001/////////////////////////////////////////////////////////////////////////////////////////////// 002// checkstyle: Checks Java source code and other text files for adherence to a set of rules. 003// Copyright (C) 2001-2023 the original author or authors. 004// 005// This library is free software; you can redistribute it and/or 006// modify it under the terms of the GNU Lesser General Public 007// License as published by the Free Software Foundation; either 008// version 2.1 of the License, or (at your option) any later version. 009// 010// This library is distributed in the hope that it will be useful, 011// but WITHOUT ANY WARRANTY; without even the implied warranty of 012// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 013// Lesser General Public License for more details. 014// 015// You should have received a copy of the GNU Lesser General Public 016// License along with this library; if not, write to the Free Software 017// Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA 018/////////////////////////////////////////////////////////////////////////////////////////////// 019 020package com.puppycrawl.tools.checkstyle.checks.coding; 021 022import com.puppycrawl.tools.checkstyle.StatelessCheck; 023import com.puppycrawl.tools.checkstyle.api.AbstractCheck; 024import com.puppycrawl.tools.checkstyle.api.DetailAST; 025import com.puppycrawl.tools.checkstyle.api.TokenTypes; 026 027/** 028 * <p> 029 * Checks that the clone method is not overridden from the 030 * Object class. 031 * </p> 032 * <p> 033 * This check is almost exactly the same as the {@code NoFinalizerCheck}. 034 * </p> 035 * <p> 036 * See 037 * <a href="https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/Object.html#clone()"> 038 * Object.clone()</a> 039 * </p> 040 * <p> 041 * Rationale: The clone method relies on strange, hard to follow rules that 042 * are difficult to get right and do not work in all situations. In some cases, 043 * either a copy constructor or a static factory method can be used instead of 044 * the clone method to return copies of an object. For more information on rules 045 * for the clone method and its issues, see Effective Java: 046 * Programming Language Guide First Edition by Joshua Bloch pages 45-52. 047 * </p> 048 * <p> 049 * Below are some rules/reasons why the clone method should be avoided. 050 * </p> 051 * <ul> 052 * <li> 053 * Classes supporting the clone method should implement the Cloneable 054 * interface but the Cloneable interface does not include the clone method. 055 * As a result, it doesn't enforce the method override. 056 * </li> 057 * <li> 058 * The Cloneable interface forces the Object's clone method to work 059 * correctly. Without implementing it, the Object's clone method will 060 * throw a CloneNotSupportedException. 061 * </li> 062 * <li> 063 * Non-final classes must return the object returned from a call to 064 * super.clone(). 065 * </li> 066 * <li> 067 * Final classes can use a constructor to create a clone which is different 068 * from non-final classes. 069 * </li> 070 * <li> 071 * If a super class implements the clone method incorrectly all subclasses 072 * calling super.clone() are doomed to failure. 073 * </li> 074 * <li> 075 * If a class has references to mutable objects then those object 076 * references must be replaced with copies in the clone method 077 * after calling super.clone(). 078 * </li> 079 * <li> 080 * The clone method does not work correctly with final mutable object 081 * references because final references cannot be reassigned. 082 * </li> 083 * <li> 084 * If a super class overrides the clone method then all subclasses must 085 * provide a correct clone implementation. 086 * </li> 087 * </ul> 088 * 089 * <p>Two alternatives to the clone method, in some cases, is a copy constructor 090 * or a static factory method to return copies of an object. Both of these 091 * approaches are simpler and do not conflict with final fields. They do not 092 * force the calling client to handle a CloneNotSupportedException. They also 093 * are typed therefore no casting is necessary. Finally, they are more 094 * flexible since they can take interface types rather than concrete classes. 095 * </p> 096 * <p>Sometimes a copy constructor or static factory is not an acceptable 097 * alternative to the clone method. The example below highlights the 098 * limitation of a copy constructor (or static factory). Assume 099 * Square is a subclass for Shape. 100 * </p> 101 * <pre> 102 * Shape s1 = new Square(); 103 * System.out.println(s1 instanceof Square); //true 104 * </pre> 105 * <p> 106 * ...assume at this point the code knows nothing of s1 being a Square 107 * that's the beauty of polymorphism but the code wants to copy 108 * the Square which is declared as a Shape, its super type... 109 * </p> 110 * <pre> 111 * Shape s2 = new Shape(s1); //using the copy constructor 112 * System.out.println(s2 instanceof Square); //false 113 * </pre> 114 * <p> 115 * The working solution (without knowing about all subclasses and doing many 116 * casts) is to do the following (assuming correct clone implementation). 117 * </p> 118 * <pre> 119 * Shape s2 = s1.clone(); 120 * System.out.println(s2 instanceof Square); //true 121 * </pre> 122 * <p> 123 * Just keep in mind if this type of polymorphic cloning is required 124 * then a properly implemented clone method may be the best choice. 125 * </p> 126 * <p>Much of this information was taken from Effective Java: 127 * Programming Language Guide First Edition by Joshua Bloch 128 * pages 45-52. Give Bloch credit for writing an excellent book. 129 * </p> 130 * <p> 131 * To configure the check: 132 * </p> 133 * <pre> 134 * <module name="NoClone"/> 135 * </pre> 136 * <p>Example: </p> 137 * <pre> 138 * public class Foo { 139 * 140 * public Object clone() {return null;} // violation, overrides the clone method 141 * 142 * public Foo clone() {return null;} // violation, overrides the clone method 143 * 144 * public static Object clone(Object o) {return null;} // OK 145 * 146 * public static Foo clone(Foo o) {return null;} // OK 147 * 148 * } 149 * </pre> 150 * <p> 151 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker} 152 * </p> 153 * <p> 154 * Violation Message Keys: 155 * </p> 156 * <ul> 157 * <li> 158 * {@code avoid.clone.method} 159 * </li> 160 * </ul> 161 * 162 * @since 5.0 163 */ 164@StatelessCheck 165public class NoCloneCheck extends AbstractCheck { 166 167 /** 168 * A key is pointing to the warning message text in "messages.properties" 169 * file. 170 */ 171 public static final String MSG_KEY = "avoid.clone.method"; 172 173 @Override 174 public int[] getDefaultTokens() { 175 return getRequiredTokens(); 176 } 177 178 @Override 179 public int[] getAcceptableTokens() { 180 return getRequiredTokens(); 181 } 182 183 @Override 184 public int[] getRequiredTokens() { 185 return new int[] {TokenTypes.METHOD_DEF}; 186 } 187 188 @Override 189 public void visitToken(DetailAST ast) { 190 final DetailAST mid = ast.findFirstToken(TokenTypes.IDENT); 191 final String name = mid.getText(); 192 193 if ("clone".equals(name)) { 194 final DetailAST params = ast.findFirstToken(TokenTypes.PARAMETERS); 195 final boolean hasEmptyParamList = 196 params.findFirstToken(TokenTypes.PARAMETER_DEF) == null; 197 198 if (hasEmptyParamList) { 199 log(ast, MSG_KEY); 200 } 201 } 202 } 203 204}