ClassUtilities.java

Power Java
Limitations of reflective method lookup
by Paul Holser
Listing 6. ClassUtilities.java.


package reflection;

import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.util.*;

/**
 * Utility routines for querying Class objects.
 */

final class ClassUtilities {
  /**
   * Mapping from primitive wrapper Classes to their
   * corresponding primitive Classes.
   */
  private static final Map objectToPrimitiveMap =
    new HashMap(13);

  static {
    objectToPrimitiveMap.put(Boolean.class, Boolean.TYPE);
    objectToPrimitiveMap.put(Byte.class, Byte.TYPE);
    objectToPrimitiveMap.put(Character.class, Character.TYPE);
    objectToPrimitiveMap.put(Double.class, Double.TYPE);
    objectToPrimitiveMap.put(Float.class, Float.TYPE);
    objectToPrimitiveMap.put(Integer.class, Integer.TYPE);
    objectToPrimitiveMap.put(Long.class, Long.TYPE);
    objectToPrimitiveMap.put(Short.class, Short.TYPE);
  }

  /**
   * Mapping from primitive wrapper Classes to the sets of
   * primitive classes whose instances can be assigned an
   * instance of the first.
   */
  private static final Map primitiveWideningsMap =
    new HashMap(11);

  static {
    Set set = new HashSet();

    set.add(Short.TYPE);
    set.add(Integer.TYPE);
    set.add(Long.TYPE);
    set.add(Float.TYPE);
    set.add(Double.TYPE);
    primitiveWideningsMap.put(Byte.TYPE, set);

    set = new HashSet();

    set.add(Integer.TYPE);
    set.add(Long.TYPE);
    set.add(Float.TYPE);
    set.add(Double.TYPE);
    primitiveWideningsMap.put(Short.TYPE, set);
    primitiveWideningsMap.put(Character.TYPE, set);

    set = new HashSet();

    set.add(Long.TYPE);
    set.add(Float.TYPE);
    set.add(Double.TYPE);
    primitiveWideningsMap.put(Integer.TYPE, set);

    set = new HashSet();

    set.add(Float.TYPE);
    set.add(Double.TYPE);
    primitiveWideningsMap.put(Long.TYPE, set);

    set = new HashSet();

    set.add(Double.TYPE);
    primitiveWideningsMap.put(Float.TYPE, set);
  }

  /**
   * Do not instantiate.  Static methods only.
   */
  private ClassUtilities() {}

  /**
   * @param  name  FQN of a class, or the name of a primitive type
   * @param  loader  a ClassLoader
   * @return  the Class for the name given.  Primitive types are
   * converted to their particular Class object.  null, the empty string,
   * "null", and "void" yield Void.TYPE.  If any classes require
   * loading because of this operation, the loading is done by the
   * given class loader.  Such classes are not initialized, however.
   * @exception  ClassNotFoundException  if name names an
   * unknown class or primitive
   */
  static Class classForNameOrPrimitive(String name, ClassLoader loader)
  throws ClassNotFoundException {
    if (name == null || name.equals("") || name.equals("null")
        || name.equals("void"))
      return Void.TYPE;
 
    if (name.equals("boolean"))
      return Boolean.TYPE;
 
    if (name.equals("byte"))
      return Byte.TYPE;

    if (name.equals("char"))
      return Character.TYPE;

    if (name.equals("double"))
      return Double.TYPE;

    if (name.equals("float"))
      return Float.TYPE;

    if (name.equals("int"))
      return Integer.TYPE;

    if (name.equals("long"))
      return Long.TYPE;

    if (name.equals("short"))
      return Short.TYPE;

    return Class.forName(name, false, loader);
  }

  /**
   * @param  aClass  a Class
   * @return  true if the class is accessible, false otherwise.
   * Presently returns true if the class is declared public.
   */
  static boolean classIsAccessible(Class aClass) {
    return Modifier.isPublic(aClass.getModifiers());
  }

  /**
   * Tells whether instances of the classes in the 'rhs' array
   * could be used as parameters to a reflective method
   * invocation whose parameter list has types denoted by the
   * 'lhs' array.
   * 
   * @param  lhs  Class array representing the types of the
   * formal parameters of a method
   * @param  rhs  Class array representing the types of the
   * actual parameters of a method.  A null value or
   * Void.TYPE is considered to match a corresponding
   * Object or array class in lhs, but not a primitive.
   * @return  true if compatible, false otherwise
   */
  static boolean compatibleClasses(Class[] lhs, Class[] rhs) {
    if (lhs.length != rhs.length)
      return false;

    for (int i = 0; i < lhs.length; ++i) {
      if (rhs[i] == null || rhs[i].equals(Void.TYPE)) {
        if (lhs[i].isPrimitive())
          return false;
        else
          continue;
      }

      if (! lhs[i].isAssignableFrom(rhs[i])) {
        Class lhsPrimEquiv = primitiveEquivalentOf(lhs[i]);
        Class rhsPrimEquiv = primitiveEquivalentOf(rhs[i]);

        if (! primitiveIsAssignableFrom(lhsPrimEquiv, rhsPrimEquiv))
          return false;
      }
    }

    return true;
  }

  /**
   * @param  aClass  a Class
   * @param  methodName  name of a method
   * @param  paramTypes  Class array representing the types
   * of a method's formal parameters
   * @return  the Method with the given name and formal
   * parameter types that is in the nearest accessible class in the
   * class hierarchy, starting with aClass's superclass.  The
   * superclass and implemented interfaces of aClass are
   * searched, then their superclasses, etc. until a method is
   * found.  Returns null if there is no such method.
   */
  static Method getAccessibleMethodFrom(Class aClass, String methodName, Class[] parameterTypes) {
    // Look for overridden method in the superclass.
    Class superclass = aClass.getSuperclass();
    Method overriddenMethod = null;

    if (superclass != null && classIsAccessible(superclass)) {
      try {
        overriddenMethod =
          superclass.getMethod(methodName, parameterTypes);
      } catch (NoSuchMethodException _) {
      }

      if (overriddenMethod != null)
        return overriddenMethod;
    }

    // If here, then aClass represents Object, or an interface, or
    // the superclass did not have an override.  Check
    // implemented interfaces.

    Class[] interfaces = aClass.getInterfaces();

    for (int i = 0; i < interfaces.length; ++i) {
      overriddenMethod = null;

      if (classIsAccessible(interfaces[i])) {
        try {
          overriddenMethod =
            interfaces[i].getMethod(methodName, parameterTypes);
        } catch (NoSuchMethodException _) {
        }

        if (overriddenMethod != null)
          return overriddenMethod;
      }
    }

    overriddenMethod = null;

    // Try superclass's superclass and implemented interfaces.
    if (superclass != null) {
      overriddenMethod =
        getAccessibleMethodFrom(superclass, methodName, parameterTypes);

      if (overriddenMethod != null)
        return overriddenMethod;
    }

    // Try implemented interfaces' extended interfaces...
    for (int i = 0; i < interfaces.length; ++i) {
      overriddenMethod =
        getAccessibleMethodFrom(interfaces[i], methodName, parameterTypes);

      if (overriddenMethod != null)
        return overriddenMethod;
    }

    // Give up.
    return null;
  }

  /**
   * @param  aClass  a Class
   * @return  the class's primitive equivalent, if aClass is a
   * primitive wrapper.  If aClass is primitive, returns aClass.
   * Otherwise, returns null.
   */
  static Class primitiveEquivalentOf(Class aClass) {
    return aClass.isPrimitive()
           ? aClass
           : (Class) objectToPrimitiveMap.get(aClass);
  }

  /**
   * Tells whether an instance of the primitive class
   * represented by 'rhs' can be assigned to an instance of the
   * primitive class represented by 'lhs'.
   * 
   * @param  lhs  assignee class
   * @param  rhs  assigned class
   * @return  true if compatible, false otherwise.  If either
   * argument is <code>null</code>, or one of the parameters
   * does not represent a primitive (e.g. Byte.TYPE), returns
   * false.
   */
  static boolean primitiveIsAssignableFrom(Class lhs, Class rhs) {
    if (lhs == null || rhs == null)
      return false;

    if (! (lhs.isPrimitive() && rhs.isPrimitive()))
      return false;

    if (lhs.equals(rhs))
      return true;

    Set wideningSet = (Set) primitiveWideningsMap.get(rhs);

    if (wideningSet == null)
      return false;

    return wideningSet.contains(lhs);
  }
}

About the Author

Paul Holser is a senior consultant at Valtech, an international consulting group specializing in e-business technologies. He can be contacted at paul.holser@valtech.com.

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