The Java Language Specification, Third Edition

17.4.1 Shared Variables THREADS AND LOCKS
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Each time the evaluation of thread t generates an inter-thread action, it must
match the inter-thread action a of t that comes next in program order. If a is a read,
then further evaluation of t uses the value seen by a as determined by the memory
model.
This section provides the specification of the Java programming language
memory model except for issues dealing with final fields, which are described in
§17.5.
17.4.1 Shared Variables
Memory that can be shared between threads is called shared memory or heap
memory.
All instance fields, static fields and array elements are stored in heap memory.
In this chapter, we use the term variable to refer to both fields and array elements.
Local variables (§14.4), formal method parameters (§8.4.1) or exception handler
parameters are never shared between threads and are unaffected by the memory
model.
Two accesses to (reads of or writes to) the same variable are said to be conflicting
if at least one of the accesses is a write.
17.4.2 Actions
An inter-thread action is an action performed by one thread that can be
detected or directly influenced by another thread. There are several kinds of interthread
action that a program may perform:
• Read (normal, or non-volatile). Reading a variable.
• Write (normal, or non-volatile). Writing a variable.
• Synchronization actions, which are:
DRAFT
◆ Volatile read. A volatile read of a variable.
◆ Volatile write. A volatile write of a variable.
◆ Lock. Locking a monitor
◆ Unlock. Unlocking a monitor.
◆ The (synthetic) first and last action of a thread
◆ Actions that start a thread or detect that a thread has terminated, as
described in §17.4.4.