Categories: Java

Some links to information about thread-safety in Java…

This tutorial provides an excellent overview:

• The Official Java Tutorials: Concurrency

This article presents a lot more detail on the behaviour of the volatile keyword, how it can be safely used - and when not:

• Brian Goetz: Managing Volatility

Note that the “pattern 2” example works because (since Java 1.5) a write to a volatile variable ensures that all previous writes made by the same thread to any variable are also made visible to other threads. A write to a volatile variable is a “memory barrier” that effectively flushes cached data to memory and notifies all other CPU caches that those locations have been updated.

This article does not mention the Atomic* types from the Java standard library, which provide some limited forms of atomic read/upate/write on a single variable - ie fit somewhere between synchronized and volatile. These are discussed in the official java tutorial referenced above.

Some other references:

• Javamex: The Volatile Keyword

• Stack Overflow: The Volatile Keyword

• Linux Kernel: Memory Barriers - information on exactly what a “memory barrier” is, for various CPU architectures.