学步园

有一静态整形变量X，初始值为0，用JAVA写四个线程，二个对其加1，二个对其减一，X等于0时不进行减操作，X等于5时结束程序。

public class ThreadTest {<br /> private static Integer X = 0;<br /> public static void main(String [] args) {<br /> ThreadTest t = new ThreadTest();<br /> AddOne add = t.new AddOne();<br /> SubOne sub = t.new SubOne();</p> <p> new Thread(add).start();<br /> new Thread(add).start();<br /> new Thread(sub).start();<br /> new Thread(sub).start();<br /> }</p> <p> private synchronized void addOne() {<br /> X++;<br /> System.out.println(Thread.currentThread().getName()+" add one " + X);<br /> }<br /> private synchronized void subOne() {<br /> X--;<br /> System.out.println(Thread.currentThread().getName()+" sub one " + X);<br /> }</p> <p> class AddOne implements Runnable {<br /> public void run(){<br /> while (true) {<br /> if (X == 5) {<br /> System.out.println(Thread.currentThread().getName()+" add stop ");<br /> return;<br /> }<br /> addOne();<br /> try{<br /> Thread.sleep(1000);<br /> }catch(Exception e) {</p> <p> }<br /> }<br /> }<br /> }<br /> class SubOne implements Runnable {<br /> public void run() {<br /> while (X != 0) {<br /> if (X == 5) {<br /> System.out.println(Thread.currentThread().getName()+" sub stop ");<br /> return;<br /> }<br /> subOne();<br /> try{<br /> Thread.sleep(1000);<br /> }catch(Exception e) {</p> <p> }<br /> }<br /> }<br /> }<br /> }

上面代码有问题，run里边的X没有同步。

public class Test {
 
 private static int res=0;
 private Object lock = new Object();

 public static void main(String[] args) {
  Test t = new Test();

  Runnable inc = t.new Inc();
  Runnable dec = t.new Dec();

  Thread t1 = new Thread(inc);
  Thread t2 = new Thread(inc);

  Thread t3 = new Thread(dec);
  Thread t4 = new Thread(dec);

  t1.start();
  t2.start();
  t3.start();
  t4.start();

 }

 class Inc implements Runnable {

  public void run() {
   synchronized(lock) {

    while (true) {
     if(res == 5) return;
     else res++;
   }
 }

 class Dec implements Runnable {

  public void run() {
   synchronized(lock) {

    while (true) {
     if(res == 5) return;
     else res--;
   }
  }
 }
}