同步（Synchronization）

具有同步的多线程示例

下面是按顺序打印计数器值的相同示例，每次运行它时，都会产生相同的结果。

例子 (Example)

class PrintDemo {
   public void printCount() {
      try {
         for(int i = 5; i > 0; i--) {
            System.out.println("Counter   ---   "  + i );
         }
      } catch (Exception e) {
         System.out.println("Thread  interrupted.");
      }
   }
}
class ThreadDemo extends Thread {
   private Thread t;
   private String threadName;
   PrintDemo  PD;
   ThreadDemo(String name,  PrintDemo pd) {
      threadName = name;
      PD = pd;
   }
   public void run() {
      synchronized(PD) {
         PD.printCount();
      }
      System.out.println("Thread " +  threadName + " exiting.");
   }
   public void start () {
      System.out.println("Starting " +  threadName );
      if (t == null) {
         t = new Thread (this, threadName);
         t.start ();
      }
   }
}
public class TestThread {
   public static void main(String args[]) {
      PrintDemo PD = new PrintDemo();
      ThreadDemo T1 = new ThreadDemo("Thread - 1 ", PD);
      ThreadDemo T2 = new ThreadDemo("Thread - 2 ", PD);
      T1.start();
      T2.start();
      // wait for threads to end
      try {
         T1.join();
         T2.join();
      } catch (Exception e) {
         System.out.println("Interrupted");
      }
   }
}

每次运行此程序时都会产生相同的结果 -

输出 (Output)

Starting Thread - 1
Starting Thread - 2
Counter   ---   5
Counter   ---   4
Counter   ---   3
Counter   ---   2
Counter   ---   1
Thread Thread - 1  exiting.
Counter   ---   5
Counter   ---   4
Counter   ---   3
Counter   ---   2
Counter   ---   1
Thread Thread - 2  exiting.