为什么URLClassLoader不加载类？

Question

我已经下载了aspectJ.jar文件并将其解压缩到一个临时目录中(此jar文件仅用于测试目的)。我有以下代码：

public class TestUrlClassLoader {
    public static void main(String [] args) throws MalformedURLException, ClassNotFoundException {
        File file = new File("C:\\temp\\org\\aspectj\\lang\\JoinPoint.class");
        URLClassLoader loader = URLClassLoader.newInstance(new URL[]{file.toURI().toURL()});
        Class<?> aClass = loader.loadClass("org.aspectj.lang.JoinPoint");
        System.out.println(aClass.toString());
    }
}

存在一个包含JoinPoint类的文件，但我得到了一个ClassNotFoundException。有人能帮我吗？

Answer 1

根据URLClassLoader的javadoc，

[...] (URLClassLoader) is used to load classes and resources from a search path of URLs referring to both JAR files and directories. 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.

您应该删除JoinPoint.class的完整路径，并尝试使用要添加到类路径的目录，例如

File file = new File("C:\\temp");
URLClassLoader loader = URLClassLoader.newInstance(new URL[]{file.toURI().toURL()});

对javadoc的引用：http://docs.oracle.com/javase/7/docs/api/java/net/URLClassLoader.html