Clojure：我如何需要一个类并调用静态方法？ [英] Clojure: how do I require a class and call a static method?

问题描述

只是进入Clojure，我在命名空间，路径和类语法上遇到了一些麻烦。

Just getting into Clojure, and I'm having a little trouble with namespaces, paths and class syntax.

我首先测试并编写了一个简单的hello world函数

I started by testing and writing a simple hello world function.

test / test_subject_test.clj：

test/test_subject_test.clj:

(ns test-subject-test
  (:require [clojure.test :refer :all]
            [test-subject :refer :all]))

(deftest testit
  (testing "All good?"
    (is (= "I got this" (subject))))

src / test_subject.clj：

src/test_subject.clj:

(ns test-subject)

(defn subject []
  "I got this")

一切正常，所以我决定尝试在下一个类上调用静态方法。

That all works fine, so I decided to try calling a static method on a class next.

test / test_subject_test.clj：

test/test_subject_test.clj:

(ns test-subject-test
  (:require [clojure.test :refer :all]
            [test-subject :refer :all]
            [hello :refer :all]))

(deftest testit
  (testing "All good?"
    (is (= "I got this" (subject))))
  (testing "Try calling a static method"
    (is (= "Hello Guy!" (-handler Hello "Guy")))))

src / hello.clj：

src/hello.clj:

(ns hello
  (:gen-class :name "Hello"
              :methods [^:static [handler [String] String]]))

(defn -handler [s]
  (str "Hello " s "!"))

但是现在编译器抛出一条带有一条有趣消息的长堆栈跟踪：由以下原因引起：java.lang.RuntimeException：无法解析符号：在这种情况下，您好。我已经尝试过：require 语句和（-handler Hello）调用的一些排列，尝试不使用：refer：all 并将该方法称为（-handler（.Hello hello））我缺少有关类在Clojure中如何工作的真正基础知识。

But now the compiler throws a long stack trace with one interesting message: Caused by: java.lang.RuntimeException: Unable to resolve symbol: Hello in this context. I've tried a few permutations of the :require statement and the (-handler Hello) call, I've tried not using :refer :all and referring to the method as (-handler (.Hello hello)) but clearly I'm missing something really basic about how classes work in Clojure.

我查看了 Clojure vars和Java静态方法，但是对于我要执行的操作来说，它似乎比必需的方法复杂得多：只需调用一个静态方法即可。但是，这个问题中有一个有趣的引号，使我尝试这样做：（Hello处理程序 Guy）没有成功（无法解析符号：在这种情况下，您好）。

I had a look at Clojure vars and Java static methods, but it seems way more complex than necessary for what I am trying to do: just call a static method. However, there was one interesting quote in the question, which led me to try this: (Hello handler "Guy") without success (Unable to resolve symbol: Hello in this context).

那么，我应该如何从另一个文件中请求一个类并对其调用静态方法？

So, how should I require a class from another file and call a static method on it?

推荐答案

如果您将一个类编译到默认包中（这就是您在此处所做的操作），那么所有这些都会稍有不同，因为您无法导入这些类。我不建议您这样做。在下面的代码中，我假装您实际上写了（：gen-class：name hello.Hello ...）

All this is slightly different if you're compiling a class into the default package (which is what you're doing here) since you can't import those classes. I don't recommend you do that. In the following code I'm pretending you actually wrote (:gen-class :name "hello.Hello" ...)

要在Java类上调用方法，您应该使用完全限定的类名或首先使用 import 该类：

To call methods on a java class you should either use the fully qualified classname or first import the class:

(import 'hello.Hello)
(Hello/handler "there")

或

(hello.Hello/handler "there")

如您所见，您应该使用/调用Java类上的静态方法。

As you can see, you should use / to call static methods on java classes.

要使所有这些与gen-class一起使用，必须确保首先编译这些类。确保 的最简单方法是先 require 其名称空间：

For this all to work with gen-class you must make sure that the classes are compiled first. The simplest way to guarantee that is to require its namespace first:

(require 'hello)
(import 'hello.Hello)

因为这比使用普通Clojure函数&命名空间，如果您只想从clojure调用代码，则可能不应该使用Java interop。

Because this is all more trouble than using plain clojure functions & namespaces, you probably shouldn't be using java interop if you're only intending to call the code from clojure.

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