Added config file support

.gitignore

@@ -144,3 +144,4 @@ gradle-app.setting
 
 # End of https://www.toptal.com/developers/gitignore/api/netbeans,intellij,java,gradle,eclipse
 application/db/
+config.json

application/build.gradle

@@ -45,8 +45,11 @@ dependencies {
     implementation 'org.apache.logging.log4j:log4j-api:2.14.1'
     implementation 'org.apache.logging.log4j:log4j-core:2.14.1'
     implementation 'org.apache.logging.log4j:log4j-slf4j18-impl:2.14.1'
+
     implementation 'org.jooq:jooq:3.15.3'
 
+    implementation 'com.fasterxml.jackson.core:jackson-databind:2.12.5'
+
     testImplementation 'org.junit.jupiter:junit-jupiter-api:5.8.0'
     testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.8.0'
 }

application/src/main/java/org/togetherjava/tjbot/Application.java

@@ -5,36 +5,48 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import org.togetherjava.tjbot.commands.CommandHandler;
+import org.togetherjava.tjbot.config.Config;
 import org.togetherjava.tjbot.db.Database;
 
 import javax.security.auth.login.LoginException;
+import java.io.IOException;
 import java.nio.file.Path;
 import java.sql.SQLException;
 
-/***
+/**
  * Main class of the application. Use {@link #main(String[])} to start an instance of it.
  */
 public enum Application {
     ;
 
     private static final Logger logger = LoggerFactory.getLogger(Application.class);
+    private static final String DEFAULT_CONFIG_PATH = "config.json";
 
     /**
      * Starts the application.
      *
-     * @param args command line arguments - [the token of the bot, the path to the database]
+     * @param args command line arguments - [the path to the configuration file (optional, by
+     *        default "config.json")]
      */
     public static void main(final String[] args) {
-        if (args.length != 2) {
-            throw new IllegalArgumentException("Expected two arguments but " + args.length
-                    + " arguments were provided. The first argument must be the token of the bot"
-                    + " and the second the path to the database.");
+        if (args.length > 1) {
+            throw new IllegalArgumentException("Expected no or one argument but " + args.length
+                    + " arguments were provided. The first argument is the path to the configuration file. If no argument was provided, '"
+                    + DEFAULT_CONFIG_PATH + "' will be assumed.");
         }
-        String token = args[0];
-        String databasePath = args[1];
 
+        Path configPath = Path.of(args.length == 1 ? args[0] : DEFAULT_CONFIG_PATH);
         try {
-            runBot(token, Path.of(databasePath));
+            Config.load(configPath);
+        } catch (IOException e) {
+            logger.error("Unable to load the configuration file from path '{}'",
+                    configPath.toAbsolutePath(), e);
+            return;
+        }
+
+        try {
+            Config config = Config.getInstance();
+            runBot(config.getToken(), Path.of(config.getDatabasePath()));
         } catch (Exception t) {
             logger.error("Unknown error", t);
         }

application/src/main/java/org/togetherjava/tjbot/config/Config.java

@@ -0,0 +1,95 @@
+package org.togetherjava.tjbot.config;
+
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+import java.io.IOException;
+import java.nio.file.Path;
+import java.util.Objects;
+
+/**
+ * Configuration of the application, as singleton.
+ * <p>
+ * Create instances using {@link #load(Path)} and then access them with {@link #getInstance()}.
+ */
+public final class Config {
+
+    private static Config config;
+
+    private final String token;
+    private final String databasePath;
+    private final String projectWebsite;
+    private final String discordGuildInvite;
+
+    @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
+    private Config(@JsonProperty("token") String token,
+            @JsonProperty("databasePath") String databasePath,
+            @JsonProperty("projectWebsite") String projectWebsite,
+            @JsonProperty("discordGuildInvite") String discordGuildInvite) {
+        this.token = token;
+        this.databasePath = databasePath;
+        this.projectWebsite = projectWebsite;
+        this.discordGuildInvite = discordGuildInvite;
+    }
+
+    /**
+     * Loads the configuration from the given file. Will override any previously loaded data.
+     * <p>
+     * Access the instance using {@link #getInstance()}.
+     *
+     * @param path the configuration file, as JSON object
+     * @throws IOException if the file could not be loaded
+     */
+    public static void load(Path path) throws IOException {
+        config = new ObjectMapper().readValue(path.toFile(), Config.class);
+    }
+
+    /**
+     * Gets the singleton instance of the configuration.
+     * <p>
+     * Must be loaded beforehand using {@link #load(Path)}.
+     *
+     * @return the previously loaded configuration
+     */
+    public static Config getInstance() {
+        return Objects.requireNonNull(config,
+                "can not get the configuration before it has been loaded");
+    }
+
+    /**
+     * Gets the token of the Discord bot to connect this application to.
+     *
+     * @return the Discord bot token
+     */
+    public String getToken() {
+        return token;
+    }
+
+    /**
+     * Gets the path where the database of the application is located at.
+     *
+     * @return the path of the database
+     */
+    public String getDatabasePath() {
+        return databasePath;
+    }
+
+    /**
+     * Gets a URL of the project's website, for example to tell the user where he can contribute.
+     *
+     * @return the website of the project
+     */
+    public String getProjectWebsite() {
+        return projectWebsite;
+    }
+
+    /**
+     * Gets an invite-URL to join the Discord guild this application is connected to.
+     *
+     * @return an invite-URL for this Discord guild
+     */
+    public String getDiscordGuildInvite() {
+        return discordGuildInvite;
+    }
+}