JVM Tuning and Tools

Common JVM Flags

Flag	Meaning
`-Xms1g -Xmx1g`	initial and max heap (same value avoids resizing)
`-XX:MaxRAMPercentage=75`	heap as % of container memory (Docker/Kubernetes)
`-Xss512k`	thread stack size
`-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/dumps`	write a heap dump when OOM happens
`-Xlog:gc*:file=gc.log`	GC log to a file
`-XX:+UseZGC`	choose a collector

JDK Tools

Tool	Use
`jps`	list running Java processes
`jcmd <pid> GC.heap_info`, `Thread.print`, `GC.heap_dump`	Swiss-army knife
`jstack <pid>`	thread dump (find deadlocks, stuck threads)
`jstat -gc <pid> 1000`	GC statistics every second
JFR + JDK Mission Control	low-overhead profiling in production
VisualVM, Eclipse MAT	heap dump analysis (find leaks)

Example

jcmd 4321 GC.heap_info

Example output

4321:
 garbage-first heap   total 524288K, used 187340K
  region size 1024K, 52 young (53248K), 6 survivors (6144K)
 Metaspace       used 61234K, committed 62016K, reserved 1114112K

Reading Runtime Info in Code

Example

Runtime and ManagementFactory expose JVM details. (Exact memory numbers vary by machine, so this prints only stable facts.)

import java.lang.management.*;

public class Main {
    public static void main(String[] args) {
        Runtime rt = Runtime.getRuntime();
        System.out.println("Java version: " + Runtime.version().feature());
        System.out.println("Processors > 0: " + (rt.availableProcessors() > 0));
        System.out.println("max >= total >= free: " + (rt.maxMemory() >= rt.totalMemory() && rt.totalMemory() >= rt.freeMemory()));

        RuntimeMXBean jvm = ManagementFactory.getRuntimeMXBean();
        System.out.println("VM vendor present: " + !jvm.getVmVendor().isEmpty());
        ThreadMXBean threads = ManagementFactory.getThreadMXBean();
        System.out.println("Live threads >= 1: " + (threads.getThreadCount() >= 1));
    }
}

Output

Java version: 21
Processors > 0: true
max >= total >= free: true
VM vendor present: true
Live threads >= 1: true