如何在Clojure中实现Java接口

Question

如何创建实现此接口的Clojure对象,然后从Java代码调用？

public interface Doer {
   public String doSomethin(String input);
}

Doer clojureDoer = ?;

String output = clojureDoer.doSomethin(input);

Answer 1

reify非常适合实现接口 - proxy重载,旧,慢,所以应尽可能避免使用.实现看起来像:

(reify Doer
  (doSomethin [this input]
    (...whatever...)))

请注意,关于使用的现有答案reify具有不正确的语法,如果您决定最后使用代理:proxy采用隐式proxy参数,而不是命名的第一个参数.

Answer 2

从Clojure 1.6开始,首选方法如下.假设您在类路径上有Clojure 1.6 jar和以下clojure文件(或其编译的等效文件):

(ns my.clojure.namespace
  (:import [my.java.package Doer]))

(defn reify-doer
  "Some docstring about what this specific implementation of Doer
  does differently than the other ones. For example, this one does
  not actually do anything but print the given string to stdout."
  []
  (reify
    Doer
    (doSomethin [this in] (println in))))

然后,从Java,您可以按如下方式访问它:

package my.other.java.package.or.maybe.the.same.one;

import my.java.package.Doer;
import clojure.lang.IFn;
import clojure.java.api.Clojure;

public class ClojureDoerUser {
    // First, we need to instruct the JVM to compile/load our
    // Clojure namespace. This should, obviously, only be done once.
    static {
        IFn require = Clojure.var("clojure.core", "require");
        require.invoke(Clojure.read("my.clojure.namespace"));
        // Clojure.var() does a somewhat expensive lookup; if we had more than
        // one Clojure namespace to load, so as a general rule its result should
        // always be saved into a variable.
        // The call to Clojure.read is necessary because require expects a Clojure
        // Symbol, for which there is no more direct official Clojure API.
    }

    // We can now lookup the function we want from our Clojure namespace.
    private static IFn doerFactory = Clojure.var("my.clojure.namespace", "reify-doer");

    // Optionally, we can wrap the doerFactory IFn into a Java wrapper,
    // to isolate the rest of the code from our Clojure dependency.
    // And from the need to typecast, as IFn.invoke() returns Object.
    public static Doer createDoer() {
        return (Doer) doerFactory.invoke();
    }
    public static void main(String[] args) {
        Doer doer = (Doer) doerFactory.invoke();
        doer.doSomethin("hello, world");
    }
}

Answer 3

有代理

看proxy宏.Clojure Docs有一些例子.它也包含在Java Interop页面上.

(proxy [Doer] []
  (doSomethin [input]
    (str input " went through proxy")))

proxy返回一个实现的对象Doer.现在,要使用Java访问它,您必须使用它gen-class来使您的Clojure代码可以从Java调用.它涵盖了"从java调用clojure"问题的答案.

有了gen-class

(ns doer-clj
  (:gen-class
    :name DoerClj
    :implements [Doer]
    :methods [[doSomethin [String] String]]))

(defn -doSomethin
  [_ input]
  (str input " went through Clojure"))

现在保存为doer_clj.clj,mkdir classes并通过调用REPL进行编译(require 'doer-clj) (compile 'doer-clj).您应该DoerClj.class已准备好在classes目录中使用Java

Answer 4

有关这个问题的更一般性的看法,当您需要某种Java-interop时,此图表可能非常有用:

https://github.com/cemerick/clojure-type-selection-flowchart