In this tutorial we’re going to show how to use Clojure proxy to mock Java classes and provide own functionality for methods.

Calls superclass default constructor:

(def my-list
  (proxy [ArrayList] []
    (get [i] 42)))

Let’s use it:

user> (.get my-list 10)
42

Superclass has no no-arg constructor:

(proxy [FileInputStream] []
        (read []
          42))

Produces an exception, because there’s no no-arg constructor in FileInputStream class:

CompilerException java.lang.IllegalArgumentException: No matching ctor found for class user.proxy$java.io.FileInputStream$ff19274a

We have to provide obligatory constructor parameter (here “/proc/version”), that will be passed to superclass:

;; To count calls and return predictive values:
(def counter (atom 0))

(def my-mock
  (proxy [FileInputStream] ["/proc/version"]
    (read []
      (swap! counter inc))))

Now our version of read method may be called as expected:

user> (.read my-mock)
1
user> (.read my-mock)
2
user> (.read my-mock)
3

(def counter (atom 0))

(def my-mock
  (proxy [FileInputStream] ["/proc/version"]
    (read
      ([] (swap! counter inc)) ; read()
      ([^bytes arr]            ; read(byte[])
       (aset-byte arr 0 42)
       (swap! counter inc)))))

read(byte[]) can be called like this:

user> (.read my-mock (byte-array 3))
1
user> (.read my-mock (byte-array 3))
2

• proxy macro
• ClojureDocs – proxy