为什么Java对象中存在内部碎片，即使每个字段都是4字节对齐的？ [英] Why is there internal fragmentation in a Java object even if every field is 4-byte aligned?
问题描述
简介：
我使用了 JOL（Java对象布局）工具，用于分析Java对象的内部和外部碎片以供研究之用。
I used the JOL (Java Object Layout) tool to analyze the internal and external fragmentation of Java objects for research purpose.
在这样做时，我偶然发现了以下内容：
While doing so, I stumbled across the following:
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），但仍处于偏移56添加对齐间隙（ 56 8（对齐/填充间隙））。我在Java 9中做了相同的测试，并且对象布局发生了一些变化，alignemnt / padding间隙仍然存在，但是甚至还有12个字节。
What's bothering me in this case is that every field is 4 byte aligned (see OFFSET column), but still at offset 56 an alignment gap gets added (56 8 (alignment/padding gap)). I did the same test in Java 9, and there the object layout changed a little bit, the alignemnt/padding gap still exists, but is even 12 bytes large.
为什么这是怎么回事？为什么它是8字节大，我看到的所有其他对象都是4字节对齐的？我自己找不到解释。
Why is this happening? And why is it 8 bytes large, all other objects I saw are 4-byte aligned interally? I couldn't find an explanation myself.
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（带压缩oops的ParallelOldGC）
Using default settings (ParallelOldGC with compressed oops)
推荐答案
有时，HotSpot会注入在Java级别上不可见的内部特定于VM的字段。在 ClassLoader 的情况下，这是指向 ClassLoaderData ：
Sometimes HotSpot injects internal VM-specific fields that are not visible on Java level. In ClassLoader case this is a pointer to ClassLoaderData:
#define CLASSLOADER_INJECTED_FIELDS(macro) \
macro(java_lang_ClassLoader, loader_data, intptr_signature, false)
这是 intptr_t 类型的一个字段，即恰好8个字节长。
This is one field of intptr_t type, i.e. exactly 8 bytes long.
以下是HotSpot的来源 describe ClassLoaderData：
Here is how HotSpot sources describe ClassLoaderData:
// A ClassLoaderData identifies the full set of class types that a class
// loader's name resolution strategy produces for a given configuration of the
// class loader.
// Class types in the ClassLoaderData may be defined by from class file binaries
// provided by the class loader, or from other class loader it interacts with
// according to its name resolution strategy.
//
// Class loaders that implement a deterministic name resolution strategy
// (including with respect to their delegation behavior), such as the boot, the
// extension, and the system loaders of the JDK's built-in class loader
// hierarchy, always produce the same linkset for a given configuration.
//
// ClassLoaderData carries information related to a linkset (e.g.,
// metaspace holding its klass definitions).
// The System Dictionary and related data structures (e.g., placeholder table,
// loader constraints table) as well as the runtime representation of classes
// only reference ClassLoaderData.
//
// Instances of java.lang.ClassLoader holds a pointer to a ClassLoaderData that
// that represent the loader's "linking domain" in the JVM.
//
// The bootstrap loader (represented by NULL) also has a ClassLoaderData,
// the singleton class the_null_class_loader_data().
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