src/java/org/apache/bcel/generic/ReferenceType.java

package org.apache.bcel.generic;

/* ====================================================================
 * The Apache Software License, Version 1.1
 *
 * Copyright (c) 2001 The Apache Software Foundation.  All rights
 * reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:
 *       "This product includes software developed by the
 *        Apache Software Foundation (http://www.apache.org/)."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Apache" and "Apache Software Foundation" and
 *    "Apache BCEL" must not be used to endorse or promote products
 *    derived from this software without prior written permission. For
 *    written permission, please contact apache@apache.org.
 *
 * 5. Products derived from this software may not be called "Apache",
 *    "Apache BCEL", nor may "Apache" appear in their name, without
 *    prior written permission of the Apache Software Foundation.
 *
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 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
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 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of the Apache Software Foundation.  For more
 * information on the Apache Software Foundation, please see
 * <http://www.apache.org/>.
 */
import org.apache.bcel.Constants;
import org.apache.bcel.Repository;
import org.apache.bcel.classfile.JavaClass;

/** 
 * Super class for objects and arrays.
 *
 * @version $Id$
 * @author  <A HREF="mailto:markus.dahm@berlin.de">M. Dahm</A>
 */
public class ReferenceType extends Type {
  protected ReferenceType(byte t, String s) {
    super(t, s);
  }

  /** Class is non-abstract but not instantiable from the outside
   */
  ReferenceType() {
    super(Constants.T_OBJECT, "<null object>");
  }

  /**
   * Return true iff this type is castable to another type t as defined in
   * the JVM specification.  The case where this is Type.NULL is not
   * defined (see the CHECKCAST definition in the JVM specification).
   * However, because e.g. CHECKCAST doesn't throw a
   * ClassCastException when casting a null reference to any Object,
   * true is returned in this case.
   */
  public boolean isCastableTo(Type t){
    if(this.equals(Type.NULL))
      return true;		// If this is ever changed in isAssignmentCompatible()

    return isAssignmentCompatibleWith(t); /* Yes, it's true: It's the same definition.
					   * See vmspec2 AASTORE / CHECKCAST definitions.
					   */
  }

  /**
   * Return true iff this is assignment compatible with another type t
   * as defined in the JVM specification; see the AASTORE definition
   * there.
   */
  public boolean isAssignmentCompatibleWith(Type t) {
    if(!(t instanceof ReferenceType))
      return false;

    ReferenceType T = (ReferenceType)t;

    if(this.equals(Type.NULL))
      return true; // This is not explicitely stated, but clear. Isn't it?

    /* If this is a class type then
     */
    if((this instanceof ObjectType) && (((ObjectType) this).referencesClass())) {
      /* If T is a class type, then this must be the same class as T,
         or this must be a subclass of T;
      */
      if((T instanceof ObjectType) && (((ObjectType) T).referencesClass())) {
	if(this.equals(T))
	  return true;

	if(Repository.instanceOf( ((ObjectType) this).getClassName(),
				  ((ObjectType) T).getClassName()))
	  return true;
      }

      /* If T is an interface type, this must implement interface T.
       */
      if ((T instanceof ObjectType) && (((ObjectType) T).referencesInterface())) {
	if (Repository.implementationOf( ((ObjectType) this).getClassName(),
					 ((ObjectType) T).getClassName() ))
	  return true;
      }
    }

    /* If this is an interface type, then:
     */
    if ((this instanceof ObjectType) && (((ObjectType) this).referencesInterface())){
      /* If T is a class type, then T must be Object (§2.4.7).
       */
      if ((T instanceof ObjectType) && (((ObjectType) T).referencesClass())){
	if (T.equals(Type.OBJECT)) return true;
      }

      /* If T is an interface type, then T must be the same interface
         as this or a superinterface of this (§2.13.2).
      */
      if ((T instanceof ObjectType) && (((ObjectType) T).referencesInterface())){
	if (this.equals(T)) return true;
	if (Repository.implementationOf( ((ObjectType) this).getClassName(),
					 ((ObjectType) T).getClassName() )) return true;
      }
    }

    /* If this is an array type, namely, the type SC[], that is, an
       array of components of type SC, then:
    */
    if(this instanceof ArrayType){
      /* If T is a class type, then T must be Object (§2.4.7).
       */
      if ((T instanceof ObjectType) && (((ObjectType) T).referencesClass())){
	if (T.equals(Type.OBJECT)) return true;
      }

      /* If T is an array type TC[], that is, an array of components
         of type TC, then one of the following must be true:
      */
      if (T instanceof ArrayType) {
	/* TC and SC are the same primitive type (§2.4.1).
	 */
	Type sc = ((ArrayType) this).getElementType();
	Type tc = ((ArrayType) this).getElementType();

	if (sc instanceof BasicType && tc instanceof BasicType && sc.equals(tc))
	  return true;

	/* TC and SC are reference types (§2.4.6), and type SC is
           assignable to TC by these runtime rules.*/
	if (tc instanceof ReferenceType && sc instanceof ReferenceType &&
	    ((ReferenceType) sc).isAssignmentCompatibleWith((ReferenceType) tc)) return true;
      }

      /* If T is an interface type, T must be one of the interfaces implemented by arrays (§2.15). */
      // TODO: Check if this is still valid or find a way to dynamically find out which
      // interfaces arrays implement. However, as of the JVM specification edition 2, there
      // are at least two different pages where assignment compatibility is defined and
      // on one of them "interfaces implemented by arrays" is exchanged with "'Cloneable' or
      // 'java.io.Serializable'"
      if ((T instanceof ObjectType) && (((ObjectType) T).referencesInterface())){
	for (int ii=0; ii<Constants.INTERFACES_IMPLEMENTED_BY_ARRAYS.length; ii++){
	  if (T.equals(new ObjectType(Constants.INTERFACES_IMPLEMENTED_BY_ARRAYS[ii]))) return true;
  }
      }
    }
    return false; // default.
  }

 /**
   * This commutative operation returns the first common superclass (narrowest ReferenceType
   * referencing a class, not an interface).
   * If one of the types is a superclass of the other, the former is returned.
   * If "this" is Type.NULL, then t is returned.
   * If t is Type.NULL, then "this" is returned.                              
   * If "this" equals t ['this.equals(t)'] "this" is returned.
   * If "this" or t is an ArrayType, then Type.OBJECT is returned.
   * If "this" or t is a ReferenceType referencing an interface, then Type.OBJECT is returned.
   * If not all of the two classes' superclasses cannot be found, "null" is returned.
   * See the JVM specification edition 2, "§4.9.2 The Bytecode Verifier".
   */
  public ReferenceType firstCommonSuperclass(ReferenceType t){
    if (this.equals(Type.NULL)) return t;

    if (t.equals(Type.NULL)) return this;                                     

    if (this.equals(t)) return this;

    // TODO: This sounds a little arbitrary. On the other hand, there is
    // no object referenced by Type.NULL so we can also say all the objects
    // referenced by Type.NULL were derived from java.lang.Object.
    // However, the Java Language's "instanceof" operator proves us wrong:
    // "null" is not referring to an instance of java.lang.Object :)

    if ((this instanceof ArrayType) || (t instanceof ArrayType))
      return Type.OBJECT;
    // TODO: Is there a proof of OBJECT being the direct ancestor of every ArrayType?

    if ( ((this instanceof ObjectType) && ((ObjectType) this).referencesInterface()) ||
	 ((   t instanceof ObjectType) && ((ObjectType)    t).referencesInterface()) )
      return Type.OBJECT;
    // TODO: The above line is correct comparing to the vmspec2. But one could
    // make class file verification a bit stronger here by using the notion of
    // superinterfaces or even castability or assignment compatibility.


    // this and t are ObjectTypes, see above.
    ObjectType thiz  = (ObjectType) this;
    ObjectType other = (ObjectType) t;
    JavaClass[] thiz_sups  = Repository.getSuperClasses( thiz.getClassName());
    JavaClass[] other_sups = Repository.getSuperClasses(other.getClassName());

    if ((thiz_sups == null) || (other_sups==null)){
      return null;
    }

    // Waaahh...
    JavaClass[] this_sups = new JavaClass[thiz_sups.length+1];
    JavaClass[] t_sups    = new JavaClass[other_sups.length+1];
    System.arraycopy( thiz_sups, 0, this_sups, 1,  thiz_sups.length);
    System.arraycopy(other_sups, 0, t_sups   , 1, other_sups.length);
    this_sups[0] = Repository.lookupClass(thiz.getClassName());
    t_sups[0]    = Repository.lookupClass(other.getClassName());

    for (int i=0; i<t_sups.length; i++){
      for (int j=0; j<this_sups.length; j++){
	if (this_sups[j].equals(t_sups[i])) return new ObjectType(this_sups[j].getClassName());
      }
    }

    // Huh? Did you ask for Type.OBJECT's superclass??
    return null;
  }
}