扩展 Java 类并强制现有库使用扩展类

Question

The situation is as follows: I'm using some libraries that use a specific class to load and access configuration parameters. The config loader class is implemented in one of the libraries. What I did is: to extend the config loader class so that it fits my requirements and is able to load different config sources:

public class BetterConfigLoader extends OldConfigLoader {
    ...
}

Now I want to make the existing libraries use my compatible BetterConfigLoader without applying changes to the libraries or without the need to recompile them. Is there a way of best practice to accomplish this?

Answer 1

This depends on how your libraries refer to the configuration class. Do they have a hardcoded new OldConfigLoader().configure() ?

Or do they use some kind of SPI technique, eg checking for existence of a resource named META-INF/services/ConfigLoader , and only if not existing, falling back to the default configuration class?

Instead of checking such a resource, they could as well check a System Property. In these cases, you can set your special class by creating a matching SPI resource, or by setting the System Property.

In case of hardcoded reference, you are out of luck.

Answer 2

Instead of extending OldConfigLoader you can replace it. Start with the source code of OldConfigLoader, change it to suit your needs and then make sure your version is higher in the classpath then the original. Make sure your implementation has the same classname and is in the same package as the original.