加载类路径中不存在的类 [英] Loading classes not present in the classpath

问题描述

假设我已经使用 Groovyc 编译了一个 Groovy 脚本，它在文件系统中生成了一个或多个 .class 文件.在 Java 应用程序中，如何将这些类动态添加到类路径以加载它们并调用它们的方法?目标是预编译 Groovy 脚本并将它们存储到数据库中，因此可以从脚本的编译版本执行评估.

Let's say I've compiled a Groovy script using Groovyc, which has generated one or more .class files in the file system. From a Java application, how do I add those classes to the classpath dynamically in order to load them and call their methods? The goal is to pre-compile Groovy scripts and store them into the database, so evaluation can be performed from compiled versions of the scripts.

推荐答案

您可以创建URLClassLoader 从目录加载新类:

You can create an instance of URLClassLoader to load new classes from a directory:

URL dirUrl = new URL("file:/" + "path_to_dir" + "/");             // 1
URLClassLoader cl = new URLClassLoader(new URL[] {dirUrl},
                             getClass().class.getClassLoader());  // 2
Class loadedClass = cl.loadClass("com.xyz.MyClass");
MyClass obj = (MyClass) loadedClass.newInstance();
obj.doSomething();

第 1 行创建 .class 文件所在目录的 URL.

Line 1 creates the URL to the directory where the .class files reside.

第 2 行创建一个新的 URLClassLoader 实例.第一个参数是要用作源的 URL 数组.您可以在数组中指定多个目录 URL.第二个参数是将成为这个新类加载器的父类的类加载器.我们将执行上述代码的类的类加载器作为这个参数传递.

Line 2 creates a new URLClassLoader instance. First argument is an array of URLs to be used as the source. You can specify multiple directory URLs within the array. Second argument is the classloader that will become the parent of this new classloader. We pass the classloader of the class executing the above code as this argument.

子类加载器加载的类可以访问父类加载器加载的类.

The classes loaded by a child classloader can access the classes loaded by the parent classloader.

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