在内存中保留最大整数数组所需的空间 [英] Space required to keep largest integer array in memory

问题描述

我正在审查一种算法，它保留一个整数数组，输入的大小是动态的.因此，根据我的计算，它最多可能需要花费

I am reviewing an algorithm and it keeps an array of integers, the size of input is dynamic. So according to my calculations it can take as much as

  integer MAX_VALUE  * int size  = ?   
      2^31 integers  * 4 bytes   = ?
2147483648 integers  * 4 bytes   = 8 Gigabytes

此计算正确吗?JVM会使用这么多连续的空间来存储int数组还是还有其他需要考虑的事情?

Is this calculation correct ? would the JVM use this much contiguous space to store the int array or are there other things that one needs to consider ?

推荐答案

数组的理论大小为:

• numberOfElementsInTheArray * 4个字节

• numberOfElementsInTheArray * 4 bytes

12个字节的标头( int [] 是一个对象).实际上，标头的大小取决于您使用的标志以及JVM版本上的您正在运行此

12 bytes of headers (int[] is an Object). Actually the size of headers depends on the flags you used and on the JVM version you are running this

4个字节以保持数组的 length 长度

4 bytes to keep the length of the array

填充.

例如:(我将为此使用 JOL ):

For example: (I am going to use JOL for this):

    int [] x = new int[10];
    for(int i=0;i<10;++i){
        x[i] = 9999;
    }
    System.out.println(GraphLayout.parseInstance((Object)x).toPrintable()); 

将输出:

 [I@7a81197dd object externals:
      ADDRESS       SIZE TYPE PATH                           VALUE
    70fe45268         56 [I                                  [9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999]

所以它有 56个字节:

• 值本身为40(10个整数* 4个字节)
• 12个标题
• 4个长度
• 0填充

如果将此数组更改为 Integer ，则情况将发生巨大变化. Integer 是一个对象，因此您将在数组内部存储一个引用(可以是 4 或 8 字节，具体取决于 UseCompressedOops 标志)，再加上每个 Integer 实例将需要2个标头(每个 Integer 是一个对象).

If you change this array to Integer, things change dramatically. Integer is an Object so you will store a reference inside the array (which could be 4 or 8 bytes, depending on UseCompressedOops flag), plus each of the Integer instances will require 2 headers (each Integer is an Object).

    Integer[] y = new Integer[10];
    for(int i=0;i<10;++i){
        y[i] = 9999;
    }

    System.out.println(GraphLayout.parseInstance((Object)y).toFootprint());

将显示:

   [Ljava.lang.Integer;@369f73a2d footprint:
 COUNT       AVG       SUM   DESCRIPTION
     1        56        56   [Ljava.lang.Integer;
    10        16       160   java.lang.Integer
    11                 216   (total)

总共 216字节:

每个引用
• 4个字节(我已打开 UseCompressedOop )，总共40个字节
• 该数组的12个字节的标头
• 数组的4个字节长
• 0字节填充

• 4 bytes for each reference (I have UseCompressedOop turned on), 40 bytes total
• 12 bytes headers of the array
• 4 bytes length of the array
• 0 bytes padding

该数组中的每个引用都指向一个 Integer ，每个对象将具有 16个字节:

Each reference from that array points to an Integer, each of those Objects will have 16 bytes:

• 内部保存的 int 的4个字节
• 12个字节的标头
• 0字节填充

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