java 多线程 6 : 同步锁（Lock）

java5开始可以显示定义同步锁对象来实现同步，这种机制下，同步锁由对象充当
Lock比同步代码块和同步方法更加灵活
在实现线程安全的机制中，比较常用的是ReentrantLock（可重入锁）。使用该Lock对象可以显示的加锁，释放锁

ReentrantLock 代码格式如下：

class X{
    //定义锁对象
    private final ReentrantLock lock = new ReentrantLock();
    //...
    //定义需要保证线程安全的方法
    public void method() {
        //加锁
        lock.lock();
        try {
            //需要保证线程安全的代码
        }
        finally {//使用finally来保证释放锁
            lock.unlock();
        }
    }
}

public class Account {

    private String account;//账号
    private double balance;//余额
    //定义锁对象
    private final ReentrantLock lock = new ReentrantLock();
    public Account() {
        super();
    }
    public Account(String account, double balance) {
        super();
        this.account = account;
        this.balance = balance;
    }
    
    public void drawMoney(double drawMoney) {
        //加锁
        lock.lock();
        try {
            if(balance >= drawMoney) {
                System.out.println(Thread.currentThread().getName() + "取钱成功！吐出钞票：" + drawMoney);
                try {
                    Thread.sleep(1);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                balance -= drawMoney;
                System.out.println("	余额为：" + balance);
            }else {
                System.out.println(Thread.currentThread().getName() + "取钱失败！余额不足");
            }
        }
        finally {//放锁
            lock.unlock();
        }
    }
    
    public String getAccount() {
        return account;
    }
    public void setAccount(String account) {
        this.account = account;
    }
    public double getBalance() {
        return balance;
    }
    public void setBalance(double balance) {
        this.balance = balance;
    }
}

public class DrawThread extends Thread{
    private Account account;//模拟账户
    private double drawMoney;//当前取钱线程想要取的钱数
    public DrawThread(String name , Account account, double drawMoney) {
        super(name);
        this.account = account;
        this.drawMoney = drawMoney;
    }
    
    public void run() {
        account.drawMoney(drawMoney);
    }
    
    public static void main(String[] args) {
        Account account = new Account("123456" , 1000);
        new DrawThread("A", account, 800).start();
        new DrawThread("B", account, 800).start();
    }
}