17.3

Rules about Variables

THREADS AND LOCKS

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 perfor 

mance. 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

.

17.3   Rules about Variables

Let

T

 be a thread and

V

 be a variable. There are certain constraints on the actions

performed by

T

 with respect to

V

:

An

use

 or

assign

 by

T

 of

V

 is permitted only when dictated by execution by

T

of the Java program according to the standard Java execution model. For

example, an occurrence of

V

 as an operand of the

+

 operator requires that a

single

use

 action occur on

V

; an occurrence of

V

 as the left hand operand of

the assignment operator

=

 requires that a single

assign

 action occur. All

use

and

assign

 actions by a given thread must occur in the order specified by the

program being executed by the thread. If the following rules forbid

T

 to per 

form a required

use

 as its next action, it may be necessary for

T

 to perform a

load

first in order to make progress.

A

store

 action by

T

 on

V

 must intervene between an

assign

 by

T

 of

V

 and a

subsequent

load

 by

T

 of

V

. (Less formally: a thread is not permitted to lose its

most recent assign.)

An

assign

 action by

T

 on

V

 must intervene between a

load

 or

store

 by

T

 of

V

and a subsequent

store

 by

T

 of

V

. (Less formally: a thread is not permitted to

write data from its working memory back to main memory for no reason.)

After a thread is created, it must perform an

assign

 or

load

 action on a vari 

able before performing a

use

 or

store

 action on that variable. (Less formally: a

new thread starts with an empty working memory.)

404