java - 64位VM不启动指针压缩，导致-8内存对齐

Question

maven pom.xml

<dependency>
    <groupId>org.openjdk.jol</groupId>
    <artifactId>jol-core</artifactId>
    <version>0.10</version>
</dependency>

Java 类:

public class MyObjectData {

    private  int i=66;
    private  long l=6L;
    private String string=new String("aaaa");

}

public class ObjectTest {

    @Test
    public  void test1() {
        Layouter l;
        //64 bit vm object distribution, pointer compression not enabled
        l = new HotSpotLayouter(new X86_64_DataModel());
        System.out.println("***** " + l);
        System.out.println(ClassLayout.parseInstance(new MyObjectData(),l).toPrintable());
        System.out.println("==============================================");
    }

}

运行结果:

OFFSET  SIZE               TYPE DESCRIPTION                               VALUE
      0     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)                           01 23 01 f8 (00000001 00100011 00000001 11111000) (-134143231)
     12     4                    (object header)                           42 00 00 00 (01000010 00000000 00000000 00000000) (66)
     16     8               long MyObjectData.l                            6
     24     4                int MyObjectData.i                            66
     28     4                    (alignment/padding gap)
     32     8   java.lang.String MyObjectData.string                       (object)
     40    -8                    (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 4 bytes internal + -8 bytes external = -4 bytes total`

不明白为什么64位VM在检查对象的内存使用情况时没有启动指针压缩，导致-8内存对齐。

Answer 1

I don't understand why the 64 bit VM did not start pointer compression when checking the memory usage of the object ...

JVM 不会动态启动指针压缩；例如“...在检查对象的内存使用情况时没有开始指针压缩...”

相反，它决定在(64 位)JVM 启动时是否使用压缩的 oop。它根据命令行选项来决定；即 UseCompressedOops、ObjectAlignmentInBytes 和最大堆大小。最大堆大小由 -xmx 给定或由特定于平台的默认算法确定。

所以...您需要查看 JVM 的(实际和默认)JVM 选项，以找出压缩 oops(显然)无效的原因。使用 -XX:+PrintCommandLineFlags 来查找。