技術標籤：javasynchronized同步

本篇是多執行緒不安全例項，模擬的是兩人取錢的過程，若兩人同時取100，會造成餘額為負的結果，說明多執行緒存在安全問題。

首先是不安全的例項。

package lesson04;

public class TestBank {
    public static void main(String[] args) {
        Account account = new Account("結婚基金", 100);
        DrawingMoney you = new DrawingMoney(50,0,account);
        DrawingMoney wife = new DrawingMoney(100,0,account);
        new Thread(you).start();
        new Thread(wife).start();
    }
}
//賬戶
class Account {
     String name;//卡名
     int money;//卡內餘額
    //有參構造
    public Account(String name,int money) {
        this.name = name;
        this.money = money;
    }
}
//銀行取錢，模擬多執行緒
class DrawingMoney implements Runnable {
    int drawingMoney ;
    int haveMoney;
    Account account;
    //構造器
    public DrawingMoney(int drawingMoney, int haveMoney, Account account) {
        this.drawingMoney = drawingMoney;
        this.haveMoney = haveMoney;
        this.account = account;
    }

    @Override
    public void run() {
        if (account.money - drawingMoney <= 0 ) {
            System.out.println("卡里餘額不足");
            return;
        }
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        account.money = account.money-drawingMoney;
        haveMoney = haveMoney+drawingMoney;
        System.out.println(Thread.currentThread().getName()+"手裡有"+haveMoney);
        System.out.println(account.name+"卡餘額為"+account.money);

    }
}


 

執行結果：

這裡就引入了synchronized同步機制，使用鎖加佇列的方式確保了多執行緒操作的安全。這裡包括兩種：

1、synchronized方法：同步方法：publicsynchronized void method(args){};

2、synchronized方法體：synchronized（Obj）{}

Obj稱之為同步監視器，可以是任何物件，同步方法中無需指定同步監視器，因為同步方法的監視器就是this，這個物件的本身，或者是classl類

下面使用synchronized方法體對程式碼進行改造，使執行緒安全。

package lesson04;

public class TestBank {
    public static void main(String[] args) {
        Account account = new Account("結婚基金", 100);
        DrawingMoney you = new DrawingMoney(50,0,account);
        DrawingMoney wife = new DrawingMoney(100,0,account);
        new Thread(you).start();
        new Thread(wife).start();
    }
}
//賬戶
class Account {
     String name;//卡名
     int money;//卡內餘額
    //有參構造
    public Account(String name,int money) {
        this.name = name;
        this.money = money;
    }
}
//銀行取錢，模擬多執行緒
class DrawingMoney implements Runnable {
    int drawingMoney ;
    int haveMoney;
    Account account;
    //構造器
    public DrawingMoney(int drawingMoney, int haveMoney, Account account) {
        this.drawingMoney = drawingMoney;
        this.haveMoney = haveMoney;
        this.account = account;
    }
    //取錢
    //synchronized預設鎖的是this
    @Override
    public void run() {
        //鎖的物件就是變化的量，增刪改的物件
        synchronized (account){
            if (account.money - drawingMoney <= 0 ) {
                System.out.println("卡里餘額不足");
                return;
            }
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            account.money = account.money-drawingMoney;
            haveMoney = haveMoney+drawingMoney;
            System.out.println(Thread.currentThread().getName()+"手裡有"+haveMoney);
            System.out.println(account.name+"卡餘額為"+account.money);
        }


    }
}


 

執行結果：