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.design;
  
  import java.util.ArrayDeque;
  import java.util.Deque;
  import java.util.regex.Pattern;
  
  import com.puppycrawl.tools.checkstyle.FileStatefulCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
  import com.puppycrawl.tools.checkstyle.api.DetailAST;
  import com.puppycrawl.tools.checkstyle.api.TokenTypes;
  
  /**
   * <p>
   * Ensures that exception classes (classes with names conforming to some pattern
   * and explicitly extending classes with names conforming to other
   * pattern) are immutable, that is, that they have only final fields.
   * </p>
   * <p>
   * The current algorithm is very simple: it checks that all members of exception are final.
   * The user can still mutate an exception's instance (e.g. Throwable has a method called
   * {@code setStackTrace} which changes the exception's stack trace). But, at least, all
   * information provided by this exception type is unchangeable.
   * </p>
   * <p>
   * Rationale: Exception instances should represent an error
   * condition. Having non-final fields not only allows the state to be
   * modified by accident and therefore mask the original condition but
   * also allows developers to accidentally forget to set the initial state.
   * In both cases, code catching the exception could draw incorrect
   * conclusions based on the state.
   * </p>
   * <ul>
   * <li>
   * Property {@code format} - Specify pattern for exception class names.
   * Type is {@code java.util.regex.Pattern}.
   * Default value is {@code "^.*Exception$|^.*Error$|^.*Throwable$"}.
   * </li>
   * <li>
   * Property {@code extendedClassNameFormat} - Specify pattern for extended class names.
   * Type is {@code java.util.regex.Pattern}.
   * Default value is {@code "^.*Exception$|^.*Error$|^.*Throwable$"}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;MutableException&quot;/&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class FirstClass extends Exception {
   *   private int code; // OK, class name doesn't match with default pattern
   *
   *   public FirstClass() {
   *     code = 1;
   *   }
   * }
   *
   * class MyException extends Exception {
   *   private int code; // violation, The field 'code' must be declared final
   *
   *   public MyException() {
   *     code = 2;
   *   }
   * }
   *
   * class MyThrowable extends Throwable {
   *    final int code; // OK
   *    String message; // violation, The field 'message' must be declared final
   *
   *    public MyThrowable(int code, String message) {
   *      this.code = code;
   *      this.message = message;
   *    }
   * }
   *
   * class BadException extends java.lang.Exception {
   *   int code; // violation, The field 'code' must be declared final
   *
   *   public BadException(int code) {
   *     this.code = code;
   *   }
   * }
   * </pre>
   * <p>
   * To configure the check so that it checks for class name that ends
   * with 'Exception':
   * </p>
   * <pre>
   * &lt;module name=&quot;MutableException&quot;&gt;
   *   &lt;property name=&quot;format&quot; value=&quot;^.*Exception$&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class FirstClass extends Exception {
   *   private int code; // OK, class name doesn't match with given pattern
   *
   *   public FirstClass() {
   *     code = 1;
   *   }
   * }
   *
   * class MyException extends Exception {
   *   private int code; // violation, The field 'code' must be declared final
   *
   *   public MyException() {
   *     code = 2;
   *   }
   * }
   *
   * class MyThrowable extends Throwable {
   *   final int code; // OK, class name doesn't match with given pattern
   *   String message; // OK, class name doesn't match with given pattern
   *
   *   public MyThrowable(int code, String message) {
   *     this.code = code;
   *     this.message = message;
   *   }
   * }
   *
   * class BadException extends java.lang.Exception {
   *   int code; // violation, The field 'code' must be declared final
   *
   *   public BadException(int code) {
   *     this.code = code;
   *   }
   * }
   * </pre>
   * <p>
   * To configure the check so that it checks for type name that is used in
   * 'extends' and ends with 'Throwable':
   * </p>
   * <pre>
   * &lt;module name=&quot;MutableException&quot;&gt;
   *   &lt;property name=&quot;extendedClassNameFormat&quot; value=&quot;^.*Throwable$&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>Example:</p>
   * <pre>
   * class FirstClass extends Exception {
   *   private int code; // OK, extended class name doesn't match with given pattern
   *
   *   public FirstClass() {
   *     code = 1;
   *   }
   * }
   *
   * class MyException extends Exception {
   *   private int code; // OK, extended class name doesn't match with given pattern
   *
   *   public MyException() {
   *     code = 2;
   *   }
   * }
   *
   * class MyThrowable extends Throwable {
   *   final int code; // OK
   *   String message; // violation, The field 'message' must be declared final
   *
   *   public MyThrowable(int code, String message) {
   *     this.code = code;
   *     this.message = message;
   *   }
   * }
   *
   * class BadException extends java.lang.Exception {
   *   int code; // OK, extended class name doesn't match with given pattern
   *
   *   public BadException(int code) {
   *     this.code = code;
   *   }
   * }
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code mutable.exception}
   * </li>
   * </ul>
   *
   * @since 3.2
   */
  @FileStatefulCheck
  public final class MutableExceptionCheck extends AbstractCheck {
  
      /**
       * A key is pointing to the warning message text in "messages.properties"
       * file.
       */
      public static final String MSG_KEY = "mutable.exception";
  
      /** Default value for format and extendedClassNameFormat properties. */
      private static final String DEFAULT_FORMAT = "^.*Exception$|^.*Error$|^.*Throwable$";
      /** Stack of checking information for classes. */
      private final Deque<Boolean> checkingStack = new ArrayDeque<>();
      /** Specify pattern for extended class names. */
      private Pattern extendedClassNameFormat = Pattern.compile(DEFAULT_FORMAT);
      /** Should we check current class or not. */
      private boolean checking;
      /** Specify pattern for exception class names. */
      private Pattern format = extendedClassNameFormat;
  
      /**
       * Setter to specify pattern for extended class names.
       *
       * @param extendedClassNameFormat a {@code String} value
       */
      public void setExtendedClassNameFormat(Pattern extendedClassNameFormat) {
          this.extendedClassNameFormat = extendedClassNameFormat;
      }
  
      /**
       * Setter to specify pattern for exception class names.
       *
       * @param pattern the new pattern
       */
      public void setFormat(Pattern pattern) {
          format = pattern;
      }
  
      @Override
      public int[] getDefaultTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public int[] getRequiredTokens() {
          return new int[] {TokenTypes.CLASS_DEF, TokenTypes.VARIABLE_DEF};
      }
  
      @Override
      public int[] getAcceptableTokens() {
          return getRequiredTokens();
      }
  
      @Override
      public void visitToken(DetailAST ast) {
          switch (ast.getType()) {
              case TokenTypes.CLASS_DEF:
                  visitClassDef(ast);
                  break;
              case TokenTypes.VARIABLE_DEF:
                  visitVariableDef(ast);
                  break;
              default:
                  throw new IllegalStateException(ast.toString());
          }
      }
  
      @Override
      public void leaveToken(DetailAST ast) {
          if (ast.getType() == TokenTypes.CLASS_DEF) {
              leaveClassDef();
          }
      }
  
      /**
       * Called when we start processing class definition.
       *
       * @param ast class definition node
       */
      private void visitClassDef(DetailAST ast) {
          checkingStack.push(checking);
          checking = isNamedAsException(ast) && isExtendedClassNamedAsException(ast);
      }
  
      /** Called when we leave class definition. */
      private void leaveClassDef() {
          checking = checkingStack.pop();
      }
  
      /**
       * Checks variable definition.
       *
       * @param ast variable def node for check
       */
      private void visitVariableDef(DetailAST ast) {
          if (checking && ast.getParent().getType() == TokenTypes.OBJBLOCK) {
              final DetailAST modifiersAST =
                  ast.findFirstToken(TokenTypes.MODIFIERS);
  
              if (modifiersAST.findFirstToken(TokenTypes.FINAL) == null) {
                  log(ast, MSG_KEY, ast.findFirstToken(TokenTypes.IDENT).getText());
              }
          }
      }
  
      /**
       * Checks that a class name conforms to specified format.
       *
       * @param ast class definition node
       * @return true if a class name conforms to specified format
       */
      private boolean isNamedAsException(DetailAST ast) {
          final String className = ast.findFirstToken(TokenTypes.IDENT).getText();
          return format.matcher(className).find();
      }
  
      /**
       * Checks that if extended class name conforms to specified format.
       *
       * @param ast class definition node
       * @return true if extended class name conforms to specified format
       */
      private boolean isExtendedClassNamedAsException(DetailAST ast) {
          boolean result = false;
          final DetailAST extendsClause = ast.findFirstToken(TokenTypes.EXTENDS_CLAUSE);
          if (extendsClause != null) {
              DetailAST currentNode = extendsClause;
              while (currentNode.getLastChild() != null) {
                  currentNode = currentNode.getLastChild();
              }
              final String extendedClassName = currentNode.getText();
              result = extendedClassNameFormat.matcher(extendedClassName).matches();
          }
          return result;
      }
  
  }
  