {{ThreadLocal}} object to lazily create a {{Helper}} instance.

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 is consequently guaranteed to be fully constructed before becoming visible. In this case, there are no further requirements to ensure that the double-checked locking idiom does not result in 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

*CON27-EX1:* The noncompliant form of the double-checked locking idiom can be used for for 32-bit primitive values (for example, {{int}} or {{float}}) \[[Pugh 04|AA. Java References#Pugh 04]\]. Note that it does not work for {{long}} or {{double}} because unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic (see [VNA05-J. Ensure atomicity when reading and writing 64-bit values|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.

Automated Detection

The Coverity Prevent Version 5.0 DOUBLE_CHECK_LOCK checker can detect the instance where a variable is being checked for initialization outside of a synchronized section, then checking again once inside the section. The double checked lock idiom can result in race conditions in a multi-threaded application, and is unnecessary in a single-threaded one.

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] 
\[[JLS 05|AA. Java References#JLS 05]\] Section 12.4, "Initialization of Classes and Interfaces"
\[[Pugh 04|AA. Java References#Pugh 04]\]
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 48: "Synchronize access to shared mutable data"
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 71: "Use lazy initialization judiciously"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"

...