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）分别手持登机牌和身份证接受工作人员的检查。
二 实战1 FlightSecurity
2 FightSecurityTest
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。
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