Programming Tutorials

Transient vs Volatile modifiers in Java

By: Reema sen in Java Tutorials on 2007-09-10  

Java defines two interesting type modifiers: transient and volatile. These modifiers are used to handle somewhat specialized situations.

When an instance variable is declared as transient, then its value need not persist when an object is stored. For example:

class T {
  transient int a; // will not persist
  int b; // will persist
}

Here, if an object of type T is written to a persistent storage area, the contents of a would not be saved, but the contents of b would. The volatile modifier tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of your program. One of these situations involves multithreaded programs. In a multithreaded program, sometimes, two or more threads share the same instance variable. For efficiency considerations, each thread can keep its own, private copy of such a shared variable. The real (or master) copy of the variable is updated at various times, such as when a synchronized method is entered. While this approach works fine, it may be inefficient at times. In some cases, all that really matters is that the master copy of a variable always reflects its current state. To ensure this, simply specify the variable as volatile, which tells the compiler that it must always use the master copy of a volatile variable (or, at least, always keep any private copies up to date with the master copy, and vice versa). Also, accesses to the master variable must be executed in the precise order in which they are executed on any private copy.

Note: volatile in Java has, more or less, the same meaning that it has in C/C++.

This is an extract from the book 'Java Complete Reference'. For more details please refer to the book