关于java:别再乱升级-MySQL-驱动了

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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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