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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 aj.org.objectweb.asm.ClassVisitor;
import aj.org.objectweb.asm.MethodVisitor;
import aj.org.objectweb.asm.Opcodes;

/**
 * A {@link ClassVisitor} that merges <clinit> methods into a single one. All the existing
 * <clinit> methods are renamed, and a new one is created, which calls all the renamed
 * methods.
 *
 * @author Eric Bruneton
 */
public class StaticInitMerger extends ClassVisitor {

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

  /** The prefix to use to rename the existing <clinit> methods. */
  private final String renamedClinitMethodPrefix;

  /** The number of <clinit> methods visited so far. */
  private int numClinitMethods;

  /** The MethodVisitor for the merged <clinit> method. */
  private MethodVisitor mergedClinitVisitor;

  /**
   * Constructs a new {@link StaticInitMerger}. Subclasses must not use this constructor.
   * Instead, they must use the {@link #StaticInitMerger(int, String, ClassVisitor)} version.
   *
   * @param prefix the prefix to use to rename the existing <clinit> methods.
   * @param classVisitor the class visitor to which this visitor must delegate method calls. May be
   *     null.
   */
  public StaticInitMerger(final String prefix, final ClassVisitor classVisitor) {
    this(/* latest api = */ Opcodes.ASM9, prefix, classVisitor);
  }

  /**
   * Constructs a new {@link StaticInitMerger}.
   *
   * @param api the ASM API version implemented by this visitor. Must be one of the {@code
   *     ASM}x values in {@link Opcodes}.
   * @param prefix the prefix to use to rename the existing <clinit> methods.
   * @param classVisitor the class visitor to which this visitor must delegate method calls. May be
   *     null.
   */
  protected StaticInitMerger(final int api, final String prefix, final ClassVisitor classVisitor) {
    super(api, classVisitor);
    this.renamedClinitMethodPrefix = prefix;
  }

  @Override
  public void visit(
      final int version,
      final int access,
      final String name,
      final String signature,
      final String superName,
      final String[] interfaces) {
    super.visit(version, access, name, signature, superName, interfaces);
    this.owner = name;
  }

  @Override
  public MethodVisitor visitMethod(
      final int access,
      final String name,
      final String descriptor,
      final String signature,
      final String[] exceptions) {
    MethodVisitor methodVisitor;
    if ("".equals(name)) {
      int newAccess = Opcodes.ACC_PRIVATE + Opcodes.ACC_STATIC;
      String newName = renamedClinitMethodPrefix + numClinitMethods++;
      methodVisitor = super.visitMethod(newAccess, newName, descriptor, signature, exceptions);

      if (mergedClinitVisitor == null) {
        mergedClinitVisitor = super.visitMethod(newAccess, name, descriptor, null, null);
      }
      mergedClinitVisitor.visitMethodInsn(Opcodes.INVOKESTATIC, owner, newName, descriptor, false);
    } else {
      methodVisitor = super.visitMethod(access, name, descriptor, signature, exceptions);
    }
    return methodVisitor;
  }

  @Override
  public void visitEnd() {
    if (mergedClinitVisitor != null) {
      mergedClinitVisitor.visitInsn(Opcodes.RETURN);
      mergedClinitVisitor.visitMaxs(0, 0);
    }
    super.visitEnd();
  }
}