package com.ankurm.flagsdemo;

import com.sun.management.HotSpotDiagnosticMXBean;
import com.sun.management.OperatingSystemMXBean;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

import java.lang.management.GarbageCollectorMXBean;
import java.lang.management.ManagementFactory;
import java.lang.management.MemoryMXBean;
import java.lang.management.MemoryUsage;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

/**
 * Prints what the JVM actually resolved its container-relevant flags to, at
 * runtime, inside whatever cgroup it is running under. This is the live
 * version of what docs/output/*.txt captures from -XX:+PrintFlagsFinal; the
 * two should agree, and the post at ankurm.com cross-checks both.
 *
 * <p>Explained in the post:
 * https://ankurm.com/jvm-flags-for-spring-boot-in-containers-jdk-25-27/
 *
 * <p><strong>Delete this endpoint before shipping to a real service.</strong>
 * It leaks heap sizing and cgroup limits to anyone who can reach it.
 */
@RestController
public class JvmDiagnosticsController {

    private static final List<String> FLAGS_OF_INTEREST = List.of(
            "MaxRAMPercentage", "MinRAMPercentage", "InitialRAMPercentage",
            "MaxRAM", "MaxHeapSize", "InitialHeapSize", "MinHeapSize",
            "UseContainerSupport", "ActiveProcessorCount",
            "UseG1GC", "UseParallelGC", "UseZGC", "UseSerialGC",
            "UseCompactObjectHeaders", "ParallelGCThreads", "ConcGCThreads"
    );

    @GetMapping("/internal/jvm-flags")
    public Map<String, Object> jvmFlags() {
        Map<String, Object> out = new LinkedHashMap<>();

        OperatingSystemMXBean os =
                (OperatingSystemMXBean) ManagementFactory.getOperatingSystemMXBean();
        Map<String, Object> hostView = new LinkedHashMap<>();
        hostView.put("availableProcessors", Runtime.getRuntime().availableProcessors());
        hostView.put("totalMemorySizeBytes", os.getTotalMemorySize());
        hostView.put("freeMemorySizeBytes", os.getFreeMemorySize());
        out.put("osMXBeanView", hostView);

        MemoryMXBean mem = ManagementFactory.getMemoryMXBean();
        MemoryUsage heap = mem.getHeapMemoryUsage();
        Map<String, Object> heapView = new LinkedHashMap<>();
        heapView.put("initBytes", heap.getInit());
        heapView.put("usedBytes", heap.getUsed());
        heapView.put("committedBytes", heap.getCommitted());
        heapView.put("maxBytes", heap.getMax());
        out.put("heap", heapView);

        List<String> gcNames = ManagementFactory.getGarbageCollectorMXBeans()
                .stream().map(GarbageCollectorMXBean::getName).toList();
        out.put("garbageCollectors", gcNames);

        HotSpotDiagnosticMXBean diag =
                ManagementFactory.getPlatformMXBean(HotSpotDiagnosticMXBean.class);
        Map<String, String> flagView = new LinkedHashMap<>();
        for (String name : FLAGS_OF_INTEREST) {
            try {
                flagView.put(name, diag.getVMOption(name).getValue());
            } catch (IllegalArgumentException notThisJvm) {
                flagView.put(name, "N/A on this build");
            }
        }
        out.put("resolvedFlags", flagView);

        out.put("jvmInputArguments",
                ManagementFactory.getRuntimeMXBean().getInputArguments());
        out.put("javaVersion", System.getProperty("java.version"));
        out.put("javaVmName", System.getProperty("java.vm.name"));

        return out;
    }
}