THREADS AND LOCKS

Locks and Synchronization

17.13

Finally, suppose that

to

 and

fro

 are both

synchronized

:

class SynchSynchSimple {

int a = 1, b = 2;

synchronized void to() {

a = 3;

b = 4;

}

synchronized void fro() {

System.out.println("a= " + a + ", b=" + b);

}

}

In this case, the actions of method

fro

 cannot be interleaved with the actions

of method

to

, and so

fro

 will print either  

a=1, b=2

  or  

a=3, b=4

 .

17.12   Threads

They plant dead trees for living, and the dead

They string together with a living thread . . .

But in no hush they string it . . . With a laugh, . . .

They bring the telephone and telegraph.

 Robert Frost,

The Line gang

 (1920)

Threads are created and managed by the built in classes

Thread

 ( 20.20) and

ThreadGroup

 ( 20.21). Creating a

Thread

 object creates a thread and that is the

only way to create a thread. When the thread is created, it is not yet active; it

begins to run when its

start

 method ( 20.20.14) is called.

Every thread has a

priority

. When there is competition for processing

resources, threads with higher priority are generally executed in preference to

threads with lower priority. Such preference is not, however, a guarantee that the

highest priority thread will always be running, and thread priorities cannot be used

to reliably implement mutual exclusion.

17.13   Locks and Synchronization

There is a lock associated with every object. The Java language does not provide a

way to perform separate

lock

 and

unlock

 actions; instead, they are implicitly per 

formed by high level constructs that arrange always to pair such actions correctly.

(We note, however, that the Java Virtual Machine provides separate

monitorenter

and

monitorexit

 instructions that implement the

lock

 and

unlock

 actions.)

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