Setting JAVA_OPTS to set Heap Memory arg

Dockerfile

FROM openjdk:11
COPY . /usr/src/myapp
WORKDIR /usr/src/myapp
RUN javac HeapMemoryHttpServer.java
ENV JAVA_OPTS=""
CMD java $JAVA_OPTS -cp .  HeapMemoryHttpServer

heapmeminfo.hcl

job "example" {

  group "java" {
    network {
      port "db" {
        to     = 8080
      }
    }

    task "app" {
      driver = "docker"

      config {
        image          = "ranjandas/heapmeminfo"
        ports          = ["db"]
        auth_soft_fail = true
      }

      identity {
        env  = true
        file = true
      }

      env {
        JAVA_OPTS = "-Xms50M -Xmx50M"
      }

      resources {
        cpu    = 500
        memory = 2048
      }
    }
  }
}

HeapMemoryHttpServer.java

import com.sun.net.httpserver.HttpServer;
import com.sun.net.httpserver.HttpHandler;
import com.sun.net.httpserver.HttpExchange;

import java.io.IOException;
import java.io.OutputStream;
import java.net.InetSocketAddress;

public class HeapMemoryHttpServer {

    public static void main(String[] args) throws IOException {
        // Set up the HTTP server on port 8080
        HttpServer server = HttpServer.create(new InetSocketAddress(8080), 0);
        server.createContext("/memory", new MemoryHandler());
        server.setExecutor(null); // Default executor
        server.start();

        System.out.println("Server started on port 8080");
    }

    // Handler that processes HTTP requests
    static class MemoryHandler implements HttpHandler {
        @Override
        public void handle(HttpExchange exchange) throws IOException {
            // Get the runtime instance
            Runtime runtime = Runtime.getRuntime();

            // Get initial heap size (Xms) in bytes and convert to megabytes
            long xms = runtime.totalMemory();

            // Get maximum heap size (Xmx) in bytes and convert to megabytes
            long xmx = runtime.maxMemory();

            // Convert bytes to megabytes
            long xmsInMb = xms / (1024 * 1024);
            long xmxInMb = xmx / (1024 * 1024);

            // Prepare response string
            String response = "Initial Heap Size (Xms): " + xmsInMb + " MB\n" +
                              "Maximum Heap Size (Xmx): " + xmxInMb + " MB\n";

            // Print memory info to stdout
            System.out.println(response);

            // Respond over HTTP
            exchange.sendResponseHeaders(200, response.getBytes().length);
            OutputStream os = exchange.getResponseBody();
            os.write(response.getBytes());
            os.close();
        }
    }
}