BranchInstruction xref

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    1   /*
    2    * Copyright  2000-2004 The Apache Software Foundation
    3    *
    4    *  Licensed under the Apache License, Version 2.0 (the "License");
    5    *  you may not use this file except in compliance with the License.
    6    *  You may obtain a copy of the License at
    7    *
    8    *      http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   *  Unless required by applicable law or agreed to in writing, software
    11   *  distributed under the License is distributed on an "AS IS" BASIS,
    12   *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13   *  See the License for the specific language governing permissions and
    14   *  limitations under the License.
    15   *
    16   */
    17  package org.apache.bcel.generic;
    18
    19  import java.io.DataOutputStream;
    20  import java.io.IOException;
    21  import org.apache.bcel.util.ByteSequence;
    22
    23  /***
    24   * Abstract super class for branching instructions like GOTO, IFEQ, etc..
    25   * Branch instructions may have a variable length, namely GOTO, JSR,
    26   * LOOKUPSWITCH and TABLESWITCH.
    27   *
    28   * @see InstructionList
    29   * @version $Id: BranchInstruction.java 386056 2006-03-15 11:31:56Z tcurdt $
    30   * @author  <A HREF="mailto:m.dahm@gmx.de">M. Dahm</A>
    31   */
    32  public abstract class BranchInstruction extends Instruction implements InstructionTargeter {
    33
    34      protected int index; // Branch target relative to this instruction
    35      protected InstructionHandle target; // Target object in instruction list
    36      protected int position; // Byte code offset
    37
    38
    39      /***
    40       * Empty constructor needed for the Class.newInstance() statement in
    41       * Instruction.readInstruction(). Not to be used otherwise.
    42       */
    43      BranchInstruction() {
    44      }
    45
    46
    47      /*** Common super constructor
    48       * @param opcode Instruction opcode
    49       * @param target instruction to branch to
    50       */
    51      protected BranchInstruction(short opcode, InstructionHandle target) {
    52          super(opcode, (short) 3);
    53          setTarget(target);
    54      }
    55
    56
    57      /***
    58       * Dump instruction as byte code to stream out.
    59       * @param out Output stream
    60       */
    61      public void dump( DataOutputStream out ) throws IOException {
    62          out.writeByte(opcode);
    63          index = getTargetOffset();
    64          if (Math.abs(index) >= 32767) {
    65              throw new ClassGenException("Branch target offset too large for short");
    66          }
    67          out.writeShort(index); // May be negative, i.e., point backwards
    68      }
    69
    70
    71      /***
    72       * @param _target branch target
    73       * @return the offset to  `target' relative to this instruction
    74       */
    75      protected int getTargetOffset( InstructionHandle _target ) {
    76          if (_target == null) {
    77              throw new ClassGenException("Target of " + super.toString(true)
    78                      + " is invalid null handle");
    79          }
    80          int t = _target.getPosition();
    81          if (t < 0) {
    82              throw new ClassGenException("Invalid branch target position offset for "
    83                      + super.toString(true) + ":" + t + ":" + _target);
    84          }
    85          return t - position;
    86      }
    87
    88
    89      /***
    90       * @return the offset to this instruction's target
    91       */
    92      protected int getTargetOffset() {
    93          return getTargetOffset(target);
    94      }
    95
    96
    97      /***
    98       * Called by InstructionList.setPositions when setting the position for every
    99       * instruction. In the presence of variable length instructions `setPositions'
    100      * performs multiple passes over the instruction list to calculate the
    101      * correct (byte) positions and offsets by calling this function.
    102      *
    103      * @param offset additional offset caused by preceding (variable length) instructions
    104      * @param max_offset the maximum offset that may be caused by these instructions
    105      * @return additional offset caused by possible change of this instruction's length
    106      */
    107     protected int updatePosition( int offset, int max_offset ) {
    108         position += offset;
    109         return 0;
    110     }
    111
    112
    113     /***
    114      * Long output format:
    115      *
    116      * &lt;position in byte code&gt;
    117      * &lt;name of opcode&gt; "["&lt;opcode number&gt;"]"
    118      * "("&lt;length of instruction&gt;")"
    119      * "&lt;"&lt;target instruction&gt;"&gt;" "@"&lt;branch target offset&gt;
    120      *
    121      * @param verbose long/short format switch
    122      * @return mnemonic for instruction
    123      */
    124     public String toString( boolean verbose ) {
    125         String s = super.toString(verbose);
    126         String t = "null";
    127         if (verbose) {
    128             if (target != null) {
    129                 if (target.getInstruction() == this) {
    130                     t = "<points to itself>";
    131                 } else if (target.getInstruction() == null) {
    132                     t = "<null instruction!!!?>";
    133                 } else {
    134                     t = target.getInstruction().toString(false); // Avoid circles
    135                 }
    136             }
    137         } else {
    138             if (target != null) {
    139                 index = getTargetOffset();
    140                 t = "" + (index + position);
    141             }
    142         }
    143         return s + " -> " + t;
    144     }
    145
    146
    147     /***
    148      * Read needed data (e.g. index) from file. Conversion to a InstructionHandle
    149      * is done in InstructionList(byte[]).
    150      *
    151      * @param bytes input stream
    152      * @param wide wide prefix?
    153      * @see InstructionList
    154      */
    155     protected void initFromFile( ByteSequence bytes, boolean wide ) throws IOException {
    156         length = 3;
    157         index = bytes.readShort();
    158     }
    159
    160
    161     /***
    162      * @return target offset in byte code
    163      */
    164     public final int getIndex() {
    165         return index;
    166     }
    167
    168
    169     /***
    170      * @return target of branch instruction
    171      */
    172     public InstructionHandle getTarget() {
    173         return target;
    174     }
    175
    176
    177     /***
    178      * Set branch target
    179      * @param target branch target
    180      */
    181     public void setTarget( InstructionHandle target ) {
    182         notifyTarget(this.target, target, this);
    183         this.target = target;
    184     }
    185
    186
    187     /***
    188      * Used by BranchInstruction, LocalVariableGen, CodeExceptionGen
    189      */
    190     static final void notifyTarget( InstructionHandle old_ih, InstructionHandle new_ih,
    191             InstructionTargeter t ) {
    192         if (old_ih != null) {
    193             old_ih.removeTargeter(t);
    194         }
    195         if (new_ih != null) {
    196             new_ih.addTargeter(t);
    197         }
    198     }
    199
    200
    201     /***
    202      * @param old_ih old target
    203      * @param new_ih new target
    204      */
    205     public void updateTarget( InstructionHandle old_ih, InstructionHandle new_ih ) {
    206         if (target == old_ih) {
    207             setTarget(new_ih);
    208         } else {
    209             throw new ClassGenException("Not targeting " + old_ih + ", but " + target);
    210         }
    211     }
    212
    213
    214     /***
    215      * @return true, if ih is target of this instruction
    216      */
    217     public boolean containsTarget( InstructionHandle ih ) {
    218         return (target == ih);
    219     }
    220
    221
    222     /***
    223      * Inform target that it's not targeted anymore.
    224      */
    225     void dispose() {
    226         setTarget(null);
    227         index = -1;
    228         position = -1;
    229     }
    230 }