java Thread编程(三) 同步的两种不同实现方式

1，创建需要同步的对象（方式一）

package concurrency;

public class Bank {
    
    private double amount;
    
    public Bank(double amount){
        this.amount = amount;
    }
    
    public double getAmount() {
        return amount;
    }
    
    public void setAmount(double amount) {
        this.amount = amount;
    }
    
    public synchronized void addAmount(double money){
        System.out.println("当前金额为： "+amount);
        amount = amount + money;
        System.out.println("增加"+money+"后的金额为："+amount);
    }
    
    public synchronized void substract(double money){
        System.out.println("当前金额为： "+amount);
        amount = amount - money;
        System.out.println("减少"+money+"后的金额为："+amount);
    }
}

2，创建两个线程对象

package concurrency;

public class AddMoneyThread implements Runnable{
    private Bank bank;
    private double money;
    
    public AddMoneyThread(Bank bank,double money){
        this.bank = bank;
        this.money = money;
    }
    
    /** 
     * @see java.lang.Runnable#run()
     */
    @Override
    public void run() {
        for(int i=0;i<10;i++){
            bank.addAmount(money);
        }
    }
}

package concurrency;

public class SubstractMoneyThread implements Runnable{
    private Bank bank;
    private double money;
    
    public SubstractMoneyThread(Bank bank,double money){
        this.bank = bank;
        this.money = money;
    }
    
    /** 
     * @see java.lang.Runnable#run()
     */
    @Override
    public void run() {
        for(int i=0;i<10;i++){
            bank.substract(money);
        }
    }
}

3，创建测试类

package concurrency;

public class ConcurrencyTest {

    public static void main(String[] args) {
        Bank bank = new Bank(1000);

        AddMoneyThread addThread = new AddMoneyThread(bank, 10);
        SubstractMoneyThread subThread = new SubstractMoneyThread(bank, 10);
        
        Thread addT = new Thread(addThread);
        Thread subT = new Thread(subThread);
        addT.start();
        subT.start();
    }

}

4，创建需要同步的对象（方式二）

package concurrency;

public class Bank {
    
    private double amount;
    
    public Bank(double amount){
        this.amount = amount;
    }
    
    public double getAmount() {
        return amount;
    }
    
    public void setAmount(double amount) {
        this.amount = amount;
    }
    
    public void addAmount(double money){
        synchronized(this){
            System.out.println("当前金额为： "+amount);
            amount = amount + money;
            System.out.println("增加"+money+"后的金额为："+amount);
        }
    }
    
    public void substract(double money){
        synchronized(this){
            System.out.println("当前金额为： "+amount);
            amount = amount - money;
            System.out.println("减少"+money+"后的金额为："+amount);
        }
    }
}

1，synchronized作为一种同步手段，解决java多线程的执行有序性和内 存可见性

2，一个线程执行临界区代码过程如下：
(1) 获得同步锁
(2)清空工作内存
(3) 从主存拷贝变量副本到工作内存
(4) 对这些变量计算
(5) 将变量从工作内存写回到主存
(6) 释放锁

 

有几个问题不是很清楚。

我给方法上或者同步对象上加了锁，但是发现synchronized所包围的块是一个原子操作？不知道是什么原因？

个人的理解是：同步对象被加了锁，同步对象只能被一个线程访问。

该对象中的方法也只能被一个线程访问？？？？

求指点！