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The AspectJ weaver applies aspects to Java classes. It can be used as a Java agent in order to apply load-time weaving (LTW) during class-loading and also contains the AspectJ runtime classes.
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// 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 aj.org.objectweb.asm.commons;

import java.io.ByteArrayOutputStream;
import java.io.DataOutput;
import java.io.DataOutputStream;
import java.io.IOException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import aj.org.objectweb.asm.ClassVisitor;
import aj.org.objectweb.asm.FieldVisitor;
import aj.org.objectweb.asm.MethodVisitor;
import aj.org.objectweb.asm.Opcodes;

/**
 * A {@link ClassVisitor} that adds a serial version unique identifier to a class if missing. A
 * typical usage of this class is:
 *
 *  *   ClassWriter classWriter = new ClassWriter(...);
 *   ClassVisitor svuidAdder = new SerialVersionUIDAdder(classWriter);
 *   ClassVisitor classVisitor = new MyClassAdapter(svuidAdder);
 *   new ClassReader(orginalClass).accept(classVisitor, 0);
 * 
 *
 * The SVUID algorithm can be found at https://docs.oracle.com/javase/10/docs/specs/serialization/class.html#stream-unique-identifiers:
 *
 * 
The serialVersionUID is computed using the signature of a stream of bytes that reflect the
 * class definition. The National Institute of Standards and Technology (NIST) Secure Hash Algorithm
 * (SHA-1) is used to compute a signature for the stream. The first two 32-bit quantities are used
 * to form a 64-bit hash. A java.lang.DataOutputStream is used to convert primitive data types to a
 * sequence of bytes. The values input to the stream are defined by the Java Virtual Machine (VM)
 * specification for classes.
 *
 * 
The sequence of items in the stream is as follows:
 *
 * 

 *   The class name written using UTF encoding.
 *   
The class modifiers written as a 32-bit integer.
 *   
The name of each interface sorted by name written using UTF encoding.
 *   
For each field of the class sorted by field name (except private static and private
 *       transient fields):
 *       
 *         The name of the field in UTF encoding.
 *         
The modifiers of the field written as a 32-bit integer.
 *         
The descriptor of the field in UTF encoding
 *       
 *   
If a class initializer exists, write out the following:
 *       
 *         The name of the method, <clinit>, in UTF encoding.
 *         
The modifier of the method, STATIC, written as a 32-bit integer.
 *         
The descriptor of the method, ()V, in UTF encoding.
 *       
 *   
For each non-private constructor sorted by method name and signature:
 *       
 *         The name of the method, <init>, in UTF encoding.
 *         
The modifiers of the method written as a 32-bit integer.
 *         
The descriptor of the method in UTF encoding.
 *       
 *   
For each non-private method sorted by method name and signature:
 *       
 *         The name of the method in UTF encoding.
 *         
The modifiers of the method written as a 32-bit integer.
 *         
The descriptor of the method in UTF encoding.
 *       
 *   
The SHA-1 algorithm is executed on the stream of bytes produced by DataOutputStream and
 *       produces five 32-bit values sha[0..4].
 *   
The hash value is assembled from the first and second 32-bit values of the SHA-1 message
 *       digest. If the result of the message digest, the five 32-bit words H0 H1 H2 H3 H4, is in an
 *       array of five int values named sha, the hash value would be computed as follows: long hash
 *       = ((sha[0] >>> 24) & 0xFF) | ((sha[0] >>> 16) & 0xFF) << 8
 *       | ((sha[0] >>> 8) & 0xFF) << 16 | ((sha[0] >>> 0) & 0xFF)
 *       << 24 | ((sha[1] >>> 24) & 0xFF) << 32 | ((sha[1] >>> 16)
 *       & 0xFF) << 40 | ((sha[1] >>> 8) & 0xFF) << 48 | ((sha[1]
 *       >>> 0) & 0xFF) << 56;
 * 
 *
 * @author Rajendra Inamdar, Vishal Vishnoi
 */
// DontCheck(AbbreviationAsWordInName): can't be renamed (for backward binary compatibility).
public class SerialVersionUIDAdder extends ClassVisitor {

  /** The JVM name of static initializer methods. */
  private static final String CLINIT = "";

  /** A flag that indicates if we need to compute SVUID. */
  private boolean computeSvuid;

  /** Whether the class already has a SVUID. */
  private boolean hasSvuid;

  /** The class access flags. */
  private int access;

  /** The internal name of the class. */
  private String name;

  /** The interfaces implemented by the class. */
  private String[] interfaces;

  /** The fields of the class that are needed to compute the SVUID. */
  private Collection svuidFields;

  /** Whether the class has a static initializer. */
  private boolean hasStaticInitializer;

  /** The constructors of the class that are needed to compute the SVUID. */
  private Collection svuidConstructors;

  /** The methods of the class that are needed to compute the SVUID. */
  private Collection svuidMethods;

  /**
   * Constructs a new {@link SerialVersionUIDAdder}. Subclasses must not use this
   * constructor. Instead, they must use the {@link #SerialVersionUIDAdder(int, ClassVisitor)}
   * version.
   *
   * @param classVisitor a {@link ClassVisitor} to which this visitor will delegate calls.
   * @throws IllegalStateException If a subclass calls this constructor.
   */
  public SerialVersionUIDAdder(final ClassVisitor classVisitor) {
    this(/* latest api = */ Opcodes.ASM9, classVisitor);
    if (getClass() != SerialVersionUIDAdder.class) {
      throw new IllegalStateException();
    }
  }

  /**
   * Constructs a new {@link SerialVersionUIDAdder}.
   *
   * @param api the ASM API version implemented by this visitor. Must be one of the {@code
   *     ASM}x values in {@link Opcodes}.
   * @param classVisitor a {@link ClassVisitor} to which this visitor will delegate calls.
   */
  protected SerialVersionUIDAdder(final int api, final ClassVisitor classVisitor) {
    super(api, classVisitor);
  }

  // -----------------------------------------------------------------------------------------------
  // Overridden methods
  // -----------------------------------------------------------------------------------------------

  @Override
  public void visit(
      final int version,
      final int access,
      final String name,
      final String signature,
      final String superName,
      final String[] interfaces) {
    // Get the class name, access flags, and interfaces information (step 1, 2 and 3) for SVUID
    // computation.
    computeSvuid = (access & Opcodes.ACC_ENUM) == 0;

    if (computeSvuid) {
      this.name = name;
      this.access = access;
      this.interfaces = interfaces.clone();
      this.svuidFields = new ArrayList<>();
      this.svuidConstructors = new ArrayList<>();
      this.svuidMethods = new ArrayList<>();
    }

    super.visit(version, access, name, signature, superName, interfaces);
  }

  @Override
  public MethodVisitor visitMethod(
      final int access,
      final String name,
      final String descriptor,
      final String signature,
      final String[] exceptions) {
    // Get constructor and method information (step 5 and 7). Also determine if there is a class
    // initializer (step 6).
    if (computeSvuid) {
      if (CLINIT.equals(name)) {
        hasStaticInitializer = true;
      }
      // Collect the non private constructors and methods. Only the ACC_PUBLIC, ACC_PRIVATE,
      // ACC_PROTECTED, ACC_STATIC, ACC_FINAL, ACC_SYNCHRONIZED, ACC_NATIVE, ACC_ABSTRACT and
      // ACC_STRICT flags are used.
      int mods =
          access
              & (Opcodes.ACC_PUBLIC
                  | Opcodes.ACC_PRIVATE
                  | Opcodes.ACC_PROTECTED
                  | Opcodes.ACC_STATIC
                  | Opcodes.ACC_FINAL
                  | Opcodes.ACC_SYNCHRONIZED
                  | Opcodes.ACC_NATIVE
                  | Opcodes.ACC_ABSTRACT
                  | Opcodes.ACC_STRICT);

      if ((access & Opcodes.ACC_PRIVATE) == 0) {
        if ("".equals(name)) {
          svuidConstructors.add(new Item(name, mods, descriptor));
        } else if (!CLINIT.equals(name)) {
          svuidMethods.add(new Item(name, mods, descriptor));
        }
      }
    }

    return super.visitMethod(access, name, descriptor, signature, exceptions);
  }

  @Override
  public FieldVisitor visitField(
      final int access,
      final String name,
      final String desc,
      final String signature,
      final Object value) {
    // Get the class field information for step 4 of the algorithm. Also determine if the class
    // already has a SVUID.
    if (computeSvuid) {
      if ("serialVersionUID".equals(name)) {
        // Since the class already has SVUID, we won't be computing it.
        computeSvuid = false;
        hasSvuid = true;
      }
      // Collect the non private fields. Only the ACC_PUBLIC, ACC_PRIVATE, ACC_PROTECTED,
      // ACC_STATIC, ACC_FINAL, ACC_VOLATILE, and ACC_TRANSIENT flags are used when computing
      // serialVersionUID values.
      if ((access & Opcodes.ACC_PRIVATE) == 0
          || (access & (Opcodes.ACC_STATIC | Opcodes.ACC_TRANSIENT)) == 0) {
        int mods =
            access
                & (Opcodes.ACC_PUBLIC
                    | Opcodes.ACC_PRIVATE
                    | Opcodes.ACC_PROTECTED
                    | Opcodes.ACC_STATIC
                    | Opcodes.ACC_FINAL
                    | Opcodes.ACC_VOLATILE
                    | Opcodes.ACC_TRANSIENT);
        svuidFields.add(new Item(name, mods, desc));
      }
    }

    return super.visitField(access, name, desc, signature, value);
  }

  @Override
  public void visitInnerClass(
      final String innerClassName,
      final String outerName,
      final String innerName,
      final int innerClassAccess) {
    // Handles a bizarre special case. Nested classes (static classes declared inside another class)
    // that are protected have their access bit set to public in their class files to deal with some
    // odd reflection situation. Our SVUID computation must do as the JVM does and ignore access
    // bits in the class file in favor of the access bits of the InnerClass attribute.
    if ((name != null) && name.equals(innerClassName)) {
      this.access = innerClassAccess;
    }
    super.visitInnerClass(innerClassName, outerName, innerName, innerClassAccess);
  }

  @Override
  public void visitEnd() {
    // Add the SVUID field to the class if it doesn't have one.
    if (computeSvuid && !hasSvuid) {
      try {
        addSVUID(computeSVUID());
      } catch (IOException e) {
        throw new IllegalStateException("Error while computing SVUID for " + name, e);
      }
    }

    super.visitEnd();
  }

  // -----------------------------------------------------------------------------------------------
  // Utility methods
  // -----------------------------------------------------------------------------------------------

  /**
   * Returns true if the class already has a SVUID field. The result of this method is only valid
   * when visitEnd has been called.
   *
   * @return true if the class already has a SVUID field.
   */
  // DontCheck(AbbreviationAsWordInName): can't be renamed (for backward binary compatibility).
  public boolean hasSVUID() {
    return hasSvuid;
  }

  /**
   * Adds a final static serialVersionUID field to the class, with the given value.
   *
   * @param svuid the serialVersionUID field value.
   */
  // DontCheck(AbbreviationAsWordInName): can't be renamed (for backward binary compatibility).
  protected void addSVUID(final long svuid) {
    FieldVisitor fieldVisitor =
        super.visitField(
            Opcodes.ACC_FINAL + Opcodes.ACC_STATIC, "serialVersionUID", "J", null, svuid);
    if (fieldVisitor != null) {
      fieldVisitor.visitEnd();
    }
  }

  /**
   * Computes and returns the value of SVUID.
   *
   * @return the serial version UID.
   * @throws IOException if an I/O error occurs.
   */
  // DontCheck(AbbreviationAsWordInName): can't be renamed (for backward binary compatibility).
  protected long computeSVUID() throws IOException {
    long svuid = 0;

    try (ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        DataOutputStream dataOutputStream = new DataOutputStream(byteArrayOutputStream)) {

      // 1. The class name written using UTF encoding.
      dataOutputStream.writeUTF(name.replace('/', '.'));

      // 2. The class modifiers written as a 32-bit integer.
      int mods = access;
      if ((mods & Opcodes.ACC_INTERFACE) != 0) {
        mods =
            svuidMethods.isEmpty() ? (mods & ~Opcodes.ACC_ABSTRACT) : (mods | Opcodes.ACC_ABSTRACT);
      }
      dataOutputStream.writeInt(
          mods
              & (Opcodes.ACC_PUBLIC
                  | Opcodes.ACC_FINAL
                  | Opcodes.ACC_INTERFACE
                  | Opcodes.ACC_ABSTRACT));

      // 3. The name of each interface sorted by name written using UTF encoding.
      Arrays.sort(interfaces);
      for (String interfaceName : interfaces) {
        dataOutputStream.writeUTF(interfaceName.replace('/', '.'));
      }

      // 4. For each field of the class sorted by field name (except private static and private
      // transient fields):
      //   1. The name of the field in UTF encoding.
      //   2. The modifiers of the field written as a 32-bit integer.
      //   3. The descriptor of the field in UTF encoding.
      // Note that field signatures are not dot separated. Method and constructor signatures are dot
      // separated. Go figure...
      writeItems(svuidFields, dataOutputStream, false);

      // 5. If a class initializer exists, write out the following:
      //   1. The name of the method, , in UTF encoding.
      //   2. The modifier of the method, ACC_STATIC, written as a 32-bit integer.
      //   3. The descriptor of the method, ()V, in UTF encoding.
      if (hasStaticInitializer) {
        dataOutputStream.writeUTF(CLINIT);
        dataOutputStream.writeInt(Opcodes.ACC_STATIC);
        dataOutputStream.writeUTF("()V");
      }

      // 6. For each non-private constructor sorted by method name and signature:
      //   1. The name of the method, , in UTF encoding.
      //   2. The modifiers of the method written as a 32-bit integer.
      //   3. The descriptor of the method in UTF encoding.
      writeItems(svuidConstructors, dataOutputStream, true);

      // 7. For each non-private method sorted by method name and signature:
      //   1. The name of the method in UTF encoding.
      //   2. The modifiers of the method written as a 32-bit integer.
      //   3. The descriptor of the method in UTF encoding.
      writeItems(svuidMethods, dataOutputStream, true);

      dataOutputStream.flush();

      // 8. The SHA-1 algorithm is executed on the stream of bytes produced by DataOutputStream and
      // produces five 32-bit values sha[0..4].
      byte[] hashBytes = computeSHAdigest(byteArrayOutputStream.toByteArray());

      // 9. The hash value is assembled from the first and second 32-bit values of the SHA-1 message
      // digest. If the result of the message digest, the five 32-bit words H0 H1 H2 H3 H4, is in an
      // array of five int values named sha, the hash value would be computed as follows:
      for (int i = Math.min(hashBytes.length, 8) - 1; i >= 0; i--) {
        svuid = (svuid << 8) | (hashBytes[i] & 0xFF);
      }
    }

    return svuid;
  }

  /**
   * Returns the SHA-1 message digest of the given value.
   *
   * @param value the value whose SHA message digest must be computed.
   * @return the SHA-1 message digest of the given value.
   */
  // DontCheck(AbbreviationAsWordInName): can't be renamed (for backward binary compatibility).
  protected byte[] computeSHAdigest(final byte[] value) {
    try {
      return MessageDigest.getInstance("SHA").digest(value);
    } catch (NoSuchAlgorithmException e) {
      throw new UnsupportedOperationException(e);
    }
  }

  /**
   * Sorts the items in the collection and writes it to the given output stream.
   *
   * @param itemCollection a collection of items.
   * @param dataOutputStream where the items must be written.
   * @param dotted whether package names must use dots, instead of slashes.
   * @exception IOException if an error occurs.
   */
  private static void writeItems(
      final Collection itemCollection,
      final DataOutput dataOutputStream,
      final boolean dotted)
      throws IOException {
    Item[] items = itemCollection.toArray(new Item[0]);
    Arrays.sort(items);
    for (Item item : items) {
      dataOutputStream.writeUTF(item.name);
      dataOutputStream.writeInt(item.access);
      dataOutputStream.writeUTF(dotted ? item.descriptor.replace('/', '.') : item.descriptor);
    }
  }

  // -----------------------------------------------------------------------------------------------
  // Inner classes
  // -----------------------------------------------------------------------------------------------

  private static final class Item implements Comparable {

    final String name;
    final int access;
    final String descriptor;

    Item(final String name, final int access, final String descriptor) {
      this.name = name;
      this.access = access;
      this.descriptor = descriptor;
    }

    @Override
    public int compareTo(final Item item) {
      int result = name.compareTo(item.name);
      if (result == 0) {
        result = descriptor.compareTo(item.descriptor);
      }
      return result;
    }

    @Override
    public boolean equals(final Object other) {
      if (other instanceof Item) {
        return compareTo((Item) other) == 0;
      }
      return false;
    }

    @Override
    public int hashCode() {
      return name.hashCode() ^ descriptor.hashCode();
    }
  }
}