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…
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