Memory Overhead of Java Objects

First, each Java object has two implicit references: one to its monitor (lock), the other to its method dispatch table. Each reference occupies 4 bytes, so that is 8 bytes overhead.

Second, byte alignment needs to be taken into consideration. On a 32-bit machine, object needs to be aligned at 4-byte boundary. On a 64-bit machine, object needs to be aligned at 8-byte boundary. Nowadays, 64-bit machines become popular. For instance, my laptop has a 64-bit Intel Core 2 Duo CPU T7500.

So on my laptop if I create an object that has only one byte field, then the actual size of the object will be 16 bytes. That is 93.75% overhead. If I create an object that has three int fields, then the actual size of the object will be 24 bytes. That is 50% overhead.

So be very careful when creating a huge number of small objects, because significant extra memory will be required for object overhead.

How to measure the size of an object

Write a simple program consisting of an infinite loop. Inside the loop, create the object whose size is to be measured. Then use "jmap -histo pid" to measure the size. pid is the process id of the Java program. Because the loop is infinite, it gives plenty of time for jmap to connect to the Java process.