.NET: Db4o.Configure().TestConstructors(false)
will prevent db4o from creating a test instance of persistent classes upon opening a database file.
Upon system startup, db4o attempts to create a test instance of all persistent classes, to ensure that a public zero-parameter constructor is present. This process can take some time, if a large number of classes are present in the database file. For the best possible startup performance this feature can be turned off.
In any case it's always good practice to create a zero-parameter constructor. If this is not possible because a class from a third party is used, it may be a good idea to write a translator that translates the third party class to one's own class.