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If a method modifies a static field, you must synchronize access to this field, even if the method is typically used only by a single thread. It is not possible for clients to perform external synchronization on such a method because there can be no guarantee that unrelated clients will do likewise.

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 \[[API 06|AA. Java References#API 06]\]:

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.

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-threadThis guideline also applies to classes that explicitly document their lack of thread-safety. Documented design intent is irrelevant when dealing with untrusted code because an attacker can always chose to ignore the documentation.

Noncompliant Code Example

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