Page History

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... 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.

This makes the originally proposed double-checked locking pattern insecure. The guideline See also CON26-J. Do not publish partially initialized objects further discusses the possibility of a non-null reference that refers to a partially initialized object.

Compliant Solution (

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Volatile)

This compliant solution declares the Helper object as volatile and consequently, uses the correct form of the double-checked locking idiom.

// 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 (

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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;
  }
}

Wiki MarkupVariables 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. This approach should not be confused with eager initialization because in this case, the Java Language Specification guarantees lazy initialization of the class when it is first used \[[JLS 05|AA. Java References#JLS 05]\].

"Today, the double-check idiom is the technique of choice for lazily initializing an instance field. While you can apply the double-check idiom to {{static}} fields as well, there is no reason to do so: the lazy initialization holder class idiom is a better choice." \[[Bloch 08|AA. Java References#Bloch 08]\].  

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Compliant Solution (Initialize-on-demand Holder Class Idiom)

This compliant solution uses the initialize-on-demand holder class idiom that implicitly incorporates lazy initialization . It uses by declaring a static variable as suggested in the previous compliant solution and declares it 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 {{Holderhelper}} classfield is deferred until the {{getInstance()}} method is called,. followingThis which the {{helper}} field idiom is initialized.a Thebetter onlychoice limitationthan ofthe thisdouble methodchecked islocking thatidiom itfor workslazily only forinitializing {{static}} fields and not for instance fields \[[Bloch 0108|AA. Java References#Bloch 0108]\]. This idiomHowever, isthis aidiom bettercannot choicebe thanused the double checked locking idiom for to lazily initializing {{static}}initialize instance fields \[[Bloch 0801|AA. Java References#Bloch 0801]\]. 

Compliant Solution (ThreadLocal

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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 {
  // If perThreadInstance.get() returns a non-null value, this thread
  // has done synchronization needed to see initialization of helper
  private final ThreadLocal<Foo> perThreadInstance = new ThreadLocal<Foo>();
  private Helper helper = null;

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

  private final synchronized void createHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    // Any non-null value wouldcan be doused as thean argument here
    perThreadInstance.set(this);
  }
}

Compliant Solution (java.util.concurrent utilities)

This compliant solution uses an AtomicReference wrapper around the Helper object. It uses the standard compareAndSet (CAS) functionality to set a newly created Helper object if helperRef is null. Otherwise, it simply returns the already created instance. (Tom Hawtin, JMM Mailing List)

// Uses atomic utilities
final class Foo {
  private final AtomicReference<Helper> helperRef =
    new AtomicReference<Helper>();

  public Helper getHelper() {
    Helper helper = helperRef.get();
    if (helper != null) {
      return helper;
    }
    Helper newHelper = new Helper();
    return helperRef.compareAndSet(null, newHelper) ?
           newHelper :
           helperRef.get();
  }
}

While this code ensures that only one Helper object is prevented from being garbage collected, it allows multiple Helper objects to be created. If constructing multiple Helper objects is infeasible or expensive, this solution may not be appropriate.

Compliant Solution (immutable)

In this compliant solution the Foo class is unchanged, but the Helper class is made immutable. Consequently, the Helper class is guaranteed to be fully constructed before becoming visible. The object must be truly immutable; it is not sufficient for the program to refrain from modifying the 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) {
      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
  }
}

Note that if class Foo were mutable, the Helper field would need to be declared volatile as recommended in CON09-J. Ensure visibility of shared references to immutable objects. Also, the getHelper() method is an instance method and the accessibility of the helper field is private. This allows safe publication of the Helper object, in that a thread cannot observe a partially initialized Foo object (CON26-J. Do not publish partially initialized objects). The class Helper is also compliant with CON26-J. Do not publish partially initialized objects and consequently, it cannot be observed to be in a partially initialized state.

Exceptions

EX1: Explicitly synchronized code (that uses method synchronization or proper block synchronization, enclosing all initialization statements) does not require the use of double-checked locking.

to set()
    perThreadInstance.set(this);
  }
}

Exceptions

*EX2CON22-EX1:* "Although the \[The noncompliant form of the\] double-checked locking idiom cannotcan be used for references to objects, it can work for 32-bit primitive values (e.g.for example, {{int's}} or {{float's)}}) \[[Pugh 04|AA. Java References#Pugh 04]\]. Note that it does not work for {{long's}} or double's, since{{double}} because unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic." \[[Pugh 04|AA. Java References#Pugh 04]\]. (See(see [CON25-J. Ensure atomicity when reading and writing 64-bit values] for more information.) 

Risk Assessment

Using incorrect forms of the double checked locking idiom can lead to synchronization issuesproblems.

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\[[API 06|AA. Java References#API 06]\] 
\[[JLS 05|AA. Java References#JLS 05]\] 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"

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