最近系統間歇性很慢，比如登錄，經常超時、無反應，查看log出現下述兩種異常，
異常情況1

The last packet successfully received from the server was 995,202 milliseconds ago. The last packet sent successfully to the server was 995,202 milliseconds ago.at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174)at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)at com.mysql.cj.jdbc.ConnectionImpl.setAutoCommit(ConnectionImpl.java:2056)at com.zaxxer.hikari.pool.ProxyConnection.setAutoCommit(ProxyConnection.java:388)at com.zaxxer.hikari.pool.HikariProxyConnection.setAutoCommit(HikariProxyConnection.java)at org.springframework.jdbc.datasource.DataSourceTransactionManager.doBegin(DataSourceTransactionManager.java:284)... 44 common frames omitted

異常情況2

Caused by: java.sql.SQLNonTransientConnectionException: No operations allowed after connection closed.at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:110)at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97)at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:89)at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:63)at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:73)at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:73)at com.mysql.cj.jdbc.ConnectionImpl.setNetworkTimeout(ConnectionImpl.java:2490)at com.zaxxer.hikari.pool.PoolBase.setNetworkTimeout(PoolBase.java:550)at com.zaxxer.hikari.pool.PoolBase.isConnectionAlive(PoolBase.java:165)at com.zaxxer.hikari.pool.HikariPool.getConnection(HikariPool.java:179)... 30 common frames omitted

懷疑數據庫鏈接存在死鏈，后端是springboot使用的默認數據庫連接池hikari，配置如下

spring:datasource:driver: com.mysql.cj.jdbc.Driverurl: jdbc:mysql://x.x.x.x:3306/dbname?useUnicode=true&characterEncoding=utf-8username: xxpassword: xxxxhikari:maximumPoolSize: 20maxLifetime: 60000minimumIdle: 10connectionTestQuery: SELECT 'x'

查看源碼得知，com.zaxxer.hikari.pool.HikariPool#getConnection(long)獲取連接時，判斷連接是否存活，

com.zaxxer.hikari.pool.PoolBase#isConnectionAlive boolean isConnectionAlive(final Connection connection)try {setNetworkTimeout(connection, validationTimeout);final int validationSeconds = (int) Math.max(1000L, validationTimeout) / 1000;//如果不配置connectionTestQuery，才會使用jdbc校驗機制if (isUseJdbc4Validation) {return connection.isValid(validationSeconds);}} com.zaxxer.hikari.pool.PoolBase#PoolBasePoolBase(final HikariConfig config){this.config = config;this.isUseJdbc4Validation = config.getConnectionTestQuery() == null;}com.mysql.cj.jdbc.ConnectionImpl#isValidpublic boolean isValid(int timeout) throws SQLException {//發送心跳包pingInternal(false, timeout * 1000);return true;}

繼續往下跟到了com.mysql.cj.protocol.a.NativeProtocol#sendCommand

public final NativePacketPayload sendCommand(Message queryPacket, boolean skipCheck, int timeoutMillis) {//發送心跳包，debug得知發送的十六進制0x0e,也就是數字14send(queryPacket, queryPacket.getPosition());}

總結

至此真相大白，因為配置了connectionTestQuery，所以沒有使用jdbc的校驗機制，修改方法就是去掉connectionTestQuery，jdbc的校驗機制是在獲取連接時，校驗合法性，比定時校驗效率要高；
另外，jdbc的校驗相比上層庫校驗效率更高，因為是tcp層的心跳包而不是mysql引擎解析的sql語句，所以druid、hikari層的校驗可以去掉了。

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總結

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