CLASSES

Field Modifiers

8.3.1

class Point {

int x, y;

transient float rho, theta;

}

were saved to persistent storage by a system service, then only the fields

x

 and

y

would be saved. This specification does not yet specify details of such services;

we intend to provide them in a future version of this specification.

8.3.1.4

volatile

Fields

As described in  17, the Java language allows threads that access shared variables

to keep private working copies of the variables; this allows a more efficient imple 

mentation of multiple threads. These working copies need be reconciled with the

master copies in the shared main memory only at prescribed synchronization

points, namely when objects are locked or unlocked. As a rule, to ensure that

shared variables are consistently and reliably updated, a thread should ensure that

it has exclusive use of such variables by obtaining a lock that, conventionally,

enforces mutual exclusion for those shared variables.

Java provides a second mechanism that is more convenient for some purposes:

a field may be declared

volatile

, in which case a thread must reconcile its work 

ing copy of the field with the master copy every time it accesses the variable.

Moreover, operations on the master copies of one or more volatile variables on

behalf of a thread are performed by the main memory in exactly the order that the

thread requested.

If, in the following example, one thread repeatedly calls the method

one

 (but

no more than

Integer.MAX_VALUE

 ( 20.7.2) times in all), and another thread

repeatedly calls the method

two

:

class Test {

static int i = 0, j = 0;

static void one() { i++; j++; }

static void two() {

System.out.println("i=" + i + " j=" + j);

}

}

then method

two

 could occasionally print a value for

j

 that is greater than the

value of

i

, because the example includes no synchronization and, under the rules

explained in  17, the shared values of

i

 and

j

 might be updated out of order.

One way to prevent this out or order behavior would be to declare methods

one

 and

two

 to be

synchronized

 ( 8.4.3.5):

147