如何做JVM内存占用测试

代码：

import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.lang.reflect.Method;
import java.nio.ByteBuffer;

/**
 * 每个方法的参数m都是表示对应区间分配多少M内存
 * @author afei
 * @date 2018-09-28
 * @since 1.0.0
 */
public class MemoryTest {
    private static final int _1m = 1024*1024;

    private static final long THREAD_SLEEP_MS = 10*1000;

    public static void main(String[] args) throws Exception{
        youngAllocate(1000);
        oldAllocate(1000);
        metaspaceAllocate(200000);
        directMemoryAllocate(400);
        // threadStackAllocate(400);
        Thread.sleep(60000);
    }

    /**
     * @param count 重复定义的MyCalc对象数量
     */
    private static void metaspaceAllocate(int count) throws Exception {
        System.out.println("metaspace object count: " + count);

        Method declaredMethod = ClassLoader.class.getDeclaredMethod("defineClass",
                new Class[]{String.class, byte[].class, int.class, int.class});
        declaredMethod.setAccessible(true);

        File classFile = new File("/app/afei/MyCalc.class");
        byte[] bcs = new byte[(int) classFile.length()];
        try(InputStream is = new FileInputStream(classFile);){
            // 将文件流读进byte数组
            while (is.read(bcs)!=-1){
            }
        }

        int outputCount = count/10;
        for (int i=1; i<=count; i++){
            try {
                // 重复定义MyCalc这个类
                declaredMethod.invoke(
                        MemoryTest.class.getClassLoader(),
                        new Object[]{"MyCalc", bcs, 0, bcs.length});
            }catch (Throwable e){
                // 重复定义类会抛出LinkageError: attempted  duplicate class definition for name: "MyCalc"
                // System.err.println(e.getCause().getLocalizedMessage());
            }
            if (i>=outputCount && i%outputCount==0){
                System.out.println("i = "+i);
            }
        }
        System.out.println("metaspace end");
    }

    /**
     * @param m 分配多少M direct memory
     */
    private static void directMemoryAllocate(int m){
        System.out.println("direct memory: "+m+"m");
        for (int i = 0; i < m; i++) {
            ByteBuffer.allocateDirect(_1m);
        }
        System.out.println("direct memory end");
    }

    /**
     * @param m 给young区分配多少M的数据
     */
    private static void youngAllocate(int m){
        System.out.println("young: "+m+"m");
        for (int i = 0; i < m; i++) {
            byte[] test = new byte[_1m];
        }
        System.out.println("young end");
    }

    /**
     * 需要配置参数: -XX:PretenureSizeThreshold=2M, 并且结合CMS
     * @param m 给old区分配多少M的数据
     */
    private static void oldAllocate(int m){
        System.out.println("old:   "+m+"m");
        for (int i = 0; i < m/5; i++) {
            byte[] test = new byte[5*_1m];
        }
        System.out.println("old end");
    }

    // 需要配置参数: -Xss10240k, 这里的实验以失败告终
    private static void threadStackAllocate(int m){
        int threadCount = m/10;
        System.out.println("thread stack count:"+threadCount);
        for (int i = 0; i < threadCount; i++) {
            new Thread(() -> {
                System.out.println("thread name: " + Thread.currentThread().getName());
                try {
                    while(true) {
                        Thread.sleep(THREAD_SLEEP_MS);
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();
        }
        System.out.println("thread stack end:"+threadCount);
    }
}

步骤参考：https://www.jianshu.com/p/0cbc4e44c596