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For both single-module and multi-module libraries, any public class (and its public and protected methods) should be considered part of the API unless the class's package name contains a segment named "impl". For example, a public class with a qualified name of "net.shibboleth.utilities.java.support.security.Foo" is part of the API. A public class with a qualified name of "net.shibboleth.utilities.java.support.security.impl.Foo" is not part of the API. The unqualified name of the class itself is not a consideration, though it would be highly unusual to see an implementation API class called "FooImpl".
Multi-module libraries are now more common for the "major" software products, and result in a number of jars that collectively contain the code. In such cases, the modules are sometimes given a designation of "API" or "Implementation" This is deducible from the name of the module (and the name of the jar file), such that modules containing "-api" in the name are API modules and modules containing "-impl" in the name are Implementation modules. However, we follow the rule outlined above, and so all packages in an "-impl" module will contain "impl" in the name and no packages in an "-api" module will.
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