JAVA如何查看HttpRequest和HttpResponse占用的内存大小？

Question

如题，“如何输出HttpRequest和HttpResponse对象占内存的大小”？
网上说，利用java.lang.Instrumentation的API来计算，我调用了提供的一个工具类，但是报NullPointerException异常：

进一步分析，应该是实例化Instrumentation的premain()方法没有被JVM调用。
应该通过“ -javaagent”命令来调用，不知道应该如何调。

Answer 1

Compile the following class and put it in ObjectSizeFetcherAgent.jar

import java.lang.instrument.Instrumentation;
public class ObjectSizeFetcher {
    private static Instrumentation instrumentation;
    public static void premain(String args, Instrumentation inst) {
        instrumentation = inst;
    }
    public static long getObjectSize(Object o) {
        return instrumentation.getObjectSize(o);
    }
}

Add to MANIFEST.MF:

Premain-Class: ObjectSizeFetcher

Then write a test class

public class C {
    private int x;
    private int y;

    public static void main(String [] args) {
        System.out.println(ObjectSizeFetcher.getObjectSize(new C()));
    }
}

Execute like this in the command line:

java -javaagent:ObjectSizeFetcherAgent.jar C