Create FlexConnect project

.gitignore

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+bin/
+target/
+settings.json
+pom.xml.tag
+pom.xml.releaseBackup
+pom.xml.versionsBackup
+pom.xml.next
+release.properties
+dependency-reduced-pom.xml
+buildNumber.properties
+.mvn/timing.properties
+.mvn/wrapper/maven-wrapper.jar
+.classpath
+.project
+.factorypath
+*.iml
+*.settings/
+.vscode/

pom.xml

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+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+  
+  <groupId>fi.flexplex</groupId>
+  <artifactId>connect</artifactId>
+  <version>1.0.0-SNAPSHOT</version>
+  <name>flexconnect</name>
+  
+  <properties>
+    <skipTests>true</skipTests>
+  </properties>
+  
+  <build>
+    <finalName>FlexConnect</finalName>
+    <resources>
+      <resource>
+        <directory>${basedir}/src/main/resources</directory>
+        <includes>
+          <include>plugin.yml</include>
+          <include>config.yml</include>
+        </includes>
+      </resource>
+    </resources>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.7.0</version>
+        <configuration>
+          <source>16</source>
+          <target>16</target>
+        </configuration>
+      </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-shade-plugin</artifactId>
+        <version>3.2.4</version>
+        <executions>
+          <execution>
+            <phase>package</phase>
+            <goals>
+              <goal>shade</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+  
+  <repositories>
+    <repository>
+      <id>paper</id>
+      <url>https://papermc.io/repo/repository/maven-public/</url>
+    </repository>
+  </repositories>
+  
+  <dependencies>
+    <dependency>
+      <groupId>com.destroystokyo.paper</groupId>
+      <artifactId>paper-api</artifactId>
+      <version>1.16.4-R0.1-SNAPSHOT</version>
+      <scope>provided</scope>
+    </dependency>
+  </dependencies>
+  
+</project>

src/main/java/fi/flexplex/connect/FlexConnect.java

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+package fi.flexplex.connect;
+
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.util.Properties;
+
+import org.bukkit.configuration.file.YamlConfiguration;
+import org.bukkit.plugin.java.JavaPlugin;
+
+public final class FlexConnect extends JavaPlugin {
+
+    private String token = "";
+
+    public String getToken() {
+        return this.token;
+    }
+
+    @Override
+    public void onEnable() {
+
+        // Plugin own configs
+        this.saveDefaultConfig();
+        this.token = this.getConfig().getString("token");
+        if (this.token.equals("default")) {
+            this.getLogger().warning("Token for FlexPlex server network has not been set into plugins config file. Please request token from FlexPlex staff members if you don't already have one!");
+        }
+
+        boolean configsModified = false;
+
+        // Check online mode
+        if (this.getServer().getOnlineMode()) {
+            final Properties properties = new Properties();
+            try {
+                properties.load(new FileInputStream("server.properties"));
+                properties.setProperty("online-mode", "false");
+                properties.store(new FileOutputStream("server.properties"), null);
+            } catch (final IOException e) {
+                e.printStackTrace();
+            }
+            this.getLogger().warning("Automatic changes has been made to server.properties file.");
+            configsModified = true;
+        }
+
+        // Check paper config
+        boolean paperConfigModified = false;
+        final YamlConfiguration paperConfig = this.getServer().spigot().getPaperConfig();
+        if (!paperConfig.getBoolean("settings.velocity-support.enabled")) {
+            paperConfig.set("settings.velocity-support.enabled", true);
+            paperConfigModified = true;
+        }
+        if (!paperConfig.getBoolean("settings.velocity-support.enabled")) {
+            paperConfig.set("settings.velocity-support.online-mode", true);
+            paperConfigModified = true;
+        }
+        if (!paperConfig.getString("settings.velocity-support.secret").equals("flexplex")) {
+            paperConfig.set("settings.velocity-support.secret", "flexplex");
+            paperConfigModified = true;
+        }
+        if (paperConfigModified) {
+            try {
+                paperConfig.save("paper.yml");
+            } catch (final IOException e) {
+                e.printStackTrace();
+            }
+            this.getLogger().warning("Automatic changes has been made to paper.yml config file.");
+            configsModified = true;
+        }
+        
+        // Actions if configs were modified
+        if (configsModified) {
+            this.getLogger().warning("Changes has been made to config files. Server will now shutdown automatically.");
+            this.getServer().shutdown();
+            return;
+        }
+
+        // Register listener
+        this.getServer().getPluginManager().registerEvents(new FlexConnectListener(this), this);
+    }
+
+}

src/main/java/fi/flexplex/connect/FlexConnectListener.java

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+package fi.flexplex.connect;
+
+import com.destroystokyo.paper.profile.ProfileProperty;
+
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.Listener;
+import org.bukkit.event.player.AsyncPlayerPreLoginEvent;
+
+public final class FlexConnectListener implements Listener {
+    
+    private final FlexConnect flexConnect;
+
+    public FlexConnectListener(final FlexConnect flexConnect) {
+        this.flexConnect = flexConnect;
+    }
+
+    @EventHandler
+    public void onAsyncPlayerPreLogin(final AsyncPlayerPreLoginEvent event) {
+        // Authenticate players
+        for (final ProfileProperty property : event.getPlayerProfile().getProperties()) {
+            if (property.getName().equals("flexplex-token") && property.getValue().equals(flexConnect.getToken())) {
+                // Authentication success
+                return;
+            }
+        }
+        // Authentication failed
+        event.disallow(AsyncPlayerPreLoginEvent.Result.KICK_OTHER, "Access denied");
+    }
+
+}

src/main/resources/config.yml

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+token: "default"

src/main/resources/plugin.yml

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+name: FlexConnect
+version: 1.0
+api-version: 1.13
+description: Connect's your server to FlexPlex network.
+author: FlexPlex
+website: https://flexplex.fi
+main: fi.flexplex.connect.FlexConnect