Constant.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.classfile;

import java.io.DataInput;
import java.io.DataOutputStream;
import java.io.IOException;
import java.util.Objects;

import org.apache.bcel.Const;
import org.apache.bcel.util.BCELComparator;

/**
 * Abstract superclass for classes to represent the different constant types in the constant pool of a class file. The
 * classes keep closely to the JVM specification.
 */
public abstract class Constant implements Cloneable, Node {

    static final Constant[] EMPTY_ARRAY = {};

    private static BCELComparator<Constant> bcelComparator = new BCELComparator<Constant>() {

        @Override
        public boolean equals(final Constant a, final Constant b) {
            return a == b || a != null && b != null && Objects.equals(a.toString(), b.toString());
        }

        @Override
        public int hashCode(final Constant o) {
            return o != null ? Objects.hashCode(o.toString()) : 0;
        }
    };

    /**
     * @return Comparison strategy object.
     */
    public static BCELComparator<Constant> getComparator() {
        return bcelComparator;
    }

    /**
     * Reads one constant from the given input, the type depends on a tag byte.
     *
     * @param dataInput Input stream
     * @return Constant object
     * @throws IOException if an I/O error occurs reading from the given {@code dataInput}.
     * @throws ClassFormatException if the next byte is not recognized
     * @since 6.0 made public
     */
    public static Constant readConstant(final DataInput dataInput) throws IOException, ClassFormatException {
        final byte b = dataInput.readByte(); // Read tag byte
        switch (b) {
        case Const.CONSTANT_Class:
            return new ConstantClass(dataInput);
        case Const.CONSTANT_Fieldref:
            return new ConstantFieldref(dataInput);
        case Const.CONSTANT_Methodref:
            return new ConstantMethodref(dataInput);
        case Const.CONSTANT_InterfaceMethodref:
            return new ConstantInterfaceMethodref(dataInput);
        case Const.CONSTANT_String:
            return new ConstantString(dataInput);
        case Const.CONSTANT_Integer:
            return new ConstantInteger(dataInput);
        case Const.CONSTANT_Float:
            return new ConstantFloat(dataInput);
        case Const.CONSTANT_Long:
            return new ConstantLong(dataInput);
        case Const.CONSTANT_Double:
            return new ConstantDouble(dataInput);
        case Const.CONSTANT_NameAndType:
            return new ConstantNameAndType(dataInput);
        case Const.CONSTANT_Utf8:
            return ConstantUtf8.getInstance(dataInput);
        case Const.CONSTANT_MethodHandle:
            return new ConstantMethodHandle(dataInput);
        case Const.CONSTANT_MethodType:
            return new ConstantMethodType(dataInput);
        case Const.CONSTANT_Dynamic:
            return new ConstantDynamic(dataInput);
        case Const.CONSTANT_InvokeDynamic:
            return new ConstantInvokeDynamic(dataInput);
        case Const.CONSTANT_Module:
            return new ConstantModule(dataInput);
        case Const.CONSTANT_Package:
            return new ConstantPackage(dataInput);
        default:
            throw new ClassFormatException("Invalid byte tag in constant pool: " + b);
        }
    }

    /**
     * @param comparator Comparison strategy object
     */
    public static void setComparator(final BCELComparator<Constant> comparator) {
        bcelComparator = comparator;
    }

    /*
     * In fact this tag is redundant since we can distinguish different 'Constant' objects by their type, i.e., via
     * 'instanceof'. In some places we will use the tag for switch()es anyway.
     *
     * First, we want match the specification as closely as possible. Second we need the tag as an index to select the
     * corresponding class name from the 'CONSTANT_NAMES' array.
     */
    /**
     * @deprecated (since 6.0) will be made private; do not access directly, use getter/setter
     */
    @java.lang.Deprecated
    protected byte tag; // TODO should be private & final

    Constant(final byte tag) {
        this.tag = tag;
    }

    /**
     * Called by objects that are traversing the nodes of the tree implicitly defined by the contents of a Java class.
     * I.e., the hierarchy of methods, fields, attributes, etc. spawns a tree of objects.
     *
     * @param v Visitor object
     */
    @Override
    public abstract void accept(Visitor v);

    @Override
    public Object clone() {
        try {
            return super.clone();
        } catch (final CloneNotSupportedException e) {
            throw new UnsupportedOperationException("Clone Not Supported", e); // never happens
        }
    }

    /**
     * @return deep copy of this constant
     */
    public Constant copy() {
        try {
            return (Constant) super.clone();
        } catch (final CloneNotSupportedException e) {
            // TODO should this throw?
        }
        return null;
    }

    public abstract void dump(DataOutputStream file) throws IOException;

    /**
     * Returns value as defined by given BCELComparator strategy. By default two Constant objects are said to be equal when
     * the result of toString() is equal.
     *
     * @see Object#equals(Object)
     */
    @Override
    public boolean equals(final Object obj) {
        return obj instanceof Constant && bcelComparator.equals(this, (Constant) obj);
    }

    /**
     * @return Tag of constant, i.e., its type. No setTag() method to avoid confusion.
     */
    public final byte getTag() {
        return tag;
    }

    /**
     * Returns value as defined by given BCELComparator strategy. By default return the hash code of the result of
     * toString().
     *
     * @see Object#hashCode()
     */
    @Override
    public int hashCode() {
        return bcelComparator.hashCode(this);
    }

    /**
     * @return String representation.
     */
    @Override
    public String toString() {
        return Const.getConstantName(tag) + "[" + tag + "]";
    }
}