Calculate size of Object in Java

This question already has an answer here:

In Java, what is the best way to determine the size of an object? 24 answers

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You can use the java.lang.instrumentation package:

http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/Instrumentation.html

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.

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);
    }
}

Use getObjectSize:

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

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

source for example:

In Java, what is the best way to determine the size of an object?

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看看https://github.com/DimitrisAndreou/memory-measurer Guava在内部使用它，而ObjectGraphMeasurer特别直接使用开箱即用，没有任何特殊的命令行参数。

import objectexplorer.ObjectGraphMeasurer;

public class Measurer {

  public static void main(String[] args) {
    Set<Integer> hashset = new HashSet<Integer>();
    Random random = new Random();
    int n = 10000;
    for (int i = 1; i <= n; i++) {
      hashset.add(random.nextInt());
    }
    System.out.println(ObjectGraphMeasurer.measure(hashset));
  }
}

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The java.lang.instrument.Instrumentation class provides a nice way to get the size of a Java Object, but it requires you to define a premain and run your program with a java agent. This is very boring when you do not need any agent and then you have to provide a dummy Jar agent to your application.

So I got an alternative solution using the Unsafe class from the sun.misc . So, considering the objects heap alignment according to the processor architecture and calculating the maximum field offset, you can measure the size of a Java Object. In the example below I use an auxiliary class UtilUnsafe to get a reference to the sun.misc.Unsafe object.

private static final int NR_BITS = Integer.valueOf(System.getProperty("sun.arch.data.model"));
private static final int BYTE = 8;
private static final int WORD = NR_BITS/BYTE;
private static final int MIN_SIZE = 16; 

public static int sizeOf(Class src){
    //
    // Get the instance fields of src class
    // 
    List<Field> instanceFields = new LinkedList<Field>();
    do{
        if(src == Object.class) return MIN_SIZE;
        for (Field f : src.getDeclaredFields()) {
            if((f.getModifiers() & Modifier.STATIC) == 0){
                instanceFields.add(f);
            }
        }
        src = src.getSuperclass();
    }while(instanceFields.isEmpty());
    //
    // Get the field with the maximum offset
    //  
    long maxOffset = 0;
    for (Field f : instanceFields) {
        long offset = UtilUnsafe.UNSAFE.objectFieldOffset(f);
        if(offset > maxOffset) maxOffset = offset; 
    }
    return  (((int)maxOffset/WORD) + 1)*WORD; 
}
class UtilUnsafe {
    public static final sun.misc.Unsafe UNSAFE;

    static {
        Object theUnsafe = null;
        Exception exception = null;
        try {
            Class<?> uc = Class.forName("sun.misc.Unsafe");
            Field f = uc.getDeclaredField("theUnsafe");
            f.setAccessible(true);
            theUnsafe = f.get(uc);
        } catch (Exception e) { exception = e; }
        UNSAFE = (sun.misc.Unsafe) theUnsafe;
        if (UNSAFE == null) throw new Error("Could not obtain access to sun.misc.Unsafe", exception);
    }
    private UtilUnsafe() { }
}

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