druid的版本：1.1.20
這個問題在druid的github源碼上一直是個open的issue，見3488和3357等
druid的配置，注意timeBetweenEvictionRunsMillis參數，這是造成卡的原因。這個參默認值爲60 * 1000L（60秒），單位是毫秒
# Druid配置
spring.datasource.druid.initial-size=5
spring.datasource.druid.min-idle=5
spring.datasource.druid.maxActive=20
spring.datasource.druid.maxWait=60000
spring.datasource.druid.timeBetweenEvictionRunsMillis=60000
spring.datasource.druid.minEvictableIdleTimeMillis=300000
spring.datasource.druid.validationQuery=SELECT 1 FROM DUAL
spring.datasource.druid.testWhileIdle=true
spring.datasource.druid.testOnBorrow=false
spring.datasource.druid.testOnReturn=false
spring.datasource.druid.poolPreparedStatements=true
spring.datasource.druid.maxPoolPreparedStatementPerConnectionSize=20
spring.datasource.druid.filters=stat,wall,log4j
spring.datasource.druid.connectionProperties=druid.stat.mergeSql\=true;druid.stat.slowSqlMillis\=5000
spring.datasource.druid.web-stat-filter.enabled=true
spring.datasource.druid.web-stat-filter.url-pattern=/*
spring.datasource.druid.web-stat-filter.exclusions=*.js,*.gif,*.jpg,*.bmp,*.png,*.css,*.ico,/druid/*
spring.datasource.druid.stat-view-servlet.url-pattern=/druid/*
spring.datasource.druid.stat-view-servlet.allow=127.0.0.1,10.53.145.28
#spring.datasource.druid.stat-view-servlet.deny=10.53.145.28
spring.datasource.druid.stat-view-servlet.reset-enable=false
spring.datasource.druid.stat-view-servlet.login-username=admin
spring.datasource.druid.stat-view-servlet.login-password=123456
現象：最後在使用Druid配置多數據源，需要手動創建DruidDataSource，在創建DruidDataSource失敗後會不停的打印日誌，如下（看時間一直在變，後臺日誌一直在刷）
網上的解決方法：在配置數據源時加上setBreakAfterAcquireFailure(true);，如下
@Bean
@ConfigurationProperties("spring.datasource.master")
public DataSource masterDataSource() {
DruidDataSource dataSource = DruidDataSourceBuilder.create().build();
dataSource.setBreakAfterAcquireFailure(true);
return dataSource;
}
原理：看報錯信息類DruidDataSource.java:2715，下面有一段源碼，在2721行處當錯誤次數大於connectionErrorRetryAttempts且timeBetweenConnectErrorMillis 大於0，connectionErrorRetryAttempts默認值爲1，timeBetweenConnectErrorMillis 默認是500，可以通過配置項spring.datasource.druid.timeBetweenEvictionRunsMillis配置，從源碼的2733行可以看到設置breakAfterAcquireFailure爲true則會break。日誌狂刷的問題就終止了，但是這次拿到的connection是空的。爲下一個問題埋了坑。
這個解決了日誌狂刷的問題，但是程序卡住了，沒響應沒返回
2019-12-23 18:59:55.806 INFO 19724 --- [nio-8087-exec-1] o.a.c.c.C.[.[localhost].[/rwdemo] : Initializing Spring DispatcherServlet 'dispatcherServlet'
2019-12-23 18:59:55.806 INFO 19724 --- [nio-8087-exec-1] o.s.web.servlet.DispatcherServlet : Initializing Servlet 'dispatcherServlet'
2019-12-23 18:59:55.815 INFO 19724 --- [nio-8087-exec-1] o.s.web.servlet.DispatcherServlet : Completed initialization in 9 ms
切換到slave2
feeId = 1
2019-12-23 18:59:55.967 INFO 19724 --- [nio-8087-exec-1] com.alibaba.druid.pool.DruidDataSource : {dataSource-1} inited
2019-12-23 18:59:56.321 ERROR 19724 --- [eate-1736720965] com.alibaba.druid.pool.DruidDataSource : create connection SQLException, url: jdbc:mysql://10.53.56.7:3306/blog?useSSL=false&characterEncoding=utf8&serverTimezone=Asia/Shanghai&allowPublicKeyRetrieval=true, errorCode 1045, state 28000
java.sql.SQLException: Access denied for user 'root'@'10.53.56.1' (using password: YES)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:835) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:455) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:240) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:207) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1643) ~[druid-1.1.20.jar:1.1.20]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1709) ~[druid-1.1.20.jar:1.1.20]
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2715) ~[druid-1.1.20.jar:1.1.20]
2019-12-23 18:59:56.329 ERROR 19724 --- [eate-1736720965] com.alibaba.druid.pool.DruidDataSource : create connection SQLException, url: jdbc:mysql://10.53.56.7:3306/blog?useSSL=false&characterEncoding=utf8&serverTimezone=Asia/Shanghai&allowPublicKeyRetrieval=true, errorCode 1045, state 28000
java.sql.SQLException: Access denied for user 'root'@'10.53.56.1' (using password: YES)
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:129) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:122) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:835) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:455) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:240) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:207) ~[mysql-connector-java-8.0.13.jar:8.0.13]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1643) ~[druid-1.1.20.jar:1.1.20]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1709) ~[druid-1.1.20.jar:1.1.20]
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2715) ~[druid-1.1.20.jar:1.1.20]
2019-12-23 18:59:56.330 INFO 19724 --- [eate-1736720965] c.a.druid.pool.DruidAbstractDataSource : {dataSource-1} failContinuous is true
程序卡住了，沒返回也沒響應
使用JVM工具（jconsole或jvisualvm）查看，線程卡在以下地方等待
原因：看線程卡住的地方，在DruidDataSource.java:2786行，源碼如下
可以看到當timeBetweenEvictionRunsMillis>0時暫停timeBetweenEvictionRunsMillis，否則暫停1秒。timeBetweenEvictionRunsMillis的默認值是60 * 1000（60秒） 嘗試了以下解決方法都不行，加上以下配置
# 修改timeBetweenEvictionRunsMillis參數，減少時間
spring.datasource.druid.timeBetweenEvictionRunsMillis=6000
# DruidDataSource.java:2724行，如果設置了該配置爲true
spring.datasource.druid.fail-fast=true
spring.datasource.druid.testOnBorrow=true
結論
該問題無解，可以換其它連接池，比如springboot 2.X默認的HikariCP，DBCP或tomcat pool