17.14

Wait Sets and Notification

THREADS AND LOCKS

The

synchronized

 statement ( 14.17) computes a reference to an object; it

then attempts to perform a

lock

 action on that object and does not proceed further

until the

lock

 action has successfully completed. (A

lock

 action may be delayed

because the rules about locks can prevent the main memory from participating

until some other thread is ready to perform one or more

unlock

 actions.) After the

lock action has been performed, the body of the

synchronized

 statement is exe 

cuted. If execution of the body is ever completed, either normally or abruptly, an

unlock

 action is automatically performed on that same lock.

A

synchronized

 method ( 8.4.3.5) automatically performs a

lock

 action

when it is invoked; its body is not executed until the

lock

 action has successfully

completed. If the method is an instance method, it locks the lock associated with

the instance for which it was invoked (that is, the object that will be known as

this

 during execution of the body of the method). If the method is

static

, it

locks the lock associated with the

Class

 object that represents the class in which

the method is defined. If execution of the method's body is ever completed, either

normally or abruptly, an

unlock

 action is automatically performed on that same

lock.

Best practice is that if a variable is ever to be assigned by one thread and used

or assigned by another, then all accesses to that variable should be enclosed in

synchronized

 methods or

synchronized

 statements.

Java does not prevent, nor require detection of, deadlock conditions. Programs

where threads hold (directly or indirectly) locks on multiple objects should use

conventional techniques for deadlock avoidance, creating higher level locking

primitives that don't deadlock, if necessary.

17.14   Wait Sets and Notification

Every object, in addition to having an associated lock, has an associated

wait set

,

which is a set of threads. When an object is first created, its wait set is empty.

Wait sets are used by the methods

wait

 ( 20.1.6,  20.1.7,  20.1.8),

notify

( 20.1.9), and

notifyAll

 ( 20.1.10) of class

Object

. These methods also inter 

act with the scheduling mechanism for threads ( 20.20).

The method

wait

 should be called for an object only when the current thread

(call it

T

) has already locked the object's lock. Suppose that thread

T

 has in fact

performed

N lock

 actions that have not been matched by

unlock

 actions. The

wait

method then adds the current thread to the wait set for the object, disables the cur 

rent thread for thread scheduling purposes, and performs

N unlock

 actions to relin 

quish the lock. The thread

T

 then lies dormant until one of three things happens:

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