Java How to Program Fourth Edition - DCC

854 Multithreading Chapter 15
sumer. Also, notice that the consumer twice retrieved value –1 (the default value of
sharedInt set at line 5 of Fig. 15.6) before the producer ever assigned 1 to the
sharedInt variable. This example clearly demonstrates that access to shared data by
concurrent threads must be controlled carefully or a program may produce incorrect results.
To solve the problems of lost data and doubled data in the previous example, we will
synchronize access of the concurrent producer and consumer threads to the shared data.
Each method used by a producer or consumer to access the shared data is declared with the
synchronized keyword. When a method declared synchronized is running in an
object, the object is locked so no other synchronized method can run in that object at
the same time.
15.7 Producer/Consumer Relationship with Thread
Synchronization
The application in Fig. 15.8 demonstrates a producer and a consumer accessing a shared
cell of memory with synchronization so that the consumer only consumes after the producer
produces a value. Classes ProduceInteger (Fig. 15.8), ConsumeInteger
(Fig. 15.9) and SharedCell (Fig. 15.11) are identical to Fig. 15.4, Fig. 15.5 and
Fig. 15.7 except that they use the new class HoldIntegerSynchronized in this example.
Class HoldIntegerSynchronized (Fig. 15.10) contains two instance variables—sharedInt
(line 6) and writeable (line 7). Also, method setSharedInt
(lines 12–39) and method getSharedInt (lines 44–70) are now synchronized
methods. Objects of class HoldIntegerSynchronized have monitors, because
HoldIntegerSynchronized contains synchronized methods. Instance variable
writeable is known as the monitor’s condition variable—is a boolean used by
methods setSharedInt and getSharedInt of class HoldIntegerSynchronized.
If writeable is true, setSharedInt can place a value into variable
sharedInt, because the variable currently does not contain information. However, this
means getSharedInt currently cannot read the value of sharedInt. If writeable
is false, getSharedInt can read a value from variable sharedInt because the variable
currently does contain information. However, this means setSharedInt currently
cannot place a value into sharedInt.
1 // Fig. 15.8: ProduceInteger.java
2 // Definition of threaded class ProduceInteger
3 public class ProduceInteger extends Thread {
4 private HoldIntegerSynchronized sharedObject;
5
6 // initialize ProduceInteger thread object
7 public ProduceInteger( HoldIntegerSynchronized shared )
8 {
9 super( "ProduceInteger" );
10 sharedObject = shared;
11 }
12
Fig. Fig. Fig. 15.8 15.8 15.8 Class ProduceInteger represents the producer in a producer/
consumer relationship (part 1 of 2).