服了，头条4面：因为一个问题问题砍了我10万薪水( 二 ) 今天我们来探讨一下分布式锁

<>();//jvm锁：当多个线程并发获取分布式锁时，需要先获取jvm锁， jvm锁获取成功，则尝试获取分布式锁static Map jvmLockMap = new ConcurrentHashMap<>();/*** 获取当前线程requestid** @return*/public static String getRequestId() {String requestId = requestIdTL.get();if (requestId == null || "".equals(requestId)) {requestId = UUID.randomUUID().toString();requestIdTL.set(requestId);}log.info("requestId:{}", requestId);return requestId;}/*** 获取锁** @param lockKey锁key* @param lockTimeOut(毫秒) 持有锁的有效时间，防止死锁* @param getTimeOut(毫秒)获取锁的超时时间，这个时间内获取不到将重试* @return*/public static boolean lock(String lockKey, long lockTimeOut, int getTimeOut) throws Exception {log.info("start");boolean lockResult = false;/*** 单个jvm中可能有多个线程并发获取一个锁* 此时我们只允许一个线程去获取分布式锁* 所以如果同一个jvm中有多个线程尝试获取分布式锁，需要先获取jvm中的锁*/ReentrantLock jvmLock = new ReentrantLock();ReentrantLock oldJvmLock = jvmLockMap.putIfAbsent(lockKey, jvmLock);oldJvmLock = oldJvmLock != null ? oldJvmLock : jvmLock;boolean jvmLockSuccess = oldJvmLock.tryLock(getTimeOut, TimeUnit.MILLISECONDS);//jvm锁获取失败，则直接失败if (!jvmLockSuccess) {return lockResult;} else {//jvm锁获取成功，则继续尝试获取分布式锁try {String request_id = getRequestId();long startTime = System.currentTimeMillis();//循环尝试获取锁while (true) {//通过lockKey获取db中的记录LockModel lockModel = DbLockUtil.get(lockKey);if (Objects.isNull(lockModel)) {//记录不存在，则先插入一条DbLockUtil.insert(LockModel.builder().lock_key(lockKey).request_id("").lock_count(0).timeout(0L).version(0).build());} else {//获取请求id ，稍后请求id会放入ThreadLocal中String requestId = lockModel.getRequest_id();//如果requestId为空字符，表示锁未被占用if ("".equals(requestId)) {lockModel.setRequest_id(request_id);lockModel.setLock_count(1);lockModel.setTimeout(System.currentTimeMillis() + lockTimeOut);//并发情况下，采用cas方式更新记录if (DbLockUtil.update(lockModel) == 1) {lockResult = true;break;}} else if (request_id.equals(requestId)) {//如果requestId和表中request_id一样表示锁被当前线程持有者，此时需要加重入锁lockModel.setTimeout(System.currentTimeMillis() + lockTimeOut);lockModel.setLock_count(lockModel.getLock_count() + 1);if (DbLockUtil.update(lockModel) == 1) {lockResult = true;break;}} else {//锁不是自己的，并且已经超时了，则重置锁，继续重试if (lockModel.getTimeout() < System.currentTimeMillis()) {DbLockUtil.resetLock(lockModel);} else {//如果未超时，休眠100毫秒，继续重试if (startTime + getTimeOut > System.currentTimeMillis()) {TimeUnit.MILLISECONDS.sleep(100);} else {break;}}}}}} finally {//释放jvm锁，将其从map中异常jvmLock.unlock();jvmLockMap.remove(lockKey);}}log.info("end");return lockResult;}/*** 释放锁** @param lock_key* @throws Exception*/private static void unlock(String lock_key) throws Exception {//获取当前线程requestIdString requestId = getRequestId();LockModel lockModel = DbLockUtil.get(lock_key);//当前线程requestId和库中request_id一致} else {lockModel.setLock_count(lockModel.getLock_count() - 1);DbLockUtil.update(lockModel);}}}/*** 重置锁** @param lockModel* @return* @throws Exception*/private static int resetLock(LockModel lockModel) throws Exception {lockModel.setRequest_id("");lockModel.setLock_count(0);lockModel.setTimeout(0L);return DbLockUtil.update(lockModel);}/*** 更新lockModel信息，内部采用乐观锁来更新** @param lockModel* @return* @throws Exception*/private static int update(LockModel lockModel) throws Exception {return exec(conn -> {String sql = "UPDATE t_lock SET request_id = ?,lock_count = ?,timeout = ?,version = version + 1 WHERE lock_key = ? ANDversion = ?";PreparedStatement ps = conn.prepareStatement(sql);int colIndex = 1;ps.setString(colIndex++, lockModel.getRequest_id());ps.setInt(colIndex++, lockModel.getLock_count());ps.setLong(colIndex++, lockModel.getTimeout());ps.setString(colIndex++, lockModel.getLock_key());ps.setInt(colIndex++, lockModel.getVersion());return ps.executeUpdate();});}private static LockModel get(String lock_key) throws Exception {return exec(conn -> {String sql = "select * from t_lock t WHERE t.lock_key=?";PreparedStatement ps = conn.prepareStatement(sql);int colIndex = 1;ps.setString(colIndex++, lock_key);ResultSet rs = ps.executeQuery();if (rs.next()) {return LockModel.builder().lock_key(lock_key).request_id(rs.getString("request_id")).lock_count(rs.getInt("lock_count")).timeout(rs.getLong("timeout")).version(rs.getInt("version")).build();}return null;});}private static int insert(LockModel lockModel) throws Exception {return exec(conn -> {String sql = "insert into t_lock (lock_key, request_id, lock_count, timeout, version) VALUES (?,?,?,?,?)";PreparedStatement ps = conn.prepareStatement(sql);int colIndex = 1;ps.setString(colIndex++, lockModel.getLock_key());ps.setString(colIndex++, lockModel.getRequest_id());ps.setInt(colIndex++, lockModel.getLock_count());ps.setLong(colIndex++, lockModel.getTimeout());ps.setInt(colIndex++, lockModel.getVersion());return ps.executeUpdate();});}private staticT exec(SqlExec sqlExec) throws Exception {Connection conn = getConn();try {return sqlExec.exec(conn);} finally {closeConn(conn);}}@FunctionalInterfacepublic interface SqlExec {T exec(Connection conn) throws Exception;}@Getter@Setter@Builderpublic static class LockModel {private String lock_key;private String request_id;private Integer lock_count;private Long timeout;private Integer version;}private static final String url = "jdbc:mysql://localhost:3306/dlock?useSSL=false";//数据库地址private static final String username = "";//数据库用户名private static final String password = "";//数据库密码private static final String driver = "com.mysql.jdbc.Driver";//mysql驱动/*** 连接数据库** @return*/private static Connection getConn() {Connection conn = null;try {Class.forName(driver);//加载数据库驱动try {conn = DriverManager.getConnection(url, username, password);//连接数据库} catch (SQLException e) {e.printStackTrace();}} catch (ClassNotFoundException e) {e.printStackTrace();}return conn;}/*** 关闭数据库链接** @return*/private static void closeConn(Connection conn) {if (conn != null) {try {conn.close();//关闭数据库链接} catch (SQLException e) {e.printStackTrace();}}}}