Shulou(Shulou.com)06/03 Report--

This article mainly introduces "what are the parts of the Java object structure". In the daily operation, I believe many people have doubts about which parts of the Java object structure. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "what parts of the Java object structure?" Next, please follow the editor to study!

How big is a Java object?

To accurately calculate the memory consumed by a Java object, you first need to understand the structural representation of the Java object.

Java object structure

The representation of a Java object in Heap can be divided into three parts:

Object Header

Class Pointer

Fields

Every ordinary Java object has an object header in the heap, which is essential and records the state of the object.

32-bit is different from 64-bit footprint, in 32-bit:

Hash (25) + age (4) + lock (3) = 32bit

64-bit:

Unused (25.1) + hash (31) + age (4) + lock (3) = 64bit

We know that in Java, everything is an object; every class has a parent class, and Class Pointer is a pointer to the current object parent class, which is 4byte in a 32-bit system; in a 64-bit system, if pointer compression is turned on (- XX:+UseCompressedOops) or the maximum value of the JVM heap is less than 32G, the pointer is also 4byte, otherwise it is 8byte.

With regard to the field (Fields), it refers to the instance field of the class; that is, it does not include a static field, because this field is in shared memory and only one exists.

Let's take a 32-bit system as an example to calculate how much memory java.lang.Integer takes up:

Both Object Header and Pointer are fixed, 4-4-8 byte; if you look at the field, it's the only one that represents a numeric value:

/ * The value of the Integer. * * @ serial * / private final int value

An int occupies the 4byte in the java, so the size of the Integer is 4+4+4=12byte.

Is this the right result? Wrong! One more thing is not said: in java, the size of the heap occupied by the object is 8-bit aligned, and the above 12byte is not aligned, so the 4byte is needed. It turned out to be 16byte!

In addition, there is a special kind of object in Java, array! Yes, this object is a bit special. It has one more attribute than other objects: length. So when we calculate the length of the array, we need to add an additional length field, that is, the size of an int.

For example: int [] arr = new int [10]

The occupied heap size of arr is:

4 (object header) + 4 (pointer) + 4 (length) + 4x10 (10 int sizes) = 52byte needs 8-bit alignment, so the final size is `56byte`. Memory saving principle

After knowing the memory usage of the object, we can simply calculate the account. A java.lang.Integer occupies 16byte, while an int occupies the proportion of 4 byte int 4 int 1! In other words, the class type of an integer is 4 times the memory of the base type!

As a result, we draw the first principle of memory saving:

1) try to use the basic type instead of the packaging type.

Field types need to be carefully considered when creating tables in a database, just as the property field types in JavaBean need to be carefully considered. Don't be stingy with short,byte,boolean, and if short types let go of data, try not to use longer types.

A long has more 4byte than an int, but you have to think that if you have 100W long in memory, you will waste about 4MB space. Don't underestimate this little bit of space waste, because in any JVM running online applications, objects can reach tens of millions! Memory is saved.

So:

2) consider the field type and use small fields as much as possible on the premise of meeting the capacity.

Do you know how much memory an ArrayList collection takes if it has 10 numbers in it? Let's do the math:

There are two fields in ArrayList:

/ * The array buffer into which the elements of the ArrayList are stored. * The capacity of the ArrayList is the length of this array buffer. * / private transient Object [] elementData;/** * The size of the ArrayList (the number of elements it contains). * * @ serial * / private int size

Object Header occupies 4bytefront pointer takes up 4byte, an int field (size) occupies 12 of the array itself, and 10 Integer objects account for 10 × 16 in the array. So the whole set space size is 4+4+4+12+160=184byte.

What if we replace the set with int [], 12 times 4 10=52byte, and then 56byte.

The collection-to-array ratio is 184 56, which is over 3:1!

So our third suggestion is:

3) if possible, try to use arrays instead of collections. Basic types can be used in arrays, but only wrapper types can be placed in the collection!

If you really need to use collections, recommend a more memory-saving collection tool, fastutil. This contains most of the implementations in the JKD collection and saves memory.

4) Tips

On the basis of the above three principles, provide two tips.

Time is expressed in long/int, not Date or String.

If a short string can be exhausted or converted to ascii, it can be expressed as long or int.

The tip has something to do with the data in the specific scenario, and you can make radical optimization to save memory according to the actual situation.

Summary

There has always been some contradiction between performance and readability, and here, in order to save memory, we have to make a choice, the code is ugly and the readability is poor, but fortunately we can save some memory. When you really need to save memory, you might as well try the above principles!

At this point, the study on "what are the parts of the Java object structure" is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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