Added starter implementation.

.gitignore

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+target/
+!.mvn/wrapper/maven-wrapper.jar
+!**/src/main/**/target/
+!**/src/test/**/target/
+
+### IntelliJ IDEA ###
+.idea
+*.iws
+*.iml
+*.ipr
+
+### Eclipse ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+build/
+!**/src/main/**/build/
+!**/src/test/**/build/
+
+### VS Code ###
+.vscode/
+
+### Mac OS ###
+.DS_Store
+
+config.json

pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<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>com.andrewlalis</groupId>
+    <artifactId>mc-status-bot</artifactId>
+    <version>1.0-SNAPSHOT</version>
+
+    <properties>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>net.dv8tion</groupId>
+            <artifactId>JDA</artifactId>
+            <version>5.0.0-beta.18</version>
+        </dependency>
+
+        <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
+        <dependency>
+            <groupId>com.fasterxml.jackson.core</groupId>
+            <artifactId>jackson-databind</artifactId>
+            <version>2.16.0</version>
+        </dependency>
+        <!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-core -->
+        <dependency>
+            <groupId>com.fasterxml.jackson.core</groupId>
+            <artifactId>jackson-core</artifactId>
+            <version>2.16.0</version>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-jar-plugin</artifactId>
+                <version>3.3.0</version>
+                <configuration>
+                    <archive>
+                        <manifest>
+                            <addClasspath>true</addClasspath>
+                            <mainClass>com.andrewlalis.mc_status_bot.McStatusBot</mainClass>
+                        </manifest>
+                    </archive>
+                </configuration>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-shade-plugin</artifactId>
+                <version>3.5.0</version>
+                <executions>
+                    <execution>
+                        <phase>package</phase>
+                        <goals><goal>shade</goal></goals>
+                        <configuration>
+                            <shadedArtifactAttached>true</shadedArtifactAttached>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+</project>

src/main/java/com/andrewlalis/mc_status_bot/McStatusBot.java

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+package com.andrewlalis.mc_status_bot;
+
+import java.nio.file.Path;
+import java.util.concurrent.Executor;
+import java.util.concurrent.Executors;
+
+public class McStatusBot {
+    public static void main(String[] args) throws Exception {
+        Executor executor = Executors.newVirtualThreadPerTaskExecutor();
+        for (ServerBot bot : ServerBot.read(Path.of("config.json"))) {
+            System.out.println("Starting server status bot for " + bot.getServerIp());
+            executor.execute(bot);
+        }
+    }
+}

src/main/java/com/andrewlalis/mc_status_bot/ServerBot.java

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+package com.andrewlalis.mc_status_bot;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+import net.dv8tion.jda.api.JDA;
+import net.dv8tion.jda.api.JDABuilder;
+import net.dv8tion.jda.api.OnlineStatus;
+import net.dv8tion.jda.api.entities.Activity;
+import net.dv8tion.jda.api.entities.channel.concrete.TextChannel;
+import net.dv8tion.jda.api.utils.cache.CacheFlag;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.*;
+
+public class ServerBot implements Runnable {
+    private final JDA jda;
+    private final String serverIp;
+    private final long channelId;
+    private final ServerStatusFetcher serverStatusFetcher;
+
+    private int lastPlayerCount = -1;
+    private final Set<String> lastPlayerNames = new HashSet<>();
+
+    public ServerBot(JDA jda, String serverIp, long channelId, ServerStatusFetcher serverStatusFetcher) {
+        this.jda = jda;
+        this.serverIp = serverIp;
+        this.channelId = channelId;
+        this.serverStatusFetcher = serverStatusFetcher;
+    }
+
+    public String getServerIp() {
+        return serverIp;
+    }
+
+    public static Collection<ServerBot> read(Path jsonConfigFile) throws Exception {
+        if (Files.notExists(jsonConfigFile)) throw new IOException("File " + jsonConfigFile + " doesn't exist.");
+        ObjectMapper mapper = new ObjectMapper();
+        ObjectNode configData;
+        try (var in = Files.newInputStream(jsonConfigFile)) {
+            configData = mapper.readValue(in, ObjectNode.class);
+        }
+
+        ArrayNode serversArray = configData.withArray("servers");
+        List<ServerBot> bots = new ArrayList<>(serversArray.size());
+        ServerStatusFetcher serverStatusFetcher = new ServerStatusFetcher();
+        for (JsonNode node : serversArray) {
+            String token = node.get("discord-token").asText();
+            String serverIp = node.get("server-ip").asText();
+            long channelId = node.get("channel-id").asLong();
+            JDABuilder builder = JDABuilder.create(token, Collections.emptyList());
+            builder.disableCache(CacheFlag.ACTIVITY, CacheFlag.VOICE_STATE);
+            JDA jda = builder.build();
+            bots.add(new ServerBot(jda, serverIp, channelId, serverStatusFetcher));
+        }
+        return bots;
+    }
+
+    @Override
+    public void run() {
+        try {
+            jda.awaitReady();
+        } catch (InterruptedException e) {
+            throw new RuntimeException("Interrupted while awaiting ready status.", e);
+        }
+        while (true) {
+            try {
+                ServerStatus status = serverStatusFetcher.fetch(serverIp);
+                displayStatus(status);
+            } catch (IOException e) {
+                handleError(e);
+            }
+            try {
+                Thread.sleep(10000);
+            } catch (InterruptedException e) {
+                e.printStackTrace(System.err);
+                Thread.currentThread().interrupt();
+            }
+        }
+    }
+
+    private void displayStatus(ServerStatus status) {
+        String text = String.format("%d/%d players", status.playersOnline(), status.maxPlayers());
+        OnlineStatus onlineStatus = jda.getPresence().getStatus();
+        OnlineStatus newOnlineStatus = status.playersOnline() > 0 ? OnlineStatus.ONLINE : OnlineStatus.IDLE;
+        Activity activity = jda.getPresence().getActivity();
+        Activity newActivity = Activity.customStatus(text);
+
+        boolean shouldUpdate = onlineStatus != newOnlineStatus ||
+                activity == null ||
+                !activity.getName().equals(newActivity.getName());
+        if (shouldUpdate) {
+            jda.getPresence().setPresence(newOnlineStatus, newActivity);
+        }
+
+        if (lastPlayerCount != -1 && lastPlayerCount != status.playersOnline()) {
+            final TextChannel channel = jda.getTextChannelById(channelId);
+            for (String name : getPlayersJoined(status.playerNames())) {
+                channel.sendMessage(name + " joined the server.").queue();
+            }
+            for (String name : getPlayersLeft(status.playerNames())) {
+                channel.sendMessage(name + " left the server.").queue();
+            }
+            lastPlayerCount = status.playersOnline();
+            lastPlayerNames.clear();
+            lastPlayerNames.addAll(status.playerNames());
+        }
+    }
+
+    private Set<String> getPlayersJoined(Set<String> current) {
+        Set<String> set = new HashSet<>(current);
+        set.removeAll(lastPlayerNames);
+        return set;
+    }
+
+    private Set<String> getPlayersLeft(Set<String> current) {
+        Set<String> set = new HashSet<>(lastPlayerNames);
+        set.removeAll(current);
+        return set;
+    }
+
+    private void handleError(IOException e) {
+        if (jda.getPresence().getStatus() != OnlineStatus.DO_NOT_DISTURB) {
+            jda.getPresence().setStatus(OnlineStatus.DO_NOT_DISTURB);
+            jda.getPresence().setActivity(Activity.customStatus("Error."));
+        }
+        e.printStackTrace(System.err);
+        lastPlayerCount = -1;
+        lastPlayerNames.clear();
+    }
+}

src/main/java/com/andrewlalis/mc_status_bot/ServerStatus.java

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+package com.andrewlalis.mc_status_bot;
+
+import java.util.Set;
+
+public record ServerStatus(
+        int playersOnline,
+        int maxPlayers,
+        Set<String> playerNames
+) {}

src/main/java/com/andrewlalis/mc_status_bot/ServerStatusFetcher.java

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+package com.andrewlalis.mc_status_bot;
+
+import com.fasterxml.jackson.databind.JsonNode;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.ObjectNode;
+
+import java.io.IOException;
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.time.Duration;
+import java.util.HashSet;
+import java.util.Set;
+import java.util.concurrent.Executors;
+
+public class ServerStatusFetcher {
+    private final HttpClient client = HttpClient.newBuilder()
+            .connectTimeout(Duration.ofSeconds(3))
+            .executor(Executors.newVirtualThreadPerTaskExecutor())
+            .build();
+    private final ObjectMapper objectMapper = new ObjectMapper();
+
+    public ServerStatus fetch(String ip) throws IOException {
+        HttpRequest request = HttpRequest.newBuilder(URI.create("https://api.mcsrvstat.us/3/" + ip))
+                .GET()
+                .timeout(Duration.ofSeconds(5))
+                .build();
+        try {
+            HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
+            if (response.statusCode() != 200) throw new IOException("Non-200 status code: " + response.statusCode());
+            ObjectNode data = objectMapper.readValue(response.body(), ObjectNode.class);
+            Set<String> playerNames = new HashSet<>();
+            ArrayNode playersArray = data.get("players").withArray("list");
+            for (JsonNode node : playersArray) {
+                playerNames.add(node.get("name").asText());
+            }
+            return new ServerStatus(
+                    data.get("players").get("online").asInt(),
+                    data.get("players").get("max").asInt(),
+                    playerNames
+            );
+        } catch (IOException | InterruptedException e) {
+            throw new IOException("Failed to get server status.", e);
+        }
+    }
+}