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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…
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