17.2 Execution Order

The rules of execution order constrain the order in which certain events may occur. There are four general constraints on the relationships among actions:

• The actions performed by any one thread are totally ordered; that is, for any two actions performed by a thread, one action precedes the other.
• The actions performed by the main memory for any one variable are totally ordered; that is, for any two actions performed by the main memory on the same variable, one action precedes the other.
• The actions performed by the main memory for any one lock are totally ordered; that is, for any two actions performed by the main memory on the same lock, one action precedes the other.
• It is not permitted for an action to follow itself.

The last rule may seem trivial, but it does need to be stated separately and explicitly for completeness. Without it, it would be possible to propose a set of actions by two or more threads and precedence relationships among the actions that would satisfy all the other rules but would require an action to follow itself.

Threads do not interact directly; they communicate only through the shared main memory. The relationships between the actions of a thread and the actions of main memory are constrained in three ways:

• Each lock or unlock action is performed jointly by some thread and the main memory.
• Each load action by a thread is uniquely paired with a read action by the main memory such that the load action follows the read action.
• Each store action by a thread is uniquely paired with a write action by the main memory such that the write action follows the store action.

Most of the rules in the following sections further constrain the order in which certain actions take place. A rule may state that one action must precede or follow some other action. Note that this relationship is transitive: if action A must precede action B, and B must precede C, then A must precede C. The programmer must remember that these rules are the only constraints on the ordering of actions; if no rule or combination of rules implies that action A must precede action B, then a Java implementation is free to perform action B before action A, or to perform action B concurrently with action A. This freedom can be the key to good performance. Conversely, an implementation is not required to take advantage of all the freedoms given it.

In the rules that follow, the phrasing "B must intervene between A and C" means that action B must follow action A and precede action C.