1 ///////////////////////////////////////////////////////////////////////////////////////////////
2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
3 // Copyright (C) 2001-2026 the original author or authors.
4 //
5 // This library is free software; you can redistribute it and/or
6 // modify it under the terms of the GNU Lesser General Public
7 // License as published by the Free Software Foundation; either
8 // version 2.1 of the License, or (at your option) any later version.
9 //
10 // This library is distributed in the hope that it will be useful,
11 // but WITHOUT ANY WARRANTY; without even the implied warranty of
12 // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 // Lesser General Public License for more details.
14 //
15 // You should have received a copy of the GNU Lesser General Public
16 // License along with this library; if not, write to the Free Software
17 // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
18 ///////////////////////////////////////////////////////////////////////////////////////////////
19
20 package com.puppycrawl.tools.checkstyle.checks.coding;
21
22 import java.util.BitSet;
23
24 import com.puppycrawl.tools.checkstyle.StatelessCheck;
25 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
26 import com.puppycrawl.tools.checkstyle.api.DetailAST;
27 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
28 import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
29
30 /**
31 * <div>
32 * Detects double brace initialization.
33 * </div>
34 *
35 * <p>
36 * Rationale: Double brace initialization (set of
37 * <a href="https://docs.oracle.com/javase/specs/jls/se12/html/jls-8.html#jls-8.6">
38 * Instance Initializers</a> in class body) may look cool, but it is considered as anti-pattern
39 * and should be avoided.
40 * This is also can lead to a hard-to-detect memory leak, if the anonymous class instance is
41 * returned outside and other object(s) hold reference to it.
42 * Created anonymous class is not static, it holds an implicit reference to the outer class
43 * instance.
44 * See this
45 * <a href="https://blog.jooq.org/dont-be-clever-the-double-curly-braces-anti-pattern/">
46 * blog post</a> and
47 * <a href="https://www.baeldung.com/java-double-brace-initialization">
48 * article</a> for more details.
49 * Check ignores any comments and semicolons in class body.
50 * </p>
51 *
52 * @since 8.30
53 */
54 @StatelessCheck
55 public class AvoidDoubleBraceInitializationCheck extends AbstractCheck {
56
57 /**
58 * A key is pointing to the warning message text in "messages.properties"
59 * file.
60 */
61 public static final String MSG_KEY = "avoid.double.brace.init";
62
63 /**
64 * Set of token types that are ignored when checking class members.
65 */
66 private static final BitSet IGNORED_TYPES = TokenUtil.asBitSet(
67 TokenTypes.INSTANCE_INIT,
68 TokenTypes.SEMI,
69 TokenTypes.LCURLY,
70 TokenTypes.RCURLY
71 );
72
73 /**
74 * Creates a new {@code AvoidDoubleBraceInitializationCheck} instance.
75 */
76 public AvoidDoubleBraceInitializationCheck() {
77 // no code by default
78 }
79
80 @Override
81 public int[] getDefaultTokens() {
82 return getRequiredTokens();
83 }
84
85 @Override
86 public int[] getAcceptableTokens() {
87 return getRequiredTokens();
88 }
89
90 @Override
91 public int[] getRequiredTokens() {
92 return new int[] {TokenTypes.OBJBLOCK};
93 }
94
95 @Override
96 public void visitToken(DetailAST ast) {
97 if (ast.getParent().getType() == TokenTypes.LITERAL_NEW
98 && hasOnlyInitialization(ast)) {
99 log(ast, MSG_KEY);
100 }
101 }
102
103 /**
104 * Checks that block has at least one instance init block and no other class members.
105 *
106 * @param objBlock token to check
107 * @return true if there is at least one instance init block and no other class members,
108 * false otherwise
109 */
110 private static boolean hasOnlyInitialization(DetailAST objBlock) {
111 final boolean hasInitBlock = objBlock.findFirstToken(TokenTypes.INSTANCE_INIT) != null;
112 return hasInitBlock
113 && TokenUtil.findFirstTokenByPredicate(objBlock,
114 AvoidDoubleBraceInitializationCheck::hasMember).isEmpty();
115 }
116
117 /**
118 * Checks whether a token represents a class member other than initialization-related tokens.
119 *
120 * @param token the token to check
121 * @return true if the token represents a class member
122 */
123 private static boolean hasMember(DetailAST token) {
124 return !IGNORED_TYPES.get(token.getType());
125 }
126
127 }