As we already know, to retrieve the size of a primitive data types such as int, double,… in java is easy, Just call Integer.BYTE, Double.BYTE,… is finished. But if you want to get the size of an object of a class that is not easy.

There are several ways to get the size of an object, in this article I'll show you how to build a class to own property. Said the construction of this question but what people write and, we just copied and used only, without interest or impact on it at all.

Note added something that we normally would think the size of an object is equal to the total size of its attributes, but the reality is a bit different. In addition to the total size of the property, the size of the object:
Plus 8 bytes of the header.
It is always a multiple of 8, otherwise be rounded up to the right is a multiple 8.

This is the class size calculation objects.

package cachhoc.net.sizeobject;

import java.lang.reflect.Field;
import java.lang.reflect.Modifier;
import java.util.HashSet;
import java.util.Set;

public class ObjectSizeCalculator {

	private static final int REFERENCE_SIZE;
	private static final int HEADER_SIZE;
	private static final int LONG_SIZE = 8;
	private static final int INT_SIZE = 4;
	private static final int BYTE_SIZE = 1;
	private static final int BOOLEAN_SIZE = 1;
	private static final int CHAR_SIZE = 2;
	private static final int SHORT_SIZE = 2;
	private static final int FLOAT_SIZE = 4;
	private static final int DOUBLE_SIZE = 8;
	private static final int ALIGNMENT = 8;

	static {
		try {
			if (System.getProperties().get("java.vm.name").toString()
					.contains("64")) {
				// java.vm.name is something like
				// "Java HotSpot(TM) 64-Bit Server VM"
				REFERENCE_SIZE = 8;
				HEADER_SIZE = 16;
			} else {
				REFERENCE_SIZE = 4;
				HEADER_SIZE = 8;
			}
		} catch (Exception ex) {
			ex.printStackTrace();
			throw new AssertionError(ex);
		}
	}

	public static long sizeOf(Object o) throws IllegalAccessException {
		return sizeOf(o, new HashSet());
	}

	private static long sizeOf(Object o, Set visited)
			throws IllegalAccessException {
		if (o == null) {
			return 0;
		}
		ObjectWrapper objectWrapper = new ObjectWrapper(o);
		if (visited.contains(objectWrapper)) {
			// We have reference graph with cycles.
			return 0;
		}
		visited.add(objectWrapper);
		long size = HEADER_SIZE;
		Class clazz = o.getClass();
		if (clazz.isArray()) {
			if (clazz == long[].class) {
				long[] objs = (long[]) o;
				size += objs.length * LONG_SIZE;
			} else if (clazz == int[].class) {
				int[] objs = (int[]) o;
				size += objs.length * INT_SIZE;
			} else if (clazz == byte[].class) {
				byte[] objs = (byte[]) o;
				size += objs.length * BYTE_SIZE;
			} else if (clazz == boolean[].class) {
				boolean[] objs = (boolean[]) o;
				size += objs.length * BOOLEAN_SIZE;
			} else if (clazz == char[].class) {
				char[] objs = (char[]) o;
				size += objs.length * CHAR_SIZE;
			} else if (clazz == short[].class) {
				short[] objs = (short[]) o;
				size += objs.length * SHORT_SIZE;
			} else if (clazz == float[].class) {
				float[] objs = (float[]) o;
				size += objs.length * FLOAT_SIZE;
			} else if (clazz == double[].class) {
				double[] objs = (double[]) o;
				size += objs.length * DOUBLE_SIZE;
			} else {
				Object[] objs = (Object[]) o;
				for (int i = 0; i < objs.length; i++) {
					size += sizeOf(objs[i], visited) + REFERENCE_SIZE;
				}
			}
			size += INT_SIZE;
		} else {
			Field[] fields = o.getClass().getDeclaredFields();
			for (int i = 0; i < fields.length; i++) {
				if (Modifier.isStatic(fields[i].getModifiers())) {
					continue;
				}
				fields[i].setAccessible(true);
				String fieldType = fields[i].getGenericType().toString();
				if (fieldType.equals("long")) {
					size += LONG_SIZE;
				} else if (fieldType.equals("int")) {
					size += INT_SIZE;
				} else if (fieldType.equals("byte")) {
					size += BYTE_SIZE;
				} else if (fieldType.equals("boolean")) {
					size += BOOLEAN_SIZE;
				} else if (fieldType.equals("char")) {
					size += CHAR_SIZE;
				} else if (fieldType.equals("short")) {
					size += SHORT_SIZE;
				} else if (fieldType.equals("float")) {
					size += FLOAT_SIZE;
				} else if (fieldType.equals("double")) {
					size += DOUBLE_SIZE;
				} else {
					size += sizeOf(fields[i].get(o), visited) + REFERENCE_SIZE;
				}
			}
		}
		if ((size % ALIGNMENT) != 0) {
			size = ALIGNMENT * (size / ALIGNMENT) + ALIGNMENT;
		}
		return size;
	}

	private static final class ObjectWrapper {

		private Object object;

		public ObjectWrapper(Object object) {
			this.object = object;
		}

		public boolean equals(Object obj) {
			if (obj == this) {
				return true;
			}
			if ((obj == null) || (obj.getClass() != ObjectWrapper.class)) {
				return false;
			}
			return object == ((ObjectWrapper) obj).object;
		}

		public int hashCode() {
			int hash = 3;
			hash = 47 * hash + System.identityHashCode(object);
			return hash;
		}
	}
}

We ran a test object size some:

package cachhoc.net.sizeobject;

import java.util.HashMap;

public class SizeObject {
	// 8 byte header
	int x; // 4 byte
	double y; // 8 byte
	String s; // 0 byte vì s đang null

	public static void main(String[] args) {
		SizeObject so1 = new SizeObject();
		SizeObject so2 = new SizeObject();
		SizeObject so3 = new SizeObject();
		so2.s = "nguyenvanquan";
		so3.s = "nguyenvanquan7826";

		try {

			System.out.println("size of int Object = "
					+ ObjectSizeCalculator.sizeOf(new Integer(5)));
			System.out.println("size of array int[3] = "
					+ ObjectSizeCalculator.sizeOf(new int[] { 1, 2, 3 }));
			System.out.println("size of HashMap = "
					+ ObjectSizeCalculator.sizeOf(new HashMap(512)));

			System.out.println("size of String\"nguyenvanquan7826\" = "
					+ ObjectSizeCalculator.sizeOf(new String(
							"nguyenvanquan7826")));

			System.out.println("size of Object so1 = "
					+ ObjectSizeCalculator.sizeOf(so1));
			System.out.println("size of Object so2 = "
					+ ObjectSizeCalculator.sizeOf(so2));
			System.out.println("size of Object so3 = "
					+ ObjectSizeCalculator.sizeOf(so3));

		} catch (IllegalAccessException e) {
			e.printStackTrace();
		}
	}
}

The result will be:

size of int Object = 16
size of array int[3] = 24
size of HashMap = 32
size of String"nguyenvanquan7826" = 64
size of Object so1 = 24
size of Object so2 = 80
size of Object so3 = 88

Reference Articles in: kyryloholodnov.wordpress.com