== Why is this an issue?

Double-checked locking can be used for lazy initialization of ``++volatile++`` fields, but only if field assignment is the last step in the ``++synchronized++`` block. Otherwise you run the risk of threads accessing a half-initialized object.


=== Noncompliant code example

[source,java]
----
public class MyClass {

  private volatile List<String> strings;

  public List<String> getStrings() {
    if (strings == null) {  // check#1
      synchronized(MyClass.class) {
        if (strings == null) {
          strings = new ArrayList<>();  // Noncompliant
          strings.add("Hello");  //When threadA gets here, threadB can skip the synchronized block because check#1 is false
          strings.add("World");
        }
      }
    }
    return strings;
  }
}
----


=== Compliant solution

[source,java]
----
public class MyClass {

  private volatile List<String> strings;

  public List<String> getStrings() {
    if (strings == null) {  // check#1
      synchronized(MyClass.class) {
        if (strings == null) {
          List<String> tmpList = new ArrayList<>();
          tmpList.add("Hello");
          tmpList.add("World");
          strings = tmpList;
        }
      }
    }
    return strings;
  }
}
----


== Resources

* https://wiki.sei.cmu.edu/confluence/x/6zdGBQ[CERT, LCK10-J.] - Use a correct form of the double-checked locking idiom

=== Related rules

* S2168 - Double-checked locking should not be used


ifdef::env-github,rspecator-view[]

'''
== Implementation Specification
(visible only on this page)

=== Message

Fully initialize this new "xxx" before assigning it to the field.


'''
== Comments And Links
(visible only on this page)

=== on 15 Jun 2015, 18:07:41 Ann Campbell wrote:
\[~tamas.vajk] I've targeted this to C# because I think it might be relevant.

=== on 17 Aug 2015, 06:56:13 Tamas Vajk wrote:
\[~ann.campbell.2] This seems very specific (to Java), I wouldn't target C#. There might be minor differences in the Java and C# memory model when using ``++volatile++``, which I'm not aware of.

=== on 17 Aug 2015, 16:08:28 Ann Campbell wrote:
Thanks for the review [~tamas.vajk]. Un-targeted.

endif::env-github,rspecator-view[]
