How to include a config file in the "META-INF/services" folder of a JAR using Maven

I'm working on a Java project in Eclipse, which is built using Maven. I'm using some recycled code from an older project, and one of these classes looks for a file in the META-INF/services folder of the JAR with a particular name, then parses the text of this file. In this particular example, it looks for a file with the name of a Java interface, then grabs the class name of the implementation from inside the file.

So basically what I'm trying to do is include a file in the "META-INF/services" folder of the JAR with a file name (X), and one line of text (Y). I'm guessing this should be done using Maven, maybe by specifying an attribute in the POM file, but my research hasn't turned up anything. I know you're not supposed to hard-code or manually type out any META files, so I'm not sure what to do here.

Solution

Create a new source folder with the location src/main/resources, then create your META-INF/services folder in there and drop in your fully-qualified class name (FQCN) file. This should copy them into the jar file automatically. So for implementations of an interface with a FQCN of com.acme.MyInterface, you'll have:

Project
| src
| | main
|   | java
|     | [your source code]
|   | resources
|     | META-INF
|       | services
|         | com.acme.MyInterface

Note that com.acme.MyInterface is the name of the file, not a directory structure like a Java package. The name of the file is the FQCN of the interface that you're implementing, and in it, you'll have the FQCN of each implementation on its own line, e.g.:

com.example.MyInterfaceImpl
com.example.AnotherMyInterfaceImpl

It's worth noting that this applies to Gradle projects with the default source sets as well.

Once you do this, you can load all the implementations of the interface using ServiceLoader:

ServiceLoader<MyInterface> loader = ServiceLoader.load(MyInterface.class);
for (MyInterface service : loader) {
    // Prints com.example.MyInterfaceImpl and com.example.AnotherMyInterfaceImpl
    System.out.println(service.class.getName());
}

Some things to note:

All the implementations must have a no-args constructor
Applications that use modules or custom classloaders may have to use the overloads of ServiceLoader.load

If these conditions won't work for you, then you may want to switch to another system, e.g. a CDI-style framework like Spring, EJB, etc.