10.2.1 Implementing Interfaces with `proxy`

Implementing Interfaces with proxy in Clojure: A Detailed Guide

When bridging the gap between Java and Clojure, one of the vital tools you can utilize is the proxy macro. It enables developers to create anonymous classes in Clojure that can either implement interfaces or extend classes, thus leveraging the vast Java ecosystem within Clojure programs.

Understanding the proxy Macro

The proxy macro provides a powerful mechanism for creating instances of anonymous classes in Clojure. These anonymous classes can implement multiple interfaces and override methods as needed.

In its simplest form, the proxy macro syntax is:

(proxy [interface-names*] [constructor-arguments]
  (method-name [args*] method-body)
  ...)

• interface-names*: A vector of interfaces (and optionally a superclass) that the proxy class should implement or extend.
• constructor-arguments: A vector of arguments that are passed to the superclass constructor. If there is no superclass, this vector can be empty.
• method-name: The name of the method to override.
• args*: The parameters for the overridden method.
• method-body: The implementation of the method.

Example: Implementing a Java Interface with proxy

Consider implementing an instance of Java’s Runnable interface, which requires an implementation of the run method.

(def my-runnable
  (proxy [Runnable] []
    (run []
      (println "Running in a separate thread!"))))

(.start (Thread. my-runnable))

In this example, the proxy macro creates an anonymous class implementing Runnable. The run method is overridden to print a message.

Overriding Methods: Tips and Tricks

When using proxy to override methods, it helps to understand a few key principles:

• Argument Matching: Ensure the overridden method in the proxy class has the same arguments as the method being overridden.
• Super Method Invocation: Utilize (proxy-super method-name args*) if you need to call a method from the superclass within your overridden method.

Advantages and Use Cases

Using proxy offers several advantages:

• Interface Implementation: Easily implement multiple interfaces within a single proxy object.
• Leverage Java Libraries: Seamlessly use Java libraries and frameworks that rely on Java interfaces or abstract classes.

Comparison with Java Approach

Both Java and Clojure provide means to work with interfaces and abstract classes, but Clojure’s proxy is particularly effective for quick implementations of single-use interfaces without needing a separate class definition.

Here is a Java equivalent using anonymous classes:

Runnable myRunnable = new Runnable() {
    @Override
    public void run() {
        System.out.println("Running in a separate thread!");
    }
};

new Thread(myRunnable).start();

Chapter Summary

The proxy macro provides Clojure developers with a way to interoperate with Java’s class system effectively. By understanding and utilizing proxy, you can extend the capabilities of your Clojure applications, integrating them seamlessly with existing Java codebases.

To reinforce your learning, try creating proxies for different Java interfaces and observe how Clojure’s approach simplifies the implementation process.