介绍:
我使用JOL(Java Object Layout)工具来分析 Java 对象的内部和外部碎片以用于研究目的。
这样做时,我偶然发现了以下内容:
x@pc:~/Util$ java -jar jol-cli-0.9-full.jar internals sun.reflect.DelegatingClassLoader
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
Instantiated the sample instance via sun.reflect.DelegatingClassLoader(java.lang.ClassLoader)
sun.reflect.DelegatingClassLoader object internals:
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) 3b 13 00 f8 (00111011 00010011 00000000 11111000) (-134212805)
12 4 java.lang.ClassLoader ClassLoader.parent null
16 4 java.util.concurrent.ConcurrentHashMap ClassLoader.parallelLockMap null
20 4 java.util.Map ClassLoader.package2certs (object)
24 4 java.util.Vector ClassLoader.classes (object)
28 4 java.security.ProtectionDomain ClassLoader.defaultDomain (object)
32 4 java.util.Set ClassLoader.domains (object)
36 4 java.util.HashMap ClassLoader.packages (object)
40 4 java.util.Vector ClassLoader.nativeLibraries (object)
44 4 java.lang.Object ClassLoader.assertionLock (object)
48 4 java.util.Map ClassLoader.packageAssertionStatus null
52 4 java.util.Map ClassLoader.classAssertionStatus null
56 8 (alignment/padding gap)
64 1 boolean ClassLoader.defaultAssertionStatus false
65 7 (loss due to the next object alignment)
Instance size: 72 bytes
Space losses: 8 bytes internal + 7 bytes external = 15 bytes total
问题:
在这种情况下困扰我的是每个字段都是 4 字节对齐的(参见 OFFSET 列),但仍然在偏移量 56 处添加了对齐间隙(56 8 (alignment/padding gap)
)。我在 Java 9 中进行了相同的测试,对象布局发生了一些变化,对齐/填充间隙仍然存在,但甚至有 12 个字节大。
为什么会这样?为什么它有 8 个字节大,我看到的所有其他对象都是 4 个字节的内部对齐?我自己找不到解释。
我的系统:
openjdk version "1.8.0_151"
OpenJDK Runtime Environment (build 1.8.0_151-8u151-b12-0ubuntu0.16.04.2-b12)
OpenJDK 64-Bit Server VM (build 25.151-b12, mixed mode)
使用默认设置(带有压缩 oops 的 ParallelOldGC)