org.ow2.asm/asm-tree/4.2 : org/objectweb/asm/util/CheckMethodAdapter.java

CheckMethodAdapter
http://asm.objectweb.org/asm-tree/: A very small and fast Java bytecode manipulation framework (ObjectWeb)
Eric Bruneton
Eugene Kuleshov
Remi Forax
ASM: a very small and fast Java bytecode manipulation framework
Copyright (c) 2000-2011 INRIA, France Telecom
All rights reserved.
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.
3. Neither the name of the copyright holders nor the names of its
   contributors may be used to endorse or promote products derived from
   this software without specific prior written permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
THE POSSIBILITY OF SUCH DAMAGE.
/***
 * ASM: a very small and fast Java bytecode manipulation framework
 * Copyright (c) 2000-2011 INRIA, France Telecom
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of its
 *    contributors may be used to endorse or promote products derived from
 *    this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */
package org.objectweb.asm.util;

import java.io.PrintWriter;
import java.io.StringWriter;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.objectweb.asm.AnnotationVisitor;
import org.objectweb.asm.Attribute;
import org.objectweb.asm.Handle;
import org.objectweb.asm.Label;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.Type;
import org.objectweb.asm.tree.MethodNode;
import org.objectweb.asm.tree.analysis.Analyzer;
import org.objectweb.asm.tree.analysis.BasicValue;
import org.objectweb.asm.tree.analysis.BasicVerifier;

A MethodVisitor that checks that its methods are properly used. More precisely this method adapter checks each instruction individually, i.e., each visit method checks some preconditions based only on its arguments - such as the fact that the given opcode is correct for a given visit method. This adapter can also perform some basic data flow checks (more precisely those that can be performed without the full class hierarchy - see BasicVerifier). For instance in a method whose signature is void m (), the invalid instruction IRETURN, or the invalid sequence IADD L2I will be detected if the data flow checks are enabled. These checks are enabled by using the CheckMethodAdapter(int, String, String, MethodVisitor, Map<Label,Integer>) constructor. They are not performed if any other constructor is used. 
Author: Eric Bruneton/**
 * A {@link MethodVisitor} that checks that its methods are properly used. More
 * precisely this method adapter checks each instruction individually, i.e.,
 * each visit method checks some preconditions based <i>only</i> on its
 * arguments - such as the fact that the given opcode is correct for a given
 * visit method. This adapter can also perform some basic data flow checks (more
 * precisely those that can be performed without the full class hierarchy - see
 * {@link org.objectweb.asm.tree.analysis.BasicVerifier}). For instance in a
 * method whose signature is <tt>void m ()</tt>, the invalid instruction
 * IRETURN, or the invalid sequence IADD L2I will be detected if the data flow
 * checks are enabled. These checks are enabled by using the
 * {@link #CheckMethodAdapter(int,String,String,MethodVisitor,Map)} constructor.
 * They are not performed if any other constructor is used.
 * 
 * @author Eric Bruneton
 */
public class CheckMethodAdapter extends MethodVisitor {

    The class version number.
/**
     * The class version number.
     */
    public int version;

    The access flags of the method.
/**
     * The access flags of the method.
     */
    private int access;

    true if the visitCode method has been called.
/**
     * <tt>true</tt> if the visitCode method has been called.
     */
    private boolean startCode;

    true if the visitMaxs method has been called.
/**
     * <tt>true</tt> if the visitMaxs method has been called.
     */
    private boolean endCode;

    true if the visitEnd method has been called.
/**
     * <tt>true</tt> if the visitEnd method has been called.
     */
    private boolean endMethod;

    Number of visited instructions.
/**
     * Number of visited instructions.
     */
    private int insnCount;

    The already visited labels. This map associate Integer values to pseudo
code offsets.
/**
     * The already visited labels. This map associate Integer values to pseudo
     * code offsets.
     */
    private final Map<Label, Integer> labels;

    The labels used in this method. Every used label must be visited with
visitLabel before the end of the method (i.e. should be in #labels).
/**
     * The labels used in this method. Every used label must be visited with
     * visitLabel before the end of the method (i.e. should be in #labels).
     */
    private Set<Label> usedLabels;

    Number of visited frames in expanded form.
/**
     * Number of visited frames in expanded form.
     */
    private int expandedFrames;

    Number of visited frames in compressed form.
/**
     * Number of visited frames in compressed form.
     */
    private int compressedFrames;

    Number of instructions before the last visited frame.
/**
     * Number of instructions before the last visited frame.
     */
    private int lastFrame = -1;

    The exception handler ranges. Each pair of list element contains the
start and end labels of an exception handler block.
/**
     * The exception handler ranges. Each pair of list element contains the
     * start and end labels of an exception handler block.
     */
    private List<Label> handlers;

    Code of the visit method to be used for each opcode.
/**
     * Code of the visit method to be used for each opcode.
     */
    private static final int[] TYPE;

    The Label.status field.
/**
     * The Label.status field.
     */
    private static Field labelStatusField;

    static {
        String s = "BBBBBBBBBBBBBBBBCCIAADDDDDAAAAAAAAAAAAAAAAAAAABBBBBBBBDD"
                + "DDDAAAAAAAAAAAAAAAAAAAABBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB"
                + "BBBBBBBBBBBBBBBBBBBJBBBBBBBBBBBBBBBBBBBBHHHHHHHHHHHHHHHHD"
                + "KLBBBBBBFFFFGGGGAECEBBEEBBAMHHAA";
        TYPE = new int[s.length()];
        for (int i = 0; i < TYPE.length; ++i) {
            TYPE[i] = s.charAt(i) - 'A' - 1;
        }
    }

    // code to generate the above string
    // public static void main (String[] args) {
    // int[] TYPE = new int[] {
    // 0, //NOP
    // 0, //ACONST_NULL
    // 0, //ICONST_M1
    // 0, //ICONST_0
    // 0, //ICONST_1
    // 0, //ICONST_2
    // 0, //ICONST_3
    // 0, //ICONST_4
    // 0, //ICONST_5
    // 0, //LCONST_0
    // 0, //LCONST_1
    // 0, //FCONST_0
    // 0, //FCONST_1
    // 0, //FCONST_2
    // 0, //DCONST_0
    // 0, //DCONST_1
    // 1, //BIPUSH
    // 1, //SIPUSH
    // 7, //LDC
    // -1, //LDC_W
    // -1, //LDC2_W
    // 2, //ILOAD
    // 2, //LLOAD
    // 2, //FLOAD
    // 2, //DLOAD
    // 2, //ALOAD
    // -1, //ILOAD_0
    // -1, //ILOAD_1
    // -1, //ILOAD_2
    // -1, //ILOAD_3
    // -1, //LLOAD_0
    // -1, //LLOAD_1
    // -1, //LLOAD_2
    // -1, //LLOAD_3
    // -1, //FLOAD_0
    // -1, //FLOAD_1
    // -1, //FLOAD_2
    // -1, //FLOAD_3
    // -1, //DLOAD_0
    // -1, //DLOAD_1
    // -1, //DLOAD_2
    // -1, //DLOAD_3
    // -1, //ALOAD_0
    // -1, //ALOAD_1
    // -1, //ALOAD_2
    // -1, //ALOAD_3
    // 0, //IALOAD
    // 0, //LALOAD
    // 0, //FALOAD
    // 0, //DALOAD
    // 0, //AALOAD
    // 0, //BALOAD
    // 0, //CALOAD
    // 0, //SALOAD
    // 2, //ISTORE
    // 2, //LSTORE
    // 2, //FSTORE
    // 2, //DSTORE
    // 2, //ASTORE
    // -1, //ISTORE_0
    // -1, //ISTORE_1
    // -1, //ISTORE_2
    // -1, //ISTORE_3
    // -1, //LSTORE_0
    // -1, //LSTORE_1
    // -1, //LSTORE_2
    // -1, //LSTORE_3
    // -1, //FSTORE_0
    // -1, //FSTORE_1
    // -1, //FSTORE_2
    // -1, //FSTORE_3
    // -1, //DSTORE_0
    // -1, //DSTORE_1
    // -1, //DSTORE_2
    // -1, //DSTORE_3
    // -1, //ASTORE_0
    // -1, //ASTORE_1
    // -1, //ASTORE_2
    // -1, //ASTORE_3
    // 0, //IASTORE
    // 0, //LASTORE
    // 0, //FASTORE
    // 0, //DASTORE
    // 0, //AASTORE
    // 0, //BASTORE
    // 0, //CASTORE
    // 0, //SASTORE
    // 0, //POP
    // 0, //POP2
    // 0, //DUP
    // 0, //DUP_X1
    // 0, //DUP_X2
    // 0, //DUP2
    // 0, //DUP2_X1
    // 0, //DUP2_X2
    // 0, //SWAP
    // 0, //IADD
    // 0, //LADD
    // 0, //FADD
    // 0, //DADD
    // 0, //ISUB
    // 0, //LSUB
    // 0, //FSUB
    // 0, //DSUB
    // 0, //IMUL
    // 0, //LMUL
    // 0, //FMUL
    // 0, //DMUL
    // 0, //IDIV
    // 0, //LDIV
    // 0, //FDIV
    // 0, //DDIV
    // 0, //IREM
    // 0, //LREM
    // 0, //FREM
    // 0, //DREM
    // 0, //INEG
    // 0, //LNEG
    // 0, //FNEG
    // 0, //DNEG
    // 0, //ISHL
    // 0, //LSHL
    // 0, //ISHR
    // 0, //LSHR
    // 0, //IUSHR
    // 0, //LUSHR
    // 0, //IAND
    // 0, //LAND
    // 0, //IOR
    // 0, //LOR
    // 0, //IXOR
    // 0, //LXOR
    // 8, //IINC
    // 0, //I2L
    // 0, //I2F
    // 0, //I2D
    // 0, //L2I
    // 0, //L2F
    // 0, //L2D
    // 0, //F2I
    // 0, //F2L
    // 0, //F2D
    // 0, //D2I
    // 0, //D2L
    // 0, //D2F
    // 0, //I2B
    // 0, //I2C
    // 0, //I2S
    // 0, //LCMP
    // 0, //FCMPL
    // 0, //FCMPG
    // 0, //DCMPL
    // 0, //DCMPG
    // 6, //IFEQ
    // 6, //IFNE
    // 6, //IFLT
    // 6, //IFGE
    // 6, //IFGT
    // 6, //IFLE
    // 6, //IF_ICMPEQ
    // 6, //IF_ICMPNE
    // 6, //IF_ICMPLT
    // 6, //IF_ICMPGE
    // 6, //IF_ICMPGT
    // 6, //IF_ICMPLE
    // 6, //IF_ACMPEQ
    // 6, //IF_ACMPNE
    // 6, //GOTO
    // 6, //JSR
    // 2, //RET
    // 9, //TABLESWITCH
    // 10, //LOOKUPSWITCH
    // 0, //IRETURN
    // 0, //LRETURN
    // 0, //FRETURN
    // 0, //DRETURN
    // 0, //ARETURN
    // 0, //RETURN
    // 4, //GETSTATIC
    // 4, //PUTSTATIC
    // 4, //GETFIELD
    // 4, //PUTFIELD
    // 5, //INVOKEVIRTUAL
    // 5, //INVOKESPECIAL
    // 5, //INVOKESTATIC
    // 5, //INVOKEINTERFACE
    // -1, //INVOKEDYNAMIC
    // 3, //NEW
    // 1, //NEWARRAY
    // 3, //ANEWARRAY
    // 0, //ARRAYLENGTH
    // 0, //ATHROW
    // 3, //CHECKCAST
    // 3, //INSTANCEOF
    // 0, //MONITORENTER
    // 0, //MONITOREXIT
    // -1, //WIDE
    // 11, //MULTIANEWARRAY
    // 6, //IFNULL
    // 6, //IFNONNULL
    // -1, //GOTO_W
    // -1 //JSR_W
    // };
    // for (int i = 0; i < TYPE.length; ++i) {
    // System.out.print((char)(TYPE[i] + 1 + 'A'));
    // }
    // System.out.println();
    // }

    Constructs a new CheckMethodAdapter object. This method adapter will not perform any data flow check (see CheckMethodAdapter(int, String, String, MethodVisitor, Map<Label,Integer>)). Subclasses must not use this constructor. Instead, they must use the CheckMethodAdapter(int, MethodVisitor, Map<Label,Integer>) version. 
Params: mv – 
           the method visitor to which this adapter must delegate calls./**
     * Constructs a new {@link CheckMethodAdapter} object. This method adapter
     * will not perform any data flow check (see
     * {@link #CheckMethodAdapter(int,String,String,MethodVisitor,Map)}).
     * <i>Subclasses must not use this constructor</i>. Instead, they must use
     * the {@link #CheckMethodAdapter(int, MethodVisitor, Map)} version.
     * 
     * @param mv
     *            the method visitor to which this adapter must delegate calls.
     */
    public CheckMethodAdapter(final MethodVisitor mv) {
        this(mv, new HashMap<Label, Integer>());
    }

    Constructs a new CheckMethodAdapter object. This method adapter will not perform any data flow check (see CheckMethodAdapter(int, String, String, MethodVisitor, Map<Label,Integer>)). Subclasses must not use this constructor. Instead, they must use the CheckMethodAdapter(int, MethodVisitor, Map<Label,Integer>) version. 
Params: mv – 
           the method visitor to which this adapter must delegate calls.
labels – 
           a map of already visited labels (in other methods)./**
     * Constructs a new {@link CheckMethodAdapter} object. This method adapter
     * will not perform any data flow check (see
     * {@link #CheckMethodAdapter(int,String,String,MethodVisitor,Map)}).
     * <i>Subclasses must not use this constructor</i>. Instead, they must use
     * the {@link #CheckMethodAdapter(int, MethodVisitor, Map)} version.
     * 
     * @param mv
     *            the method visitor to which this adapter must delegate calls.
     * @param labels
     *            a map of already visited labels (in other methods).
     */
    public CheckMethodAdapter(final MethodVisitor mv,
            final Map<Label, Integer> labels) {
        this(Opcodes.ASM4, mv, labels);
    }

    Constructs a new CheckMethodAdapter object. This method adapter will not perform any data flow check (see CheckMethodAdapter(int, String, String, MethodVisitor, Map<Label,Integer>)). 
Params: mv – 
           the method visitor to which this adapter must delegate calls.
labels – 
           a map of already visited labels (in other methods)./**
     * Constructs a new {@link CheckMethodAdapter} object. This method adapter
     * will not perform any data flow check (see
     * {@link #CheckMethodAdapter(int,String,String,MethodVisitor,Map)}).
     * 
     * @param mv
     *            the method visitor to which this adapter must delegate calls.
     * @param labels
     *            a map of already visited labels (in other methods).
     */
    protected CheckMethodAdapter(final int api, final MethodVisitor mv,
            final Map<Label, Integer> labels) {
        super(api, mv);
        this.labels = labels;
        this.usedLabels = new HashSet<Label>();
        this.handlers = new ArrayList<Label>();
    }

    Constructs a new CheckMethodAdapter object. This method adapter will perform basic data flow checks. For instance in a method whose signature is void m (), the invalid instruction IRETURN, or the
invalid sequence IADD L2I will be detected.
Params: access – 
           the method's access flags.
name – 
           the method's name.
desc –  the method's descriptor (see Type).
cmv – 
           the method visitor to which this adapter must delegate calls.
labels – 
           a map of already visited labels (in other methods)./**
     * Constructs a new {@link CheckMethodAdapter} object. This method adapter
     * will perform basic data flow checks. For instance in a method whose
     * signature is <tt>void m ()</tt>, the invalid instruction IRETURN, or the
     * invalid sequence IADD L2I will be detected.
     * 
     * @param access
     *            the method's access flags.
     * @param name
     *            the method's name.
     * @param desc
     *            the method's descriptor (see {@link Type Type}).
     * @param cmv
     *            the method visitor to which this adapter must delegate calls.
     * @param labels
     *            a map of already visited labels (in other methods).
     */
    public CheckMethodAdapter(final int access, final String name,
            final String desc, final MethodVisitor cmv,
            final Map<Label, Integer> labels) {
        this(new MethodNode(access, name, desc, null, null) {
            @Override
            public void visitEnd() {
                Analyzer<BasicValue> a = new Analyzer<BasicValue>(
                        new BasicVerifier());
                try {
                    a.analyze("dummy", this);
                } catch (Exception e) {
                    if (e instanceof IndexOutOfBoundsException
                            && maxLocals == 0 && maxStack == 0) {
                        throw new RuntimeException(
                                "Data flow checking option requires valid, non zero maxLocals and maxStack values.");
                    }
                    e.printStackTrace();
                    StringWriter sw = new StringWriter();
                    PrintWriter pw = new PrintWriter(sw, true);
                    CheckClassAdapter.printAnalyzerResult(this, a, pw);
                    pw.close();
                    throw new RuntimeException(e.getMessage() + ' '
                            + sw.toString());
                }
                accept(cmv);
            }
        }, labels);
        this.access = access;
    }

    @Override
    public AnnotationVisitor visitAnnotation(final String desc,
            final boolean visible) {
        checkEndMethod();
        checkDesc(desc, false);
        return new CheckAnnotationAdapter(super.visitAnnotation(desc, visible));
    }

    @Override
    public AnnotationVisitor visitAnnotationDefault() {
        checkEndMethod();
        return new CheckAnnotationAdapter(super.visitAnnotationDefault(), false);
    }

    @Override
    public AnnotationVisitor visitParameterAnnotation(final int parameter,
            final String desc, final boolean visible) {
        checkEndMethod();
        checkDesc(desc, false);
        return new CheckAnnotationAdapter(super.visitParameterAnnotation(
                parameter, desc, visible));
    }

    @Override
    public void visitAttribute(final Attribute attr) {
        checkEndMethod();
        if (attr == null) {
            throw new IllegalArgumentException(
                    "Invalid attribute (must not be null)");
        }
        super.visitAttribute(attr);
    }

    @Override
    public void visitCode() {
        if ((access & Opcodes.ACC_ABSTRACT) != 0) {
            throw new RuntimeException("Abstract methods cannot have code");
        }
        startCode = true;
        super.visitCode();
    }

    @Override
    public void visitFrame(final int type, final int nLocal,
            final Object[] local, final int nStack, final Object[] stack) {
        if (insnCount == lastFrame) {
            throw new IllegalStateException(
                    "At most one frame can be visited at a given code location.");
        }
        lastFrame = insnCount;
        int mLocal;
        int mStack;
        switch (type) {
        case Opcodes.F_NEW:
        case Opcodes.F_FULL:
            mLocal = Integer.MAX_VALUE;
            mStack = Integer.MAX_VALUE;
            break;

        case Opcodes.F_SAME:
            mLocal = 0;
            mStack = 0;
            break;

        case Opcodes.F_SAME1:
            mLocal = 0;
            mStack = 1;
            break;

        case Opcodes.F_APPEND:
        case Opcodes.F_CHOP:
            mLocal = 3;
            mStack = 0;
            break;

        default:
            throw new IllegalArgumentException("Invalid frame type " + type);
        }

        if (nLocal > mLocal) {
            throw new IllegalArgumentException("Invalid nLocal=" + nLocal
                    + " for frame type " + type);
        }
        if (nStack > mStack) {
            throw new IllegalArgumentException("Invalid nStack=" + nStack
                    + " for frame type " + type);
        }

        if (type != Opcodes.F_CHOP) {
            if (nLocal > 0 && (local == null || local.length < nLocal)) {
                throw new IllegalArgumentException(
                        "Array local[] is shorter than nLocal");
            }
            for (int i = 0; i < nLocal; ++i) {
                checkFrameValue(local[i]);
            }
        }
        if (nStack > 0 && (stack == null || stack.length < nStack)) {
            throw new IllegalArgumentException(
                    "Array stack[] is shorter than nStack");
        }
        for (int i = 0; i < nStack; ++i) {
            checkFrameValue(stack[i]);
        }
        if (type == Opcodes.F_NEW) {
            ++expandedFrames;
        } else {
            ++compressedFrames;
        }
        if (expandedFrames > 0 && compressedFrames > 0) {
            throw new RuntimeException(
                    "Expanded and compressed frames must not be mixed.");
        }
        super.visitFrame(type, nLocal, local, nStack, stack);
    }

    @Override
    public void visitInsn(final int opcode) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 0);
        super.visitInsn(opcode);
        ++insnCount;
    }

    @Override
    public void visitIntInsn(final int opcode, final int operand) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 1);
        switch (opcode) {
        case Opcodes.BIPUSH:
            checkSignedByte(operand, "Invalid operand");
            break;
        case Opcodes.SIPUSH:
            checkSignedShort(operand, "Invalid operand");
            break;
        // case Constants.NEWARRAY:
        default:
            if (operand < Opcodes.T_BOOLEAN || operand > Opcodes.T_LONG) {
                throw new IllegalArgumentException(
                        "Invalid operand (must be an array type code T_...): "
                                + operand);
            }
        }
        super.visitIntInsn(opcode, operand);
        ++insnCount;
    }

    @Override
    public void visitVarInsn(final int opcode, final int var) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 2);
        checkUnsignedShort(var, "Invalid variable index");
        super.visitVarInsn(opcode, var);
        ++insnCount;
    }

    @Override
    public void visitTypeInsn(final int opcode, final String type) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 3);
        checkInternalName(type, "type");
        if (opcode == Opcodes.NEW && type.charAt(0) == '[') {
            throw new IllegalArgumentException(
                    "NEW cannot be used to create arrays: " + type);
        }
        super.visitTypeInsn(opcode, type);
        ++insnCount;
    }

    @Override
    public void visitFieldInsn(final int opcode, final String owner,
            final String name, final String desc) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 4);
        checkInternalName(owner, "owner");
        checkUnqualifiedName(version, name, "name");
        checkDesc(desc, false);
        super.visitFieldInsn(opcode, owner, name, desc);
        ++insnCount;
    }

    @Override
    public void visitMethodInsn(final int opcode, final String owner,
            final String name, final String desc) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 5);
        if (opcode != Opcodes.INVOKESPECIAL || !"<init>".equals(name)) {
            checkMethodIdentifier(version, name, "name");
        }
        checkInternalName(owner, "owner");
        checkMethodDesc(desc);
        super.visitMethodInsn(opcode, owner, name, desc);
        ++insnCount;
    }

    @Override
    public void visitInvokeDynamicInsn(String name, String desc, Handle bsm,
            Object... bsmArgs) {
        checkStartCode();
        checkEndCode();
        checkMethodIdentifier(version, name, "name");
        checkMethodDesc(desc);
        if (bsm.getTag() != Opcodes.H_INVOKESTATIC
                && bsm.getTag() != Opcodes.H_NEWINVOKESPECIAL) {
            throw new IllegalArgumentException("invalid handle tag "
                    + bsm.getTag());
        }
        for (int i = 0; i < bsmArgs.length; i++) {
            checkLDCConstant(bsmArgs[i]);
        }
        super.visitInvokeDynamicInsn(name, desc, bsm, bsmArgs);
        ++insnCount;
    }

    @Override
    public void visitJumpInsn(final int opcode, final Label label) {
        checkStartCode();
        checkEndCode();
        checkOpcode(opcode, 6);
        checkLabel(label, false, "label");
        checkNonDebugLabel(label);
        super.visitJumpInsn(opcode, label);
        usedLabels.add(label);
        ++insnCount;
    }

    @Override
    public void visitLabel(final Label label) {
        checkStartCode();
        checkEndCode();
        checkLabel(label, false, "label");
        if (labels.get(label) != null) {
            throw new IllegalArgumentException("Already visited label");
        }
        labels.put(label, new Integer(insnCount));
        super.visitLabel(label);
    }

    @Override
    public void visitLdcInsn(final Object cst) {
        checkStartCode();
        checkEndCode();
        checkLDCConstant(cst);
        super.visitLdcInsn(cst);
        ++insnCount;
    }

    @Override
    public void visitIincInsn(final int var, final int increment) {
        checkStartCode();
        checkEndCode();
        checkUnsignedShort(var, "Invalid variable index");
        checkSignedShort(increment, "Invalid increment");
        super.visitIincInsn(var, increment);
        ++insnCount;
    }

    @Override
    public void visitTableSwitchInsn(final int min, final int max,
            final Label dflt, final Label... labels) {
        checkStartCode();
        checkEndCode();
        if (max < min) {
            throw new IllegalArgumentException("Max = " + max
                    + " must be greater than or equal to min = " + min);
        }
        checkLabel(dflt, false, "default label");
        checkNonDebugLabel(dflt);
        if (labels == null || labels.length != max - min + 1) {
            throw new IllegalArgumentException(
                    "There must be max - min + 1 labels");
        }
        for (int i = 0; i < labels.length; ++i) {
            checkLabel(labels[i], false, "label at index " + i);
            checkNonDebugLabel(labels[i]);
        }
        super.visitTableSwitchInsn(min, max, dflt, labels);
        for (int i = 0; i < labels.length; ++i) {
            usedLabels.add(labels[i]);
        }
        ++insnCount;
    }

    @Override
    public void visitLookupSwitchInsn(final Label dflt, final int[] keys,
            final Label[] labels) {
        checkEndCode();
        checkStartCode();
        checkLabel(dflt, false, "default label");
        checkNonDebugLabel(dflt);
        if (keys == null || labels == null || keys.length != labels.length) {
            throw new IllegalArgumentException(
                    "There must be the same number of keys and labels");
        }
        for (int i = 0; i < labels.length; ++i) {
            checkLabel(labels[i], false, "label at index " + i);
            checkNonDebugLabel(labels[i]);
        }
        super.visitLookupSwitchInsn(dflt, keys, labels);
        usedLabels.add(dflt);
        for (int i = 0; i < labels.length; ++i) {
            usedLabels.add(labels[i]);
        }
        ++insnCount;
    }

    @Override
    public void visitMultiANewArrayInsn(final String desc, final int dims) {
        checkStartCode();
        checkEndCode();
        checkDesc(desc, false);
        if (desc.charAt(0) != '[') {
            throw new IllegalArgumentException(
                    "Invalid descriptor (must be an array type descriptor): "
                            + desc);
        }
        if (dims < 1) {
            throw new IllegalArgumentException(
                    "Invalid dimensions (must be greater than 0): " + dims);
        }
        if (dims > desc.lastIndexOf('[') + 1) {
            throw new IllegalArgumentException(
                    "Invalid dimensions (must not be greater than dims(desc)): "
                            + dims);
        }
        super.visitMultiANewArrayInsn(desc, dims);
        ++insnCount;
    }

    @Override
    public void visitTryCatchBlock(final Label start, final Label end,
            final Label handler, final String type) {
        checkStartCode();
        checkEndCode();
        checkLabel(start, false, "start label");
        checkLabel(end, false, "end label");
        checkLabel(handler, false, "handler label");
        checkNonDebugLabel(start);
        checkNonDebugLabel(end);
        checkNonDebugLabel(handler);
        if (labels.get(start) != null || labels.get(end) != null
                || labels.get(handler) != null) {
            throw new IllegalStateException(
                    "Try catch blocks must be visited before their labels");
        }
        if (type != null) {
            checkInternalName(type, "type");
        }
        super.visitTryCatchBlock(start, end, handler, type);
        handlers.add(start);
        handlers.add(end);
    }

    @Override
    public void visitLocalVariable(final String name, final String desc,
            final String signature, final Label start, final Label end,
            final int index) {
        checkStartCode();
        checkEndCode();
        checkUnqualifiedName(version, name, "name");
        checkDesc(desc, false);
        checkLabel(start, true, "start label");
        checkLabel(end, true, "end label");
        checkUnsignedShort(index, "Invalid variable index");
        int s = labels.get(start).intValue();
        int e = labels.get(end).intValue();
        if (e < s) {
            throw new IllegalArgumentException(
                    "Invalid start and end labels (end must be greater than start)");
        }
        super.visitLocalVariable(name, desc, signature, start, end, index);
    }

    @Override
    public void visitLineNumber(final int line, final Label start) {
        checkStartCode();
        checkEndCode();
        checkUnsignedShort(line, "Invalid line number");
        checkLabel(start, true, "start label");
        super.visitLineNumber(line, start);
    }

    @Override
    public void visitMaxs(final int maxStack, final int maxLocals) {
        checkStartCode();
        checkEndCode();
        endCode = true;
        for (Label l : usedLabels) {
            if (labels.get(l) == null) {
                throw new IllegalStateException("Undefined label used");
            }
        }
        for (int i = 0; i < handlers.size();) {
            Integer start = labels.get(handlers.get(i++));
            Integer end = labels.get(handlers.get(i++));
            if (start == null || end == null) {
                throw new IllegalStateException(
                        "Undefined try catch block labels");
            }
            if (end.intValue() <= start.intValue()) {
                throw new IllegalStateException(
                        "Emty try catch block handler range");
            }
        }
        checkUnsignedShort(maxStack, "Invalid max stack");
        checkUnsignedShort(maxLocals, "Invalid max locals");
        super.visitMaxs(maxStack, maxLocals);
    }

    @Override
    public void visitEnd() {
        checkEndMethod();
        endMethod = true;
        super.visitEnd();
    }

    // -------------------------------------------------------------------------

    Checks that the visitCode method has been called.
/**
     * Checks that the visitCode method has been called.
     */
    void checkStartCode() {
        if (!startCode) {
            throw new IllegalStateException(
                    "Cannot visit instructions before visitCode has been called.");
        }
    }

    Checks that the visitMaxs method has not been called.
/**
     * Checks that the visitMaxs method has not been called.
     */
    void checkEndCode() {
        if (endCode) {
            throw new IllegalStateException(
                    "Cannot visit instructions after visitMaxs has been called.");
        }
    }

    Checks that the visitEnd method has not been called.
/**
     * Checks that the visitEnd method has not been called.
     */
    void checkEndMethod() {
        if (endMethod) {
            throw new IllegalStateException(
                    "Cannot visit elements after visitEnd has been called.");
        }
    }

    Checks a stack frame value.
Params: value – 
           the value to be checked./**
     * Checks a stack frame value.
     * 
     * @param value
     *            the value to be checked.
     */
    void checkFrameValue(final Object value) {
        if (value == Opcodes.TOP || value == Opcodes.INTEGER
                || value == Opcodes.FLOAT || value == Opcodes.LONG
                || value == Opcodes.DOUBLE || value == Opcodes.NULL
                || value == Opcodes.UNINITIALIZED_THIS) {
            return;
        }
        if (value instanceof String) {
            checkInternalName((String) value, "Invalid stack frame value");
            return;
        }
        if (!(value instanceof Label)) {
            throw new IllegalArgumentException("Invalid stack frame value: "
                    + value);
        } else {
            usedLabels.add((Label) value);
        }
    }

    Checks that the type of the given opcode is equal to the given type.
Params: opcode – 
           the opcode to be checked.
type – 
           the expected opcode type./**
     * Checks that the type of the given opcode is equal to the given type.
     * 
     * @param opcode
     *            the opcode to be checked.
     * @param type
     *            the expected opcode type.
     */
    static void checkOpcode(final int opcode, final int type) {
        if (opcode < 0 || opcode > 199 || TYPE[opcode] != type) {
            throw new IllegalArgumentException("Invalid opcode: " + opcode);
        }
    }

    Checks that the given value is a signed byte.
Params: value – 
           the value to be checked.
msg – 
           an message to be used in case of error./**
     * Checks that the given value is a signed byte.
     * 
     * @param value
     *            the value to be checked.
     * @param msg
     *            an message to be used in case of error.
     */
    static void checkSignedByte(final int value, final String msg) {
        if (value < Byte.MIN_VALUE || value > Byte.MAX_VALUE) {
            throw new IllegalArgumentException(msg
                    + " (must be a signed byte): " + value);
        }
    }

    Checks that the given value is a signed short.
Params: value – 
           the value to be checked.
msg – 
           an message to be used in case of error./**
     * Checks that the given value is a signed short.
     * 
     * @param value
     *            the value to be checked.
     * @param msg
     *            an message to be used in case of error.
     */
    static void checkSignedShort(final int value, final String msg) {
        if (value < Short.MIN_VALUE || value > Short.MAX_VALUE) {
            throw new IllegalArgumentException(msg
                    + " (must be a signed short): " + value);
        }
    }

    Checks that the given value is an unsigned short.
Params: value – 
           the value to be checked.
msg – 
           an message to be used in case of error./**
     * Checks that the given value is an unsigned short.
     * 
     * @param value
     *            the value to be checked.
     * @param msg
     *            an message to be used in case of error.
     */
    static void checkUnsignedShort(final int value, final String msg) {
        if (value < 0 || value > 65535) {
            throw new IllegalArgumentException(msg
                    + " (must be an unsigned short): " + value);
        }
    }

    Checks that the given value is an Integer, aFloat, a Long, a Double or a String. 
Params: cst – 
           the value to be checked./**
     * Checks that the given value is an {@link Integer}, a{@link Float}, a
     * {@link Long}, a {@link Double} or a {@link String}.
     * 
     * @param cst
     *            the value to be checked.
     */
    static void checkConstant(final Object cst) {
        if (!(cst instanceof Integer) && !(cst instanceof Float)
                && !(cst instanceof Long) && !(cst instanceof Double)
                && !(cst instanceof String)) {
            throw new IllegalArgumentException("Invalid constant: " + cst);
        }
    }

    void checkLDCConstant(final Object cst) {
        if (cst instanceof Type) {
            int s = ((Type) cst).getSort();
            if (s != Type.OBJECT && s != Type.ARRAY && s != Type.METHOD) {
                throw new IllegalArgumentException("Illegal LDC constant value");
            }
            if (s != Type.METHOD && (version & 0xFFFF) < Opcodes.V1_5) {
                throw new IllegalArgumentException(
                        "ldc of a constant class requires at least version 1.5");
            }
            if (s == Type.METHOD && (version & 0xFFFF) < Opcodes.V1_7) {
                throw new IllegalArgumentException(
                        "ldc of a method type requires at least version 1.7");
            }
        } else if (cst instanceof Handle) {
            if ((version & 0xFFFF) < Opcodes.V1_7) {
                throw new IllegalArgumentException(
                        "ldc of a handle requires at least version 1.7");
            }
            int tag = ((Handle) cst).getTag();
            if (tag < Opcodes.H_GETFIELD || tag > Opcodes.H_INVOKEINTERFACE) {
                throw new IllegalArgumentException("invalid handle tag " + tag);
            }
        } else {
            checkConstant(cst);
        }
    }

    Checks that the given string is a valid unqualified name.
Params: version – 
           the class version.
name – 
           the string to be checked.
msg – 
           a message to be used in case of error./**
     * Checks that the given string is a valid unqualified name.
     * 
     * @param version
     *            the class version.
     * @param name
     *            the string to be checked.
     * @param msg
     *            a message to be used in case of error.
     */
    static void checkUnqualifiedName(int version, final String name,
            final String msg) {
        if ((version & 0xFFFF) < Opcodes.V1_5) {
            checkIdentifier(name, msg);
        } else {
            for (int i = 0; i < name.length(); ++i) {
                if (".;[/".indexOf(name.charAt(i)) != -1) {
                    throw new IllegalArgumentException("Invalid " + msg
                            + " (must be a valid unqualified name): " + name);
                }
            }
        }
    }

    Checks that the given string is a valid Java identifier.
Params: name – 
           the string to be checked.
msg – 
           a message to be used in case of error./**
     * Checks that the given string is a valid Java identifier.
     * 
     * @param name
     *            the string to be checked.
     * @param msg
     *            a message to be used in case of error.
     */
    static void checkIdentifier(final String name, final String msg) {
        checkIdentifier(name, 0, -1, msg);
    }

    Checks that the given substring is a valid Java identifier.
Params: name – 
           the string to be checked.
start – 
           index of the first character of the identifier (inclusive).
end – 
           index of the last character of the identifier (exclusive). -1
           is equivalent to name.length() if name is not
           null.
msg – 
           a message to be used in case of error./**
     * Checks that the given substring is a valid Java identifier.
     * 
     * @param name
     *            the string to be checked.
     * @param start
     *            index of the first character of the identifier (inclusive).
     * @param end
     *            index of the last character of the identifier (exclusive). -1
     *            is equivalent to <tt>name.length()</tt> if name is not
     *            <tt>null</tt>.
     * @param msg
     *            a message to be used in case of error.
     */
    static void checkIdentifier(final String name, final int start,
            final int end, final String msg) {
        if (name == null || (end == -1 ? name.length() <= start : end <= start)) {
            throw new IllegalArgumentException("Invalid " + msg
                    + " (must not be null or empty)");
        }
        if (!Character.isJavaIdentifierStart(name.charAt(start))) {
            throw new IllegalArgumentException("Invalid " + msg
                    + " (must be a valid Java identifier): " + name);
        }
        int max = end == -1 ? name.length() : end;
        for (int i = start + 1; i < max; ++i) {
            if (!Character.isJavaIdentifierPart(name.charAt(i))) {
                throw new IllegalArgumentException("Invalid " + msg
                        + " (must be a valid Java identifier): " + name);
            }
        }
    }

    Checks that the given string is a valid Java identifier.
Params: version – 
           the class version.
name – 
           the string to be checked.
msg – 
           a message to be used in case of error./**
     * Checks that the given string is a valid Java identifier.
     * 
     * @param version
     *            the class version.
     * @param name
     *            the string to be checked.
     * @param msg
     *            a message to be used in case of error.
     */
    static void checkMethodIdentifier(int version, final String name,
            final String msg) {
        if (name == null || name.length() == 0) {
            throw new IllegalArgumentException("Invalid " + msg
                    + " (must not be null or empty)");
        }
        if ((version & 0xFFFF) >= Opcodes.V1_5) {
            for (int i = 0; i < name.length(); ++i) {
                if (".;[/<>".indexOf(name.charAt(i)) != -1) {
                    throw new IllegalArgumentException("Invalid " + msg
                            + " (must be a valid unqualified name): " + name);
                }
            }
            return;
        }
        if (!Character.isJavaIdentifierStart(name.charAt(0))) {
            throw new IllegalArgumentException(
                    "Invalid "
                            + msg
                            + " (must be a '<init>', '<clinit>' or a valid Java identifier): "
                            + name);
        }
        for (int i = 1; i < name.length(); ++i) {
            if (!Character.isJavaIdentifierPart(name.charAt(i))) {
                throw new IllegalArgumentException(
                        "Invalid "
                                + msg
                                + " (must be '<init>' or '<clinit>' or a valid Java identifier): "
                                + name);
            }
        }
    }

    Checks that the given string is a valid internal class name.
Params: name – 
           the string to be checked.
msg – 
           a message to be used in case of error./**
     * Checks that the given string is a valid internal class name.
     * 
     * @param name
     *            the string to be checked.
     * @param msg
     *            a message to be used in case of error.
     */
    static void checkInternalName(final String name, final String msg) {
        if (name == null || name.length() == 0) {
            throw new IllegalArgumentException("Invalid " + msg
                    + " (must not be null or empty)");
        }
        if (name.charAt(0) == '[') {
            checkDesc(name, false);
        } else {
            checkInternalName(name, 0, -1, msg);
        }
    }

    Checks that the given substring is a valid internal class name.
Params: name – 
           the string to be checked.
start – 
           index of the first character of the identifier (inclusive).
end – 
           index of the last character of the identifier (exclusive). -1
           is equivalent to name.length() if name is not
           null.
msg – 
           a message to be used in case of error./**
     * Checks that the given substring is a valid internal class name.
     * 
     * @param name
     *            the string to be checked.
     * @param start
     *            index of the first character of the identifier (inclusive).
     * @param end
     *            index of the last character of the identifier (exclusive). -1
     *            is equivalent to <tt>name.length()</tt> if name is not
     *            <tt>null</tt>.
     * @param msg
     *            a message to be used in case of error.
     */
    static void checkInternalName(final String name, final int start,
            final int end, final String msg) {
        int max = end == -1 ? name.length() : end;
        try {
            int begin = start;
            int slash;
            do {
                slash = name.indexOf('/', begin + 1);
                if (slash == -1 || slash > max) {
                    slash = max;
                }
                checkIdentifier(name, begin, slash, null);
                begin = slash + 1;
            } while (slash != max);
        } catch (IllegalArgumentException _) {
            throw new IllegalArgumentException(
                    "Invalid "
                            + msg
                            + " (must be a fully qualified class name in internal form): "
                            + name);
        }
    }

    Checks that the given string is a valid type descriptor.
Params: desc – 
           the string to be checked.
canBeVoid – 
           true if V can be considered valid./**
     * Checks that the given string is a valid type descriptor.
     * 
     * @param desc
     *            the string to be checked.
     * @param canBeVoid
     *            <tt>true</tt> if <tt>V</tt> can be considered valid.
     */
    static void checkDesc(final String desc, final boolean canBeVoid) {
        int end = checkDesc(desc, 0, canBeVoid);
        if (end != desc.length()) {
            throw new IllegalArgumentException("Invalid descriptor: " + desc);
        }
    }

    Checks that a the given substring is a valid type descriptor.
Params: desc – 
           the string to be checked.
start – 
           index of the first character of the identifier (inclusive).
canBeVoid – 
           true if V can be considered valid.
Returns: the index of the last character of the type decriptor, plus one./**
     * Checks that a the given substring is a valid type descriptor.
     * 
     * @param desc
     *            the string to be checked.
     * @param start
     *            index of the first character of the identifier (inclusive).
     * @param canBeVoid
     *            <tt>true</tt> if <tt>V</tt> can be considered valid.
     * @return the index of the last character of the type decriptor, plus one.
     */
    static int checkDesc(final String desc, final int start,
            final boolean canBeVoid) {
        if (desc == null || start >= desc.length()) {
            throw new IllegalArgumentException(
                    "Invalid type descriptor (must not be null or empty)");
        }
        int index;
        switch (desc.charAt(start)) {
        case 'V':
            if (canBeVoid) {
                return start + 1;
            } else {
                throw new IllegalArgumentException("Invalid descriptor: "
                        + desc);
            }
        case 'Z':
        case 'C':
        case 'B':
        case 'S':
        case 'I':
        case 'F':
        case 'J':
        case 'D':
            return start + 1;
        case '[':
            index = start + 1;
            while (index < desc.length() && desc.charAt(index) == '[') {
                ++index;
            }
            if (index < desc.length()) {
                return checkDesc(desc, index, false);
            } else {
                throw new IllegalArgumentException("Invalid descriptor: "
                        + desc);
            }
        case 'L':
            index = desc.indexOf(';', start);
            if (index == -1 || index - start < 2) {
                throw new IllegalArgumentException("Invalid descriptor: "
                        + desc);
            }
            try {
                checkInternalName(desc, start + 1, index, null);
            } catch (IllegalArgumentException _) {
                throw new IllegalArgumentException("Invalid descriptor: "
                        + desc);
            }
            return index + 1;
        default:
            throw new IllegalArgumentException("Invalid descriptor: " + desc);
        }
    }

    Checks that the given string is a valid method descriptor.
Params: desc – 
           the string to be checked./**
     * Checks that the given string is a valid method descriptor.
     * 
     * @param desc
     *            the string to be checked.
     */
    static void checkMethodDesc(final String desc) {
        if (desc == null || desc.length() == 0) {
            throw new IllegalArgumentException(
                    "Invalid method descriptor (must not be null or empty)");
        }
        if (desc.charAt(0) != '(' || desc.length() < 3) {
            throw new IllegalArgumentException("Invalid descriptor: " + desc);
        }
        int start = 1;
        if (desc.charAt(start) != ')') {
            do {
                if (desc.charAt(start) == 'V') {
                    throw new IllegalArgumentException("Invalid descriptor: "
                            + desc);
                }
                start = checkDesc(desc, start, false);
            } while (start < desc.length() && desc.charAt(start) != ')');
        }
        start = checkDesc(desc, start + 1, true);
        if (start != desc.length()) {
            throw new IllegalArgumentException("Invalid descriptor: " + desc);
        }
    }

    Checks that the given label is not null. This method can also check that
the label has been visited.
Params: label – 
           the label to be checked.
checkVisited – 
           true to check that the label has been visited.
msg – 
           a message to be used in case of error./**
     * Checks that the given label is not null. This method can also check that
     * the label has been visited.
     * 
     * @param label
     *            the label to be checked.
     * @param checkVisited
     *            <tt>true</tt> to check that the label has been visited.
     * @param msg
     *            a message to be used in case of error.
     */
    void checkLabel(final Label label, final boolean checkVisited,
            final String msg) {
        if (label == null) {
            throw new IllegalArgumentException("Invalid " + msg
                    + " (must not be null)");
        }
        if (checkVisited && labels.get(label) == null) {
            throw new IllegalArgumentException("Invalid " + msg
                    + " (must be visited first)");
        }
    }

    Checks that the given label is not a label used only for debug purposes.
Params: label – 
           the label to be checked./**
     * Checks that the given label is not a label used only for debug purposes.
     * 
     * @param label
     *            the label to be checked.
     */
    private static void checkNonDebugLabel(final Label label) {
        Field f = getLabelStatusField();
        int status = 0;
        try {
            status = f == null ? 0 : ((Integer) f.get(label)).intValue();
        } catch (IllegalAccessException e) {
            throw new Error("Internal error");
        }
        if ((status & 0x01) != 0) {
            throw new IllegalArgumentException(
                    "Labels used for debug info cannot be reused for control flow");
        }
    }

    Returns the Field object corresponding to the Label.status field.
Returns: the Field object corresponding to the Label.status field./**
     * Returns the Field object corresponding to the Label.status field.
     * 
     * @return the Field object corresponding to the Label.status field.
     */
    private static Field getLabelStatusField() {
        if (labelStatusField == null) {
            labelStatusField = getLabelField("a");
            if (labelStatusField == null) {
                labelStatusField = getLabelField("status");
            }
        }
        return labelStatusField;
    }

    Returns the field of the Label class whose name is given.
Params: name – 
           a field name.
Returns: the field of the Label class whose name is given, or null./**
     * Returns the field of the Label class whose name is given.
     * 
     * @param name
     *            a field name.
     * @return the field of the Label class whose name is given, or null.
     */
    private static Field getLabelField(final String name) {
        try {
            Field f = Label.class.getDeclaredField(name);
            f.setAccessible(true);
            return f;
        } catch (NoSuchFieldException e) {
            return null;
        }
    }
}
Params:	desc – the string to be checked. start – index of the first character of the identifier (inclusive). canBeVoid – `true` if `V` can be considered valid.
Returns:	the index of the last character of the type decriptor, plus one.
Params:	name – a field name.
Returns:	the field of the Label class whose name is given, or null.
/

org.ow2.asm/ asm-tree/ 4.2/ org/objectweb/asm/util/CheckMethodAdapter.java
