http://asm.ow2.io/: ASM, a very small and fast Java bytecode manipulation framework (OW2)
  • Eric Bruneton
  • Eugene Kuleshov
  • Remi Forax 
// ASM: a very small and fast Java bytecode manipulation framework
// Copyright (c) 2000-2011 INRIA, France Telecom
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//
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package org.objectweb.asm;


A non standard class, field, method or Code attribute, as defined in the Java Virtual Machine
Specification (JVMS).

Author:Eric Bruneton, Eugene Kuleshov
See Also:
/**
 * A non standard class, field, method or Code attribute, as defined in the Java Virtual Machine
 * Specification (JVMS).
 *
 * @see <a href= "https://docs.oracle.com/javase/specs/jvms/se9/html/jvms-4.html#jvms-4.7">JVMS
 *     4.7</a>
 * @see <a href= "https://docs.oracle.com/javase/specs/jvms/se9/html/jvms-4.html#jvms-4.7.3">JVMS
 *     4.7.3</a>
 * @author Eric Bruneton
 * @author Eugene Kuleshov
 */
public class Attribute {

  
The type of this attribute, also called its name in the JVMS. 
/** The type of this attribute, also called its name in the JVMS. */
  public final String type;

  
The raw content of this attribute, only used for unknown attributes (see isUnknown()). The 6 header bytes of the attribute (attribute_name_index and attribute_length) are not
included.

/**
   * The raw content of this attribute, only used for unknown attributes (see {@link #isUnknown()}).
   * The 6 header bytes of the attribute (attribute_name_index and attribute_length) are <i>not</i>
   * included.
   */
  private byte[] content;

  
The next attribute in this attribute list (Attribute instances can be linked via this field to store a list of class, field, method or Code attributes). May be null. 
/**
   * The next attribute in this attribute list (Attribute instances can be linked via this field to
   * store a list of class, field, method or Code attributes). May be {@literal null}.
   */
  Attribute nextAttribute;

  
Constructs a new empty attribute.

Params:
  • type – the type of the attribute.
/**
   * Constructs a new empty attribute.
   *
   * @param type the type of the attribute.
   */
  protected Attribute(final String type) {
    this.type = type;
  }

  
Returns true if this type of attribute is unknown. This means that the attribute content can't be parsed to extract constant pool references, labels, etc. Instead, the attribute content is read as an opaque byte array, and written back as is. This can lead to invalid attributes, if the content actually contains constant pool references, labels, or other symbolic references that need to be updated when there are changes to the constant pool, the method bytecode, etc. The default implementation of this method always returns true. 
Returns:true if this type of attribute is unknown.
/**
   * Returns {@literal true} if this type of attribute is unknown. This means that the attribute
   * content can't be parsed to extract constant pool references, labels, etc. Instead, the
   * attribute content is read as an opaque byte array, and written back as is. This can lead to
   * invalid attributes, if the content actually contains constant pool references, labels, or other
   * symbolic references that need to be updated when there are changes to the constant pool, the
   * method bytecode, etc. The default implementation of this method always returns {@literal true}.
   *
   * @return {@literal true} if this type of attribute is unknown.
   */
  public boolean isUnknown() {
    return true;
  }

  
Returns true if this type of attribute is a Code attribute. 
Returns:true if this type of attribute is a Code attribute.
/**
   * Returns {@literal true} if this type of attribute is a Code attribute.
   *
   * @return {@literal true} if this type of attribute is a Code attribute.
   */
  public boolean isCodeAttribute() {
    return false;
  }

  
Returns the labels corresponding to this attribute.

Returns:the labels corresponding to this attribute, or null if this attribute is not a Code attribute that contains labels.
/**
   * Returns the labels corresponding to this attribute.
   *
   * @return the labels corresponding to this attribute, or {@literal null} if this attribute is not
   *     a Code attribute that contains labels.
   */
  protected Label[] getLabels() {
    return new Label[0];
  }

  
Reads a type attribute. This method must return a new Attribute object, of type type, corresponding to the 'length' bytes starting at 'offset', in the given ClassReader. 
Params:
  • classReader – the class that contains the attribute to be read.
  • offset – index of the first byte of the attribute's content in ClassReader. The 6 attribute header bytes (attribute_name_index and attribute_length) are not taken into account here.
  • length – the length of the attribute's content (excluding the 6 attribute header bytes).
  • charBuffer – the buffer to be used to call the ClassReader methods requiring a
    'charBuffer' parameter.
  • codeAttributeOffset – index of the first byte of content of the enclosing Code attribute in ClassReader, or -1 if the attribute to be read is not a Code attribute. The 6 attribute header bytes (attribute_name_index and attribute_length) are not taken into account here.
  • labels – the labels of the method's code, or null if the attribute to be read is not a Code attribute.
Returns:a new Attribute object corresponding to the specified bytes.
/**
   * Reads a {@link #type} attribute. This method must return a <i>new</i> {@link Attribute} object,
   * of type {@link #type}, corresponding to the 'length' bytes starting at 'offset', in the given
   * ClassReader.
   *
   * @param classReader the class that contains the attribute to be read.
   * @param offset index of the first byte of the attribute's content in {@link ClassReader}. The 6
   *     attribute header bytes (attribute_name_index and attribute_length) are not taken into
   *     account here.
   * @param length the length of the attribute's content (excluding the 6 attribute header bytes).
   * @param charBuffer the buffer to be used to call the ClassReader methods requiring a
   *     'charBuffer' parameter.
   * @param codeAttributeOffset index of the first byte of content of the enclosing Code attribute
   *     in {@link ClassReader}, or -1 if the attribute to be read is not a Code attribute. The 6
   *     attribute header bytes (attribute_name_index and attribute_length) are not taken into
   *     account here.
   * @param labels the labels of the method's code, or {@literal null} if the attribute to be read
   *     is not a Code attribute.
   * @return a <i>new</i> {@link Attribute} object corresponding to the specified bytes.
   */
  protected Attribute read(
      final ClassReader classReader,
      final int offset,
      final int length,
      final char[] charBuffer,
      final int codeAttributeOffset,
      final Label[] labels) {
    Attribute attribute = new Attribute(type);
    attribute.content = new byte[length];
    System.arraycopy(classReader.classFileBuffer, offset, attribute.content, 0, length);
    return attribute;
  }

  
Returns the byte array form of the content of this attribute. The 6 header bytes
(attribute_name_index and attribute_length) must not be added in the returned
ByteVector.

Params:
  • classWriter – the class to which this attribute must be added. This parameter can be used
    to add the items that corresponds to this attribute to the constant pool of this class.
  • code – the bytecode of the method corresponding to this Code attribute, or null if this attribute is not a Code attribute. Corresponds to the 'code' field of the Code attribute.
  • codeLength – the length of the bytecode of the method corresponding to this code
    attribute, or 0 if this attribute is not a Code attribute. Corresponds to the 'code_length'
    field of the Code attribute.
  • maxStack – the maximum stack size of the method corresponding to this Code attribute, or
    -1 if this attribute is not a Code attribute.
  • maxLocals – the maximum number of local variables of the method corresponding to this code
    attribute, or -1 if this attribute is not a Code attribute.
Returns:the byte array form of this attribute.
/**
   * Returns the byte array form of the content of this attribute. The 6 header bytes
   * (attribute_name_index and attribute_length) must <i>not</i> be added in the returned
   * ByteVector.
   *
   * @param classWriter the class to which this attribute must be added. This parameter can be used
   *     to add the items that corresponds to this attribute to the constant pool of this class.
   * @param code the bytecode of the method corresponding to this Code attribute, or {@literal null}
   *     if this attribute is not a Code attribute. Corresponds to the 'code' field of the Code
   *     attribute.
   * @param codeLength the length of the bytecode of the method corresponding to this code
   *     attribute, or 0 if this attribute is not a Code attribute. Corresponds to the 'code_length'
   *     field of the Code attribute.
   * @param maxStack the maximum stack size of the method corresponding to this Code attribute, or
   *     -1 if this attribute is not a Code attribute.
   * @param maxLocals the maximum number of local variables of the method corresponding to this code
   *     attribute, or -1 if this attribute is not a Code attribute.
   * @return the byte array form of this attribute.
   */
  protected ByteVector write(
      final ClassWriter classWriter,
      final byte[] code,
      final int codeLength,
      final int maxStack,
      final int maxLocals) {
    return new ByteVector(content);
  }

  
Returns the number of attributes of the attribute list that begins with this attribute.

Returns:the number of attributes of the attribute list that begins with this attribute.
/**
   * Returns the number of attributes of the attribute list that begins with this attribute.
   *
   * @return the number of attributes of the attribute list that begins with this attribute.
   */
  final int getAttributeCount() {
    int count = 0;
    Attribute attribute = this;
    while (attribute != null) {
      count += 1;
      attribute = attribute.nextAttribute;
    }
    return count;
  }

  
Returns the total size in bytes of all the attributes in the attribute list that begins with
this attribute. This size includes the 6 header bytes (attribute_name_index and
attribute_length) per attribute. Also adds the attribute type names to the constant pool.

Params:
  • symbolTable – where the constants used in the attributes must be stored.
Returns:the size of all the attributes in this attribute list. This size includes the size of
    the attribute headers.
/**
   * Returns the total size in bytes of all the attributes in the attribute list that begins with
   * this attribute. This size includes the 6 header bytes (attribute_name_index and
   * attribute_length) per attribute. Also adds the attribute type names to the constant pool.
   *
   * @param symbolTable where the constants used in the attributes must be stored.
   * @return the size of all the attributes in this attribute list. This size includes the size of
   *     the attribute headers.
   */
  final int computeAttributesSize(final SymbolTable symbolTable) {
    final byte[] code = null;
    final int codeLength = 0;
    final int maxStack = -1;
    final int maxLocals = -1;
    return computeAttributesSize(symbolTable, code, codeLength, maxStack, maxLocals);
  }

  
Returns the total size in bytes of all the attributes in the attribute list that begins with
this attribute. This size includes the 6 header bytes (attribute_name_index and
attribute_length) per attribute. Also adds the attribute type names to the constant pool.

Params:
  • symbolTable – where the constants used in the attributes must be stored.
  • code – the bytecode of the method corresponding to these Code attributes, or 
    null if they are not Code attributes. Corresponds to the 'code' field of the Code attribute.
  • codeLength – the length of the bytecode of the method corresponding to these code
    attributes, or 0 if they are not Code attributes. Corresponds to the 'code_length' field of
    the Code attribute.
  • maxStack – the maximum stack size of the method corresponding to these Code attributes, or
    -1 if they are not Code attributes.
  • maxLocals – the maximum number of local variables of the method corresponding to these
    Code attributes, or -1 if they are not Code attribute.
Returns:the size of all the attributes in this attribute list. This size includes the size of
    the attribute headers.
/**
   * Returns the total size in bytes of all the attributes in the attribute list that begins with
   * this attribute. This size includes the 6 header bytes (attribute_name_index and
   * attribute_length) per attribute. Also adds the attribute type names to the constant pool.
   *
   * @param symbolTable where the constants used in the attributes must be stored.
   * @param code the bytecode of the method corresponding to these Code attributes, or {@literal
   *     null} if they are not Code attributes. Corresponds to the 'code' field of the Code
   *     attribute.
   * @param codeLength the length of the bytecode of the method corresponding to these code
   *     attributes, or 0 if they are not Code attributes. Corresponds to the 'code_length' field of
   *     the Code attribute.
   * @param maxStack the maximum stack size of the method corresponding to these Code attributes, or
   *     -1 if they are not Code attributes.
   * @param maxLocals the maximum number of local variables of the method corresponding to these
   *     Code attributes, or -1 if they are not Code attribute.
   * @return the size of all the attributes in this attribute list. This size includes the size of
   *     the attribute headers.
   */
  final int computeAttributesSize(
      final SymbolTable symbolTable,
      final byte[] code,
      final int codeLength,
      final int maxStack,
      final int maxLocals) {
    final ClassWriter classWriter = symbolTable.classWriter;
    int size = 0;
    Attribute attribute = this;
    while (attribute != null) {
      symbolTable.addConstantUtf8(attribute.type);
      size += 6 + attribute.write(classWriter, code, codeLength, maxStack, maxLocals).length;
      attribute = attribute.nextAttribute;
    }
    return size;
  }

  
Returns the total size in bytes of all the attributes that correspond to the given field,
method or class access flags and signature. This size includes the 6 header bytes
(attribute_name_index and attribute_length) per attribute. Also adds the attribute type names
to the constant pool.

Params:
  • symbolTable – where the constants used in the attributes must be stored.
  • accessFlags – some field, method or class access flags.
  • signatureIndex – the constant pool index of a field, method of class signature.
Returns:the size of all the attributes in bytes. This size includes the size of the attribute
    headers.
/**
   * Returns the total size in bytes of all the attributes that correspond to the given field,
   * method or class access flags and signature. This size includes the 6 header bytes
   * (attribute_name_index and attribute_length) per attribute. Also adds the attribute type names
   * to the constant pool.
   *
   * @param symbolTable where the constants used in the attributes must be stored.
   * @param accessFlags some field, method or class access flags.
   * @param signatureIndex the constant pool index of a field, method of class signature.
   * @return the size of all the attributes in bytes. This size includes the size of the attribute
   *     headers.
   */
  static int computeAttributesSize(
      final SymbolTable symbolTable, final int accessFlags, final int signatureIndex) {
    int size = 0;
    // Before Java 1.5, synthetic fields are represented with a Synthetic attribute.
    if ((accessFlags & Opcodes.ACC_SYNTHETIC) != 0
        && symbolTable.getMajorVersion() < Opcodes.V1_5) {
      // Synthetic attributes always use 6 bytes.
      symbolTable.addConstantUtf8(Constants.SYNTHETIC);
      size += 6;
    }
    if (signatureIndex != 0) {
      // Signature attributes always use 8 bytes.
      symbolTable.addConstantUtf8(Constants.SIGNATURE);
      size += 8;
    }
    // ACC_DEPRECATED is ASM specific, the ClassFile format uses a Deprecated attribute instead.
    if ((accessFlags & Opcodes.ACC_DEPRECATED) != 0) {
      // Deprecated attributes always use 6 bytes.
      symbolTable.addConstantUtf8(Constants.DEPRECATED);
      size += 6;
    }
    return size;
  }

  
Puts all the attributes of the attribute list that begins with this attribute, in the given
byte vector. This includes the 6 header bytes (attribute_name_index and attribute_length) per
attribute.

Params:
  • symbolTable – where the constants used in the attributes must be stored.
  • output – where the attributes must be written.
/**
   * Puts all the attributes of the attribute list that begins with this attribute, in the given
   * byte vector. This includes the 6 header bytes (attribute_name_index and attribute_length) per
   * attribute.
   *
   * @param symbolTable where the constants used in the attributes must be stored.
   * @param output where the attributes must be written.
   */
  final void putAttributes(final SymbolTable symbolTable, final ByteVector output) {
    final byte[] code = null;
    final int codeLength = 0;
    final int maxStack = -1;
    final int maxLocals = -1;
    putAttributes(symbolTable, code, codeLength, maxStack, maxLocals, output);
  }

  
Puts all the attributes of the attribute list that begins with this attribute, in the given
byte vector. This includes the 6 header bytes (attribute_name_index and attribute_length) per
attribute.

Params:
  • symbolTable – where the constants used in the attributes must be stored.
  • code – the bytecode of the method corresponding to these Code attributes, or 
    null if they are not Code attributes. Corresponds to the 'code' field of the Code attribute.
  • codeLength – the length of the bytecode of the method corresponding to these code
    attributes, or 0 if they are not Code attributes. Corresponds to the 'code_length' field of
    the Code attribute.
  • maxStack – the maximum stack size of the method corresponding to these Code attributes, or
    -1 if they are not Code attributes.
  • maxLocals – the maximum number of local variables of the method corresponding to these
    Code attributes, or -1 if they are not Code attribute.
  • output – where the attributes must be written.
/**
   * Puts all the attributes of the attribute list that begins with this attribute, in the given
   * byte vector. This includes the 6 header bytes (attribute_name_index and attribute_length) per
   * attribute.
   *
   * @param symbolTable where the constants used in the attributes must be stored.
   * @param code the bytecode of the method corresponding to these Code attributes, or {@literal
   *     null} if they are not Code attributes. Corresponds to the 'code' field of the Code
   *     attribute.
   * @param codeLength the length of the bytecode of the method corresponding to these code
   *     attributes, or 0 if they are not Code attributes. Corresponds to the 'code_length' field of
   *     the Code attribute.
   * @param maxStack the maximum stack size of the method corresponding to these Code attributes, or
   *     -1 if they are not Code attributes.
   * @param maxLocals the maximum number of local variables of the method corresponding to these
   *     Code attributes, or -1 if they are not Code attribute.
   * @param output where the attributes must be written.
   */
  final void putAttributes(
      final SymbolTable symbolTable,
      final byte[] code,
      final int codeLength,
      final int maxStack,
      final int maxLocals,
      final ByteVector output) {
    final ClassWriter classWriter = symbolTable.classWriter;
    Attribute attribute = this;
    while (attribute != null) {
      ByteVector attributeContent =
          attribute.write(classWriter, code, codeLength, maxStack, maxLocals);
      // Put attribute_name_index and attribute_length.
      output.putShort(symbolTable.addConstantUtf8(attribute.type)).putInt(attributeContent.length);
      output.putByteArray(attributeContent.data, 0, attributeContent.length);
      attribute = attribute.nextAttribute;
    }
  }

  
Puts all the attributes that correspond to the given field, method or class access flags and
signature, in the given byte vector. This includes the 6 header bytes (attribute_name_index and
attribute_length) per attribute.

Params:
  • symbolTable – where the constants used in the attributes must be stored.
  • accessFlags – some field, method or class access flags.
  • signatureIndex – the constant pool index of a field, method of class signature.
  • output – where the attributes must be written.
/**
   * Puts all the attributes that correspond to the given field, method or class access flags and
   * signature, in the given byte vector. This includes the 6 header bytes (attribute_name_index and
   * attribute_length) per attribute.
   *
   * @param symbolTable where the constants used in the attributes must be stored.
   * @param accessFlags some field, method or class access flags.
   * @param signatureIndex the constant pool index of a field, method of class signature.
   * @param output where the attributes must be written.
   */
  static void putAttributes(
      final SymbolTable symbolTable,
      final int accessFlags,
      final int signatureIndex,
      final ByteVector output) {
    // Before Java 1.5, synthetic fields are represented with a Synthetic attribute.
    if ((accessFlags & Opcodes.ACC_SYNTHETIC) != 0
        && symbolTable.getMajorVersion() < Opcodes.V1_5) {
      output.putShort(symbolTable.addConstantUtf8(Constants.SYNTHETIC)).putInt(0);
    }
    if (signatureIndex != 0) {
      output
          .putShort(symbolTable.addConstantUtf8(Constants.SIGNATURE))
          .putInt(2)
          .putShort(signatureIndex);
    }
    if ((accessFlags & Opcodes.ACC_DEPRECATED) != 0) {
      output.putShort(symbolTable.addConstantUtf8(Constants.DEPRECATED)).putInt(0);
    }
  }

  
A set of attribute prototypes (attributes with the same type are considered equal). 
/** A set of attribute prototypes (attributes with the same type are considered equal). */
  static final class Set {

    private static final int SIZE_INCREMENT = 6;

    private int size;
    private Attribute[] data = new Attribute[SIZE_INCREMENT];

    void addAttributes(final Attribute attributeList) {
      Attribute attribute = attributeList;
      while (attribute != null) {
        if (!contains(attribute)) {
          add(attribute);
        }
        attribute = attribute.nextAttribute;
      }
    }

    Attribute[] toArray() {
      Attribute[] result = new Attribute[size];
      System.arraycopy(data, 0, result, 0, size);
      return result;
    }

    private boolean contains(final Attribute attribute) {
      for (int i = 0; i < size; ++i) {
        if (data[i].type.equals(attribute.type)) {
          return true;
        }
      }
      return false;
    }

    private void add(final Attribute attribute) {
      if (size >= data.length) {
        Attribute[] newData = new Attribute[data.length + SIZE_INCREMENT];
        System.arraycopy(data, 0, newData, 0, size);
        data = newData;
      }
      data[size++] = attribute;
    }
  }
}