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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, 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(

Instead of initializing a member object using a constructor, sometimes a technique called lazy initialization is 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 \[[Bloch 05|AA. Java References#Bloch 05]\].

A class or an instance method is used for lazy initialization, depending on whether the member object is static or not. The method checks whether the instance has already been created and if not, creates it. If the instance already exists, it simply returns it. This is shown below:

// 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 multithreading scenario, the initialization must be synchronized so that two or more threads do not create multiple instances of the member object. The code shown below is safe for execution in a multithreaded environment, albeit slower than the previous, single threaded code example.

// 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 (DCL) idiom is used to provide lazy initialization in multithreaded code. In a multithreading scenario, traditional lazy initialization is supplemented by reducing the cost of synchronization for each method access by limiting the synchronization to the case where the instance is required to be created and forgoing it when retrieving an already created instance.

The double-checked locking pattern uses block synchronization instead of method synchronization. It strives to make the previous code example faster by installing a null check before attempting to synchronize. This makes expensive synchronization necessary only for initialization, and dispensable for the common case of retrieving the value of helper. The noncompliant code example shows the originally proposed DCL pattern.

Noncompliant Code Example

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 the Java Memory Model (discussion reference) \[[Pugh 04|AA. Java References#Pugh 04]\]:

... 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 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 (volatile)

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;
  }
  // ...
}

Noncompliant Code Example

The double-checked locking pattern uses block synchronization rather than method synchronization and installs an additional null reference check before attempting synchronization. This noncompliant code example uses an incorrect 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
// BrokenDouble-checked under JDK 1.4 and earlierlocking idiom
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 \[[Manson 04|AA. Java References#Manson 04]\] 

"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." \[[Bloch 08|AA. Java References#Bloch 08]\].  

Compliant Solution (static initialization)

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

 }

  // 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.

This code also violates 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

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. Static initializers also exhibit these properties. 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 \[[JLS 05|AA. Java References#JLS 05]\].

...

new Helper();
        }
      }
    }
    return helper;
  }
}

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 . It uses by declaring a static variable as suggested in the previous compliant solution. The variable is declared within a static Holder 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 {{Holder}} class is deferred until the {{the static helper field is deferred until the getInstance()}} method is called, following which the {{helper}} field is initialized. The only limitation of this method is that it works only for {{static}} fields and not for instance fields \[[Bloch 01|AA. Java References#Bloch 01]\]. This idiom is a better choice than the double checked locking idiom for lazily initializing {{static}} fields \[[Bloch 08|AA. Java References#Bloch 08]\].

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.

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]) uses a ThreadLocal object to 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().

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 noncompliant code example, the Helper class is made immutable by declaring its fields final. The JMM guarantees that immutable objects are fully constructed before they 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

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

  public Helper getHelper() {
    if (perThreadInstance.get() == null) {
      createHelper();
    }
    return helper;
  }
        
  private final void createHelper() {
    synchronized(this) {
      if (helper == null) {
    this.n = n;
  helper = new Helper();}
 
  // Other fields and }
methods, all fields are final
}
 
final //class Any non-null value would do as the argument here
      perThreadInstance.set(perThreadInstance);
    }
  }
}

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 be inappropriate unless a weak reference is used to hold Helper.

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.

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;                   // Third read of helper
  }
}

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

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 (helperh == null) {
          helperh = new Helper(42);
 //   If the helper is null, create ahelper new= instanceh;
        }
      }
    }
    return helper; // If}
 helper is non-null, return its instanceh;
  }
}

Note that if class Foo were mutable, the Helper field would need to be declared volatile as shown in CON09-J. Ensure visibility of shared references to immutable objects. Also, the method getHelper() 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, cannot be observed to be in a partially initialized state.

Exceptions

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

*EX2:* "Although the \[noncompliant form of the\] double-checked locking idiom cannot be used for references to objects, it can work for 32-bit primitive values (e.g., int's or float's). Note that it does not work for long's or double's, since unsynchronized reads/writes of 64-bit primitives are not guaranteed to be atomic." \[[Pugh 04|AA. Java References#Pugh 04]\]. 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 issues.

Automated Detection

TODO

Related Vulnerabilities

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

References

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

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).

Risk Assessment

Using incorrect forms of the double-checked locking idiom can lead to synchronization problems and can expose partially initialized objects.

Automated Detection

Related Guidelines

Bibliography

...

Image Added Image Added Image AddedCON21-J. Use thread pools to enable graceful degradation of service during traffic bursts      11. Concurrency (CON)      CON23-J. Address the shortcomings of the Singleton design pattern