ReferenceType.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */
package org.apache.bcel.generic;

import org.apache.bcel.Const;
import org.apache.bcel.Repository;
import org.apache.bcel.classfile.JavaClass;

/**
 * Super class for object and array types.
 */
public abstract class ReferenceType extends Type {

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

    protected ReferenceType(final byte t, final String s) {
        super(t, s);
    }

    /**
     * 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 NULL, then t
     * is returned. If t is NULL, then "this" is returned. If "this" equals t ['this.equals(t)'] "this" is returned. If
     * "this" or t is an ArrayType, then {@link #OBJECT} is returned. If "this" or t is a ReferenceType referencing an
     * interface, then {@link #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".
     *
     * @deprecated use getFirstCommonSuperclass(ReferenceType t) which has slightly changed semantics.
     * @throws ClassNotFoundException on failure to find superclasses of this type, or the type passed as a parameter
     */
    @Deprecated
    public ReferenceType firstCommonSuperclass(final ReferenceType t) throws ClassNotFoundException {
        if (equals(NULL)) {
            return t;
        }
        if (t.equals(NULL) || equals(t)) {
            return this;
            /*
             * TODO: Above sounds a little arbitrary. On the other hand, there is no object referenced by {@link #NULL} so we can also
             * say all the objects referenced by {@link #NULL} were derived from {@link Object}. However, the Java Language's
             * "instanceof" operator proves us wrong: "null" is not referring to an instance of {@link Object} :)
             */
        }
        if (this instanceof ArrayType || t instanceof ArrayType) {
            return OBJECT;
            // TODO: Is there a proof of {@link #OBJECT} being the direct ancestor of every ArrayType?
        }
        return getFirstCommonSuperclassInternal(t);
    }

    /**
     * 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 NULL, then t
     * is returned. If t is NULL, then "this" is returned. If "this" equals t ['this.equals(t)'] "this" is returned. If
     * "this" or t is an ArrayType, then {@link #OBJECT} is returned; unless their dimensions match. Then an ArrayType of the
     * same number of dimensions is returned, with its basic type being the first common super class of the basic types of
     * "this" and t. If "this" or t is a ReferenceType referencing an interface, then {@link #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".
     *
     * @throws ClassNotFoundException on failure to find superclasses of this type, or the type passed as a parameter
     */
    public ReferenceType getFirstCommonSuperclass(final ReferenceType t) throws ClassNotFoundException {
        if (equals(NULL)) {
            return t;
        }
        if (t.equals(NULL) || equals(t)) {
            return this;
            /*
             * TODO: Above sounds a little arbitrary. On the other hand, there is no object referenced by {@link #NULL} so we can also
             * say all the objects referenced by {@link #NULL} were derived from {@link Object}. However, the Java Language's
             * "instanceof" operator proves us wrong: "null" is not referring to an instance of {@link Object} :)
             */
        }
        /* This code is from a bug report by Konstantin Shagin <konst@cs.technion.ac.il> */
        if (this instanceof ArrayType && t instanceof ArrayType) {
            final ArrayType arrType1 = (ArrayType) this;
            final ArrayType arrType2 = (ArrayType) t;
            if (arrType1.getDimensions() == arrType2.getDimensions() && arrType1.getBasicType() instanceof ObjectType
                && arrType2.getBasicType() instanceof ObjectType) {
                return new ArrayType(((ObjectType) arrType1.getBasicType()).getFirstCommonSuperclass((ObjectType) arrType2.getBasicType()),
                    arrType1.getDimensions());
            }
        }
        if (this instanceof ArrayType || t instanceof ArrayType) {
            return OBJECT;
            // TODO: Is there a proof of {@link #OBJECT} being the direct ancestor of every ArrayType?
        }
        return getFirstCommonSuperclassInternal(t);
    }

    private ReferenceType getFirstCommonSuperclassInternal(final ReferenceType t) throws ClassNotFoundException {
        if (this instanceof ObjectType && ((ObjectType) this).referencesInterfaceExact()
            || t instanceof ObjectType && ((ObjectType) t).referencesInterfaceExact()) {
            return 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.
        final ObjectType thiz = (ObjectType) this;
        final ObjectType other = (ObjectType) t;
        final JavaClass[] thizSups = Repository.getSuperClasses(thiz.getClassName());
        final JavaClass[] otherSups = Repository.getSuperClasses(other.getClassName());
        if (thizSups == null || otherSups == null) {
            return null;
        }
        // Waaahh...
        final JavaClass[] thisSups = new JavaClass[thizSups.length + 1];
        final JavaClass[] tSups = new JavaClass[otherSups.length + 1];
        System.arraycopy(thizSups, 0, thisSups, 1, thizSups.length);
        System.arraycopy(otherSups, 0, tSups, 1, otherSups.length);
        thisSups[0] = Repository.lookupClass(thiz.getClassName());
        tSups[0] = Repository.lookupClass(other.getClassName());
        for (final JavaClass tSup : tSups) {
            for (final JavaClass thisSup : thisSups) {
                if (thisSup.equals(tSup)) {
                    return ObjectType.getInstance(thisSup.getClassName());
                }
            }
        }
        // Huh? Did you ask for OBJECT's superclass??
        return null;
    }

    /**
     * Return true iff this is assignment compatible with another type t as defined in the JVM specification; see the
     * AASTORE definition there.
     *
     * @throws ClassNotFoundException if any classes or interfaces required to determine assignment compatibility can't be
     *         found
     */
    public boolean isAssignmentCompatibleWith(final Type t) throws ClassNotFoundException {
        if (!(t instanceof ReferenceType)) {
            return false;
        }
        final ReferenceType T = (ReferenceType) t;
        if (equals(NULL)) {
            return true; // This is not explicitly stated, but clear. Isn't it?
        }
        /*
         * If this is a class type then
         */
        if (this instanceof ObjectType && ((ObjectType) this).referencesClassExact()) {
            /*
             * 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).referencesClassExact()
                && (equals(T) || 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).referencesInterfaceExact()
                && Repository.implementationOf(((ObjectType) this).getClassName(), ((ObjectType) T).getClassName())) {
                return true;
            }
        }
        /*
         * If this is an interface type, then:
         */
        if (this instanceof ObjectType && ((ObjectType) this).referencesInterfaceExact()) {
            /*
             * If T is a class type, then T must be Object (�2.4.7).
             */
            if (T instanceof ObjectType && ((ObjectType) T).referencesClassExact() && T.equals(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).referencesInterfaceExact()
                && (equals(T) || 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).referencesClassExact() && T.equals(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).
                 */
                final Type sc = ((ArrayType) this).getElementType();
                final Type tc = ((ArrayType) T).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(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).referencesInterfaceExact()) {
                for (final String element : Const.getInterfacesImplementedByArrays()) {
                    if (T.equals(ObjectType.getInstance(element))) {
                        return true;
                    }
                }
            }
        }
        return false; // default.
    }

    /**
     * Return true iff this type is castable to another type t as defined in the JVM specification. The case where this is
     * {@link #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.
     *
     * @throws ClassNotFoundException if any classes or interfaces required to determine assignment compatibility can't be
     *         found
     */
    public boolean isCastableTo(final Type t) throws ClassNotFoundException {
        if (equals(NULL)) {
            return t instanceof ReferenceType; // If this is ever changed in isAssignmentCompatible()
        }
        return isAssignmentCompatibleWith(t);
        /*
         * Yes, it's true: It's the same definition. See vmspec2 AASTORE / CHECKCAST definitions.
         */
    }
}