获取指定package下指定类的子类：

    /**
     * 获取指定package下 superType 的子类
     * @param packageName 包名
     * @param superType 指定类
     * @return 指定package下 superType 的所有子类
     */
    public static Set<Class<? extends Class<?>>> getChildrenClasses(String packageName, Class<?> superType) {
        ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<Class<?>>();
        resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
        Set<Class<? extends Class<?>>> classes = resolverUtil.getClasses();
//        Set<Class<? extends Class<?>>> results = new HashSet<Class<? extends Class<?>>>();
//        for (Class<? extends Class<?>> clazz : classes) {
//            //过滤掉 匿名内部类、接口 以及 抽象类
//            if (!clazz.isAnonymousClass() && !clazz.isInterface() && !Modifier.isAbstract(clazz.getModifiers())) {
//                results.add(clazz);
//            }
//        }
        return classes;
    }
}

ResolverUtil 类：

import java.io.IOException;
import java.lang.annotation.Annotation;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;

/**
 * 根据指定的条件查找指定包下的类，其中使用的条件由 Test 接口表示
 */
public class ResolverUtil<T> {
  /*
   * An instance of Log to use for logging in this class.
   */
  private static final Log log = LogFactory.getLog(ResolverUtil.class);

  /**
   * A simple interface that specifies how to test classes to determine if they
   * are to be included in the results produced by the ResolverUtil.
   */
  public static interface Test {
    /**
     * Will be called repeatedly with candidate classes. Must return True if a class
     * is to be included in the results, false otherwise.
     */
    boolean matches(Class<?> type);
  }

  /**
   * A Test that checks to see if each class is assignable to the provided class. Note
   * that this test will match the parent type itself if it is presented for matching.
   */
  public static class IsA implements Test {
    private Class<?> parent;

    /** Constructs an IsA test using the supplied Class as the parent class/interface. */
    public IsA(Class<?> parentType) {
      this.parent = parentType;
    }

    /** Returns true if type is assignable to the parent type supplied in the constructor. */
    @Override
    public boolean matches(Class<?> type) {
      return type != null && parent.isAssignableFrom(type);
    }

    @Override
    public String toString() {
      return "is assignable to " + parent.getSimpleName();
    }
  }

  /**
   * A Test that checks to see if each class is annotated with a specific annotation. If it
   * is, then the test returns true, otherwise false.
   */
  public static class AnnotatedWith implements Test {
    private Class<? extends Annotation> annotation;

    /** Constructs an AnnotatedWith test for the specified annotation type. */
    public AnnotatedWith(Class<? extends Annotation> annotation) {
      this.annotation = annotation;
    }

    /** Returns true if the type is annotated with the class provided to the constructor. */
    @Override
    public boolean matches(Class<?> type) {
      return type != null && type.isAnnotationPresent(annotation);
    }

    @Override
    public String toString() {
      return "annotated with @" + annotation.getSimpleName();
    }
  }

  /** The set of matches being accumulated. */
  private Set<Class<? extends T>> matches = new HashSet<Class<? extends T>>();

  /**
   * The ClassLoader to use when looking for classes. If null then the ClassLoader returned
   * by Thread.currentThread().getContextClassLoader() will be used.
   */
  private ClassLoader classloader;

  /**
   * Provides access to the classes discovered so far. If no calls have been made to
   * any of the {@code find()} methods, this set will be empty.
   *
   * @return the set of classes that have been discovered.
   */
  public Set<Class<? extends T>> getClasses() {
    return matches;
  }

  /**
   * Returns the classloader that will be used for scanning for classes. If no explicit
   * ClassLoader has been set by the calling, the context class loader will be used.
   *
   * @return the ClassLoader that will be used to scan for classes
   */
  public ClassLoader getClassLoader() {
    return classloader == null ? Thread.currentThread().getContextClassLoader() : classloader;
  }

  /**
   * Sets an explicit ClassLoader that should be used when scanning for classes. If none
   * is set then the context classloader will be used.
   *
   * @param classloader a ClassLoader to use when scanning for classes
   */
  public void setClassLoader(ClassLoader classloader) {
    this.classloader = classloader;
  }

  /**
   * Attempts to discover classes that are assignable to the type provided. In the case
   * that an interface is provided this method will collect implementations. In the case
   * of a non-interface class, subclasses will be collected.  Accumulated classes can be
   * accessed by calling {@link #getClasses()}.
   *
   * @param parent the class of interface to find subclasses or implementations of
   * @param packageNames one or more package names to scan (including subpackages) for classes
   */
  public ResolverUtil<T> findImplementations(Class<?> parent, String... packageNames) {
    if (packageNames == null) {
      return this;
    }

    Test test = new IsA(parent);
    for (String pkg : packageNames) {
      find(test, pkg);
    }

    return this;
  }

  /**
   * Attempts to discover classes that are annotated with the annotation. Accumulated
   * classes can be accessed by calling {@link #getClasses()}.
   *
   * @param annotation the annotation that should be present on matching classes
   * @param packageNames one or more package names to scan (including subpackages) for classes
   */
  public ResolverUtil<T> findAnnotated(Class<? extends Annotation> annotation, String... packageNames) {
    if (packageNames == null) {
      return this;
    }

    Test test = new AnnotatedWith(annotation);
    for (String pkg : packageNames) {
      find(test, pkg);
    }

    return this;
  }

  /**
   * Scans for classes starting at the package provided and descending into subpackages.
   * Each class is offered up to the Test as it is discovered, and if the Test returns
   * true the class is retained.  Accumulated classes can be fetched by calling
   * {@link #getClasses()}.
   *
   * @param test an instance of {@link Test} that will be used to filter classes
   * @param packageName the name of the package from which to start scanning for
   *        classes, e.g. {@code net.sourceforge.stripes}
   */
  public ResolverUtil<T> find(Test test, String packageName) {
    String path = getPackagePath(packageName);

    try {
      List<String> children = VFS.getInstance().list(path);
      for (String child : children) {
        if (child.endsWith(".class")) {
          addIfMatching(test, child);
        }
      }
    } catch (IOException ioe) {
      log.error("Could not read package: " + packageName, ioe);
    }

    return this;
  }

  /**
   * Converts a Java package name to a path that can be looked up with a call to
   * {@link ClassLoader#getResources(String)}.
   * 
   * @param packageName The Java package name to convert to a path
   */
  protected String getPackagePath(String packageName) {
    return packageName == null ? null : packageName.replace('.', '/');
  }

  /**
   * Add the class designated by the fully qualified class name provided to the set of
   * resolved classes if and only if it is approved by the Test supplied.
   *
   * @param test the test used to determine if the class matches
   * @param fqn the fully qualified name of a class
   */
  @SuppressWarnings("unchecked")
  protected void addIfMatching(Test test, String fqn) {
    try {
      String externalName = fqn.substring(0, fqn.indexOf('.')).replace('/', '.');
      ClassLoader loader = getClassLoader();
      if (log.isDebugEnabled()) {
        log.debug("Checking to see if class " + externalName + " matches criteria [" + test + "]");
      }

      Class<?> type = loader.loadClass(externalName);
      if (test.matches(type)) {
        matches.add((Class<T>) type);
      }
    } catch (Throwable t) {
      log.warn("Could not examine class '" + fqn + "'" + " due to a " +
          t.getClass().getName() + " with message: " + t.getMessage());
    }
  }
}

VFS 类：在调用 ResolverUtil#find(Test test, String packageName) 方法时，需要查找 packageName 包下的所有资源，就是通过调用 VFS 来实现的，具体实现如下：

package org.apache.ibatis.io;

import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.URL;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

import org.apache.ibatis.logging.Log;
import org.apache.ibatis.logging.LogFactory;

/**
 * 1.VFS 是虚拟文件系统通用API，不需关心什么存储媒介
 * 2.优先使用自定义实现VFS，最后使用mybatis内置的 JBoss6VFS, DefaultVFS
 * 3.VFS 是单例模式，根据顺序：自定义实现VFS > 内置原则：实例VFS
 */
public abstract class VFS {
  /**
   * 日志
   */
  private static final Log log = LogFactory.getLog(VFS.class);

  /**
   *  Mybatis 内置实现：JBoss6VFS, DefaultVFS
   */
  public static final Class<?>[] IMPLEMENTATIONS = { JBoss6VFS.class, DefaultVFS.class };

  /**
   *  用户自定义实现的 VFS 的类
   */
  public static final List<Class<? extends VFS>> USER_IMPLEMENTATIONS = new ArrayList<Class<? extends VFS>>();

  /** Singleton instance. */
  private static VFS instance;

  /**
   * Get the singleton {@link VFS} instance. If no {@link VFS} implementation can be found for the
   * current environment, then this method returns null.
   */
  @SuppressWarnings("unchecked")
  public static VFS getInstance() {
    if (instance != null) {
      return instance;
    }

    // Try the user implementations first, then the built-ins
    List<Class<? extends VFS>> impls = new ArrayList<Class<? extends VFS>>();
    //先加入用户自定义实现的 VFS
    impls.addAll(USER_IMPLEMENTATIONS);
    //加载 mybatis 内置实现的 VFS
    impls.addAll(Arrays.asList((Class<? extends VFS>[]) IMPLEMENTATIONS));

    // Try each implementation class until a valid one is found
    VFS vfs = null;
    for (int i = 0; vfs == null || !vfs.isValid(); i++) {
      //循环遍历，直到找到有效 VFS 实现类
      Class<? extends VFS> impl = impls.get(i);
      try {
        //实例化
        vfs = impl.newInstance();
        if (vfs == null || !vfs.isValid()) {
          if (log.isDebugEnabled()) {
            log.debug("VFS implementation " + impl.getName() +
              " is not valid in this environment.");
          }
        }
      } catch (InstantiationException e) {
        log.error("Failed to instantiate " + impl, e);
        return null;
      } catch (IllegalAccessException e) {
        log.error("Failed to instantiate " + impl, e);
        return null;
      }
    }

    if (log.isDebugEnabled()) {
      log.debug("Using VFS adapter " + vfs.getClass().getName());
    }
    VFS.instance = vfs;
    return VFS.instance;
  }

  /**
   * Adds the specified class to the list of {@link VFS} implementations. Classes added in this
   * manner are tried in the order they are added and before any of the built-in implementations.
   * 
   * @param clazz The {@link VFS} implementation class to add.
   */
  public static void addImplClass(Class<? extends VFS> clazz) {
    if (clazz != null) {
      USER_IMPLEMENTATIONS.add(clazz);
    }
  }

  /**
   * 用当前线程绑定的类加载器加载目标类
   */
  protected static Class<?> getClass(String className) {
    try {
      return Thread.currentThread().getContextClassLoader().loadClass(className);
//      return ReflectUtil.findClass(className);
    } catch (ClassNotFoundException e) {
      if (log.isDebugEnabled()) {
        log.debug("Class not found: " + className);
      }
      return null;
    }
  }

  /**
   * 通过类、方法名、方法参数类型获取对应参数
   */
  protected static Method getMethod(Class<?> clazz, String methodName, Class<?>... parameterTypes) {
    if (clazz == null) {
      return null;
    }
    try {
      return clazz.getMethod(methodName, parameterTypes);
    } catch (SecurityException e) {
      log.error("Security exception looking for method " + clazz.getName() + "." + methodName + ".  Cause: " + e);
      return null;
    } catch (NoSuchMethodException e) {
      log.error("Method not found " + clazz.getName() + "." + methodName + "." + methodName + ".  Cause: " + e);
      return null;
    }
  }

  /**
   * 调用方法
   */
  @SuppressWarnings("unchecked")
  protected static <T> T invoke(Method method, Object object, Object... parameters)
      throws IOException, RuntimeException {
    try {
      return (T) method.invoke(object, parameters);
    } catch (IllegalArgumentException e) {
      throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
      throw new RuntimeException(e);
    } catch (InvocationTargetException e) {
      if (e.getTargetException() instanceof IOException) {
        throw (IOException) e.getTargetException();
      } else {
        throw new RuntimeException(e);
      }
    }
  }

  /**
   * 在特定路径下加载所有资源 URL 返回
   */
  protected static List<URL> getResources(String path) throws IOException {
    return Collections.list(Thread.currentThread().getContextClassLoader().getResources(path));
  }

  /**
   * 判断 是否有效
   */
  public abstract boolean isValid();

  /**
   * 递归列出URL的下所有子的资源列表
   */
  protected abstract List<String> list(URL url, String forPath) throws IOException;

  /**
   * 列出某个路径所有 URL 对应 子资源
   */
  public List<String> list(String path) throws IOException {
    List<String> names = new ArrayList<String>();
    for (URL url : getResources(path)) {
      names.addAll(list(url, path));
    }
    return names;
  }
}