某天日志中连续都是如下的异常：
2017-09-06 21:39:52.936 ERROR [DruidDataSource.java:1974] create connection holder error
org.postgresql.util.PSQLException: ERROR: current transaction is aborted, commands ignored until end of transaction block
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2476) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2189) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:300) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:428) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:354) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:301) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeCachedSql(PgStatement.java:287) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:264) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgConnection.execSQLQuery(PgConnection.java:416) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgConnection.execSQLQuery(PgConnection.java:410) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgConnection.getTransactionIsolation(PgConnection.java:793) ~[postgresql-42.1.1.jar:42.1.1]
at com.alibaba.druid.filter.FilterChainImpl.connection_getTransactionIsolation(FilterChainImpl.java:362) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.filter.FilterAdapter.connection_getTransactionIsolation(FilterAdapter.java:871) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.filter.FilterChainImpl.connection_getTransactionIsolation(FilterChainImpl.java:359) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.proxy.jdbc.ConnectionProxyImpl.getTransactionIsolation(ConnectionProxyImpl.java:260) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidConnectionHolder.<init>(DruidConnectionHolder.java:145) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidConnectionHolder.<init>(DruidConnectionHolder.java:84) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidDataSource.put(DruidDataSource.java:1964) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2221) [druid-1.1.0.jar:1.1.0]
Caused by: org.postgresql.util.PSQLException: ERROR: canceling statement due to user request
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2476) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2189) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:300) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:428) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:354) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:301) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeCachedSql(PgStatement.java:287) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeWithFlags(PgStatement.java:264) ~[postgresql-42.1.1.jar:42.1.1]
at org.postgresql.jdbc.PgStatement.executeQuery(PgStatement.java:231) ~[postgresql-42.1.1.jar:42.1.1]
at com.alibaba.druid.pool.vendor.PGValidConnectionChecker.isValidConnection(PGValidConnectionChecker.java:65) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidAbstractDataSource.validateConnection(DruidAbstractDataSource.java:1293) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1539) ~[druid-1.1.0.jar:1.1.0]
at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2178) [druid-1.1.0.jar:1.1.0]
最后查找资料，查看druid源代码，终于找到了原因，在数据库建立连接时，会检查连接状况：
(!= ) {
result = ;
Exception error = ;
{
result = .isValidConnection(conn, , );
} (Exception ex) {
error = ex;
}
(!result) {
SQLException sqlError = error != ? SQLException(, error) : SQLException();
sqlError;
}
;
}
由于我们使用的是postgres数据库，在PGValidConnectionChecker代码中
int queryTimeout = validationQueryTimeout < 0 ? defaultQueryTimeout : validationQueryTimeout;
Statement stmt = ;
ResultSet rs = ;
{
stmt = conn.createStatement();
stmt.setQueryTimeout(queryTimeout);
rs = stmt.executeQuery(validateQuery);
;
此处validationQueryTime默认值-1，defaultQueryTimeout则为1，这就造成执行sql语句需要在1秒内返回，偏偏那时候我们数据库正处于非常复杂的计算中，没有及时响应，就抛出了
如下异常Caused by: org.postgresql.util.PSQLException: ERROR: canceling statement due to user request，然后更有趣的事情发生了
这段代码是数据源建立连接的时候，测试连接是否通顺运行的，抛出异常后，
由于此处result默认为true，抛出异常也没有更改result为false，然后该连接会被正常使用，由于本身该连接使用过程中已经出现过异常，再次使用就会抛出
ERROR [DruidDataSource.java:1974] create connection holder error
org.postgresql.util.PSQLException: ERROR: current transaction is aborted, commands ignored until end of transaction block
由于druid的连接池未满，所以不停的创建连接，接连不断抛出如上异常