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 java.util.BitSet;
023import java.util.function.Predicate;
024
025import com.puppycrawl.tools.checkstyle.StatelessCheck;
026import com.puppycrawl.tools.checkstyle.api.AbstractCheck;
027import com.puppycrawl.tools.checkstyle.api.DetailAST;
028import com.puppycrawl.tools.checkstyle.api.TokenTypes;
029import com.puppycrawl.tools.checkstyle.utils.TokenUtil;
030
031/**
032 * <p>
033 * Detects double brace initialization.
034 * </p>
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 * <p>
052 * To configure the check:
053 * </p>
054 * <pre>
055 * &lt;module name=&quot;AvoidDoubleBraceInitialization&quot;/&gt;
056 * </pre>
057 * <p>
058 * Which results in the following violations:
059 * </p>
060 * <pre>
061 * class MyClass {
062 *     List&lt;Integer&gt; list1 = new ArrayList&lt;&gt;() { // violation
063 *         {
064 *             add(1);
065 *         }
066 *     };
067 *     List&lt;String&gt; list2 = new ArrayList&lt;&gt;() { // violation
068 *         ;
069 *         // comments and semicolons are ignored
070 *         {
071 *             add("foo");
072 *         }
073 *     };
074 * }
075 * </pre>
076 * <p>
077 * Check only looks for double brace initialization and it ignores cases
078 * where the anonymous class has fields or methods.
079 * Though these might create the same memory issues as double brace,
080 * the extra class members can produce side effects if changed incorrectly.
081 * </p>
082 * <pre>
083 * class MyClass {
084 *     List&lt;Object&gt; list = new ArrayList&lt;&gt;() { // OK, not pure double brace pattern
085 *         private int field;
086 *         {
087 *             add(new Object());
088 *         }
089 *     };
090 * }
091 * </pre>
092 * <p>
093 * Parent is {@code com.puppycrawl.tools.checkstyle.TreeWalker}
094 * </p>
095 * <p>
096 * Violation Message Keys:
097 * </p>
098 * <ul>
099 * <li>
100 * {@code avoid.double.brace.init}
101 * </li>
102 * </ul>
103 *
104 * @since 8.30
105 */
106@StatelessCheck
107public class AvoidDoubleBraceInitializationCheck extends AbstractCheck {
108
109    /**
110     * A key is pointing to the warning message text in "messages.properties"
111     * file.
112     */
113    public static final String MSG_KEY = "avoid.double.brace.init";
114
115    /**
116     * Set of token types that are used in {@link #HAS_MEMBERS} predicate.
117     */
118    private static final BitSet IGNORED_TYPES = TokenUtil.asBitSet(
119        TokenTypes.INSTANCE_INIT,
120        TokenTypes.SEMI,
121        TokenTypes.LCURLY,
122        TokenTypes.RCURLY
123    );
124
125    /**
126     * Predicate for tokens that is used in {@link #hasOnlyInitialization(DetailAST)}.
127     */
128    private static final Predicate<DetailAST> HAS_MEMBERS =
129        token -> !IGNORED_TYPES.get(token.getType());
130
131    @Override
132    public int[] getDefaultTokens() {
133        return getRequiredTokens();
134    }
135
136    @Override
137    public int[] getAcceptableTokens() {
138        return getRequiredTokens();
139    }
140
141    @Override
142    public int[] getRequiredTokens() {
143        return new int[] {TokenTypes.OBJBLOCK};
144    }
145
146    @Override
147    public void visitToken(DetailAST ast) {
148        if (ast.getParent().getType() == TokenTypes.LITERAL_NEW
149            && hasOnlyInitialization(ast)) {
150            log(ast, MSG_KEY);
151        }
152    }
153
154    /**
155     * Checks that block has at least one instance init block and no other class members.
156     *
157     * @param objBlock token to check
158     * @return true if there is least one instance init block and no other class members,
159     *     false otherwise
160     */
161    private static boolean hasOnlyInitialization(DetailAST objBlock) {
162        final boolean hasInitBlock = objBlock.findFirstToken(TokenTypes.INSTANCE_INIT) != null;
163        return hasInitBlock
164                  && TokenUtil.findFirstTokenByPredicate(objBlock, HAS_MEMBERS).isEmpty();
165    }
166}