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How to check if you're still connected to the database with jpa

I'm trying to check the database connection with jpa using the EntitityManager/Session class.
To check both cases (connected/not connected), I simply start/deactivate the service mysql before running the code.
Asking simple sql queries to the database is out of mind since it doesn't cover all cases.
I already tried:
Session session = entityManager.unwrap(Session.class)
session.isConnected();
But that always returns true...
When I disable the mySQL service I want session.isConnected() to return false, but it always returns true;
Is there any other way to check for the database connection(using any other classes maybe)?

Short answer: don't
Long answer: if you're developing an application where this is important, you will probably be using some sort of database connection pooling framework, like the
Tomcat JDBC pool
Apache Commons DBCP
HikariCP
C3P0 Connection pool
This will manage many important aspects for you, including connection lifecycle. In tomcat jdbc for example you can specify:
testOnBorrow="true"
validationQuery="select 1 from sysibm.dual"
This on an IBM DB2 database will test whether a connection went stale every time you use it, and also swap it out for a fresh one if the select fails. Done this way, you will never have to worry about this on the level of you application code.

How to handle re-connects to the DB with ActiveJDBC

I've a Spring Boot Java application in production that uses ActiveJDBC to access a MariaDB database.
If at launch the application boots before the db server, of if the db server crashes and restarts, the apps doesn't re-estabilish the connection with the db.
ActiveJDBC is on version 1.4.13 and if possible I'd prefer not upgrading it, to avoid possible breakages. The db parameters are configured using the database.properties file and typically the usage pattern is:
try {
Base.open();
...
} finally {
Base.close();
}
Is there a way to circumvent this problem, without monitoring and relaunching the application? Maybe using connection pools? If this is the case, are there any docs or examples?

If you are using a direct JDBC connection in your database.properties file, a new connection will be open every time you execute Base.open(). This means that any old connection that is broken is not relevant anymore. If you use a JDNI pool such as:
production.jndi=java:comp/env/jdbc/acme
then you want to configure your containers' pool to ensure that every connection served from the pool is valid just before the pool serves the connection to your app. It is up to the implementation and documentation of your container/pool how to do that.
In any case, I do not think you are going to have issues.

Connection pool for dynamic database connections

The problem setup is based on a webservice (Spring/Java, Tomcat7 and MySql) where every user gets their own database, hence each request needs their own connection. As all databases are created dynamically during runtime, configuring them statically before startup is not an option.
To optimise database connection usage, an implementation of a database connection pool would be great, right?
With Java/Spring: How would I create a connection pool for dynamic databases? I am a bit struck by the lack of clean options here!
Problem: Tomcat's Connection Pool (and as far as i understand C3P0 as well) treats each new DataSource instance as a whole new connection pool -> stack-reference
Is it a good idea to create a static datasource with a generic MySql connection (without specifing the database on connection) and use a connection pool with this datasource together with adapted SQL statements?
stack-reference
What about developing a custom persistent database based datasource pool? Any experience with performance here? Any advice? Any libraries that do that?
Or would it be feasable to workaround Tomcat's DataSource problem by creating Tomcat JNDI Datasources dynamically by manipulating it's context.xml dynamically from Java?
I can't believe that there aren't more plain/simple solutions for this. Grails/Hibernate struggles with this, Java/JDBC struggles with this, ... is it such a rare use-case to separate userdata on a user basis by creating user specific databases dynamically? If so, what would be a better setup?
EDIT
Another option is the suggestion from #M.Deinum to use a single configured datasource and dynamically hotswap it for the right connection ->M.Deinum Blog and stack-reference. How does that perform with a connection pool like the ones above?

I believe that HikariCP works without having to specify a single database.

Once the databases are created in runtime, you have to create the pools also in runtime. I am afraid the spring infrastructure is not giving you any help here, as it is tuned for the usual static use case.
I'd have a map of pools:
have a Map < connectionUrlString,List< c3poPool > > map
when requesting a connection, get the corresponding c3po pool from the map
and you can get the best of both worlds, since the real connection pool for each dynamically created database is handled by a c3po instance, but you can create new instances in runtime
This works as a low-level solution. If you want to go further, you can wrap this logic into a db connection provider, and register that as a "driver". This way any part of your application requests a new connection, you can just return one connection from the existing pools (and if a totally new connection is requested, create a new pool for that).

First than all, sorry for my english, i'm improving every day.
In my experience, I had a similar situation and it was resolve with spring framework. Let me explain you how you'd solve that question.
Make a spring config file with these characteristics:
a) A resource loader: This one is the responsible of load properties from configurations files or from database, those properties will be the appropriates to establish the database connection.
b) A pool database configuration parameterized with the properties that you'll load.
Create a locator class: In this class you'll need a HashMap
Use the multi context feature of spring: The idea is assign a code to every one connection that you establish and later load that connection like an application context with spring, then in the locator class, put in the map that context and use it as frequent as you need.
I think is you follow these steps, you can create dynamic pool or database connection as you want.

Efficient way to validate a DB connection

The DB driver I am working with (for a sybase DB) does not implement a Connection.isValid(). What is the most efficient way I can validate a db connection using a query (or otherwise) in Java?

Libraries such as C3P0 and DBCP allow you to provide a validation query which is typically something very simple such as "select 1". Hence, you could take the same approach or simply use either of these libraries (my recommendation would be C3P0).
However, rather than testing the connection prior to executing your query you could simply attempt to execute and then retry the operation if it fails because the connection is invalid. Alternatively you could consider a non-pooled connection approach where the connection is created on-the-fly each time (e.g. such as Spring's DriverManagerDataSource).

I mostly agree with Adamski's comment about using "select 1 from table" as an efficient way of checking connectivity and using connection pooling (e.g. commons-dbcp, C3PO).
Application servers (e.g. Websphere) allow you to configure this validation for you so your applciation code doesn't have to know about it. You have the choice of always having the connection checked prior to using it, or the connection pool being validated when a new connections is created. You can also purge connections periodically in case they get stale.
If you're not running in an application server you can use Common DBCP with the properties described here:
http://commons.apache.org/dbcp/configuration.html
Or C3PO and take a look at using the idleConnectionTestPeriod property. This link talks about use with Hibernate but its still relevant for C3PO generally
http://community.jboss.org/wiki/HowToConfigureTheC3P0ConnectionPool

Context specific usernames for connections on a Java EE Datasource (JBoss 5.1)

We have an application that needs to access a database that is owned by a different team.
That database has security inside the database (triggers, table permissions, etc) and so we need to establish a connection to the database using the same username/password that connected to our EJB.
We're running on JBoss 5.1. Standard Java EE solutions are preferred, but JBoss specific answers will do.
At the moment our solution is
Create a datasource in JBoss with no user-id password
Require the client to pass their username/password into the EJB (the EJB is a stateful session bean (SFSB), and remembers the username/password)
The session bean creates a new connection using DataSource.getConnection(String, String)
The connection is "created" from the datasource at the start of each request (The datasource implementation might reuse an existing connection)
The main problem we have is connection pooling.
The JBoss connection pool doesn't manage separate pools for each username - they're all thrown into 1 big pool, and the username is checked after the object is retrieved from the pool (inside InternalManagedConnectionPool).
If the usernames don't match, then the connection is removed from the pool & destroyed.
This means that as soon as we have 2 users, there's a 50% chance that any connection that is put into the pool will be destroyed when it is next accessed. And as we increase the number of users, those odds get a lot worse.
We can't simply create 1 connection in the SFSB and retain it because JBoss is too smart for us, and it detects that we've left a connection open and automatically returns it to the pool for us, so the next request to the SFSB will fail with a "not associated" connection.
It would also be nice if we could simply get JBoss to create a connection as "the currently logged in user", but the solution we have is bearable.
My googling has failed to find any recommended patterns for doing this sort of thing. Everyone seems to assume that you want your datasource to use a single user for all connections (which is nice when it's possible, but I can't do that in this case)
The only solutions I can some up with are
Don't use a container provided datasource. Put the JDBC URL into a configuration value somewhere and create connections myself (possibly with the help of spring)
Bind a different Datasource implementation (possibly a custom one) into JNDI
Has anyone got any better solutions? Or pointers to recommended practices in this area?
(The database is Sybase ASE 15, but I doubt that makes any difference to the solution)

A bit more reading of the documentation has led me to what appears to be the solution.
Adding
<application-managed-security/>
into my datasource file seems to have fixed the problem of pooling the connections.
I'm not sure how I missed that the first time around.