Tag Archives: Java

TCP三次握手与四次挥手

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说明

  1. TCP在建立连接与断开连接时报文头部的标志位与序号值分别是多少呢？
  2. 利用Java的Socket对象分别实现客户端与服务端，对进通讯过程进行数据报文捕获；
  3. 分别研究两种情况，一种是服务器与客户端都在本机，另一种是分别位于两台机器上；

Java示例代码

Client

// packag[......]
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Java多线程中的线程安全问题

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多个线程访问共享数据就会产生冲突的问题，以卖票为例，多个窗口卖同一批票就会出现重复卖的问题

  public static void main(String[] args) {
      Runnable runnable = new Runnable() {
          Integer tickets = 100;
          @Override
          public void run() {
              String name = Thread.currentThread().getName();
              while(tickets-- > 0) {
                  try {
                      Thread.sleep(1);
                  } catch (InterruptedException e) {
                      e.printStackTrace();
                  }
                  System.out.println(name + "正在卖票: " + tickets);
              }
          }
      };
      new Thread(runnable).start();
      new Thread(runnable).start();
      new Thread(runnable).start();
  }

解决方案一：使用同步机制，共享部分同步执行则不会出现重复操作问题

  1. synchronized关键字实现代码块同步 
    public static void main(String[] args) {
    Object lock = new Object();
    Runnable runnable = new Runnable() {
        Integer tickets = 100;

        @Override
        public void run() {
            String name = Thread.currentThread().getName();
            while (true) {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                synchronized (lock) {
                    if (tickets <= 0) break;
                    System.out.println(name + "正在卖票: " + tickets);
                    tickets--;
                }
            }
        }
    };
    new Thread(runnable).start();
    new Thread(runnable).start();
    new Thread(runnable).start();
}
  2. synchronized同步方法实现同步机制 
    static Integer tickets = 100;
public static void main(String[] args) {
    Runnable runnable = new Runnable() {
        @Override
        public void run() {
            while (true) {
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                sellTicked();
                if (tickets <= 0) break;
            }
        }
    };
    new Thread(runnable).start();
    new Thread(runnable).start();
    new Thread(runnable).start();
}

synchronized private static void sellTicked() {
    if (tickets <= 0) return;
    String name = Thread.currentThread().getName();
    System.out.println(name + "正在卖票: " + tickets);
    tickets--;
}

解决方案二：使用锁，ReentrantLock
[……]

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