Add Checkstyle plugin and enforce header

checkstyle/checkstyle.xml

@@ -0,0 +1,14 @@
+<?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">
+    <property name="fileExtensions" value="java, xml"/>
+    <property name="charset" value="UTF-8"/>
+
+    <module name="Header">
+        <property name="fileExtensions" value="java"/>
+        <property name="headerFile" value="${checkstyle.header.file}"/>
+    </module>
+</module>

checkstyle/header.txt

@@ -0,0 +1,5 @@
+/*
+ * Copyright (C) filoghost and contributors
+ *
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ */

checkstyle/suppressions.xml

@@ -0,0 +1,8 @@
+<?xml version="1.0"?>
+<!DOCTYPE suppressions PUBLIC
+        "-//Checkstyle//DTD SuppressionFilter Configuration 1.2//EN"
+        "https://checkstyle.org/dtds/suppressions_1_2.dtd">
+
+<suppressions>
+    <suppress checks="Header" files=".*[\\/]me[\\/]filoghost[\\/]chestcommands[\\/]util[\\/]nbt[\\/].+\.java"/>
+</suppressions>

pom.xml

@@ -154,6 +154,12 @@
 					<version>3.2.3</version>
 				</plugin>
 
+				<plugin>
+					<groupId>org.apache.maven.plugins</groupId>
+					<artifactId>maven-checkstyle-plugin</artifactId>
+					<version>3.1.1</version>
+				</plugin>
+
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-enforcer-plugin</artifactId>
@@ -169,6 +175,29 @@
 		</pluginManagement>
 
 		<plugins>
+			<plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-checkstyle-plugin</artifactId>
+				<configuration>
+					<configLocation>checkstyle/checkstyle.xml</configLocation>
+					<suppressionsLocation>checkstyle/suppressions.xml</suppressionsLocation>
+					<headerLocation>checkstyle/header.txt</headerLocation>
+					<sourceDirectories>
+						<sourceDirectory>${project.basedir}</sourceDirectory>
+					</sourceDirectories>
+					<includes>pom.xml, src/main/java/**/*.java</includes>
+				</configuration>
+				<executions>
+					<execution>
+						<id>checkstyle-check</id>
+						<phase>validate</phase>
+						<goals>
+							<goal>check</goal>
+						</goals>
+					</execution>
+				</executions>
+			</plugin>
+
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-enforcer-plugin</artifactId>