> FBConnectionPoolDataSource xaDatasource = new
> FBConnectionPoolDataSource();
> xaDatasource.setType(GDSType.getType("PURE_JAVA").toString());

You can simply use

xaDatasource.setType("PURE_JAVA");

xaDatasource.setDatabase("localhost/3050:D:/Projects/test/test.fdb");

> xaDatasource.setMinConnections(0);
> xaDatasource.setMaxConnections(2);
> xaDatasource.setPingInterval(5000);
> Properties p = new Properties();
> p.setProperty(FBDriver.USER, "sysdba");
> p.setProperty(FBDriver.PASSWORD, "masterkey");
> p.setProperty("charSet", "ISO8859_1");

Where is following code?

xaDatasource.setProperties(p);

> but as soon as I try to get a connection
> xaDatasource.getXAConnection();
> the driver does not return anymore. Looking at the code I noticed that
> in PooledConnectionQueue.take() the driver wasn't able to get a
> connection because userName and password failed, although correctly
> initialized, and entered an infinite loop.

Inifine loop or a very long loop? Try to set blocking timeout (default
value is Integer.MAX_VALUE).

> FBConnectionPoolDataSource.getPooledConnection() actually is
> hardcoded to use EMPTY_USER_PASSWORD instead of the previously
> provider credentials.

that's not empty user name and password, but a constant saying that no
user and password were supplied in the method call. This object is
used to find the appropriate conection queue in the map, that it turn
uses credentials supplied to the data source.

> Getting a connection as follows works:
> xaDatasource.getXAConnection("masterkey", "sysdba");

... but your character encoding for the connection is NONE, not
ISO8859_1, right?

Roman