001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one
003 * or more contributor license agreements. See the NOTICE file
004 * distributed with this work for additional information
005 * regarding copyright ownership. The ASF licenses this file
006 * to you under the Apache License, Version 2.0 (the "License");
007 * you may not use this file except in compliance with the License.
008 * You may obtain a copy of the License at
009 *
010 * http://www.apache.org/licenses/LICENSE-2.0
011 *
012 * Unless required by applicable law or agreed to in writing, software
013 * distributed under the License is distributed on an "AS IS" BASIS,
014 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015 * See the License for the specific language governing permissions and
016 * limitations under the License.
017 */
018 /*
019 * $Id: MarkerInstruction.java 1225426 2011-12-29 04:13:08Z mrglavas $
020 */
021
022 package org.apache.xalan.xsltc.compiler.util;
023
024 import java.io.DataOutputStream;
025 import java.io.IOException;
026
027 import org.apache.bcel.Constants;
028 import org.apache.bcel.generic.ConstantPoolGen;
029 import org.apache.bcel.generic.Instruction;
030 import org.apache.bcel.generic.Visitor;
031
032 /**
033 * A special abstract dummy subclass of
034 * {@link org.apache.bcel.generic.Instruction} used to mark locations of
035 * interest in an {@link org.apache.bcel.generic.InstructionList}. It and
036 * its subclasses are only used as placeholders, and do not contribute to the
037 * actual byte code instruction stream.
038 */
039 abstract class MarkerInstruction extends Instruction {
040 /**
041 * Zero-argument constructor. Sets the opcode to an invalid value and
042 * sets the length to zero, as it will not be written as part of the
043 * generated byte code.
044 */
045 public MarkerInstruction() {
046 super(Constants.UNDEFINED, (short) 0);
047 }
048
049 /**
050 * {@link org.apache.bcel.generic.Visitor}s will know nothing about this
051 * kind of {@link org.apche.bcel.generic.Instruction}, so this method does
052 * nothing.
053 */
054 public void accept(Visitor v) {
055 }
056
057 /**
058 * The number of JVM stack entries consumed by the instruction.
059 * This instruction is just a place holder, so it does not consume any
060 * stack entries.
061 * @param cpg The {@link org.apache.bcel.generic.ConstantPoolGen} for the
062 * current {@link org.apache.bcel.generic.ClassGen}
063 * @return <code>0</code> always
064 */
065 final public int consumeStack(ConstantPoolGen cpg) {
066 return 0;
067 }
068 /**
069 * The number of JVM stack entries produced by the instruction.
070 * This instruction is just a place holder, so it does not produce any
071 * stack entries.
072 * @param cpg The {@link org.apache.bcel.generic.ConstantPoolGen} for the
073 * current {@link org.apache.bcel.generic.ClassGen}
074 * @return <code>0</code> always
075 */
076 final public int produceStack(ConstantPoolGen cpg) {
077 return 0;
078 }
079
080 /**
081 * Produce a copy of the instruction. By default a
082 * {@link MarkerInstruction} has no parameters, so the base implementation
083 * of {@link #copy()} returns the instruction itself.
084 * @return The instruction itself.
085 */
086 public Instruction copy() {
087 return this;
088 }
089 /**
090 * Dump instruction as byte code to stream out. A {@link MarkerInstruction}
091 * has no effect on the generated byte code so it is never emitted to the
092 * output stream.
093 * @param out Output stream
094 */
095 final public void dump(DataOutputStream out) throws IOException {
096 }
097 }