Transition Detector
public class TransitionDetectorMain extends Object {
private static Thread startTrueWaiter(final TransitionDetector td,
String name) {
Runnable r = new Runnable() {
public void run() {
try {
while (true) {
print("about to wait for false-to-"
+ "true transition, td=" + td);
td.waitForFalseToTrueTransition();
print("just noticed for false-to-"
+ "true transition, td=" + td);
}
} catch (InterruptedException ix) {
return;
}
}
};
Thread t = new Thread(r, name);
t.start();
return t;
}
private static Thread startFalseWaiter(final TransitionDetector td,
String name) {
Runnable r = new Runnable() {
public void run() {
try {
while (true) {
print("about to wait for true-to-"
+ "false transition, td=" + td);
td.waitForTrueToFalseTransition();
print("just noticed for true-to-"
+ "false transition, td=" + td);
}
} catch (InterruptedException ix) {
return;
}
}
};
Thread t = new Thread(r, name);
t.start();
return t;
}
private static void print(String msg) {
String name = Thread.currentThread().getName();
System.err.println(name + ": " + msg);
}
public static void main(String[] args) {
try {
TransitionDetector td = new TransitionDetector(false);
Thread threadA = startTrueWaiter(td, "threadA");
Thread threadB = startFalseWaiter(td, "threadB");
Thread.sleep(200);
print("td=" + td + ", about to set to 'false'");
td.setValue(false);
Thread.sleep(200);
print("td=" + td + ", about to set to 'true'");
td.setValue(true);
Thread.sleep(200);
print("td=" + td + ", about to pulse value");
td.pulseValue();
Thread.sleep(200);
threadA.interrupt();
threadB.interrupt();
} catch (InterruptedException x) {
x.printStackTrace();
}
}
}
class TransitionDetector extends Object {
private boolean value;
private Object valueLock;
private Object falseToTrueLock;
private Object trueToFalseLock;
public TransitionDetector(boolean initialValue) {
value = initialValue;
valueLock = new Object();
falseToTrueLock = new Object();
trueToFalseLock = new Object();
}
public void setValue(boolean newValue) {
synchronized (valueLock) {
if (newValue != value) {
value = newValue;
if (value) {
notifyFalseToTrueWaiters();
} else {
notifyTrueToFalseWaiters();
}
}
}
}
public void pulseValue() {
// Sync on valueLock to be sure that no other threads
// get into setValue() between these two setValue()
// calls.
synchronized (valueLock) {
setValue(!value);
setValue(!value);
}
}
public boolean isTrue() {
synchronized (valueLock) {
return value;
}
}
public void waitForFalseToTrueTransition() throws InterruptedException {
synchronized (falseToTrueLock) {
falseToTrueLock.wait();
}
}
private void notifyFalseToTrueWaiters() {
synchronized (falseToTrueLock) {
falseToTrueLock.notifyAll();
}
}
public void waitForTrueToFalseTransition() throws InterruptedException {
synchronized (trueToFalseLock) {
trueToFalseLock.wait();
}
}
private void notifyTrueToFalseWaiters() {
synchronized (trueToFalseLock) {
trueToFalseLock.notifyAll();
}
}
public String toString() {
return String.valueOf(isTrue());
}
}
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