001///////////////////////////////////////////////////////////////////////////////////////////////
002// checkstyle: Checks Java source code and other text files for adherence to a set of rules.
003// Copyright (C) 2001-2026 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 java.util.BitSet;
023
024import com.puppycrawl.tools.checkstyle.StatelessCheck;
025import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
026import com.puppycrawl.tools.checkstyle.api.DetailAST;
027import com.puppycrawl.tools.checkstyle.api.TokenTypes;
028import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
029
030/**
031 * <div>
032 * Detects double brace initialization.
033 * </div>
034 *
035 * <p>
036 * Rationale: Double brace initialization (set of
037 * <a href="https://docs.oracle.com/javase/specs/jls/se12/html/jls-8.html#jls-8.6">
038 * Instance Initializers</a> in class body) may look cool, but it is considered as anti-pattern
039 * and should be avoided.
040 * This is also can lead to a hard-to-detect memory leak, if the anonymous class instance is
041 * returned outside and other object(s) hold reference to it.
042 * Created anonymous class is not static, it holds an implicit reference to the outer class
043 * instance.
044 * See this
045 * <a href="https://blog.jooq.org/dont-be-clever-the-double-curly-braces-anti-pattern/">
046 * blog post</a> and
047 * <a href="https://www.baeldung.com/java-double-brace-initialization">
048 * article</a> for more details.
049 * Check ignores any comments and semicolons in class body.
050 * </p>
051 *
052 * @since 8.30
053 */
054@StatelessCheck
055public class AvoidDoubleBraceInitializationCheck extends AbstractCheck {
056
057    /**
058     * A key is pointing to the warning message text in "messages.properties"
059     * file.
060     */
061    public static final String MSG_KEY = "avoid.double.brace.init";
062
063    /**
064     * Set of token types that are ignored when checking class members.
065     */
066    private static final BitSet IGNORED_TYPES = TokenUtil.asBitSet(
067        TokenTypes.INSTANCE_INIT,
068        TokenTypes.SEMI,
069        TokenTypes.LCURLY,
070        TokenTypes.RCURLY
071    );
072
073    /**
074     * Creates a new {@code AvoidDoubleBraceInitializationCheck} instance.
075     */
076    public AvoidDoubleBraceInitializationCheck() {
077        // no code by default
078    }
079
080    @Override
081    public int[] getDefaultTokens() {
082        return getRequiredTokens();
083    }
084
085    @Override
086    public int[] getAcceptableTokens() {
087        return getRequiredTokens();
088    }
089
090    @Override
091    public int[] getRequiredTokens() {
092        return new int[] {TokenTypes.OBJBLOCK};
093    }
094
095    @Override
096    public void visitToken(DetailAST ast) {
097        if (ast.getParent().getType() == TokenTypes.LITERAL_NEW
098            && hasOnlyInitialization(ast)) {
099            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}