Characteristics of Thread-Safe Programs

1. It is usually easier to design thread safety at an early design stage than to modify existing code.

2. A program consisting of thread-safe classes may not be thread-safe and a thread-safe program may contain some classes that are not thread-safe. (jcip p.15)

3. We do not yet have a formal definition of thread safety in general and we do not have formal techniques for stating a complete definition of thread safety for a given class. This poor state of affairs in the world of so-called computer science is mainly due to the lack of a technique for formal and complete definitions of program correctness. If any such techniques exist, they are not in common use, at least not in commercial software. Techniques that I studied for this purpose, in the 1980's for example, where often more complex than the code that they were describing.

4. In theory, a correct class means that the class conforms to its specification (what it is supposed to do). And frequently, classes specifications are vague (i.e., informal and incomplete) and single-threaded correctness is often assumed when no incorrect behavior is observed. In many cases, the source code of the class is the specification. In the best cases, the correctness of a class is defined by associated testing classes and test cases (data sets for testing) that define the input and matching output of each of the methods of the class. The validity of correctness-by-testing is dependent on the correctness of the testing classes and data sets. This is like a house of cards.

5. The definition of thread-safe class from the JCiP book: A class is thread-safe when it behaves correctly for a single thread and it continues to behave correctly when used by multiple threads, and with no additional synchronization or other coordination on the part of the calling code.

6. My current definition of a thread-safe class: A class is deemed thread-safe when it complies with the Mandatory Basic Rules in this blog.