Operations that may seem safe are not, when threads are present
// : c13:SerialNumberChecker.java
// Operations that may seem safe are not,
// when threads are present.
// From 'Thinking in Java, 3rd ed.' (c) Bruce Eckel 2002
// www.BruceEckel.com. See copyright notice in CopyRight.txt.
// Reuses storage so we don't run out of memory:
class CircularSet {
private int[] array;
private int len;
private int index = 0;
public CircularSet(int size) {
array = new int[size];
len = size;
// Initialize to a value not produced
// by the SerialNumberGenerator:
for (int i = 0; i < size; i++)
array[i] = -1;
}
public synchronized void add(int i) {
array[index] = i;
// Wrap index and write over old elements:
index = ++index % len;
}
public synchronized boolean contains(int val) {
for (int i = 0; i < len; i++)
if (array[i] == val)
return true;
return false;
}
}
public class SerialNumberChecker {
private static CircularSet serials = new CircularSet(1000);
static class SerialChecker extends Thread {
SerialChecker() {
start();
}
public void run() {
while (true) {
int serial = SerialNumberGenerator.nextSerialNumber();
if (serials.contains(serial)) {
System.out.println("Duplicate: " + serial);
System.exit(0);
}
serials.add(serial);
}
}
}
public static void main(String[] args) {
for (int i = 0; i < 10; i++)
new SerialChecker();
// Stop after 4 seconds:
new Timeout(4000, "No duplicates detected");
}
} ///:~
class SerialNumberGenerator {
private static volatile int serialNumber = 0;
public static int nextSerialNumber() {
return serialNumber++;
}
} ///:~
class Timeout extends java.util.Timer {
public Timeout(int delay, final String msg) {
super(true); // Daemon thread
schedule(new TimerTask() {
public void run() {
System.out.println(msg);
System.exit(0);
}
}, delay);
}
} ///:~
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