用Java计算对象的大小 [英] Calculate size of Object in Java

问题描述

我想记录一个对象为一个项目占用了多少内存(希望以字节为单位)(我正在比较数据结构的大小)，似乎在 Java 中没有方法可以做到这一点.据说，C/C++ 有 sizeOf() 方法，但这在 Java 中是不存在的.我尝试在创建对象之前和之后使用 Runtime.getRuntime().freeMemory() 记录 JVM 中的空闲内存，然后记录差异，但它只会给出 0 或 131304，而在之间，而不考虑结构中的元素数量.请帮忙！

I want to record how much memory (in bytes, hopefully) an object takes up for a project (I'm comparing sizes of data structures) and it seems like there is no method to do this in Java. Supposedly, C/C++ has sizeOf() method, but this is nonexistant in Java. I tried recording the free memory in the JVM with Runtime.getRuntime().freeMemory() before and after creating the object and then recording the difference, but it would only give 0 or 131304, and nothing in between, regardless of the number of elements in the structure. Help please!

推荐答案

您可以使用 java.lang.instrumentation 包.

You can use the java.lang.instrumentation package.

它有一个方法可用于获取对象大小的实现特定近似值，以及与对象相关的开销.

It has a method that can be used to get the implementation specific approximation of object size, as well as overhead associated with the object.

谢尔盖链接的答案有一个很好的例子，我会在这里重新发布，但你应该已经从他的评论中看到了:

The answer that Sergey linked has a great example, which I'll repost here, but you should have already looked at from his comment:

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);
    }
}

使用getObjectSize:

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

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

来源

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