java/org/apache/tomcat/util/bcel/classfile/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.tomcat.util.bcel.classfile;

import java.io.DataInput;
import java.io.IOException;

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

/**
 * 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.
 *
 * @author  <A HREF="mailto:m.dahm@gmx.de">M. Dahm</A>
 */
public abstract class Constant {

    /* 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.
     */
    protected final byte tag;


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


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


    /**
     * Read one constant from the given input, the type depends on a tag byte.
     *
     * @param input Input stream
     * @return Constant object
     */
    static Constant readConstant(final DataInput input) throws IOException,
            ClassFormatException {
        final byte b = input.readByte(); // Read tag byte
        int skipSize;
        switch (b) {
            case Const.CONSTANT_Class:
                return new ConstantClass(input);
            case Const.CONSTANT_Integer:
                return new ConstantInteger(input);
            case Const.CONSTANT_Float:
                return new ConstantFloat(input);
            case Const.CONSTANT_Long:
                return new ConstantLong(input);
            case Const.CONSTANT_Double:
                return new ConstantDouble(input);
            case Const.CONSTANT_Utf8:
                return ConstantUtf8.getInstance(input);
            case Const.CONSTANT_String:
            case Const.CONSTANT_MethodType:
            case Const.CONSTANT_Module:
            case Const.CONSTANT_Package:
                skipSize = 2; // unsigned short
                break;
            case Const.CONSTANT_MethodHandle:
                skipSize = 3; // unsigned byte, unsigned short
                break;
            case Const.CONSTANT_Fieldref:
            case Const.CONSTANT_Methodref:
            case Const.CONSTANT_InterfaceMethodref:
            case Const.CONSTANT_NameAndType:
            case Const.CONSTANT_InvokeDynamic:
                skipSize = 4; // unsigned short, unsigned short
                break;
            default:
                throw new ClassFormatException("Invalid byte tag in constant pool: " + b);
        }
        Utility.skipFully(input, skipSize);
        return null;
    }


    @Override
    public String toString() {
        return "[" + tag + "]";
    }
}