code: ///////////////////////////////////////////////////////////////////////////////////////////////
  // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
  // Copyright (C) 2001-2023 the original author or authors.
  //
  // This library is free software; you can redistribute it and/or
  // modify it under the terms of the GNU Lesser General Public
  // License as published by the Free Software Foundation; either
  // version 2.1 of the License, or (at your option) any later version.
  //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.checks.javadoc;
  
  import java.util.Arrays;
  import java.util.Set;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  import java.util.stream.Collectors;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailNode;
  import com.puppycrawl.tools.checkstyle.api.JavadocTokenTypes;
  
  /**
   * <p>
   * Checks that a
   * <a href="https://docs.oracle.com/en/java/javase/11/docs/specs/doc-comment-spec.html#block-tags">
   * javadoc block tag</a> appears only at the beginning of a line, ignoring
   * leading asterisks and white space. A block tag is a token that starts with
   * {@code @} symbol and is preceded by a whitespace. This check ignores block
   * tags in comments and inside inline tags {&#64;code } and {&#64;literal }.
   * </p>
   * <p>
   * Rationale: according to
   * <a href="https://docs.oracle.com/en/java/javase/11/docs/specs/doc-comment-spec.html#block-tags">
   * the specification</a> all javadoc block tags should be placed at the beginning
   * of a line. Tags that are not placed at the beginning are treated as plain text.
   * To recognize intentional tag placement to text area it is better to escape the
   * {@code @} symbol, and all non-escaped tags should be located at the beginning
   * of the line. See NOTE section for details on how to escape.
   * </p>
   * <p>
   * To place a tag explicitly as text, escape the {@code @} symbol with HTML entity
   * &amp;#64; or place it inside {@code {@code }}, for example:
   * </p>
   * <pre>
   * &#47;**
   *  * &amp;#64;serial literal in {&#64;code &#64;serial} Javadoc tag.
   *  *&#47;
   * </pre>
   * <ul>
   * <li>
   * Property {@code tags} - Specify the javadoc tags to process.
   * Type is {@code java.lang.String[]}.
   * Default value is {@code author, deprecated, exception, hidden, param, provides,
   * return, see, serial, serialData, serialField, since, throws, uses, version}.
   * </li>
   * <li>
   * Property {@code violateExecutionOnNonTightHtml} - Control when to print violations
   * if the Javadoc being examined by this check violates the tight html rules defined at
   * <a href="https://checkstyle.org/writingjavadocchecks.html#Tight-HTML_rules">Tight-HTML Rules</a>.
   * Type is {@code boolean}.
   * Default value is {@code false}.
   * </li>
   * </ul>
   * <p>
   * To configure the default check:
   * </p>
   * <pre>
   * &lt;module name="JavadocBlockTagLocation"/&gt;
   * </pre>
   * <p>
   * Example:
   * </p>
   * <pre>
   * &#47;**
   *  * Escaped tag &amp;#64;version (OK)
   *  * Plain text with {&#64;code &#64;see} (OK)
   *  * A @custom tag (OK)
   *  * <!-- @see commented out (OK) -->
   *  * email@author (OK)
   *  * (@param in parentheses) (OK)
   *  * '@param in single quotes' (OK)
   *  * &#64;since 1.0 (OK)
   *  * text &#64;return (violation)
   *  * * &#64;param (violation)
   * +* &#64;serial (violation)
   *  * &#64;see first (OK) &#64;see second (violation)
   *  *&#47;
   * public int field;
   * </pre>
   * <p>
   * To configure the check to verify tags from
   * <a href="https://openjdk.org/jeps/8068562">JEP 8068562</a> only:
   * </p>
   * <pre>
   * &lt;module name="JavadocBlockTagLocation"&gt;
   *   &lt;property name="tags" value="apiNote, implSpec, implNote"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * To configure the check to verify all default tags and some custom tags in addition:
   * </p>
   * <pre>
   * &lt;module name="JavadocBlockTagLocation"&gt;
   *   &lt;!-- default tags --&gt;
   *   &lt;property name="tags" value="author, deprecated, exception, hidden"/&gt;
   *   &lt;property name="tags" value="param, provides, return, see, serial"/&gt;
   *   &lt;property name="tags" value="serialData, serialField, since, throws"/&gt;
   *   &lt;property name="tags" value="uses, version"/&gt;
   *   &lt;!-- additional tags used in the project --&gt;
   *   &lt;property name="tags" value="noinspection"/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code javadoc.blockTagLocation}
   * </li>
   * <li>
   * {@code javadoc.missed.html.close}
   * </li>
   * <li>
   * {@code javadoc.parse.rule.error}
   * </li>
   * <li>
   * {@code javadoc.wrong.singleton.html.tag}
   * </li>
   * </ul>
   *
   * @since 8.24
   */
  @StatelessCheck
  public class JavadocBlockTagLocationCheck extends AbstractJavadocCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties" file.
       */
      public static final String MSG_BLOCK_TAG_LOCATION = "javadoc.blockTagLocation";
  
      /**
       * This regexp is used to extract the javadoc tags.
       */
      private static final Pattern JAVADOC_BLOCK_TAG_PATTERN = Pattern.compile("\\s@(\\w+)");
  
      /**
       * Block tags from Java 11
       * <a href="https://docs.oracle.com/en/java/javase/11/docs/specs/doc-comment-spec.html">
       * Documentation Comment Specification</a>.
       */
      private static final String[] DEFAULT_TAGS = {
          "author",
          "deprecated",
          "exception",
          "hidden",
          "param",
          "provides",
          "return",
          "see",
          "serial",
          "serialData",
          "serialField",
          "since",
          "throws",
          "uses",
          "version",
      };
  
      /**
       * Specify the javadoc tags to process.
       */
      private Set<String> tags;
  
      /**
       * Creates a new {@code JavadocBlockTagLocationCheck} instance with default settings.
       */
      public JavadocBlockTagLocationCheck() {
          setTags(DEFAULT_TAGS);
      }
  
      /**
       * Setter to specify the javadoc tags to process.
       *
       * @param values user's values.
       */
      public final void setTags(String... values) {
          tags = Arrays.stream(values).collect(Collectors.toSet());
      }
  
      /**
       * The javadoc tokens that this check must be registered for. According to
       * <a href="https://docs.oracle.com/en/java/javase/11/docs/specs/doc-comment-spec.html#block-tags">
       * the specs</a> each block tag must appear at the beginning of a line, otherwise
       * it will be interpreted as a plain text. This check looks for a block tag
       * in the javadoc text, thus it needs the {@code TEXT} tokens.
       *
       * @return the javadoc token set this must be registered for.
       * @see JavadocTokenTypes
       */
      @Override
      public int[] getRequiredJavadocTokens() {
          return new int[] {
              JavadocTokenTypes.TEXT,
          };
      }
  
      @Override
      public int[] getAcceptableJavadocTokens() {
          return getRequiredJavadocTokens();
      }
  
      @Override
      public int[] getDefaultJavadocTokens() {
          return getRequiredJavadocTokens();
      }
  
      @Override
      public void visitJavadocToken(DetailNode ast) {
          if (!isCommentOrInlineTag(ast.getParent())) {
              final Matcher tagMatcher = JAVADOC_BLOCK_TAG_PATTERN.matcher(ast.getText());
              while (tagMatcher.find()) {
                  final String tagName = tagMatcher.group(1);
                  if (tags.contains(tagName)) {
                      log(ast.getLineNumber(), MSG_BLOCK_TAG_LOCATION, tagName);
                  }
              }
          }
      }
  
      /**
       * Checks if the node can contain an unescaped block tag without violation.
       *
       * @param node to check
       * @return {@code true} if node is {@code @code}, {@code @literal} or HTML comment.
       */
      private static boolean isCommentOrInlineTag(DetailNode node) {
          return node.getType() == JavadocTokenTypes.JAVADOC_INLINE_TAG
                  || node.getType() == JavadocTokenTypes.HTML_COMMENT;
      }
  
  }
  