GcMemDemo.java

import java.util.*;

/**
 * A demo Java application to see how GC works.
 * 
 * @author Thanh Nguyen
 */
public class GcMemDemo {
    static Map<Long, byte[]> buffer = new HashMap<>();

    static String format(double bytes, int digits) {
        String[] dictionary = { "bytes", "KB", "MB" };
        int index = 0;
        for (index = 0; index < dictionary.length-1; index++) {
            if (bytes < 1024) {
                break;
            }
            bytes = bytes / 1024;
        }
        return String.format("%." + digits + "f", bytes) + " " + dictionary[index];
    }

    static void printMemInfo() {
        Runtime runtime = Runtime.getRuntime();
        System.out.println("Memory info:");
        long total = runtime.totalMemory();
        long free = runtime.freeMemory();
        long freePercent = (free*100/total);
        System.out.println("\tUsed : " + buffer.size() + " - " + format(buffer.size()*1024*1024, 2));
        System.out.println("\tMax  : " + format(runtime.maxMemory(), 2));
        System.out.println("\tTotal: " + format(total, 2));
        System.out.println("\tFree : " + format(free, 2) + " ("+freePercent+"%)");
    }

    static void printMemInfo(long beforeTotal, long beforeFree) {
        Runtime runtime = Runtime.getRuntime();
        long beforeFreePersent = (beforeFree*100/beforeTotal);
        long afterTotal = runtime.totalMemory();
        long afterFree = runtime.freeMemory();
        long afterFreePersent = (afterFree*100/afterTotal);
        System.out.println("Memory info:");
        System.out.println("\tUsed : " + buffer.size() + " - " + format(buffer.size()*1024*1024, 2));
        System.out.println("\tMax  : " + format(runtime.maxMemory(), 2));
        System.out.println("\tTotal: " + format(beforeTotal, 2) + " --> " + format(afterTotal, 2));
        System.out.println("\tFree : " + format(beforeFree, 2) + " ("+beforeFreePersent+"%) --> " + format(afterFree, 2) + " ("+afterFreePersent+"%)");
    }

    static void printMenu() {
        System.out.println("========================================");
        System.out.println("1. Print memory info");        
        System.out.println("2. Allocate 100Mb memory");
        System.out.println("3. Dispose all allocated memory");
        System.out.println("4. Dispose and System.gc()");
        System.out.println("Q. Quit");
        System.out.println();
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        boolean done  = false;
        String input = "1";
        while (!done) {
            printMenu();
            System.out.print("input> ");
            input = scanner.nextLine();

            if ("Q".equalsIgnoreCase(input)) {
                done = true;
            } else if ("1".equals(input)) {
                printMemInfo();
            } else if ("2".equals(input)) {
                Runtime runtime = Runtime.getRuntime();
                long beforeTotal = runtime.totalMemory();
                long beforeFree = runtime.freeMemory();
                long index = System.nanoTime();
                for ( int i = 0; i < 100; i++) {
                    buffer.put(index+i, new byte[1024*1024]);
                }
                printMemInfo(beforeTotal, beforeFree);
            } else if ("3".equals(input)) {
                Runtime runtime = Runtime.getRuntime();
                long beforeTotal = runtime.totalMemory();
                long beforeFree = runtime.freeMemory();
                buffer = new HashMap<>();
                printMemInfo(beforeTotal, beforeFree);
            } else if ("4".equals(input)) {
                Runtime runtime = Runtime.getRuntime();
                long beforeTotal = runtime.totalMemory();
                long beforeFree = runtime.freeMemory();
                buffer = new HashMap<>();
                System.gc();
                printMemInfo(beforeTotal, beforeFree);
            }
        }
    }
}

run_g1gc.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseG1GC -Xms16m -Xmx1234m GcMemDemo

run_g1gc_small.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseG1GC -Xms16m -Xmx1234m -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:-ShrinkHeapInSteps GcMemDemo

run_parallelgc.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseParallelGC -Xms16m -Xmx1234m GcMemDemo

run_parallelgc_small.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseParallelGC -Xms16m -Xmx1234m -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:-ShrinkHeapInSteps GcMemDemo

run_serialgc.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseSerialGC -Xms16m -Xmx1234m GcMemDemo

run_serialgc_small.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseSerialGC -Xms16m -Xmx1234m -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:-ShrinkHeapInSteps GcMemDemo

run_shenandoahgc.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseShenandoahGC -Xms16m -Xmx1234m GcMemDemo

run_shenandoahgc_small.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UseShenandoahGC -Xms16m -Xmx1234m -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:-ShrinkHeapInSteps GcMemDemo

run_zgc.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -Xms16m -Xmx1234m GcMemDemo

run_zgc_small.sh

#!/bin/sh

rm *.class
java -version
javac GcMemDemo.java
java -server -XX:+ExitOnOutOfMemoryError -XX:+CrashOnOutOfMemoryError -XX:+UnlockExperimentalVMOptions -XX:+UseZGC -Xms16m -Xmx1234m -XX:MinHeapFreeRatio=5 -XX:MaxHeapFreeRatio=10 -XX:-ShrinkHeapInSteps GcMemDemo