performance2JvmFlagsGuide

1package pkg19performance;2 3/*4 * performance2JvmFlagsGuide.java5 * ------------------------------6 * Essential JVM flags for performance tuning and diagnostics.7 *8 * DEFINITION:9 *   JVM flags control heap size, GC algorithm, logging, and diagnostics.10 *   Architects choose flags based on workload: latency vs throughput vs memory.11 */12public class performance2JvmFlagsGuide {13 14    public static void main(String[] args) {15        System.out.println("=== Heap ===");16        System.out.println("-Xms512m -Xmx2g          initial / max heap");17        System.out.println("-XX:MaxMetaspaceSize=256m class metadata cap");18 19        System.out.println("\n=== GC selection (pick ONE collector) ===");20        System.out.println("-XX:+UseG1GC              G1 (default on most JDKs, balanced)");21        System.out.println("-XX:+UseZGC               ZGC (low latency, large heaps)");22        System.out.println("-XX:+UseShenandoahGC      Shenandoah (low pause, concurrent)");23        System.out.println("-XX:+UseParallelGC         throughput-oriented");24 25        System.out.println("\n=== GC / JVM logging (Java 9+) ===");26        System.out.println("-Xlog:gc:stdout            GC events to console");27        System.out.println("-Xlog:gc*:file=gc.log      detailed GC log file");28 29        System.out.println("\n=== Diagnostics ===");30        System.out.println("-XX:+HeapDumpOnOutOfMemoryError");31        System.out.println("-XX:HeapDumpPath=./dumps   heap dump on OOM");32        System.out.println("-XX:StartFlightRecording=duration=60s,filename=app.jfr");33 34        System.out.println("\n=== Current runtime ===");35        Runtime rt = Runtime.getRuntime();36        System.out.println("Processors : " + rt.availableProcessors());37        System.out.println("Max heap   : " + rt.maxMemory() / 1024 / 1024 + " MB");38        System.out.println("Free heap  : " + rt.freeMemory() / 1024 / 1024 + " MB");39    }40}