Java 机场安检

一 场景描述

乘飞机时，在进入登机口之前必须经过安全检查，安检口类似于独木桥，每次只能通过一个人，工作人员除了检查你的登机牌外，还有一些其它检查。在此模拟一个非线程安全的安检口类，旅客（线程）分别手持登机牌和SFZ接受工作人员的检查。

二 实战

1 FlightSecurity

package concurrent.FlightSecurity;
public class FlightSecurity {
    private int count;
    // 登机牌
    private String boardingPass = "null";
    // SFZ
    private String idCard = "null";
    public void pass(String boardingPass,String idCard){
        this.boardingPass = boardingPass;
        this.idCard = idCard;
        this.count++;
        check();
    }
    private void check() {
        // 简单测试，当登机牌和SFZ首字母不相同时则表示检查不通过
        if(boardingPass.charAt(0)!=idCard.charAt(0)){
            throw new RuntimeException("======Excepton======"+toString());
        }
    }
    public String toString(){
        return "The " +count +" paaaenges,boardingPass [" + boardingPass + "],idCard [" + idCard +"]";
    }
}

2 FightSecurityTest

package concurrent.FlightSecurity;
public class FightSecurityTest {
    // 旅客线程
    static class Passengers extends Thread{
        // 机场安检类
        private final FlightSecurity fightSecurity;
        // 旅客的SFZ
        private final String idCard;
        // 旅客登机牌
        private final String boardingPass;
        Passengers(FlightSecurity fightSecurity, String idCard, String boardingPass) {
            this.fightSecurity = fightSecurity;
            this.idCard = idCard;
            this.boardingPass = boardingPass;
        }
        @Override
        public void run() {
            while(true){
                // 旅客不断地过安检
                fightSecurity.pass(boardingPass,idCard);
            }
        }
    }
    public static void main(String[] args) {
        // 定义三个旅客，SFZ和登机牌首字母相同
        final FlightSecurity flightSecurity= new FlightSecurity();
        new Passengers(flightSecurity,"A","A").start();
        new Passengers(flightSecurity,"B","B").start();
        new Passengers(flightSecurity,"C","C").start();
    }
}

三 测试

1 测试结果1

Exception in thread "Thread-1" Exception in thread "Thread-0" java.lang.RuntimeException: ======Excepton======The 356 paaaenges,boardingPass [B],idCard [A]
    at concurrent.FlightSecurity.FlightSecurity.check(FlightSecurity.java:20)
    at concurrent.FlightSecurity.FlightSecurity.pass(FlightSecurity.java:14)
    at concurrent.FlightSecurity.FightSecurityTest$Passengers.run(FightSecurityTest.java:24)
java.lang.RuntimeException: ======Excepton======The 356 paaaenges,boardingPass [B],idCard [A]
    at concurrent.FlightSecurity.FlightSecurity.check(FlightSecurity.java:20)
    at concurrent.FlightSecurity.FlightSecurity.pass(FlightSecurity.java:14)
    at concurrent.FlightSecurity.FightSecurityTest$Passengers.run(FightSecurityTest.java:24)

2 测试结果2

Exception in thread "Thread-0" Exception in thread "Thread-1" java.lang.RuntimeException: ======Excepton======The 953 paaaenges,boardingPass [C],idCard [C]
    at concurrent.FlightSecurity.FlightSecurity.check(FlightSecurity.java:20)
    at concurrent.FlightSecurity.FlightSecurity.pass(FlightSecurity.java:14)
    at concurrent.FlightSecurity.FightSecurityTest$Passengers.run(FightSecurityTest.java:24)
java.lang.RuntimeException: ======Excepton======The 1039 paaaenges,boardingPass [C],idCard [C]
    at concurrent.FlightSecurity.FlightSecurity.check(FlightSecurity.java:20)
    at concurrent.FlightSecurity.FlightSecurity.pass(FlightSecurity.java:14)
    at concurrent.FlightSecurity.FightSecurityTest$Passengers.run(FightSecurityTest.java:24)

四 问题分析

在多线程情况下调用 pass 方法，虽然参数的传递百分百保证就是这两个值，但是在 pass 方法中对这两个值的赋值很有可能交叉，不能保证原子性操作。

解决的方法是给 pass 方法加上 synchronized 关键字。

public synchronized void pass(String boardingPass,String idCard)

五 说明

在 Java 中经常会听到线程安全的类和线程非安全的类，所谓线程安全的类是指多个线程同时进行操作时，不会引起数据不一致问题，反之则是线程非安全的类，在线程安全的类中经常会看到 synchronized 关键字的身影。