Page History

/

Protecting the fields of a class is unnecessary when it is designed for single-threaded use. Such classes are required to document their lack of thread-safety. For example, the Java Platform, Standard Edition 6 API Specification for the {{java.lang.StringBuilder}} class states \[[API 06|AA. Java References#API 06]\]:

{quote}
This class is designed for use as a drop-in replacement for {{StringBuffer}} in places where the string buffer was being used by a single thread (as is generally the case). Where possible, it is recommended that this class be used in preference to {{StringBuffer}} as it will be faster under most implementations.
{quote}

Multithreaded clients must synchronize accesses to classes whose documentation explicitly specifies the class is not thread-safe or fails to provide any assurances that the class is safe-thread.

Classes that use {{static}} fields that are both publicly accessible and mutable must always protect accesses to their fields, regardless of their documentation. This is because there is no guarantee that all clients will synchronize externally when accessing the field. Because a {{static}} field is shared by all clients, unrelated clients may violate the contract by not performing suitable locking.


h2. Noncompliant Code Example

This noncompliant code example does not synchronize access to the {{static}} field {{counter}}. 

{code:bgColor=#FFCCCC}
/** This class is not thread-safe! */
public final class CountHits {
  private static int counter;
  
  public void incrementCounter() {
    counter++;
  }
}
{code}

{mc} This class is not thread-safe because it violates [CON01

/ atomic]. {mc}
The class relies on clients to externally synchronize the object and this class documents its lack of thread-safety. This code is noncompliant, not because it violates [CON01-J. Ensure that compound operations on shared variables are atomic], and not because it documents its lack of thread-safety, but rather because it has an accessible mutable static field {{counter}}.  Untrusted code is free to modify {{CountHits.counter}} with no respect for this class's documented lack of thread-safety.


h2. Compliant Solution

This compliant solution uses a static private final lock to protect the {{counter}} field and consequently, does not depend on any external synchronization. This is recommended by [CON04-J. Use private final lock objects to synchronize classes that may interact with untrusted code].

{code:bgColor=#ccccff}
/** This class is thread-safe */
public final class CountHits {
  private static int counter;

  private static final Object lock = new Object();

  public void incrementCounter() {
    synchronized (lock) {
      counter++;
    }
  }
}
{code}


h2. Risk Assessment

Failing to protect classes containing accessible static members can result in unexpected results when a client fails to obey the classes' synchronization policy.

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON32- 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+CON32-J].

h2. References

\[[API 06|AA. Java References#API 06]\] 
\[[Bloch 08|AA. Java References#Bloch 08]\] Item 67: "Avoid excessive synchronization"

h2. Issue Tracking

{tasklist:Review List}
||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||
|T|M|F|1270153006582|1270216402886|svoboda|and specifies its lack of thread-safety in the documentation. => rephrase to : and documents its lack of thread-safety.  <---- FIXED ~DS|
|T|M|F|1270152984383|1270219726077|rcs|protecting fields is a weak word. Should be changed to synchronizing access to fields.|
|F|M|F|1270152995003|          | |and not because it documents its lack of thread-safety; suggest removing this|
|F|M|F|1270153016807|          | |The title can also be Ensure classes containing publicly accessible mutable static fields are thread-safe|
|F|M|F|1270215734760|          |svoboda|"Classes that use {{static}} fields that are both publicly accessible and mutable must always protect accesses to their fields, *regardless of their documentation.*" => A class that is internally providing synchronization would not document that it is not thread-safe. So I don't understand why you added the words in bold.|
|F|M|F|1270215935982|          |svoboda|"The class relies on clients to externally synchronize the object and *this class documents* its lack of thread-safety" => "this class" occurs twice in the sentence. Should be "and documents its lack of thread safety".|
{tasklist}



----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|VOID CON06-J. Do not defer a thread that is holding a lock]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON08-J. Do not call alien methods that synchronize on the same objects as any callers in the execution chain]