以下使用 Java 的 Semaphore 类编写的并发代码进入了死锁，甚至很棘手，根据控制台输出，许可证正在被释放。
package ThreadTraining;
import java.util.concurrent.Semaphore;
public class ThreadTraining {
public static class Value {
private static int value = 0;
private static final Semaphore SEMAPHORE = new Semaphore(1);
public static synchronized void acquire() throws InterruptedException {
SEMAPHORE.acquire();
System.out.println("A thread has aquired a permit!");
}
public static synchronized void release() {
SEMAPHORE.release();
}
public static int get() {
return value;
}
public static void add() {
value++;
}
public static void subtract() {
value--;
}
}
public static class Adder extends Thread {
public Adder(String name) {
this.setName(name);
}
@Override
public void run() {
System.out.println(this.getName() + " has been created.");
boolean keepRunning = true;
while (keepRunning) {
try {
Value.acquire();
System.out.print(this.getName() + " has aquired Value's permit. --- ");
if (Value.get() > 99) {
System.out.print(this.getName() + " has finished it's job. --- ");
keepRunning = false;
} else {
System.out.print(this.getName() + " has modified value from " + Value.get() + " to ");
Value.add();
System.out.println(Value.get() + ".");
}
} catch (InterruptedException ie) {
System.err.println("This thread was interrupted.");
} finally {
System.out.println(this.getName() + " is releasing Value's permit.");
Value.release();
}
}
}
}
public static void main(String[] args) {
Thread threads[] = new Thread[3];
for (int i = 0; i < threads.length; i++) {
threads[i] = new Adder("[Adder]Thread #" + i);
}
for (Thread t : threads) {
t.start();
}
}
}
代码的控制台输出：（这是一次“幸运”的运行，它通常只打印到指定的点）
[Adder]Thread #0 has been created.
[Adder]Thread #1 has been created.
[Adder]Thread #2 has been created.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 0 to 1.
[Adder]Thread #0 is releasing Value's permit. /*NOTE: It usually prints only up to this line, hanging after the first permit-release.*/
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 1 to 2.
[Adder]Thread #0 is releasing Value's permit.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 2 to 3.
[Adder]Thread #0 is releasing Value's permit.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 3 to 4.
[Adder]Thread #0 is releasing Value's permit.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 4 to 5.
[Adder]Thread #0 is releasing Value's permit.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 5 to 6.
[Adder]Thread #0 is releasing Value's permit.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 6 to 7.
[Adder]Thread #0 is releasing Value's permit.
A thread has aquired a permit!
[Adder]Thread #0 has aquired Value's permit. --- [Adder]Thread #0 has modified value from 7 to 8.
[Adder]Thread #0 is releasing Value's permit.
背后的原因是什么？并且，如果可能的话，如何解决它？
附加信息：
这个问题是我之前的并发问题的“延续” 。
新代码非常基于这个信号量教程。