java - 自定义类加载器问题

Question

接下来的问题是:我从 here 获取了基类加载器代码。但我的类加载器在某种程度上是特定的，它必须能够从文件系统加载类(以 WinOS 为例)，因此在类加载器中必须有一些 setAdditionalPath() 方法，该方法设置路径(a文件系统上的目录)，我们将从中加载类(仅 *.class，无 jar)。这是代码，它从链接修改加载器(您可以看到，仅修改了 loadClass)，但它无法正常工作:

public void setAdditionalPath(String dir) {
            if(dir == null) {
                throw new NullPointerException("");
            }

            this.Path = dir;
        }

        public Loader(){
              super(Loader.class.getClassLoader());
        }


        public Class loadClass(String className) throws ClassNotFoundException {
          if(Path.length() != 0) {
            File file = new File(Path);

            try {
                // Convert File to an URL

         URL url = file.toURL();          
                URL[] urls = new URL[]{url};

                // Create a new class loader with the directory
                ClassLoader cl = new URLClassLoader(urls);


                ClassLoader c = cl.getSystemClassLoader();
                Class cls = c.loadClass(className);
                return cls;

            } catch (MalformedURLException e) {

            } catch (ClassNotFoundException e) {

            }

        }
            return findClass(Path);
        }

如果有人帮忙，我将不胜感激:)

Answer 1

您可以只使用提供的框架java.net.URLClassLoader 。无需自己编写。它支持从目录和 JAR 文件加载类。

Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed.

它还支持父类加载器。如果这个类加载器不适合您的要求，也许您可​​以更详细地指定您需要的内容。无论如何，您可以查看源代码并基于它派生您自己的类加载器类。

下面是一个简短的工作代码片段，它演示了如何从 URLClassLoader 按名称加载类:

    ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();

    // This URL for a directory will be searched *recursively*
    URL classes =
        new URL( "file:///D:/code/myCustomClassesAreUnderThisFolder/" );

    ClassLoader custom = 
        new URLClassLoader( new URL[] { classes }, systemClassLoader );

    // this class should be loaded from your directory
    Class< ? > clazz = custom.loadClass( "my.custom.class.Name" ); 
    // this class will be loaded as well, because you specified the system 
    // class loader as the parent
    Class< ? > clazzString = custom.loadClass( "java.lang.String" );