checkstyle.xml

<?xml version="1.0"?>
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  ~ Modifications Copyright OpenSearch Contributors. See
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<!DOCTYPE module PUBLIC
        "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
        "https://checkstyle.org/dtds/configuration_1_3.dtd">

<module name="Checker">
    <property name="charset" value="UTF-8" />

    <module name="SuppressionFilter">
        <property name="file" value="${config_loc}/checkstyle_suppressions.xml" />
    </module>

    <module name="Header">
        <property name="headerFile" value="${config_loc}/header.java.txt"/>
        <property name="fileExtensions" value="java,kt,kts"/>
    </module>

    <module name="SuppressWarningsFilter" />

    <!-- Checks Java files and forbids empty Javadoc comments. -->
    <!-- Although you can use the "JavadocStyle" rule for this, it considers Javadoc -->
    <!-- that only contains a "@return" line to be empty. -->
    <!--
    <module name="RegexpMultiline">
        <property name="id" value="EmptyJavadoc" />
        <property name="format" value="\/\*[\s\*]*\*\/" />
        <property name="fileExtensions" value="java" />
        <property name="message" value="Empty javadoc comments are forbidden" />
    </module>
    -->

    <!-- Its our official line length! See checkstyle_suppressions.xml for the files that don't pass this. For now we
      suppress the check there but enforce it everywhere else. This prevents the list from getting longer even if it is
      unfair. -->
    <module name="LineLength">
        <property name="max" value="140" />
        <property name="ignorePattern" value="^ *\* *https?://[^ ]+$" />
    </module>

    <module name="TreeWalker">
        <!-- Make the @SuppressWarnings annotations available to Checkstyle -->
        <module name="SuppressWarningsHolder" />

        <module name="AvoidStarImport" />

        <!-- Unused imports are forbidden -->
        <module name="UnusedImports" />

        <!-- Non-inner classes must be in files that match their names. -->
        <module name="OuterTypeFilename" />

        <!-- No line wraps inside of import and package statements. -->
        <module name="NoLineWrap" />

        <!-- only one statement per line should be allowed -->
        <module name="OneStatementPerLine" />

        <!-- Each java file has only one outer class -->
        <module name="OneTopLevelClass" />

        <!-- The suffix L is preferred, because the letter l (ell) is often
          hard to distinguish from the digit 1 (one). -->
        <module name="UpperEll" />

        <module name="EqualsHashCode" />

        <!-- Checks that the order of modifiers conforms to the suggestions in the
        Java Language specification, sections 8.1.1, 8.3.1 and 8.4.3. It is not that
        the standard is perfect, but having a consistent order makes the code more
        readable and no other order is compellingly better than the standard.
        The correct order is:
                public
                protected
                private
                abstract
                static
                final
                transient
                volatile
                synchronized
                native
                strictfp
        -->
        <module name="ModifierOrder" />

        <!-- Checks that we don't include modifier where they are implied. For
          example, this does not allow interface methods to be declared public
          because they are *always* public. -->
        <module name="RedundantModifier" />

        <!-- Checks that all java files have a package declaration and that it
          lines up with the directory structure. -->
        <module name="PackageDeclaration" />

        <!-- We don't use Java's builtin serialization and we suppress all warning
          about it. The flip side of that coin is that we shouldn't _try_ to use
          it. We can't outright ban it with ForbiddenApis because it complain about
          every we reference a class that implements Serializable like String or
          Exception.
          -->
        <module name="RegexpSinglelineJava">
            <property name="format" value="serialVersionUID" />
            <property name="message" value="Do not declare serialVersionUID." />
            <property name="ignoreComments" value="true" />
        </module>
        <module name="RegexpSinglelineJava">
            <property name="format" value="java\.io\.Serializable;" />
            <property name="message" value="References java.io.Serializable." />
            <property name="ignoreComments" value="true" />
        </module>
        <!-- end Orwellian suppression of Serializable -->

        <!-- Forbid equality comparisons with `true` -->
        <module name="DescendantToken">
            <property name="id" value="EqualityWithTrue" />
            <property name="tokens" value="EQUAL" />
            <property name="limitedTokens" value="LITERAL_TRUE" />
            <property name="maximumNumber" value="0" />
            <property name="maximumDepth" value="1" />
            <message key="descendant.token.max" value="Do not check for equality with 'true', since it is implied" />
        </module>

    </module>
</module>