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The code shown below is correctly synchronized, albeit slower. The double-checked locking pattern strives to make it faster.

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

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}
Please check: \[[Pugh 04|AA. Java References#Pugh 04]\]

 lists this as lazy initialization but it seems more like eager initialization.
{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}


h2. Exceptions

*EX1:* Explicitly synchronized code 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

\[[API 06|AA. Java References#API 06]\] 
\[[Pugh 04|AA. Java References#Pugh 04]\]
\[[MITRE 09|AA. Java References#MITRE 09]\] [CWE ID 609|http://cwe.mitre.org/data/definitions/609.html] "Double-Checked Locking"

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[!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]