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The double-checked locking idiom is a software design pattern used to reduce the overhead of acquiring a lock by first testing the locking criterion without actually acquiring the lock. Double-checked locking improves performance by limiting synchronization to the rare case of computing the field's value or constructing a new instance for the field to reference and by foregoing synchronization during the common case of retrieving an already-created instance or value.

Incorrect forms of the double-checked locking idiom include those that allow publication of an uninitialized or partially initialized object. Consequently, only those forms of the double-checked locking idiom that correctly establish a happens-before relationship both for the helper reference and for the complete construction of the Helper instance are permitted.

The double-checked locking idiom is frequently used to implement a singleton factory pattern that performs lazy initialization. Lazy initialization defers the construction of a member field or an object referred to by a member field until an instance is actually required rather than computing the field value or constructing the referenced object in the class's constructor. Lazy initialization helps to break harmful circularities in class and instance initialization. It also enables other optimizations [Bloch 2005].

Lazy initialization uses either a class or an instance method

Instead of initializing a member object using a constructor, _lazy initialization_ can be used to defer the construction of the member object until an instance is actually required. Lazy initialization also helps in breaking harmful circularities in class and instance initialization, and performing other optimizations \[[Bloch 2005|AA. Java References#Bloch 05]\].

A class or an instance method is used for lazy initialization, depending on whether the member object is static. The method checks whether the instance has already been created and, if not, creates it. If When the instance already exists, it the method simply returns itthe 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;
  }
  // ...
}

In a multithreaded application, Lazy initialization must be synchronized so that in multithreaded applications to prevent multiple threads do not create 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 foregoing it during the common case of retrieving an already created instance.

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

// "Double-Checkedchecked Locking"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...
}

Wiki MarkupAccording to Pugh \[ [Pugh 2004|AA. Java References#Pugh 04]\]:],

Writes ... 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 guideline This code also violates TSM03-J. Do not publish partially initialized objects.

Compliant Solution (Volatile)

This compliant solution declares the Helper 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
  }
}

Wiki MarkupIf a thread initializes the {{Helper}} object, a [When 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. \[[Pugh 2004|AA. Java References#Pugh 04]\] and \[[Manson 2004|AA. Java References#Manson 04]\]and any other thread that retrieves and returns the instance [Pugh 2004], [Manson 2004].

Compliant Solution (Static Initialization)

Wiki MarkupThis compliant solution initializes the {{helper}} field in the declaration of the static variable \ [[Manson 2006|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. 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;
  }
}

Wiki MarkupInitialization 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 \[[Bloch 2008|AA. Java References#Bloch 08]\]. However, this idiom cannot be used to lazily initialize instance fields \[[Bloch 2001|AA. Java References#Bloch 01]\called. The necessary happens-before relationships are created by the combination of the class loader's actions loading and initializing the Holder instance and the guarantees provided by the Java memory model (JMM). 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 \[ [Pugh 2004|AA. Java References#Pugh 04]\]) uses a {{ThreadLocal}} object to lazily create a {{Helper}} instanceto track whether each individual thread has participated in the synchronization that creates the needed happens-before relationships. Each thread stores a non-null value into its thread-local perThreadInstance only inside the synchronized createHelper() method; consequently, any thread that sees a null value must establish the necessary happens-before relationships by invoking createHelper().

Code Block
bgColor #ccccff
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); } }

...

Noncompliant Code Example (Immutable)

In this compliant solution, the noncompliant code example, the Helper class is immutable and consequently guaranteed to be made immutable by declaring its fields final. The JMM guarantees that immutable objects are 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 fieldthey become visible to any other thread. The block synchronization in the getHelper() method guarantees that all threads that can see a non-null value of the helper field will also see the fully initialized Helper object.

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) {            // First read of helper
      synchronized (this) {
        if (helper == null) {        // Second read of helper
          helper = new Helper(42);
        }
      }
    }
    return helper;                   // IfThird read theof helper
  is null, create a new instance
        }
      }
    }
    return helper; // If helper is non-null, return its instance
  }
}

Exceptions

}
}

However, this code is not guaranteed to succeed on all Java Virtual Machine platforms because there is no happens-before relationship between the first read and third read of helper. Consequently, it is possible for the third read of helper to obtain a stale null value (perhaps because its value was cached or reordered by the compiler), causing the getHelper() method to return a null pointer.

Compliant Solution (Immutable)

This compliant solution uses a local variable to reduce the number of unsynchronized reads of the helper field to 1. As a result, if the read of helper yields a non-null value, it is cached in a local variable that is inaccessible to other threads and is safely returned.

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() {
    Helper h = helper;       // Only unsynchronized read of helper
    if (h == null) {
      synchronized (this) {
        h = helper;          // In synchronized block, so this is safe
        if (h == null) {
          h = new Helper(42);
          helper = h;
        }
      }
    }
    return h;
  }
}

Exceptions

LCK10-J-EX0: 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. The noncompliant form establishes the necessary happens-before relationship between threads that see an initialized version of the primitive value. The second happens-before relationship (for the initialization of the fields of the referent) is of no practical value because unsynchronized reads and writes of primitive values up to 32-bits are guaranteed to be atomic. Consequently, the noncompliant form establishes the only needed happens-before relationship in this case. Note, however, that the noncompliant form fails for long and double because unsynchronized reads or writes of 64-bit primitives lack a guarantee of atomicity and consequently require a second happens-before relationship to guarantee that all threads see only fully assigned 64-bit values (see  VNA05-J. Ensure atomicity when reading and writing 64-bit values for more information). Wiki Markup*LCK10-EX1:* The noncompliant form of the double-checked locking idiom can be used for for 32-bit primitive values (for example, {{int}} or {{float}}) \[[Pugh 2004|AA. Java References#Pugh 04]\]. 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 guideline [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 to synchronization problems and can expose partially initialized objects.

References

\[[API 2006|AA. Java References#API 06]\]
\[[JLS 2005|AA. Java References#JLS 05]\] Section 12.4, "Initialization of Classes and Interfaces"
\[[Pugh 2004|AA. Java References#Pugh 04]\]
\[[Bloch 2001|AA. Java References#Bloch 01]\] Item 48: "Synchronize access to shared mutable data"
\[[Bloch 2008|AA. Java References#Bloch 08]\] Item 71: "Use lazy initialization judiciously"
\[[MITRE 2009|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"

Automated Detection

Related Guidelines

Bibliography

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Image Added Image Added THI06-J. Ensure that threads performing blocking operations can be terminated      12. Locking (LCK)      Image Modified