一 場景描述

乘飛機時，在進入登機口之前必須經(jīng)過安全檢查，安檢口類似于獨木橋，每次只能通過一個人，工作人員除了檢查你的登機牌外，還有一些其它檢查。在此模擬一個非線程安全的安檢口類，旅客（線程）分別手持登機牌和身份證接受工作人員的檢查。

二 實戰(zhàn)

1 FlightSecurity

package concurrent.FlightSecurity;
public class FlightSecurity {
    private int count;
    // 登機牌
    private String boardingPass = "null";
    // 身份證
    private String idCard = "null";
    public void pass(String boardingPass,String idCard){
        this.boardingPass = boardingPass;
        this.idCard = idCard;
        this.count++;
        check();
    }
    private void check() {
        // 簡單測試，當?shù)菣C牌和身份證首字母不相同時則表示檢查不通過
        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 {
    // 旅客線程
    static class Passengers extends Thread{
        // 機場安檢類
        private final FlightSecurity fightSecurity;
        // 旅客的身份證
        private final String idCard;
        // 旅客登機牌
        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){
                // 旅客不斷地過安檢
                fightSecurity.pass(boardingPass,idCard);
            }
        }
    }
    public static void main(String[] args) {
        // 定義三個旅客，身份證和登機牌首字母相同
        final FlightSecurity flightSecurity= new FlightSecurity();
        new Passengers(flightSecurity,"A","A").start();
        new Passengers(flightSecurity,"B","B").start();
        new Passengers(flightSecurity,"C","C").start();
    }
}

三 測試

1 測試結(jié)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 測試結(jié)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 問題分析

在多線程情況下調(diào)用 pass 方法，雖然參數(shù)的傳遞百分百保證就是這兩個值，但是在 pass 方法中對這兩個值的賦值很有可能交叉，不能保證原子性操作。

解決的方法是給 pass 方法加上 synchronized 關(guān)鍵字。

public synchronized void pass(String boardingPass,String idCard)

五 說明

在 Java 中經(jīng)常會聽到線程安全的類和線程非安全的類，所謂線程安全的類是指多個線程同時進行操作時，不會引起數(shù)據(jù)不一致問題，反之則是線程非安全的類，在線程安全的類中經(jīng)常會看到 synchronized 關(guān)鍵字的身影。

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