Java Performance Tuning

Atomic Access and Assignment

Variables shared between multiple threads (e.g., instance variables of objects) have atomic assignment guaranteed by the Java language specification for all data types except for longs and doubles. Actually, the storing of a value into a variable takes two primitive operations, a store and a write. However, the language specification also states that once a store operation occurs on a particular variable, no other store operation is allowed on that variable until the write operation has occurred. The specification allows longs and doubles to be stored in two separate sets of store+write operations, hence their exception to atomicity. A similar atomic specification applies for reading variables.

This means that access and update of variables are automatically synchronized (as long as they are not longs or doubles). If a method consists solely of a variable access or assignment, there is no need to make it synchronized for thread safety, and every reason not to do so for performance. Thread safety extends further to any set of statements that are accessing or assigning to a variable independently of any other variable values. The exclusion here precludes setting a variable that depends on the value of another variable as being thread-safe; this would be two separate operations, which is inherently not thread-safe. For example:

public void setMe(Object o) {me = o;}
public Object getMe( ) {return me;}

are thread-safe methods, with no need for synchronized ...