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 }