THREADS AND LOCKS

Example: Out of Order Writes

17.11

17.11   Example: Out of Order Writes

This example is similar to that in the preceding section, except that one method

assigns to both variables and the other method reads both variables. Consider a

class that has class variables

a

 and

b

 and methods

to

 and

fro

:

class Simple {

int a = 1, b = 2;

void to() {

a = 3;

b = 4;

}

void fro() {

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

}

}

Now suppose that two threads are created, and that one thread calls

to

 while the

other thread calls

fro

. What is the required set of actions and what are the order 

ing constraints?

Let us consider the thread that calls

to

. According to the rules, this thread

must perform an

assign

 of

a

 followed by an

assign

 of

b

. That is the bare minimum

required to execute a call to the method

to

. Because there is no synchronization, it

is at the option of the implementation whether or not to

store

 the assigned values

back to main memory! Therefore the thread that calls

fro

 may obtain either

1

 or

3

for the value of

a

, and independently may obtain either

2

 or

4

 for the value of

b

.

Now suppose that

to

 is

synchronized

 but

fro

 is not:

class SynchSimple {

int a = 1, b = 2;

synchronized void to() {

a = 3;

b = 4;

}

void fro() {

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

}

}

In this case the method

to

 will be forced to

store

 the assigned values back to main

memory before the

unlock

 action at the end of the method. The method

fro

 must,

of course, use

a

 and

b

 (in that order) and so must

load

 values for

a

 and

b

 from

main memory.

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