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MySQL "No database selected" over jdbc

We have a Java application (basically integration tests) that uses Hibernate (which uses Jdbc) to read/write data to the MySQL Database. Hibernate objects like sessions or transactions are created and configured via our own code (no Spring or other wrappers are being used). The issue is that periodically (multiple times during tests execution) we observe a "No database selected" exception. Database URL that we use for DataSource configuration already contains database name in it:
jdbc:mysql://localhost:3306/test?useSSL=false&createDatabaseIfNotExist=false&cacheServerConfiguration=true&cacheResultSetMetadata=true&useLocalSessionState=true&rewriteBatchedStatements=true&tcpNoDelay=true&tcpTrafficClass=16&alwaysSendSetIsolation=false&tcpSndBuf=1048576&tcpRcvBuf=1048576&characterEncoding=utf8&allowPublicKeyRetrieval=true
I tried to catch the Exception and test the connection's selected database by running select database() and it actually reports that the value is null on the database side.
Even more strange thing is that next queries on the same connection are executed against the normal database (so it somehow self-heals).
Does anybody know why can MySQL connections "lose" and then "restore" selected database?
Or maybe there is a way to trace the problem down. Will be grateful for any help or thought that you can provide
Versions:
Java 1.8.0_292
Mysql 5.6.31
Hibernate 5.4.2
JDBC mysql-connector-java 8.0.22

Getting Database url, name and schema version via native query

I am using EclipseLink with Oralce DB and would like to get database name, schema version and url via EntityManager. I have already tried solution mentioned in following question
Getting a JDBC connection from EclipseLink
But this approach is not working for me and I have mentioned the problem in following question:
Getting sql connection via EclipseLink
I would like to know how can I get the required data directly via native query. I have an instance of entityManager and using it, I can execute native queries and get what I want

Make schema created by Spring JDBC the default schema

I'm using Spring Boot and JDBC for my database connection. I placed schema.sql at the classpath to initialize a schema and tables.
Because the schema doesn't exist yet while connecting to the datasource, I have to configure the datasource in application.properties like so:
spring.datasource.url=jdbc:mysql://localhost:3306/
schema.sql:
CREATE DATABASE IF NOT EXISTS <schema_name>
USE <schema.name>;
CREATE TABLE...
So I select the schema after creating it. This obviously doesn't persist for too long.
How do I configure this properly? Is there a way to select a default schema after the create script or maybe change the datasource url?

With JDBC you need to use Connection.setCatalog to switch between databases. You should not use USE <databasename> as the JDBC driver itself needs to be aware of which database it is operating on.

Based on your code from the schema.sql
USE <schema.name>;
This will not work for your java environment. The schema.sql will be executed and finished, which will not cater your requirement to set the default schema.
The General approach will be to use JDBC URL as;
jdbc:mysql://localhost:3306/DB_NAME
This will set the default db as DB_NAME.
Assumptions: I am assuming that you want to connect to single node DB without any loadbalancer or failover mechanism to be used. URL may change based on these features to be configured.
If you are not specifying the DB_NAME in the URL, it means their is no default schema.
You have 2 options to access the DB in that case.
1) Always use the Connection.setCatalog() method to specify the desired database in JDBC applications, rather than the USE database statement.
2) fully specify table names using the database name (that is, SELECT dbname.tablename.colname FROM dbname.tablename...) in your SQL. Opening a connection without specifying the database to use is generally only useful when building tools that work with multiple databases, such as GUI database managers.
For more details refer to the below mysql portal for reference.
https://dev.mysql.com/doc/connector-j/en/connector-j-reference-configuration-properties.html

Capturing sql queries along with bind parameters into log files

I used p6spy/log4jdbc to capture the sql queries along with the bind parameters, connecting to Oracle database (hibernate 4.3.5 and oracle 10g is used) and push it to log files which are configured using logback.
The datasource is created by providing the proxy details required for p6spy/log4jdbc.
But, is there any way to enable/disable this feature and switch back to oracle datasource at runtime?
I guess it requires a runtime switch of datasource at runtime..?
Is there any other approach to capture the sql queries along with bind parameters with out using any external libraries like p6spy/log4jdbc..?

You can disable the logging for P6Spy by simply changing your logging configuration. Just change it from INFO to WARN or higher to disable the logging.

Does Spring embedded database support different SQL dialects?

H2 has a range of compatibility modes for various other databases such as MS SQL Server, MySQL, Oracle, etc that support different SQL dialects. However, when setting up an embedded database in Spring I do not find any corresponding setting. Does this imply that I have to use "plain" SQL without any dialect specific features if I for example use Oracle in production and H2 during test? Have I overlooked something?

which version of H2 database? per the documentation, you can set compatible mode by SQL statement (http://www.h2database.com/html/features.html#compatibility)
SET MODE PostgreSQL
just add this statement into your first sql script file loaded by Spring jdbc embedded-database

According to the H2 doc, the Oracle compatibility mode is quite limited.
For instance, you can not use PL/SQL procedures.
If you use Spring's EmbeddedDatabase, you cannot set the compatibility mode as-is; you have to implement you own EmbeddedDatabaseConfigurer and specify the compatibility mode through the JDBC URL (see below).
But also, to use the compatibility mode with H2 and Spring, you just have to set the mode in your JDBC URL (so it is not Spring related) in a classic way, using a DataSource:
jdbc:h2:~/test;MODE=Oracle
And if you use Hibernate, you have to specify the Oracle dialect instead of the H2 one.

You have 2 options:
use spring to start the H2 database as follows (check setName() to see how to pass H2 specific URL parameters to spring builder):
Spring code generates the URL as follows:
String.format("jdbc:h2:mem:%s;DB_CLOSE_DELAY=-1", databaseName)
So, in setName() you can all any H2 specific parameter in the URL.
private DataSource dataSource() {
EmbeddedDatabaseBuilder builder = new EmbeddedDatabaseBuilder();
EmbeddedDatabase db = builder
.setType(EmbeddedDatabaseType.H2)
.setName("testdb;DATABASE_TO_UPPER=false;MODE=Oracle")
.addScript("schema.sql")
.addScript("data.sql")
.build();
return db;
}
configure directly the DB URL, sth like:
org.h2.jdbcx.JdbcDataSource dataSource = new org.h2.jdbcx.JdbcDataSource();
dataSource.setURL("jdbc:h2:testdb;MODE=MySQL;DATABASE_TO_UPPER=false;INIT=runscript from 'src/test/resources/schema.sql'\;runscript from 'src/test/resources/data.sql'");
The main different is that (2) is executing scripts defined at INIT for every database connection creation and not once per DB creation! This causes various issues, like INSERTs failing due to duplicate keys etc..