Java 中的 线程同步函数

当无涯教程在程序中启动两个或多个线程时，可能会出现多个线程尝试访问同一资源，最终由于并发问题，它们可能产生无法预料的情况。如果多个线程试图在同一文件中写入数据，则它们可能会破坏数据，因为其中一个线程可以覆盖数据，或者当一个线程打开同一文件时，另一个线程可能正在关闭同一文件。

Java编程语言提供了一种非常方便的方法，可以通过使用 synchronized 块来创建线程并同步其任务，以下是同步语句的一般形式-

synchronized(objectidentifier) {
   //Access shared variables and other shared resources
}

在这里， objectidentifier 是对对象的引用，该对象 的锁与同步语句表示的监视器相关联。

未同步多线程示例

这是一个简单的示例，它可以按顺序打印计数器值，也可以不按顺序打印，并且每次运行它时，它的输出值都不一样。

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() {
      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");
      }
   }
}

每次您运行该程序时，都会产生不同的输出-

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

同步多线程示例

这是相同的示例，它按顺序打印计数器值，并且每次运行它时，它都会产生相同的输出。

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");
      }
   }
}

每次您运行该程序时，都会产生相同的输出-

链接：https://www.learnfk.comhttps://www.learnfk.com/java/java-thread-synchronization.html

来源：LearnFk无涯教程网

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.