Generic builder DSL

AbstractAppender.java

public abstract class AbstractAppender extends AbstractFilterable implements Appender {
    public abstract static class Builder<T extends AbstractAppender, B extends Builder<T, B> & org.apache.logging.log4j.core.util.Builder<T>>
        extends AbstractFilterable.Builder<T, B> {
        // ...
    }
}

AbstractFilterable.java

public abstract class AbstractFilterable extends AbstractLifeCycle implements Filterable {
    public abstract static class Builder<T extends AbstractFilterable, B extends Builder<T, B> & org.apache.logging.log4j.core.util.Builder<T>>
        implements org.apache.logging.log4j.core.util.Builder<T> {
        // ...
    }
}

MockAppender.java

public class MockAppender extends AbstractAppender {
    public static class Builder<B extends Builder<B> & org.apache.logging.log4j.core.util.Builder<MockAppender>>
        extends AbstractAppender.Builder<MockAppender, B> {
        // ...
        public MockAppender build() {
            return new MockAppender();
        }
    }

    @PluginBuilderFactory
    public static <B extends Builder<B> & org.apache.logging.log4j.core.util.Builder<MockAppender>> Builder<B> newBuilder() {
        return new Builder<>();
    }

    public static <B extends Builder<B>> MockAppender createMockAppender(final String name, ...) {
        return MockAppender.<B>newBuilder()
            .setFoo(foo)
            .setBar(bar)
            // ...
            // notice that ancestor builders are chained in order of closest parent as the generic type narrows
            .withName(name)
            .withIgnoreException(ignoreExceptions)
            .withLayout(layout)
            // with the top-most builder last
            .withFilter(filter)
            .build();
    }
}