Java分布式锁之数据库实现 - HelloWorld开发者社区

之前的文章《Java分布式锁实现》中列举了分布式锁的3种实现方式，分别是基于数据库实现，基于缓存实现和基于zookeeper实现。三种实现方式各有可取之处，本篇文章就详细讲解一下Java分布式锁之基于数据库的实现方式，也是最简单最易理解的实现方式。

首先，先来阐述下“锁”的概念，锁作为一种安全防御工具，既能上锁防止别人打开，又能让持有钥匙的人打开锁，这是锁的基本功能。那再来说一下“分布式锁”，分布式锁是在分布式系统(多个独立运行系统)内的锁，相对来说，这把锁的安全级别以及作用范围更大，所以从设计上就要考虑更多东西。

现在来说，怎么基于数据库实现这把分布式锁。其实说白了就是，把锁作为数据资源存入数据库，当持有这把锁的访问者来决定是否开锁。以下详细讲解了数据库的交易同步锁和交易重试补偿锁的实现。

一、数据库的设计

数据库锁表的表结构如下：


field	type	comment
ID	bigint	主键
OUTER\_SERIAL\_NO	varchar	流水号
CUST\_NO	char	客户号
SOURCE\_CODE	varchar	锁操作
THREAD\_NO	varchar	线程号
STATUS	char	锁状态
REMARK	varchar	备注
CREATED\_AT	timestamp	创建时间
UPDATED\_AT	timestamp	更新时间

作为锁的必要属性有5个：系统流水号，客户号，锁操作，线程号和锁状态，下面来解释一下每种属性

流水号：锁的具体指向，比如可以是产品，可以是交易流水号（后面会说到交易同步锁、交易补偿锁的使用方式）

客户号：客户的唯一标识

锁操作：客户的某种操作，比如客户取现操作，取现补偿重试操作

线程号：当前操作线程的线程号，比如取当前线程的uuid

锁状态：P处理中，F失败，Y成功

二、代码设计

代码的目录结构如下：

Java分布式锁之数据库实现

主要贴一下锁操作的核心代码实现：

锁接口定义：DbLockManager.java

/\*\* \* 锁接口 <br> \* \* @Author fugaoyang \* \*/ public interface DbLockManager {

</span><span style="color: #008000">/**</span><span style="color: #008000">
 * 加锁
 </span><span style="color: #008000">*/</span>
<span style="color: #0000ff">boolean</span><span style="color: #000000"> lock(String outerSerialNo, String custNo, LockSource source);

</span><span style="color: #008000">/**</span><span style="color: #008000">
 * 解锁
 </span><span style="color: #008000">*/</span>
<span style="color: #0000ff">void</span><span style="color: #000000"> unLock(String outerSerialNo, String custNo, LockSource source, LockStatus targetStatus);

}

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锁接口实现类：DbLockManagerImpl.java

/\*\* \* \* 数据库锁实现<br> \* \* @author fugaoyang \* \*/ @Service public class DbLockManagerImpl implements DbLockManager {

</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">final</span> Logger LOG = LoggerFactory.getLogger(<span style="color: #0000ff">this</span><span style="color: #000000">.getClass());

@Autowired
</span><span style="color: #0000ff">private</span><span style="color: #000000"> DbSyncLockMapper lockMapper;

@Transactional
</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">boolean</span><span style="color: #000000"> lock(String outerSerialNo, String custNo, LockSource source) {

    </span><span style="color: #0000ff">boolean</span> isLock = <span style="color: #0000ff">false</span><span style="color: #000000">;
    TradeSyncLock lock </span>= <span style="color: #0000ff">null</span><span style="color: #000000">;
    </span><span style="color: #0000ff">try</span><span style="color: #000000"> {
        lock </span>=<span style="color: #000000"> lockMapper.find(outerSerialNo, custNo, source.getCode());

        </span><span style="color: #0000ff">if</span> (<span style="color: #0000ff">null</span> ==<span style="color: #000000"> lock) {
            lock </span>= <span style="color: #0000ff">new</span><span style="color: #000000"> TradeSyncLock();
            createLock(lock, outerSerialNo, custNo, source);

            </span><span style="color: #0000ff">int</span> num =<span style="color: #000000"> lockMapper.insert(lock);
            </span><span style="color: #0000ff">if</span> (num == 1<span style="color: #000000">) {
                isLock </span>= <span style="color: #0000ff">true</span><span style="color: #000000">;
            }

            LOG.info(ThreadLogUtils.getLogPrefix() </span>+ "加入锁，客户号[{}]，锁类型[{}]"<span style="color: #000000">, custNo, source.getCode());
            </span><span style="color: #0000ff">return</span><span style="color: #000000"> isLock;
        }

        </span><span style="color: #008000">//</span><span style="color: #008000"> 根据交易类型进行加锁</span>
        isLock =<span style="color: #000000"> switchSynsLock(lock, source);
        LOG.info(ThreadLogUtils.getLogPrefix() </span>+ "更新锁，客户号[{}]，锁类型[{}]"<span style="color: #000000">, custNo, source.getCode());

    } </span><span style="color: #0000ff">catch</span><span style="color: #000000"> (Exception e) {
        LOG.error(ThreadLogUtils.getLogPrefix() </span>+ "交易加锁异常, 客户号：" +<span style="color: #000000"> custNo, e);
    }
    </span><span style="color: #0000ff">return</span><span style="color: #000000"> isLock;
}

@Transactional
</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">void</span><span style="color: #000000"> unLock(String outerSerialNo, String custNo, LockSource source, LockStatus targetStatus) {

    </span><span style="color: #0000ff">try</span><span style="color: #000000"> {
        TradeSyncLock lock </span>=<span style="color: #000000"> lockMapper.find(outerSerialNo, custNo, source.getCode());

        </span><span style="color: #0000ff">if</span> (<span style="color: #0000ff">null</span> !=<span style="color: #000000"> lock) {
            lockMapper.update(lock.getId(), targetStatus.getName(), LockStatus.P.getName(),
                    ThreadLogUtils.getCurrThreadUuid(), ThreadLogUtils.getCurrThreadUuid());
        }

        LOG.info(ThreadLogUtils.getLogPrefix() </span>+ "释放锁，客户号[{}]，锁类型[{}]"<span style="color: #000000">, custNo, source.getCode());
    } </span><span style="color: #0000ff">catch</span><span style="color: #000000"> (Exception e) {
        LOG.error(ThreadLogUtils.getLogPrefix() </span>+ "释放锁异常, 客户号：{}"<span style="color: #000000">, custNo, e);
    }
}

</span><span style="color: #008000">/**</span><span style="color: #008000">
 * 匹配加锁
 </span><span style="color: #008000">*/</span>
<span style="color: #0000ff">private</span> <span style="color: #0000ff">boolean</span><span style="color: #000000"> switchSynsLock(TradeSyncLock lock, LockSource source) {
    </span><span style="color: #0000ff">boolean</span> isLock = <span style="color: #0000ff">false</span><span style="color: #000000">;

    </span><span style="color: #0000ff">switch</span><span style="color: #000000"> (source) {
    </span><span style="color: #0000ff">case</span><span style="color: #000000"> WITHDRAW:
        ;
        isLock </span>=<span style="color: #000000"> tradeSynsLock(lock);
        </span><span style="color: #0000ff">break</span><span style="color: #000000">;
    </span><span style="color: #0000ff">case</span><span style="color: #000000"> WITHDRAW_RETRY:
        ;
        isLock </span>=<span style="color: #000000"> retrySynsLock(lock);
        </span><span style="color: #0000ff">break</span><span style="color: #000000">;
    </span><span style="color: #0000ff">default</span><span style="color: #000000">:
        ;
    }
    </span><span style="color: #0000ff">return</span><span style="color: #000000"> isLock;
}

</span><span style="color: #008000">/**</span><span style="color: #008000">
 * 交易同步锁
 </span><span style="color: #008000">*/</span>
<span style="color: #0000ff">private</span> <span style="color: #0000ff">boolean</span><span style="color: #000000"> tradeSynsLock(TradeSyncLock lock) {
    </span><span style="color: #008000">//</span><span style="color: #008000"> 处理中的不加锁，即不执行交易操作</span>
    <span style="color: #0000ff">if</span><span style="color: #000000"> (LockStatus.P.getName().equals(lock.getStatus())) {
        </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span><span style="color: #000000">;
    }

    </span><span style="color: #0000ff">int</span> num =<span style="color: #000000"> lockMapper.update(lock.getId(), LockStatus.P.getName(), LockStatus.S.getName(),
            ThreadLogUtils.getCurrThreadUuid(), </span><span style="color: #0000ff">null</span><span style="color: #000000">);
    </span><span style="color: #0000ff">if</span> (num == 1<span style="color: #000000">) {
        </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">true</span><span style="color: #000000">;
    }
    </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span><span style="color: #000000">;
}

</span><span style="color: #008000">/**</span><span style="color: #008000">
 * 补偿同步锁
 </span><span style="color: #008000">*/</span>
<span style="color: #0000ff">private</span> <span style="color: #0000ff">boolean</span><span style="color: #000000"> retrySynsLock(TradeSyncLock lock) {
    </span><span style="color: #008000">//</span><span style="color: #008000"> 处理中或处理完成的不加锁，即不执行补偿操作</span>
    <span style="color: #0000ff">if</span> (LockStatus.P.getName().equals(lock.getStatus()) ||<span style="color: #000000"> LockStatus.S.getName().equals(lock.getStatus())) {
        </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span><span style="color: #000000">;
    }

    </span><span style="color: #0000ff">int</span> num =<span style="color: #000000"> lockMapper.update(lock.getId(), LockStatus.P.getName(), LockStatus.F.getName(),
            ThreadLogUtils.getCurrThreadUuid(), </span><span style="color: #0000ff">null</span><span style="color: #000000">);
    </span><span style="color: #0000ff">if</span> (num == 1<span style="color: #000000">) {
        </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">true</span><span style="color: #000000">;
    }
    </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span><span style="color: #000000">;
}

</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">void</span><span style="color: #000000"> createLock(TradeSyncLock lock, String outerSerialNo, String custNo, LockSource source) {
    lock.setOuterSerialNo(outerSerialNo);
    lock.setCustNo(custNo);
    lock.setSourceCode(source.getCode());
    lock.setThreadNo(ThreadLogUtils.getCurrThreadUuid());
    lock.setStatus(LockStatus.P.getName());
    lock.setRemark(source.getDesc());
}

}

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获取当前线程号以及打印uuid工具类ThreadLogUtils.Java

/\*\* \* \* 线程处理<br> \* \* @author fugaoyang \* \*/ public class ThreadLogUtils {

</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> ThreadLogUtils instance = <span style="color: #0000ff">null</span><span style="color: #000000">;

</span><span style="color: #0000ff">private</span><span style="color: #000000"> ThreadLogUtils() {
    setInstance(</span><span style="color: #0000ff">this</span><span style="color: #000000">);
}

</span><span style="color: #008000">//</span><span style="color: #008000"> 初始化标志</span>
<span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> <span style="color: #0000ff">final</span> Object __noop = <span style="color: #0000ff">new</span><span style="color: #000000"> Object();
</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> ThreadLocal&lt;Object&gt; __flag = <span style="color: #0000ff">new</span> InheritableThreadLocal&lt;Object&gt;<span style="color: #000000">() {
    @Override
    </span><span style="color: #0000ff">protected</span><span style="color: #000000"> Object initialValue() {
        </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">null</span><span style="color: #000000">;
    }
};

</span><span style="color: #008000">//</span><span style="color: #008000"> 当前线程的UUID信息，主要用于打印日志；</span>
<span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> ThreadLocal&lt;String&gt; currLogUuid = <span style="color: #0000ff">new</span> InheritableThreadLocal&lt;String&gt;<span style="color: #000000">() {
    @Override
    </span><span style="color: #0000ff">protected</span><span style="color: #000000"> String initialValue() {
        </span><span style="color: #0000ff">return</span> UUID.randomUUID().toString()<span style="color: #008000">/*</span><span style="color: #008000"> .toUpperCase() </span><span style="color: #008000">*/</span><span style="color: #000000">;
    }
};

</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> ThreadLocal&lt;String&gt; currThreadUuid = <span style="color: #0000ff">new</span> ThreadLocal&lt;String&gt;<span style="color: #000000">() {
    @Override
    </span><span style="color: #0000ff">protected</span><span style="color: #000000"> String initialValue() {
        </span><span style="color: #0000ff">return</span><span style="color: #000000"> UUIDGenerator.getUuid();
    }
};

</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span> <span style="color: #0000ff">void</span><span style="color: #000000"> clear(Boolean isNew) {
    </span><span style="color: #0000ff">if</span><span style="color: #000000"> (isNew) {

        currLogUuid.remove();

        __flag.remove();

        currThreadUuid.remove();

    }
}

</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span><span style="color: #000000"> String getCurrLogUuid() {
    </span><span style="color: #0000ff">if</span> (!<span style="color: #000000">isInitialized()) {
        </span><span style="color: #0000ff">throw</span> <span style="color: #0000ff">new</span> IllegalStateException("TLS未初始化"<span style="color: #000000">);
    }

    </span><span style="color: #0000ff">return</span><span style="color: #000000"> currLogUuid.get();
}

</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span><span style="color: #000000"> String getCurrThreadUuid() {
    </span><span style="color: #0000ff">return</span><span style="color: #000000"> currThreadUuid.get();
}

</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span> <span style="color: #0000ff">void</span><span style="color: #000000"> clearCurrThreadUuid() {
    currThreadUuid.remove();
}

</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span><span style="color: #000000"> String getLogPrefix() {
    </span><span style="color: #0000ff">if</span> (!<span style="color: #000000">isInitialized()) {
        </span><span style="color: #0000ff">return</span> ""<span style="color: #000000">;
    }

    </span><span style="color: #0000ff">return</span> "&lt;uuid=" + getCurrLogUuid() + "&gt;"<span style="color: #000000">;
}

</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> <span style="color: #0000ff">boolean</span><span style="color: #000000"> isInitialized() {
    </span><span style="color: #0000ff">return</span> __flag.get() != <span style="color: #0000ff">null</span><span style="color: #000000">;
}

</span><span style="color: #008000">/**</span><span style="color: #008000">
 * 初始化上下文，如果已经初始化则返回false，否则返回true&lt;br/&gt;
 *
 * </span><span style="color: #808080">@return</span>
 <span style="color: #008000">*/</span>
<span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span> <span style="color: #0000ff">boolean</span><span style="color: #000000"> initialize() {
    </span><span style="color: #0000ff">if</span><span style="color: #000000"> (isInitialized()) {
        </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">false</span><span style="color: #000000">;
    }

    __flag.set(__noop);
    </span><span style="color: #0000ff">return</span> <span style="color: #0000ff">true</span><span style="color: #000000">;
}

</span><span style="color: #0000ff">private</span> <span style="color: #0000ff">static</span> <span style="color: #0000ff">void</span><span style="color: #000000"> setInstance(ThreadLogUtils instance) {
    ThreadLogUtils.instance </span>=<span style="color: #000000"> instance;
}

</span><span style="color: #0000ff">public</span> <span style="color: #0000ff">static</span><span style="color: #000000"> ThreadLogUtils getInstance() {
    </span><span style="color: #0000ff">return</span><span style="color: #000000"> instance;
}

}

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两种锁的实现的大致思路如下：

1.交易同步锁

当一个客户来取现，第一次进入时，会插入一条当前线程，状态是P，操作是取现的锁，取现成功后根据当前线程号会更新成功；

当一个客户同时多个取现操作时，只有一个取现操作会加锁成功，其它会加锁失败；

当一个客户已经在取现中，这时数据库已经有一条状态P的锁，该客户同时又做了取现，这个取现动作会尝试加锁而退出；

2.交易重试补偿锁

1.当一个客户取现加锁成功，因调用第三方支付接口超时时，后台会对该笔交易重新发起重试打款操作，这时会新加一条当前交易流水号，当前线程号，状态是P，操作是取现重试的锁，重试的支付结果是成功的话，更新该条锁数据为Y状态，否则更新该条数据为F状态；

2.当重试支付失败后，再去重试打款时，发现锁的状态是F，这时把F更新为P，继续重试，根据重试结果更新锁状态。

上面实现的是一个最基本的数据库分布式锁，满足的并发量也是基于数据库所能扛得住的，性能基本可以满足普通的交易量。

后续可以优化的部分：

1.当一个用户同时多次获取lock时，因为目前是用的乐观锁，只会有一个加锁成功，可以优化成加入while(true)循环获取lock，当失败次数到达指定次数时退出，当前的操作结束。