diff --git a/build.gradle b/build.gradle
index 3cc6fa0c4..ebac57210 100644
--- a/build.gradle
+++ b/build.gradle
@@ -30,6 +30,7 @@ allprojects {
         nanojsonVersion = "1d9e1aea9049fc9f85e68b43ba39fe7be1c1f751"
         spotbugsVersion = "4.6.0"
         junitVersion = "5.8.2"
+        checkstyleVersion = "9.3" // do not use latest version (10.0) as it requires compile SDK 11
     }
 }
 
diff --git a/checkstyle/checkstyle.xml b/checkstyle/checkstyle.xml
new file mode 100644
index 000000000..4a82f6be3
--- /dev/null
+++ b/checkstyle/checkstyle.xml
@@ -0,0 +1,189 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+          "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+          "https://checkstyle.org/dtds/configuration_1_3.dtd">
+<module name="Checker">
+    <!--
+        If you set the basedir property below, then all reported file
+        names will be relative to the specified directory. See
+        https://checkstyle.org/5.x/config.html#Checker
+
+        <property name="basedir" value="${basedir}"/>
+    -->
+    <property name="severity" value="error"/>
+
+    <property name="fileExtensions" value="java, properties, xml"/>
+
+    <!-- Excludes all 'module-info.java' files              -->
+    <!-- See https://checkstyle.org/config_filefilters.html -->
+    <module name="BeforeExecutionExclusionFileFilter">
+        <property name="fileNamePattern" value="module\-info\.java$"/>
+    </module>
+
+    <!-- https://checkstyle.org/config_filters.html#SuppressionFilter -->
+    <module name="SuppressionFilter">
+        <property name="file" value="${config_loc}/suppressions.xml" />
+        <property name="optional" value="true"/>
+    </module>
+
+    <!-- Checks that a package-info.java file exists for each package.     -->
+    <!-- See https://checkstyle.org/config_javadoc.html#JavadocPackage -->
+    <!--<module name="JavadocPackage"/>-->
+
+    <!-- Checks whether files end with a new line.                        -->
+    <!-- See https://checkstyle.org/config_misc.html#NewlineAtEndOfFile -->
+    <module name="NewlineAtEndOfFile"/>
+
+    <!-- Checks that property files contain the same keys.         -->
+    <!-- See https://checkstyle.org/config_misc.html#Translation -->
+    <module name="Translation"/>
+
+    <!-- Checks for Size Violations.                    -->
+    <!-- See https://checkstyle.org/config_sizes.html -->
+    <module name="FileLength"/>
+    <module name="LineLength">
+      <property name="max" value="100"/>
+      <property name="fileExtensions" value="java"/>
+    </module>
+
+    <!-- Checks for whitespace                               -->
+    <!-- See https://checkstyle.org/config_whitespace.html -->
+    <module name="FileTabCharacter"/>
+
+    <!-- Miscellaneous other checks.                   -->
+    <!-- See https://checkstyle.org/config_misc.html -->
+    <module name="RegexpSingleline">
+       <property name="format" value="\s+$"/>
+       <property name="minimum" value="0"/>
+       <property name="maximum" value="0"/>
+       <property name="message" value="Line has trailing spaces."/>
+    </module>
+
+    <!-- Checks for Headers                                -->
+    <!-- See https://checkstyle.org/config_header.html   -->
+    <!-- <module name="Header"> -->
+    <!--   <property name="headerFile" value="${checkstyle.header.file}"/> -->
+    <!--   <property name="fileExtensions" value="java"/> -->
+    <!-- </module> -->
+
+    <module name="SuppressWarningsFilter" />
+
+    <module name="TreeWalker">
+        <!-- Checks for Javadoc comments.                     -->
+        <!-- See https://checkstyle.org/config_javadoc.html -->
+        <module name="InvalidJavadocPosition"/>
+        <module name="JavadocMethod">
+            <property name="allowMissingParamTags" value="true"/>
+            <property name="allowMissingReturnTag" value="true"/>
+        </module>
+        <module name="JavadocType"/>
+        <!--<module name="JavadocVariable"/>-->
+        <module name="JavadocStyle">
+            <property name="checkFirstSentence" value="false"/>
+        </module>
+        <!--<module name="MissingJavadocMethod"/>-->
+
+        <!-- Checks for Naming Conventions.                  -->
+        <!-- See https://checkstyle.org/config_naming.html -->
+        <module name="ConstantName"/>
+        <module name="LocalFinalVariableName"/>
+        <module name="LocalVariableName"/>
+        <module name="MemberName">
+            <property name="format" value="^(TAG|DEBUG|[a-z][a-zA-Z0-9]*)$"/>
+        </module>
+        <module name="MethodName"/>
+        <module name="PackageName"/>
+        <module name="ParameterName"/>
+        <module name="StaticVariableName"/>
+        <module name="TypeName"/>
+
+        <!-- Checks for imports                              -->
+        <!-- See https://checkstyle.org/config_import.html -->
+        <module name="AvoidStarImport"/>
+        <module name="IllegalImport"/> <!-- defaults to sun.* packages -->
+        <module name="RedundantImport"/>
+        <module name="UnusedImports"/>
+
+        <!-- Checks for Size Violations.                    -->
+        <!-- See https://checkstyle.org/config_sizes.html -->
+        <module name="MethodLength">
+            <property name="severity" value="warning"/>
+        </module>
+        <module name="ParameterNumber">
+            <property name="severity" value="warning"/>
+        </module>
+
+        <!-- Checks for whitespace                               -->
+        <!-- See https://checkstyle.org/config_whitespace.html -->
+        <module name="EmptyForIteratorPad"/>
+        <module name="GenericWhitespace"/>
+        <module name="MethodParamPad"/>
+        <module name="NoWhitespaceAfter"/>
+        <module name="NoWhitespaceBefore"/>
+        <module name="OperatorWrap"/>
+        <module name="ParenPad"/>
+        <module name="TypecastParenPad"/>
+        <module name="WhitespaceAfter"/>
+        <module name="WhitespaceAround"/>
+
+        <!-- Modifier Checks                                    -->
+        <!-- See https://checkstyle.org/config_modifiers.html -->
+        <module name="ModifierOrder"/>
+        <module name="RedundantModifier"/>
+
+        <!-- Checks for blocks. You know, those {}'s         -->
+        <!-- See https://checkstyle.org/config_blocks.html -->
+        <module name="AvoidNestedBlocks"/>
+        <module name="EmptyBlock"/>
+        <module name="LeftCurly"/>
+        <module name="NeedBraces"/>
+        <module name="RightCurly"/>
+
+        <!-- Checks for common coding problems               -->
+        <!-- See https://checkstyle.org/config_coding.html -->
+        <module name="EmptyStatement"/>
+        <module name="EqualsHashCode">
+            <property name="severity" value="warning"/>
+        </module>
+        <module name="HiddenField">
+            <property name="ignoreConstructorParameter" value="true"/>
+            <property name="ignoreSetter" value="true"/>
+        </module>
+        <module name="IllegalInstantiation"/>
+        <module name="InnerAssignment"/>
+        <!--<module name="MagicNumber"/>-->
+        <!--<module name="MissingSwitchDefault">
+            <property name="severity" value="warning"/>
+        </module>-->
+        <module name="MultipleVariableDeclarations"/>
+        <module name="SimplifyBooleanExpression"/>
+        <module name="SimplifyBooleanReturn"/>
+        <module name="FinalLocalVariable">
+            <property name="tokens" value="VARIABLE_DEF,PARAMETER_DEF"/>
+            <property name="validateEnhancedForLoopVariable" value="true"/>
+        </module>
+
+        <!-- Checks for class design                         -->
+        <!-- See https://checkstyle.org/config_design.html -->
+        <!--<module name="DesignForExtension"/>-->
+        <module name="FinalClass"/>
+        <module name="HideUtilityClassConstructor"/>
+        <module name="InterfaceIsType"/>
+        <!--<module name="VisibilityModifier">
+            <property name="ignoreAnnotationCanonicalNames" value="State,ColumnInfo"/>
+            <property name="severity" value="warning"/>
+        </module>-->
+
+        <!-- Miscellaneous other checks.                   -->
+        <!-- See https://checkstyle.org/config_misc.html -->
+        <module name="ArrayTypeStyle"/>
+        <module name="FinalParameters"/>
+        <!--<module name="TodoComment">
+            <property name="format" value="(TODO:|FIXME:)"/>
+            <property name="severity" value="warning"/>
+        </module>-->
+        <module name="UpperEll"/>
+
+        <module name="SuppressWarningsHolder" />
+    </module>
+</module>
diff --git a/extractor/build.gradle b/extractor/build.gradle
index a9b5f7f3e..70b51a338 100644
--- a/extractor/build.gradle
+++ b/extractor/build.gradle
@@ -1,9 +1,25 @@
+plugins {
+    id 'checkstyle'
+}
+
 test {
-//  Pass on downloader type to tests for different CI jobs. See DownloaderFactory.java and ci.yml
+    // Pass on downloader type to tests for different CI jobs. See DownloaderFactory.java and ci.yml
     if (System.properties.containsKey('downloader')) {
         systemProperty('downloader', System.getProperty('downloader'))
     }
     useJUnitPlatform()
+    dependsOn checkstyleMain // run checkstyle when testing
+}
+
+checkstyle {
+    getConfigDirectory().set(rootProject.file("checkstyle"))
+    ignoreFailures false
+    showViolations true
+    toolVersion checkstyleVersion
+}
+
+checkstyleTest {
+    enabled false // do not checkstyle test files
 }
 
 dependencies {
@@ -15,6 +31,8 @@ dependencies {
     implementation "com.github.spotbugs:spotbugs-annotations:$spotbugsVersion"
     implementation 'org.nibor.autolink:autolink:0.10.0'
 
+    checkstyle "com.puppycrawl.tools:checkstyle:$checkstyleVersion"
+
     testImplementation platform("org.junit:junit-bom:$junitVersion")
     testImplementation 'org.junit.jupiter:junit-jupiter-api'
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine'