JUC (8) : synchronized little exercise

卖票练习

测试下面代码是否存在线程安全问题,并尝试改正

• 将sell方法声明为synchronized即可
• Note only paircountOne line of code to modifysynchronizedNot even enclosed.对countThe size determination must also be part of the atomic operation,否则也会导致count值异常.

import lombok.extern.slf4j.Slf4j;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Vector;

@Slf4j
public class ExerciseSell {
    
    public static void main(String[] args) throws InterruptedException {
    
        // 模拟多人买票
        TicketWindow window = new TicketWindow(1000);
        
        // So a collection of threads
        List<Thread> threadList = new ArrayList<>();
        // 用来存储买出去多少张票
        List<Integer> amountList = new Vector<>();
        
        for (int i = 0; i < 5000; i++) {
    
            Thread t = new Thread(() -> {
    
                {
     // Although it is a safe combination,But the operation is not the same object,So there is no need to lock again to ensure atomicity
                    // 买票,分析这里的竞态条件
                    int count = window.sell(randomAmount());
                    // 统计买票数
                    amountList.add(count);
                }
            });
            threadList.add(t);
            t.start();
        }
        for (Thread thread : threadList) {
    
            thread.join();
        }
        // 买出去的票求和
        log.debug("销售数量:{}", amountList.stream().mapToInt(c -> c).sum());
        // 剩余票数
        log.debug("余数:{}", window.getCount());
    }
    // Random 为线程安全
    static Random random = new Random();
    // 随机 1~5
    public static int randomAmount() {
    
        return random.nextInt(5) + 1;
    }
}
class TicketWindow {
    
    private int count;
    public TicketWindow(int count) {
    
        this.count = count;
    }
    public int getCount() {
    
        return count;
    }
    //在方法上加一个synchronized即可
    public int sell(int amount) {
    
        // 临界区 需要保护
        if (this.count >= amount) {
    
            this.count -= amount;
            return amount;
        } else {
    
            return 0;
        }
    }
}    

另外,用下面的代码行不行,为什么？

List<Integer> amountLis = new ArrayList<>();

• 不行,因为amountLis会被多个线程共享,A thread-safe implementation class must be used.

测试脚本

for /L %n in (1,1,10) do java -cp ".;C:\Users\manyh\.m2\repository\ch\qos\logback\logback-classic\1.2.3\logback-classic-1.2.3.jar;C:\Users\manyh\.m2\repository\ch\qos\logback\logback-core\1.2.3\logback-core-1.2.3.jar;C:\Users\manyh\.m2\repository\org\slf4j\slf4j-api\1.7.25\slf4j-api-1.7.25.jar" cn.xiaozheng.n4.exercise.ExerciseSell

for /L %n in (1,1,10) do E:\prolificacy\Java\OpenJDK-11.0.15_9\bin\java.exe -Dfile.encoding=UTF-8 -classpath "E:\CodeDirectory\CodeIDEA\practice-project\juc-demo\target\classes;E:\prolificacy\apache-maven-3.8.4\.maven-respository\ch\qos\logback\logback-classic\1.2.10\logback-classic-1.2.10.jar;E:\prolificacy\apache-maven-3.8.4\.maven-respository\ch\qos\logback\logback-core\1.2.10\logback-core-1.2.10.jar;E:\prolificacy\apache-maven-3.8.4\.maven-respository\org\slf4j\slf4j-api\1.7.32\slf4j-api-1.7.32.jar;E:\prolificacy\apache-maven-3.8.4\.maven-respository\org\projectlombok\lombok\1.18.20\lombok-1.18.20.jar" com.xiaozheng.day1.ExerciseSell

说明：

• Two pieces of critical section code with no causal relationship,Just need to guarantee their atomicity,Brackets are not required.

转账练习

测试下面代码是否存在线程安全问题,并尝试改正

• 将transferThe method body of the method is wrapped in a synchronized block,将当Account.classset as lock object.

public class ExerciseTransfer {
    
    public static void main(String[] args) throws InterruptedException {
    
        Account a = new Account(1000);
        Account b = new Account(1000);
        Thread t1 = new Thread(() -> {
    
            for (int i = 0; i < 1000; i++) {
    
                a.transfer(b, randomAmount());
            }
        }, "t1");
        Thread t2 = new Thread(() -> {
    
            for (int i = 0; i < 1000; i++) {
    
                b.transfer(a, randomAmount());
            }
        }, "t2");
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        // 查看转账2000次后的总金额
        log.debug("total:{}",(a.getMoney() + b.getMoney()));
    }
    // Random 为线程安全
    static Random random = new Random();
    // 随机 1~100
    public static int randomAmount() {
    
        return random.nextInt(100) +1;
    }
}
class Account {
    
    private int money;
    public Account(int money) {
    
        this.money = money;
    }
    public int getMoney() {
    
        return money;
    }
    public void setMoney(int money) {
    
        this.money = money;
    }
    public void transfer(Account target, int amount) {
    
        if (this.money > amount) {
    
            this.setMoney(this.getMoney() - amount);
            target.setMoney(target.getMoney() + amount);
        }
    }
}

这样改正行不行,为什么？

• 不行,Because different threads call this method,将会Lock different objects

public synchronized void transfer(Account target, int amount) {
    
    if (this.money > amount) {
    
        this.setMoney(this.getMoney() - amount);
        target.setMoney(target.getMoney() + amount);
    }
}

// 等价于 
public void transfer(Account target, int amount) {
    
    synchronized (this) {
    
        // 临界区
        if (this.money > amount) {
    
            this.setMoney(this.getMoney() - amount);
            target.setMoney(target.getMoney() + amount);
        }
    }
}

• 正确解决,锁Class

  public void transfer(Account target, int amount) {
        
      synchronized (Account.class) {
        
          // 临界区
          if (this.money > amount) {
        
              this.setMoney(this.getMoney() - amount);
              target.setMoney(target.getMoney() + amount);
          }
      }
  }