Java Threads, Second Edition

Thread-Unsafe Classes

In a perfect world, we would not have to write this section: in that world, every class that you used would be correctly synchronized for use by multiple threads running simultaneously, and you would be free from considering synchronization issues whenever you used someone else’s Java classes.

Welcome to the real world. In this world, there are often times when you need to use classes that are thread unsafe—classes that lack the correct synchronization to be used by multiple threads. Just because we acknowledge that these circumstances exist does not mean that you are absolved from producing thread-safe classes in your own work: we urge you to make this a better world and correctly synchronize all of your own classes.

In this section, we’ll examine two techniques that allow you to deal with classes that are not thread safe.

Explicit Synchronization

Since its inception, Java has had certain classes that are collection classes: the Hashtable class, the Vector class, and others provide aggregates of objects. These classes all have the advantage that they are thread safe: their methods contain the necessary synchronization such that two threads that simultaneously insert objects into a vector, for example, will do so without corrupting the internal state of the vector.

Java 2 formalized the notion of a collection by introducing a number of collection classes; these are classes that implement either the Collection or the Map interface. There are a number of these ...