详解JAVA如何实现乐观锁以及CAS机制【java基础】

目录

• 前言
• 问题引入
• 悲观锁解决
• 乐观锁解决
• 乐观锁改进
• CAS机制
• 总结

public class AccountUnsafe {
     // 余额
     private Integer balance;
    
     public AccountUnsafe(Integer balance) {
     	this.balance = balance;
     }
    
    @Override
     public Integer getBalance() {
     	return balance;
     }
    
     @Override
     public void withdraw(Integer amount) {
     	balance -= amount;
     }
}

public static void main(String[] args) {
        // 账户10000元
        AccountUnsafe account = new AccountUnsafe(10000);
        List ts = new ArrayList<>();
        long start = System.nanoTime();
        // 1000个线程，每次取10元
        for (int i = 0; i <1000; i++) {
            ts.add(new Thread(() -> {
                account.withdraw(10);
            }));
        }
        ts.forEach(Thread::start);
        ts.forEach(t -> {
            try {
                t.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        long end = System.nanoTime();
        // 打印账户余额和花费时间
        log.info("账户余额：{}, 花费时间: {}", account.getBalance(), (end-start)/1000_000 + " ms");
    }

@Slf4j(topic = "a.AccountCAS")
public class AccountCAS {
    // 余额
    private volatile int balance;
    // Unsafe对象
    static final Unsafe unsafe;
    // balance 字段的偏移量
    static final long BALANCE_OFFSET;
    static {
        try {
            Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");
            theUnsafe.setAccessible(true);
            unsafe = (Unsafe) theUnsafe.get(null);
            // balance 属性在 AccountCAS 对象中的偏移量，用于 Unsafe 直接访问该属性
            BALANCE_OFFSET = unsafe.objectFieldOffset(AccountCAS.class.getDeclaredField("balance"));
        } catch (NoSuchFieldException | IllegalAccessException e) {
            throw new Error(e);
        }
    }

    public AccountCAS(Integer balance) {
        this.balance = balance;
    }

    public int getBalance() {
        return balance;
    }

    public void withdraw(Integer amount) {
        // 自旋
        while (true) {
            // 获取老的余额
            int oldBalance = balance;
            // 获取新的余额
            int newBalance = oldBalance - amount;
            // 更新余额，BALANCE_OFFSET表示balance属性的偏移量， 返回true表示更新成功， false更新失败，继续更新
            if(unsafe.compareAndSwapInt(this, BALANCE_OFFSET, oldBalance, newBalance)) {
                return;
            }
        }
    }

    public static void main(String[] args) {
        // 账户10000元
        AccountCAS account = new AccountCAS(10000);
        List ts = new ArrayList<>();
        long start = System.nanoTime();
        // 1000个线程，每次取10元
        for (int i = 0; i <1000; i++) {
            ts.add(new Thread(() -> {
                account.withdraw(10);
            }));
        }
        ts.forEach(Thread::start);
        ts.forEach(t -> {
            try {
                t.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        long end = System.nanoTime();
        // 打印账户余额和花费时间
        log.info("账户余额：{}, 花费时间: {}", account.getBalance(), (end-start)/1000_000 + " ms");
    }
}