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The usual way of initializing a sub-object is to use a constructor. Sometimes it is required to limit the number of instances of the sub-object to just one (this is similar to a singleton, however, the sub-object may or may not be {{static}}). In addition, a technique called lazy initialization is used to defer the construction of the sub-object until it is actually required.

For these purposes, instead of a constructor, a class or an instance method should be used for initialization, depending on whether the sub-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:

{code:bgColor=#ccccff}
// 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;
  }
  // ...
}
{code}

In a multi-threading scenario, the initialization must be synchronized so that two or more threads do not create multiple instances of the sub-object. The code shown below is correctly synchronized, albeit slower than the previous, single threaded code example. 

{code:bgColor=#ccccff}
// Correct multithreaded version using synchronization
class Foo { 
  private Helper helper = null;
  
  public synchronized Helper getHelper() {
    if (helper == null) {
      helper = new Helper();
    }
    return helper;
  }
  // Other functions and members...
}
{code}

The double checked locking (DCL) idiom is sometimes used to provide lazy initialization in multithreaded code. In a multi-threading scenario, lazy initialization is supplemented by reducing the cost of synchronization on each method access by limiting the synchronization to the case where the instance is to be created and forgoing it when retrieving an already created instance.

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

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

{quote}
... 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. 
{quote}

This makes the originally proposed double-checked locking pattern insecure. 

h2. Noncompliant Code Example

This noncompliant code example uses the incorrect form of the double checked locking idiom. 

{code:bgColor=#FFCCCC}
// "Double-Checked Locking" idiom
class Foo { 
  private

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

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

// 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...
}

// 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();
        }
      }
    }
    return helper;
  }
  // other functions and members...
}
{code}

h2. Compliant Solution (volatile)

This compliant solution declares the {{Helper}} object as {{volatile}}.

{code:bgColor=#ccccff}
// Works with acquire/release semantics for volatile
// Broken under JDK 1.4 and earlier
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;
  }
}

final class Foo {
  // Lazy initialization
  private static class Holder {
    static Helper helper    helper = new Helper(); // If the helper is null, create a new instance
        }

  public static Helper getInstance() }{
    }
    return Holder.helper;
 // If helper is non-null, return its instance
  }
}
{code}

JDK 5.0 allows a write of a {{volatile}} variable to be reordered with respect to a previous read or write. A read of a {{volatile}} variable cannot be reordered with respect to any following read or write. Because of this, the double checked locking idiom can work when {{helper}} is declared {{volatile}}. If a thread initializes the {{Helper}} object, a happens-before relationship is established between this thread and another that retrieves and returns the instance. \[[Pugh 04|AA. Java References#Pugh 04]\] and \[[Manson 04|AA. Java References#Manson 04]\] 

h2. Compliant Solution (static eager initialization)

This compliant solution eagerly initializes the {{helper}} in the declaration of the {{static}} variable \[[Pugh 04|AA. Java References#Pugh 04]\] (sic). 

{mc}
\[[Pugh 04|AA. Java References#Pugh 04]\] lists this as lazy initialization but it is actually eager initialization and so the (sic).
{mc}

{code:bgColor=#ccccff}
class Foo {
  static final Helper helper = new Helper();

  private Helper() {
    // ...
  }

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

Variables declared {{static}} are guaranteed to be initialized and made visible to other threads immediately. Static initializers also exhibit these properties.

h2. Compliant Solution (static lazy initialization)

This compliant solution incorporates lazy initialization which makes it more productive. It also uses a {{static}} variable as suggested in the previous compliant solution. The variable is declared within a {{static}} inner, {{Holder}} class. 

{code:bgColor=#ccccff}
class Foo {
  // Lazy initialization 
  private static class Holder {
    static Helper helper = new Helper();
  }

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

This idiom is called the initialize-on-demand holder class idiom. Initialization of the {{Holder}} class is deferred until the {{getInstance()}} method is called, following which the {{helper}} is initialized. The only limitation of this method is that it works only for {{static}} fields and not instance fields. \[[Bloch 01|AA. Java References#Bloch 01]\]

h2. Exceptions

*EX1:* Explicitly synchronized code (that uses method synchronization) does not require the use of double-checked locking.

*EX2:* "Although 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." \[[Pugh 04|AA. Java References#Pugh 04]\]

h2. Risk Assessment

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

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON22- J | low | probable | medium | {color:green}{*}P4{*}{color} | {color:green}{*}L3{*}{color} |

h3. Automated Detection

TODO

h3. Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON43-J].

h2. References

\[[API 06|AA. Java References#API 06]\] 
\[[Pugh 04|AA. Java References#Pugh 04]\]
\[[Bloch 01|AA. Java References#Bloch 01]\] Item 48: "Synchronize access to shared mutable data"
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"

----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON21-J. Facilitate thread reuse by using Thread Pools]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON23-J. Address the shortcomings of the Singleton design pattern]

 }
}

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);
  }
}

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;                   // Third read of helper
  }
}

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;
  }
}