起源：https://blog.csdn.net/qq_4037...
景象
最近共事发现新服务用的驱动都是 MySQL8.0，而一些老零碎 MySQL 驱动版本较低，存在一些危险破绽，于是被动的把一些老利用的 MySQL 驱动程序降级到了 8.0。
然而降级后，在并发量较高时，查看监控打点，Druid 连接池拿到连贯并执行 SQL 的工夫大部分都超过 200ms。
本文具体的剖析了这次“破案”的全过程。
对系统进行压测，发现呈现大量线程阻塞的状况，线程 dump 信息如下：
"http-nio-5366-exec-48" #210 daemon prio=5 os_prio=0 tid=0x00000000023d0800 nid=0x3be9 waiting for monitor entry [0x00007fa4c1400000] java.lang.Thread.State: BLOCKED (on object monitor) at org.springframework.boot.web.embedded.tomcat.TomcatEmbeddedWebappClassLoader.loadClass(TomcatEmbeddedWebappClassLoader.java:66) - waiting to lock <0x0000000775af0960> (a java.lang.Object) at org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1186) at com.alibaba.druid.util.Utils.loadClass(Utils.java:220) at com.alibaba.druid.util.MySqlUtils.getLastPacketReceivedTimeMs(MySqlUtils.java:372)通过下面的信息得悉，有线程 BLOCKED 了，BLOCKED 的地位在 com.alibaba.druid.util.Utils.loadClass(Utils.java:220)，于是重点的查看这一块的代码，最终发现了问题。
根因剖析
public class MySqlUtils { public static long getLastPacketReceivedTimeMs(Connection conn) throws SQLException { if (class_connectionImpl == null && !class_connectionImpl_Error) { try { class_connectionImpl = Utils.loadClass("com.mysql.jdbc.MySQLConnection"); } catch (Throwable error){ class_connectionImpl_Error = true; } } if (class_connectionImpl == null) { return -1; } if (method_getIO == null && !method_getIO_error) { try { method_getIO = class_connectionImpl.getMethod("getIO"); } catch (Throwable error){ method_getIO_error = true; } } if (method_getIO == null) { return -1; } if (class_MysqlIO == null && !class_MysqlIO_Error) { try { class_MysqlIO = Utils.loadClass("com.mysql.jdbc.MysqlIO"); } catch (Throwable error){ class_MysqlIO_Error = true; } } if (class_MysqlIO == null) { return -1; } if (method_getLastPacketReceivedTimeMs == null && !method_getLastPacketReceivedTimeMs_error) { try { Method method = class_MysqlIO.getDeclaredMethod("getLastPacketReceivedTimeMs"); method.setAccessible(true); method_getLastPacketReceivedTimeMs = method; } catch (Throwable error){ method_getLastPacketReceivedTimeMs_error = true; } } if (method_getLastPacketReceivedTimeMs == null) { return -1; } try { Object connImpl = conn.unwrap(class_connectionImpl); if (connImpl == null) { return -1; } Object mysqlio = method_getIO.invoke(connImpl); Long ms = (Long) method_getLastPacketReceivedTimeMs.invoke(mysqlio); return ms.longValue(); } catch (IllegalArgumentException e) { throw new SQLException("getLastPacketReceivedTimeMs error", e); } catch (IllegalAccessException e) { throw new SQLException("getLastPacketReceivedTimeMs error", e); } catch (InvocationTargetException e) { throw new SQLException("getLastPacketReceivedTimeMs error", e); } }MySqlUtils 中的getLastPacketReceivedTimeMs()办法会加载com.mysql.jdbc.MySQLConnection这个类，但在 MySQL 驱动 8.0 中类名改为com.mysql.cj.jdbc.ConnectionImpl，所以 MySQL 驱动8.0 中加载不到com.mysql.jdbc.MySQLConnection
getLastPacketReceivedTimeMs()办法实现中，如果Utils.loadClass("com.mysql.jdbc.MySQLConnection")加载不到类并抛出异样，会批改变量 class_connectionImpl_Error，下次调用不会再进行加载
public class Utils { public static Class<?> loadClass(String className) { Class<?> clazz = null; if (className == null) { return null; } try { return Class.forName(className); } catch (ClassNotFoundException e) { // skip } ClassLoader ctxClassLoader = Thread.currentThread().getContextClassLoader(); if (ctxClassLoader != null) { try { clazz = ctxClassLoader.loadClass(className); } catch (ClassNotFoundException e) { // skip } } return clazz; }然而，在 Utils 的loadClass()办法中同样 catch了ClassNotFoundException，这就导致loadClass()在加载不到类的时候，并不会抛出异样，从而会导致每调用一次getLastPacketReceivedTimeMs()办法，就会加载一次 MySQLConnection 这个类
线程 dump 信息中能够看到是在调用 TomcatEmbeddedWebappClassLoader 的loadClass()办法时，导致线程阻塞的。
public class TomcatEmbeddedWebappClassLoader extends ParallelWebappClassLoader { public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException { synchronized (JreCompat.isGraalAvailable() ? this : getClassLoadingLock(name)) { Class<?> result = findExistingLoadedClass(name); result = (result != null) ? result : doLoadClass(name); if (result == null) { throw new ClassNotFoundException(name); } return resolveIfNecessary(result, resolve); } }这是因为 TomcatEmbeddedWebappClassLoader 在加载类的时候，会加synchronized锁，这就导致每调用一次getLastPacketReceivedTimeMs()办法，就会加载一次com.mysql.jdbc.MySQLConnection，而又始终加载不到，在加载类的时候会加 synchronized 锁，所以会呈现线程阻塞，性能降落的景象。
getLastPacketReceivedTimeMs()办法调用机会
public abstract class DruidAbstractDataSource extends WrapperAdapter implements DruidAbstractDataSourceMBean, DataSource, DataSourceProxy, Serializable { protected boolean testConnectionInternal(DruidConnectionHolder holder, Connection conn) { String sqlFile = JdbcSqlStat.getContextSqlFile(); String sqlName = JdbcSqlStat.getContextSqlName(); if (sqlFile != null) { JdbcSqlStat.setContextSqlFile(null); } if (sqlName != null) { JdbcSqlStat.setContextSqlName(null); } try { if (validConnectionChecker != null) { boolean valid = validConnectionChecker.isValidConnection(conn, validationQuery, validationQueryTimeout); long currentTimeMillis = System.currentTimeMillis(); if (holder != null) { holder.lastValidTimeMillis = currentTimeMillis; holder.lastExecTimeMillis = currentTimeMillis; } if (valid && isMySql) { // unexcepted branch long lastPacketReceivedTimeMs = MySqlUtils.getLastPacketReceivedTimeMs(conn); if (lastPacketReceivedTimeMs > 0) { long mysqlIdleMillis = currentTimeMillis - lastPacketReceivedTimeMs; if (lastPacketReceivedTimeMs > 0 // && mysqlIdleMillis >= timeBetweenEvictionRunsMillis) { discardConnection(holder); String errorMsg = "discard long time none received connection. " + ", jdbcUrl : " + jdbcUrl + ", jdbcUrl : " + jdbcUrl + ", lastPacketReceivedIdleMillis : " + mysqlIdleMillis; LOG.error(errorMsg); return false; } } } if (valid && onFatalError) { lock.lock(); try { if (onFatalError) { onFatalError = false; } } finally { lock.unlock(); } } return valid; } if (conn.isClosed()) { return false; } if (null == validationQuery) { return true; } Statement stmt = null; ResultSet rset = null; try { stmt = conn.createStatement(); if (getValidationQueryTimeout() > 0) { stmt.setQueryTimeout(validationQueryTimeout); } rset = stmt.executeQuery(validationQuery); if (!rset.next()) { return false; } } finally { JdbcUtils.close(rset); JdbcUtils.close(stmt); } if (onFatalError) { lock.lock(); try { if (onFatalError) { onFatalError = false; } } finally { lock.unlock(); } } return true; } catch (Throwable ex) { // skip return false; } finally { if (sqlFile != null) { JdbcSqlStat.setContextSqlFile(sqlFile); } if (sqlName != null) { JdbcSqlStat.setContextSqlName(sqlName); } } }只有DruidAbstractDataSource的testConnectionInternal()办法中会调用getLastPacketReceivedTimeMs()办法
testConnectionInternal()是用来检测连贯是否无效的，在获取连贯和偿还连贯时都有可能会调用该办法，这取决于Druid检测连贯是否无效的参数
「Druid检测连贯是否无效的参数」：
- testOnBorrow：每次获取连贯时执行validationQuery检测连贯是否无效（会影响性能）
- testOnReturn：每次偿还连贯时执行validationQuery检测连贯是否无效（会影响性能）
- testWhileIdle：申请连贯的时候检测，如果闲暇工夫大于timeBetweenEvictionRunsMillis，执行validationQuery检测连贯是否无效
利用中设置了testOnBorrow=true，每次获取连贯时，都会去抢占synchronized锁，所以性能降落的很显著
解决方案
教训证，应用Druid 1.x版本<=1.1.22会呈现该bug，解决方案就是降级至Druid 1.x版本>=1.1.23或者Druid 1.2.x版本
GitHub issue：https://github.com/alibaba/dr...
近期热文举荐：
1.1,000+ 道 Java面试题及答案整顿(2021最新版)
2.别在再满屏的 if/ else 了，试试策略模式，真香！！
3.卧槽！Java 中的 xx ≠ null 是什么新语法？
4.Spring Boot 2.5 重磅公布，光明模式太炸了！
5.《Java开发手册（嵩山版）》最新公布，速速下载！
感觉不错，别忘了顺手点赞+转发哦！