1 ///////////////////////////////////////////////////////////////////////////////////////////////
2 // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
3 // Copyright (C) 2001-2024 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 import java.util.function.Predicate;
24
25 import com.puppycrawl.tools.checkstyle.StatelessCheck;
26 import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
27 import com.puppycrawl.tools.checkstyle.api.DetailAST;
28 import com.puppycrawl.tools.checkstyle.api.TokenTypes;
29 import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
30
31 /**
32 * <p>
33 * Detects double brace initialization.
34 * </p>
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 * <p>
52 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
53 * </p>
54 * <p>
55 * Violation Message Keys:
56 * </p>
57 * <ul>
58 * <li>
59 * {@code avoid.double.brace.init}
60 * </li>
61 * </ul>
62 *
63 * @since 8.30
64 */
65 @StatelessCheck
66 public class AvoidDoubleBraceInitializationCheck extends AbstractCheck {
67
68 /**
69 * A key is pointing to the warning message text in "messages.properties"
70 * file.
71 */
72 public static final String MSG_KEY = "avoid.double.brace.init";
73
74 /**
75 * Set of token types that are used in {@link #HAS_MEMBERS} predicate.
76 */
77 private static final BitSet IGNORED_TYPES = TokenUtil.asBitSet(
78 TokenTypes.INSTANCE_INIT,
79 TokenTypes.SEMI,
80 TokenTypes.LCURLY,
81 TokenTypes.RCURLY
82 );
83
84 /**
85 * Predicate for tokens that is used in {@link #hasOnlyInitialization(DetailAST)}.
86 */
87 private static final Predicate<DetailAST> HAS_MEMBERS =
88 token -> !IGNORED_TYPES.get(token.getType());
89
90 @Override
91 public int[] getDefaultTokens() {
92 return getRequiredTokens();
93 }
94
95 @Override
96 public int[] getAcceptableTokens() {
97 return getRequiredTokens();
98 }
99
100 @Override
101 public int[] getRequiredTokens() {
102 return new int[] {TokenTypes.OBJBLOCK};
103 }
104
105 @Override
106 public void visitToken(DetailAST ast) {
107 if (ast.getParent().getType() == TokenTypes.LITERAL_NEW
108 && hasOnlyInitialization(ast)) {
109 log(ast, MSG_KEY);
110 }
111 }
112
113 /**
114 * Checks that block has at least one instance init block and no other class members.
115 *
116 * @param objBlock token to check
117 * @return true if there is least one instance init block and no other class members,
118 * false otherwise
119 */
120 private static boolean hasOnlyInitialization(DetailAST objBlock) {
121 final boolean hasInitBlock = objBlock.findFirstToken(TokenTypes.INSTANCE_INIT) != null;
122 return hasInitBlock
123 && TokenUtil.findFirstTokenByPredicate(objBlock, HAS_MEMBERS).isEmpty();
124 }
125 }