Ensure there are no unused imports with Spotless rather than Checkstyle (#5457)

CONTRIBUTING.md

@@ -33,7 +33,7 @@ You can read a description of the [building and debugging process here].
 If you want simply to build the `jenkins.war` file as fast as possible without tests, run:
 
 ```sh
-mvn -am -pl war,bom -DskipTests -Dspotbugs.skip clean install
+mvn -am -pl war,bom -DskipTests -Dspotbugs.skip -Dspotless.check.skip clean install
 ```
 
 The WAR file will be created in `war/target/jenkins.war`.

pom.xml

@@ -113,6 +113,7 @@ THE SOFTWARE.
     <access-modifier-checker.version>1.21</access-modifier-checker.version>
     <junit.jupiter.version>5.7.1</junit.jupiter.version>
     <powermock.version>2.0.9</powermock.version>
+    <spotless.version>2.10.3</spotless.version>
   </properties>
 
   <!-- Note that the 'repositories' and 'pluginRepositories' blocks below are actually copy-pasted
@@ -489,7 +490,6 @@ THE SOFTWARE.
                     <property name="illegalClasses" value="javax.annotation.MatchesPattern.Checker, javax.annotation.Nonnegative.Checker, javax.annotation.Nonnull.Checker, javax.annotation.RegEx.Checker, javax.annotation.CheckForNull, javax.annotation.CheckForSigned, javax.annotation.CheckReturnValue, javax.annotation.Detainted, javax.annotation.MatchesPattern, javax.annotation.Nonnegative, javax.annotation.Nonnull, javax.annotation.Nullable, javax.annotation.OverridingMethodsMustInvokeSuper, javax.annotation.ParametersAreNonnullByDefault, javax.annotation.ParametersAreNullableByDefault, javax.annotation.PropertyKey, javax.annotation.RegEx, javax.annotation.Signed, javax.annotation.Syntax, javax.annotation.Tainted, javax.annotation.Untainted, javax.annotation.WillClose, javax.annotation.WillCloseWhenClosed, javax.annotation.WillNotClose, javax.annotation.concurrent.GuardedBy, javax.annotation.concurrent.Immutable, javax.annotation.concurrent.NotThreadSafe, javax.annotation.concurrent.ThreadSafe, javax.annotation.meta.TypeQualifierValidator, javax.annotation.meta.When, javax.annotation.meta.Exclusive, javax.annotation.meta.Exhaustive, javax.annotation.meta.TypeQualifier, javax.annotation.meta.TypeQualifierDefault, javax.annotation.meta.TypeQualifierNickname" />
                   </module>
                   <module name="RedundantImport" />
-                  <module name="UnusedImports" />
                   <!--
                     Miscellaneous: https://checkstyle.sourceforge.io/config_misc.html
                   -->
@@ -515,6 +515,25 @@ THE SOFTWARE.
             </execution>
           </executions>
         </plugin>
+        <plugin>
+          <groupId>com.diffplug.spotless</groupId>
+          <artifactId>spotless-maven-plugin</artifactId>
+          <version>${spotless.version}</version>
+          <configuration>
+            <java>
+              <removeUnusedImports />
+            </java>
+          </configuration>
+          <executions>
+            <execution>
+              <!-- Runs in verify phase by default -->
+              <goals>
+                <!-- Can be disabled using -Dspotless.check.skip -->
+                <goal>check</goal>
+              </goals>
+            </execution>
+          </executions>
+        </plugin>
       </plugins>
     </pluginManagement>
 
@@ -612,6 +631,11 @@ THE SOFTWARE.
         <artifactId>maven-checkstyle-plugin</artifactId>
         <!-- Version specified in parent POM -->
       </plugin>
+      <plugin>
+        <groupId>com.diffplug.spotless</groupId>
+        <artifactId>spotless-maven-plugin</artifactId>
+        <version>${spotless.version}</version>
+      </plugin>
     </plugins>
 
     <extensions>

test/src/test/java/jenkins/widgets/HistoryPageFilterCaseSensitiveSearchTest.java

@@ -3,7 +3,6 @@ package jenkins.widgets;
 import java.io.IOException;
 import java.util.Arrays;
 import java.util.Collections;
-import java.util.List;
 
 import org.junit.Assert;
 import org.junit.Rule;