UseCompressedClassPointers和UseCompressedOops都是JVM对指针进行压缩的设置，比较一下两个参数的适用范围，下面使用openjdk的jol-core打印一下
1、maven引入
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.16</version>
</dependency>
2、代码
package cn.callmebuzhou.jvm;
import org.openjdk.jol.info.ClassLayout;
public class CompressedCommandTest {
public static void main(String[] args) {
System.out.println(ClassLayout.parseInstance(new T()).toPrintable());
}
static class T{
String name;
}
}
3、开始测试
首先要明确一个，默认情况下两个都会默认开启压缩
3.1、默认情况下什么参数都不加
运行结果：
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0xf800c043
12 4 java.lang.String T.name null
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
可以看到第四行类类型指针占4字节，第五行对象引用指针也是4字节，说明都进行了压缩
3.2、关闭UseCompressedClassPointers
-XX:-UseCompressedClassPointers
运行结果
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 8 (object header: class) 0x00000205ce5608d0
16 4 java.lang.String T.name null
20 4 (object alignment gap)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
打印结果可以看出来，这里类类型指针没有压缩，占8字节，对象引用指针占4字节压缩了
3.3、开启UseCompressedClassPointers
-XX:+UseCompressedClassPointers
运行结果
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 8 (object header: class) 0x00000205717908d0
16 4 java.lang.String T.name null
20 4 (object alignment gap)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
运行结果是类类型指针不做压缩，对象引用指针压缩了
3.3、关闭UseCompressedOops
-XX:-UseCompressedOops
运行结果
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 8 (object header: class) 0x0000013d53e708d0
16 8 java.lang.String T.name null
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
可以看出来，类类型指针和对象引用指针都未进行了压缩
3.4、开启UseCompressedOops
-XX:+UseCompressedOops
运行结果
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0xf800c043
12 4 java.lang.String T.name null
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
可以看出来，类类型指针和对象引用指针进行了压缩,这里有人会说，也许是UseCompressedClassPointers默认值起了效果，那我们继续往下看
3.5、关闭UseCompressedClassPointers && 开启UseCompressedOops
-XX:-UseCompressedClassPointers -XX:+UseCompressedOops
运行结果
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 8 (object header: class) 0x000001ae2de408d0
16 4 java.lang.String T.name null
20 4 (object alignment gap)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
这下就很明细了，如果开启UseCompressedOops类类型指针和对象引用指针都会做压缩，如果想类类型指针不进行压缩需要显式设置
3.6、开启UseCompressedClassPointers && 关闭UseCompressedOops
-XX:+UseCompressedClassPointers -XX:-UseCompressedOops
Java HotSpot(TM) 64-Bit Server VM warning: UseCompressedClassPointers requires UseCompressedOops
cn.callmebuzhou.jvm.CompressedCommandTest$T object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 8 (object header: class) 0x00000154fc5508d0
16 8 java.lang.String T.name null
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
Java HotSpot(TM) 64-Bit Server VM warning: UseCompressedClassPointers requires UseCompressedOops
| 参数类型 | 类类型指针 | 引用类型指针 |
|---|---|---|
| 默认情况 | 压缩 | 压缩 |
| 关闭UseCompressedClassPointers | 未压缩 | 压缩 |
| 开启UseCompressedClassPointers | 压缩 | 压缩 |
| 关闭UseCompressedOops | 未压缩 | 未压缩 |
| 开启UseCompressedOops | 压缩 | 压缩 |
| 关闭UseCompressedClassPointers && 开启UseCompressedOops | 未压缩 | 压缩 |
| 开启UseCompressedClassPointers && 关闭UseCompressedOops | 未压缩 | 未压缩 |
