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Java Object Layout

Java内存布局介绍

Java的实例对象、数组对象在内存中的组成包括如下三部分：对象头Hearder、实例数据、内存填充。示意图如下所示 img.png

对象头：其主要包括两部分数据：Mark Word、Class对象指针。特别地对于数组对象而言，其还包括了数组长度数据。在64位的HotSpot虚拟机下，Mark Word占8个字节，其记录了Hash Code、GC信息、锁信息等相关信息；而Class对象指针则指向该实例的Class对象，在开启指针压缩的情况下占用4个字节，否则占8个字节；如果其是一个数组对象，则还需要4个字节用于记录数组长度信息。这里列出64位HotSpot虚拟机Mark Word的具体含义，以供参考。需要注意的是在下图的Mark Word中，左侧为高字节，右侧为低字节 img_1.png

使用工具查看Java内存布局

依赖

<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    <version>0.16</version>
</dependency>

简单使用

-Djdk.attach.allowAttachSelf
public static void main(String[] args) {
    System.out.println(ClassLayout.parseInstance(new Node()).toPrintable());
    System.out.println(ClassLayout.parseInstance(new int[]{1,2,3,4,5}).toPrintable());
}

public static class Node {
    public long value;
}
cn.nextop.gadget.Main$Node object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000000000d (biasable; age: 1)
  8   4        (object header: class)    0x00066248
 12   4        (alignment/padding gap)   
 16   8   long Node2.value               0
Instance size: 24 bytes
Space losses: 4 bytes internal + 0 bytes external = 4 bytes total

[I object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000001 (non-biasable; age: 0)
  8   4        (object header: class)    0x00000c10
 12   4        (array length)            5
 12   4        (alignment/padding gap)   
 16  20    int [I.<elements>             N/A
 36   4        (object alignment gap)    
Instance size: 40 bytes
Space losses: 4 bytes internal + 4 bytes external = 8 bytes total

false sharing与缓存行填充

img_2.png

  1. disruptor

  2. 手动填充

public class LhsPadding
{
    protected byte
        p10, p11, p12, p13, p14, p15, p16, p17,
        p20, p21, p22, p23, p24, p25, p26, p27,
        p30, p31, p32, p33, p34, p35, p36, p37,
        p40, p41, p42, p43, p44, p45, p46, p47,
        p50, p51, p52, p53, p54, p55, p56, p57,
        p60, p61, p62, p63, p64, p65, p66, p67,
        p70, p71, p72, p73, p74, p75, p76, p77;
}

public class Value extends LhsPadding
{
    protected long value;
}

public class RhsPadding extends Value
{
    protected byte
        p90, p91, p92, p93, p94, p95, p96, p97,
        p100, p101, p102, p103, p104, p105, p106, p107,
        p110, p111, p112, p113, p114, p115, p116, p117,
        p120, p121, p122, p123, p124, p125, p126, p127,
        p130, p131, p132, p133, p134, p135, p136, p137,
        p140, p141, p142, p143, p144, p145, p146, p147,
        p150, p151, p152, p153, p154, p155, p156, p157;
}
  1. 使用Contended注解
public class Value {
@jdk.internal.vm.annotation.Contended
public long value;
}
-XX:-RestrictContended 
-XX:ContendedPaddingWidth=64 
--add-opens=java.base/jdk.internal.vm.annotation=ALL-UNNAMED
cn.nextop.gadget.Value object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000000000d (biasable; age: 1)
  8   4        (object header: class)    0x00066248
 12  68        (alignment/padding gap)   
 80   8   long Node2.value               0
 88   0        (object alignment gap)    
Instance size: 152 bytes
Space losses: 68 bytes internal + 0 bytes external = 68 bytes total
type length
mark word 12 bytes
class pointer 4 bytes
cache line alignment 64 bytes
Node2.value 8 bytes
cache line alignment 64 bytes
object alignment 4 bytes

基础类型的包装对象占用的内存空间

java.lang.Byte object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4        (object header: class)    0x00013548
 12   1   byte Byte.value                5
 13   3        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 3 bytes external = 3 bytes total
java.lang.Long object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000000000d (biasable; age: 1)
  8   4        (object header: class)    0x00013c38
 12   4        (alignment/padding gap)   
 16   8   long Long.value                5
Instance size: 24 bytes
Space losses: 4 bytes internal + 0 bytes external = 4 bytes total

避免boxing, unboxing

LongHashMap
LongFunction
...

字段重排序

static class Node {
    byte b1;
    long l1;
    byte b2;
    byte b3;
    byte b4;
    byte b5;
    byte b6;
    byte b7;
}

重排后

cn.nextop.gadget.Main$Node object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x000000000000000d (biasable; age: 1)
  8   4        (object header: class)    0x00067248
 12   1   byte Node.b1                   0
 13   1   byte Node.b2                   0
 14   1   byte Node.b3                   0
 15   1   byte Node.b4                   0
 16   8   long Node.l1                   0
 24   1   byte Node.b5                   0
 25   1   byte Node.b6                   0
 26   1   byte Node.b7                   0
 27   5        (object alignment gap)

压缩指针与object alignment

-XX:+UseCompressedOops
-XX:ObjectAlignmentInBytes=8

img_3.png

32位4字节地址可以根据地址后三位为0，那么进行位移操作后可以保存35位地址；
即可以在2^35=32g堆空间内使用压缩指针

当-XX:ObjectAlignmentInBytes=16时，那么地址后4位为0;
即可以在2^36=64g堆空间内使用压缩指针

开启与关闭指针压缩的空间对比

public static void main(String[] args) {
    System.out.println(ClassLayout.parseInstance(new Node()).toPrintable());
}

static class Node {
    Node n1;
    Node n2;
}

开启(-XX:+UseCompressedOops)

cn.nextop.gadget.Main$Node object internals:
OFF  SZ                         TYPE DESCRIPTION               VALUE
  0   8                              (object header: mark)     0x000000000000000d (biasable; age: 1)
  8   4                              (object header: class)    0x00067248
 12   4   cn.nextop.gadget.Main.Node Node.n1                   null
 16   4   cn.nextop.gadget.Main.Node Node.n2                   null
 20   4                              (object alignment gap)    
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

关闭(-XX:-UseCompressedOops)

cn.nextop.gadget.Main$Node object internals:
OFF  SZ                         TYPE DESCRIPTION               VALUE
  0   8                              (object header: mark)     0x000000000000000d (biasable; age: 1)
  8   8                              (object header: class)    0x000001fd34fa5c10
 16   8   cn.nextop.gadget.Main.Node Node.n1                   null
 24   8   cn.nextop.gadget.Main.Node Node.n2                   null
Instance size: 32 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

开启之后节省了8字节空间

示例

protected static final class Node { 
    protected final int length; protected long address;
    protected Node ( final long a, final int n ) { 
        this.address = a; this.length = n; 
    }
}
cn.nextop.gadget.XDirectMap$Node object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     0x0000000000000005 (biasable; age: 0)
  8   4        (object header: class)    0x00067248
 12   4    int Node.length               0
 16   8   long Node.address              0
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

References