Spring 2.5提供了自动在当前ClassPath搜索被标注有特定注解的类，这个特性非常有用，跟踪了一下源码，发现其实核心代码就是利用ClassLoader的方法：

public Enumeration<URL> getResources(String name)

于是自己动手，也写了一个能在ClassPath下搜索特定类的Scanner：

package com.javadou.util;


import java.io.File;
  import java.io.IOException;

import java.net.URL;

import java.util.ArrayList;

import java.util.Enumeration;

import java.util.List;

import java.util.jar.JarEntry;

import java.util.jar.JarFile;

public class ClassPathScanner {

    private static final String PROTOCOL_FILE = "file";
      private static final String PROTOCOL_JAR = "jar";

    private static final String PREFIX_FILE = "file:";

    private static final String JAR_URL_SEPERATOR = "!/";
      private static final String CLASS_FILE = ".class";

    private final String packageName;
      private final ClassFilter filter;

    public ClassPathScanner(String packageName) {
          this(packageName, null);

    }

    public ClassPathScanner(String packageName, ClassFilter filter) {
          this.packageName = packageName;

        this.filter = filter;

    }

public List<Class<?>> scan() {
          List<Class<?>> list = new ArrayList<Class<?>>();

        Enumeration<URL> en = null;

        try {

            en = getClass().getClassLoader().getResources(dotToPath(packageName));

        }

        catch(IOException e) {

            e.printStackTrace();

        }

        while (en.hasMoreElements()) {

            URL url = en.nextElement();

            if (PROTOCOL_FILE.equals(url.getProtocol())) {

                File root = new File(url.getFile());

                findInDirectory(list, root, root, packageName);

            }

            else if (PROTOCOL_JAR.equals(url.getProtocol())) {

                findInJar(list, getJarFile(url), packageName);

            }

        }

        return list;

    }

    public File getJarFile(URL url) {
          String file = url.getFile();

        if (file.startsWith(PREFIX_FILE))

            file = file.substring(PREFIX_FILE.length());

        int end = file.indexOf(JAR_URL_SEPERATOR);

        if (end!=(-1))

            file = file.substring(0, end);

        return new File(file);

    }

    void findInJar(List<Class<?>> results, File file, String packageName) {
          JarFile jarFile = null;

        String packagePath = dotToPath(packageName) + "/";

        try {

            jarFile = new JarFile(file);

            Enumeration<JarEntry> en = jarFile.entries();

            while (en.hasMoreElements()) {

                JarEntry je = en.nextElement();

                String name = je.getName();

                if (name.startsWith(packagePath) && name.endsWith(CLASS_FILE)) {

                    String className = name.substring(0, name.length() - CLASS_FILE.length());

                    add(results, pathToDot(className));

                }

            }

        }

        catch(IOException e) {

            e.printStackTrace();

        }

        finally {

            if (jarFile!=null) {

                try {

                    jarFile.close();

                }

                catch(IOException e) {}

            }

        }

    }

    void findInDirectory(List<Class<?>> results, File rootDir, File dir, String packageName) {
          File[] files = dir.listFiles();

        String rootPath = rootDir.getPath();

        for (File file : files) {

            if (file.isFile()) {

                String classFileName = file.getPath();

                if (classFileName.endsWith(CLASS_FILE)) {

                    String className = classFileName.substring(rootPath.length() - packageName.length(), classFileName.length() - CLASS_FILE.length());

                    add(results, pathToDot(className));

                }

            }

            else if (file.isDirectory()) {

                findInDirectory(results, rootDir, file, packageName);

            }

        }

    }

    void add(List<Class<?>> results, String className) {
          Class<?> clazz = null;

        try {

            clazz = Class.forName(className);

        }

        catch(ClassNotFoundException e) {

            return;

        }

        if (filter==null || filter.accept(clazz))

            results.add(clazz);

    }

    String dotToPath(String s) {
          return s.replace('.', '/');

    }

    String pathToDot(String s) {
          return s.replace('/', '.').replace('\\', '.');

    }

}

public interface ClassFilter {

    boolean accept(Class<?> clazz);

}

用法：

    public static void main(String[] args) {
          List<Class<?>> list = new ClassPathScanner(

                "org.springframework.core.io",

                new ClassFilter() {

                    public boolean accept(Class<?> clazz) {
                          return clazz.isInterface();

                    }

                }

        ).scan();

        for (Class<?> clazz : list) {

            System.out.println(clazz);

        }

    }