17.1

Terminology and Framework

THREADS AND LOCKS

memory on behalf of any one thread are performed in the same order as the corre 

sponding actions by that thread. (This is explained in greater detail below.)

A single Java thread issues a stream of

use

,

assign

,

lock

, and

unlock

 actions as

dictated by the semantics of the Java program it is executing. The underlying Java

implementation is then required additionally to perform appropriate

load

,

store

,

read

, and

write

 actions so as to obey a certain set of constraints, explained below.

If the Java implementation correctly follows these rules and the Java application

programmer follows certain other rules of programming, then data can be reliably

transferred between threads through shared variables. The rules are designed to be

 tight  enough to make this possible but  loose  enough to allow hardware and

software designers considerable freedom to improve speed and throughput

through such mechanisms as registers, queues, and caches.

Here are the detailed definitions of each of the actions:

A

use

 action (by a thread) transfers the contents of the thread's working copy

of a variable to the thread's execution engine. This action is performed when 

ever a thread executes a virtual machine instruction that uses the value of a

variable.

An

assign

 action (by a thread) transfers a value from the thread's execution

engine into the thread s working copy of a variable. This action is performed

whenever a thread executes a virtual machine instruction that assigns to a

variable.

A

read

 action (by the main memory) transmits the contents of the master copy

of a variable to a thread's working memory for use by a later

load

 action.

A

load

action (by a thread) puts a value transmitted from main memory by a

read

 action into the thread s working copy of a variable.

A

store

action (by a thread) transmits the contents of the thread's working

copy of a variable to main memory for use by a later

write

 action.

A

write

 action (by the main memory) puts a value transmitted from the

thread's working memory by a

store

 action into the master copy of a variable

in main memory.

A

lock

 action (by a thread tightly synchronized with main memory) causes a

thread to acquire one claim on a particular lock.

An

unlock

 action (by a thread tightly synchronized with main memory) causes

a thread to release one claim on a particular lock.

402