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" file.
059     */
060    public static final String MSG_KEY = "avoid.double.brace.init";
061
062    /**
063     * Set of token types that are ignored when checking class members.
064     */
065    private static final BitSet IGNORED_TYPES = TokenUtil.asBitSet(
066        TokenTypes.INSTANCE_INIT,
067        TokenTypes.SEMI,
068        TokenTypes.LCURLY,
069        TokenTypes.RCURLY
070    );
071
072    /**
073     * Creates a new {@code AvoidDoubleBraceInitializationCheck} instance.
074     */
075    public AvoidDoubleBraceInitializationCheck() {
076        // no code by default
077    }
078
079    @Override
080    public int[] getDefaultTokens() {
081        return getRequiredTokens();
082    }
083
084    @Override
085    public int[] getAcceptableTokens() {
086        return getRequiredTokens();
087    }
088
089    @Override
090    public int[] getRequiredTokens() {
091        return new int[] {TokenTypes.OBJBLOCK};
092    }
093
094    @Override
095    public void visitToken(DetailAST ast) {
096        if (ast.getParent().getType() == TokenTypes.LITERAL_NEW
097            && hasOnlyInitialization(ast)) {
098            log(ast, MSG_KEY);
099        }
100    }
101
102    /**
103     * Checks that block has at least one instance init block and no other class members.
104     *
105     * @param objBlock token to check
106     * @return true if there is at least one instance init block and no other class members,
107     *     false otherwise
108     */
109    private static boolean hasOnlyInitialization(DetailAST objBlock) {
110        final boolean hasInitBlock = objBlock.findFirstToken(TokenTypes.INSTANCE_INIT) != null;
111        return hasInitBlock
112            && TokenUtil.findFirstTokenByPredicate(objBlock,
113                AvoidDoubleBraceInitializationCheck::hasMember).isEmpty();
114    }
115
116    /**
117     * Checks whether a token represents a class member other than initialization-related tokens.
118     *
119     * @param token the token to check
120     * @return true if the token represents a class member
121     */
122    private static boolean hasMember(DetailAST token) {
123        return !IGNORED_TYPES.get(token.getType());
124    }
125
126}