Operations, or "op"s, are algorithms that fulfill a particular contract. To be an Op, an algorithm must:

• implement one FunctionalInterface.
• Have n inputs and one output.
• Be stateless; given the same inputs, an Op must produce the same output

By fulfilling this contract, Ops provide a few key benefits:

• Ease of construction: Ops require only one method to provide the needed functionality, and can be written as, for example, a lambda expression
• Ease of use: Ops By exposing only one method, Ops leave no ambiguity in the way they should be run
• Reusability and Reproducability: Without any state, Ops maintain speed, and operate the same whether run once or millions of times
• Simplicity: Ops can only provide one output; this keeps Ops small and focused, encouraging code reuse to construct more complex algorithms.

SciJava Ops SPI is currently tied to the Plugin discovery mechanisms of SciJava Common. While we may or may not switch to Spring in the future, there are three main ways of declaring Ops:

Classes can be discovered as an Op using the @Plugin annotation with type = Op.class, adding Plugin metadata as you see fit:

@Plugin(type = Op.class, name = "foo.bar")
public class FooBarOp implements Producer<String> {

	@Override
	public void create() {
		return "foobar";
	}

}

Fields can be discovered within classes annotated using the @Plugin annotation with type = OpCollection.class. The Fields themselves should then be annotated using the @OpField annotation.

@Plugin(type = OpCollection.class)
public class FieldOpCollection {

	@OpField(names = "foo.bar")
	public final Producer<String> fooBarField = () -> "foobar";
}

Just like Fields, Methods can be discovered within classes annotated using the @Plugin annotation with type = OpCollection.class. The Methods themselves should then be annotated using the @OpMethod annotation. For methods, it is important to specify the type that the method is attempting to mimic.

@Plugin(type = OpCollection.class)
public class MethodOpCollection {

	@OpMethod(names = "foo.bar", type = Producer.class)
	public static String fooBarMethod() {
		return "foobar";
	}
}