JVM 指针压缩
运用java内存对齐填充，对java内存进行8字节划分，java对象指针映射到每个划分区域上，使得4个字节（32位）表示2^32个地址，从而使4个字节指针映射32G内存空间。
1.为什么进行指针压缩：
- jvm从32位变为64位，内存带宽也增加了一倍，原来32位存储一个变量，现在可以使用64位存储，无形中浪费了大量空间。占用较大带宽，同时GC也会承受较大压力。
- .为了减少64位平台下内存的消耗，启用指针压缩功能
- 在jvm中，32位地址最大支持4G内存(2的32次方)，可以通过对对象指针的压缩编码、解码方式进行优化，使得jvm 只用32位地址就可以支持更大的内存配置(小于等于32G)
- 堆内存小于4G时，不需要启用指针压缩，jvm会直接去除高32位地址，即使用低虚拟地址空间
- 堆内存大于32G时，压缩指针会失效，会强制使用64位(即8字节)来对java对象寻址，这就会出现1的问题，所以堆内 存不要大于32G为好
2.开启关闭对象指针压缩
jdk1.6 update14开始，在64bit操作系统中，JVM支持指针压缩。
- 启用指针压缩:
-XX:+UseCompressedOops(默认开启) - 禁止指针压缩:
-XX:-UseCompressedOops
对象大小可以用jol-core包查看，引入依赖
implementation group: 'org.openjdk.jol', name: 'jol-core', version: '0.9'
package com;import org.openjdk.jol.info.ClassLayout;public class JOLSample {public static void main(String[] args){ClassLayout layout = ClassLayout.parseInstance(new Object());System.out.println(layout.toPrintable());System.out.println();ClassLayout layout1 = ClassLayout.parseInstance(new int[]{});System.out.println(layout1.toPrintable());System.out.println();ClassLayout layout2 = ClassLayout.parseInstance(new A());System.out.println(layout2.toPrintable());}public static class A {int id;String name;byte b;Object o;}
}配置关闭指针压缩，打印：
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
java.lang.Object object internals:OFFSET SIZE TYPE DESCRIPTION VALUE0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)8 4 (object header) 50 1f c4 30 (01010000 00011111 11000100 00110000) (818159440)12 4 (object header) a0 01 00 00 (10100000 00000001 00000000 00000000) (416)
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total[I object internals:OFFSET SIZE TYPE DESCRIPTION VALUE0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)8 4 (object header) 10 0c c4 30 (00010000 00001100 11000100 00110000) (818154512)12 4 (object header) a0 01 00 00 (10100000 00000001 00000000 00000000) (416)16 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)20 4 (alignment/padding gap) 24 0 int [I.<elements> N/A
Instance size: 24 bytes
Space losses: 4 bytes internal + 0 bytes external = 4 bytes totalcom.JOLSample$A object internals:OFFSET SIZE TYPE DESCRIPTION VALUE0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)8 4 (object header) 08 78 45 32 (00001000 01111000 01000101 00110010) (843413512)12 4 (object header) a0 01 00 00 (10100000 00000001 00000000 00000000) (416)16 4 int A.id 020 1 byte A.b 021 3 (alignment/padding gap) 24 8 java.lang.String A.name null32 8 java.lang.Object A.o null
Instance size: 40 bytes
Space losses: 3 bytes internal + 0 bytes external = 3 bytes total开启指针压缩
# WARNING: Unable to get Instrumentation. Dynamic Attach failed. You may add this JAR as -javaagent manually, or supply -Djdk.attach.allowAttachSelf
java.lang.Object object internals:OFFSET SIZE TYPE DESCRIPTION VALUE0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)8 4 (object header) 00 10 00 00 (00000000 00010000 00000000 00000000) (4096)12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total[I object internals:OFFSET SIZE TYPE DESCRIPTION VALUE0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)8 4 (object header) 10 0c 00 00 (00010000 00001100 00000000 00000000) (3088)12 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)16 0 int [I.<elements> N/A
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes totalcom.JOLSample$A object internals:OFFSET SIZE TYPE DESCRIPTION VALUE0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)8 4 (object header) a8 e9 0d 00 (10101000 11101001 00001101 00000000) (911784)12 4 int A.id 016 1 byte A.b 017 3 (alignment/padding gap) 20 4 java.lang.String A.name null24 4 java.lang.Object A.o null28 4 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 3 bytes internal + 4 bytes external = 7 bytes total通过指针压缩，类型指针、对象引用等由8字节转为4个字节。降低对象占用的内存大小，顺便减轻GC压力；当指针移动时，减少带宽损耗。
