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For the question of how much space the Java object occupies in memory, this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible method.

Note that this method of calculation applies to both OpenJDK and Oracle JDK versions, and other versions may be different.

On the whole, the java object consists of three parts: object header, instance data, and alignment padding. Alignment padding means that if the occupied space of the object header and the instance data is not an integral multiple of 8, you need to add pad fill until the total occupied space is a multiple of 8. The integer multiple of 8 is temporarily mentioned here, because the 64-bit virtual machine in the lucene source code is dynamically acquired (the specific reason is not clear for the time being, if, as mentioned on the Internet and books, it is not necessary to dynamically obtain the integer multiple of 8, it is better to believe that books are better than no books. Some things are better not to come to a conclusion before seeing the source code), 32 bits are fixed 8 bytes. The size of an object reference is only 4 bytes in a 64-bit jvm if the pointer is compressed to 4 bytes or 8 bytes in 32-bit jvm.

The object header size of the normal object is the size of the object reference plus 8 bytes, and the object header of the array is equal to the size of the normal object header plus the sum of 4 bytes and is aligned according to 8 bytes.

The space occupied by the byte array = the object header of the array + the sum of the number of 1 * arrays and aligned according to 8 bytes

The boolean array occupies the same space as the byte array

The occupied space of the char array = the object head of the array + the sum of the number of 2 * arrays and aligned according to 8 bytes

The occupied space of short array is the same as that of char array.

The occupied space of the int array = the object head of the array + the sum of the number of 4 * arrays and aligned according to 8 bytes

The occupied space of float array is the same as that of int array.

The occupied space of the long array = the object head of the array + the sum of the number of 8 * arrays and aligned according to 8 bytes

The occupied space of double array is the same as that of long array.

The object array is slightly different from the above array, in which the reference addresses of all objects are recorded, so the space occupied is the sum of the object head of the array + the size of the object reference * the number of arrays and aligned according to 8 bytes plus the actual space occupied by each object itself.

Now specify the space occupied by each object, except for the object header, where the instance data part (excluding static variables) includes basic types and reference types (all arrays and ordinary objects are reference types). The pointer size of the reference type has been mentioned above, and the occupied space pointing to the real object is now discussed, which is actually a recursion, such as:

Class Test {

Int a

Byte [] b=new byte [10]

}

Suppose that in the case of pointer compression in 64-bit jvm, the space occupied by the Test object = align (object header (4x8) + data (4x4)) + size (b). Note: all reference types are not in a contiguous address space with this object, so byte alignment align cannot contain the actual size of the reference object!

In lucene, in order to calculate the actual occupied space of an object, you need to implement the Accountable interface, because when there is an object reference inside the object, you need to calculate the actual occupied space of the reference object!

This is the answer to the question about how much space the Java object occupies in memory. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel for more related knowledge.

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