本文将通过JOL分析Java对象的内存布局，包括Throwable、Class、Object Header、HashCode等内容
代码托管在https://github.com/zhongmingmao/java_object_layout
Throwable
代码
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public class JOLSample_07_Exceptions {
public static void main(String[] args) throws Exception {
out.println(VM.current().details());
out.println(ClassLayout.parseClass(Throwable.class).toPrintable());
}
}
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运行结果
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| # Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# 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]
java.lang.Throwable object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 (alignment/padding gap)
16 4 java.lang.String Throwable.detailMessage N/A
20 4 java.lang.Throwable Throwable.cause N/A
24 4 java.lang.StackTraceElement[] Throwable.stackTrace N/A
28 4 java.util.List Throwable.suppressedExceptions N/A
Instance size: 32 bytes
Space losses: 4 bytes internal + 0 bytes external = 4 bytes total
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分析
- 在内存中并没有为
Throwable.backtrace分配空间，这是因为这个实例域用于处理虚拟机的内部信息（VM internal info），因此**在任何条件下都不允许用户访问**
- 下面尝试用
反射的机制访问Throwable.backtrace
反射测试代码
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| @Test(expected = NoSuchFieldException.class)
public void backtraceTest() throws NoSuchFieldException {
try {
new Throwable().getClass().getDeclaredField("backtrace");
} catch (NoSuchFieldException e) {
System.err.println(String.format("无法通过反射获得Throwable的backtrace实例域 ，%s", e));
throw e;
}
}
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在尝试通过反射获取Throwable的backtrace实例域时，会抛出NoSuchFieldException
Class
代码
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public class JOLSample_08_Class {
public static void main(String[] args) throws Exception {
out.println(VM.current().details());
out.println(ClassLayout.parseClass(Class.class).toPrintable());
}
}
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运行结果
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| # Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# 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]
java.lang.Class object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 java.lang.reflect.Constructor Class.cachedConstructor N/A
16 4 java.lang.Class Class.newInstanceCallerCache N/A
20 4 java.lang.String Class.name N/A
24 4 (alignment/padding gap)
28 4 java.lang.ref.SoftReference Class.reflectionData N/A
32 4 sun.reflect.generics.repository.ClassRepository Class.genericInfo N/A
36 4 java.lang.Object[] Class.enumConstants N/A
40 4 java.util.Map Class.enumConstantDirectory N/A
44 4 java.lang.Class.AnnotationData Class.annotationData N/A
48 4 sun.reflect.annotation.AnnotationType Class.annotationType N/A
52 4 java.lang.ClassValue.ClassValueMap Class.classValueMap N/A
56 32 (alignment/padding gap)
88 4 int Class.classRedefinedCount N/A
92 4 (loss due to the next object alignment)
Instance size: 96 bytes
Space losses: 36 bytes internal + 4 bytes external = 40 bytes total
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分析
- 在
56的位置开始有32 Bytes的的内存空间，JVM会向该内存区域注入元数据（meta-information），相关内容请看下来CPP代码，暂未弄懂其中原理
- http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/classfile/javaClasses.hpp
- http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/tip/src/share/vm/classfile/javaClasses.cpp
代码
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public class JOLSample_11_ClassWord {
public static void main(String[] args) throws Exception {
out.println(VM.current().details());
out.println(ClassLayout.parseInstance(new A()).toPrintable());
out.println(ClassLayout.parseInstance(new B()).toPrintable());
}
public static class A {
}
public static class B {
}
}
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运行结果
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| # Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# 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]
me.zhongmingmao.jol.JOLSample_11_ClassWord$A object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) a2 f0 00 f8 (10100010 11110000 00000000 11111000) (-134156126)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
me.zhongmingmao.jol.JOLSample_11_ClassWord$B 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) 32 f2 00 f8 (00110010 11110010 00000000 11111000) (-134155726)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
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分析
- 在
Hotspot JVM中，对象头（Object Header）由两部分组成Mark Word和Class Word（即「对象内存布局 - Instrumentation + sa-jdi 实例分析」中的Klass Ref）
- 上述运行结果将对象头的值都打印出来了，方便进行更详细地分析，因为
hashCode和锁信息都存储在Mark Word中
HashCode
代码
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public class JOLSample_15_IdentityHashCode {
public static void main(String[] args) throws Exception {
out.println(VM.current().details());
final A a = new A();
ClassLayout layout = ClassLayout.parseInstance(a);
out.println("**** Fresh object");
out.println(layout.toPrintable());
out.println("hashCode: " + Integer.toHexString(a.hashCode()));
out.println();
out.println("**** After identityHashCode()");
out.println(layout.toPrintable());
}
public static class A {
}
}
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运行结果
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| # Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# 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]
**** Fresh object
me.zhongmingmao.jol.JOLSample_15_IdentityHashCode$A object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) a2 f0 00 f8 (10100010 11110000 00000000 11111000) (-134156126)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
hashCode: 36aa7bc2
**** After identityHashCode()
me.zhongmingmao.jol.JOLSample_15_IdentityHashCode$A object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 01 c2 7b aa (00000001 11000010 01111011 10101010) (-1434729983)
4 4 (object header) 36 00 00 00 (00110110 00000000 00000000 00000000) (54)
8 4 (object header) a2 f0 00 f8 (10100010 11110000 00000000 11111000) (-134156126)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
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分析
- 对象尚未调用
hashCode()之前，Mark Word对应的hash全为为0
- 对象尚调用
hashCode()之后，计算得到的hash记录在Mark Word中（08~39），（Intel是小端序，低字节存储在低地址）
Mark Word的格式请参考markOop.hpp
手动计算HashCode
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| @Test
public void haseCodeTest() throws NoSuchFieldException, IllegalAccessException {
ClassLayout layout = ClassLayout.parseClass(Object.class);
Object object = new Object();
System.out.println(layout.toPrintable(object));
String realHashCode = Integer.toHexString(object.hashCode());
System.out.println(String.format("realHashCode : 0x%s\n", realHashCode));
System.out.println(layout.toPrintable(object));
Field field = Unsafe.class.getDeclaredField("theUnsafe");
field.setAccessible(true);
Unsafe unsafe = (Unsafe) field.get(null);
long hashCode = 0;
for (long index = 7; index > 0; index--) {
hashCode |= (unsafe.getByte(object, index) & 0xFF) << ((index - 1) * 8);
}
String manualHashCode = Long.toHexString(hashCode);
System.out.println(String.format("manualHashCode : 0x%s", manualHashCode));
assertEquals(realHashCode, manualHashCode);
}
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在Hotspot JVM计算对象的HashCode后，通过Mark Word可以手动计算HashCode，如果哪位大神知道个中原理，麻烦不吝赐教
