JOL
简介
JOL(Java Object Layout)是用于分析 JVM 中对象布局方案的微型工具箱。这些工具大量使用 Unsafe、JVMTI 和 Serviceability Agent (SA) 来解码实际的 对象布局、占用空间和引用。这使得JOL比其他依赖堆转储、规范假设等的工具更准确。
使用
测试类
public class SimpleObject {
private byte b;
private boolean bl;
private short s;
private char c;
private int a;
private float f;
private double d;
private long l;
private Object obj;
private Object [] objArray;
}
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测试代码
public void testPrintClassLayout(){
ClassLayout classLayout = ClassLayout.parseClass(SimpleObject.class);
System.out.println(classLayout.toPrintable());
}
public void testPrintObjectLayout(){
SimpleObject obj = new SimpleObject();
System.out.println(ClassLayout.parseInstance(obj).toPrintable());
}
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关闭指针压缩
VM参数：-XX:-UseCompressedOops
Class Layout
SimpleObject object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) N/A
8 8 (object header: class) N/A
16 8 double SimpleObject.d N/A
24 8 long SimpleObject.l N/A
32 4 int SimpleObject.a N/A
36 4 float SimpleObject.f N/A
40 2 short SimpleObject.s N/A
42 2 char SimpleObject.c N/A
44 1 byte SimpleObject.b N/A
45 1 boolean SimpleObject.bl N/A
46 2 (alignment/padding gap)
48 8 java.lang.Object SimpleObject.obj N/A
56 8 java.lang.Object[] SimpleObject.objArray N/A
Instance size: 64 bytes
Space losses: 2 bytes internal + 0 bytes external = 2 bytes total
|
Object Layout
SimpleObject object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 8 (object header: class) 0x000000001c123380
16 8 double SimpleObject.d 0.0
24 8 long SimpleObject.l 0
32 4 int SimpleObject.a 0
36 4 float SimpleObject.f 0.0
40 2 short SimpleObject.s 0
42 2 char SimpleObject.c
44 1 byte SimpleObject.b 0
45 1 boolean SimpleObject.bl false
46 2 (alignment/padding gap)
48 8 java.lang.Object SimpleObject.obj null
56 8 java.lang.Object[] SimpleObject.objArray null
Instance size: 64 bytes
Space losses: 2 bytes internal + 0 bytes external = 2 bytes total
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开启指针压缩
默认是开启的
VM参数:-XX:+UseCompressedOops
uid:8个字节
timpstamp:8个字节
Map每个Node大小
4+8+8+4
hash(4) + key(8) + value(8) +next(4)
每个用户占用16个字节
Class Layout
SimpleObject object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) N/A
8 4 (object header: class) N/A
12 4 int SimpleObject.a N/A
16 8 double SimpleObject.d N/A
24 8 long SimpleObject.l N/A
32 4 float SimpleObject.f N/A
36 2 short SimpleObject.s N/A
38 2 char SimpleObject.c N/A
40 1 byte SimpleObject.b N/A
41 1 boolean SimpleObject.bl N/A
42 2 (alignment/padding gap)
44 4 java.lang.Object SimpleObject.obj N/A
48 4 java.lang.Object[] SimpleObject.objArray N/A
52 4 (object alignment gap)
Instance size: 56 bytes
Space losses: 2 bytes internal + 4 bytes external = 6 bytes total
|
Object Layout
SimpleObject object internals:
OFF SZ TYPE DESCRIPTION VALUE
0 8 (object header: mark) 0x0000000000000001 (non-biasable; age: 0)
8 4 (object header: class) 0xf800c105
12 4 int SimpleObject.a 0
16 8 double SimpleObject.d 0.0
24 8 long SimpleObject.l 0
32 4 float SimpleObject.f 0.0
36 2 short SimpleObject.s 0
38 2 char SimpleObject.c
40 1 byte SimpleObject.b 0
41 1 boolean SimpleObject.bl false
42 2 (alignment/padding gap)
44 4 java.lang.Object SimpleObject.obj null
48 4 java.lang.Object[] SimpleObject.objArray null
52 4 (object alignment gap)
Instance size: 56 bytes
Space losses: 2 bytes internal + 4 bytes external = 6 bytes total
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指针压缩对结果的影响
以下都是基于64bit操作系统的测试结果
|
开启指针压缩 |
关闭指针压缩 |
| 对象头 |
12byte |
16byte |
| 引用类型 |
4byte |
8byte |
其他
对象的大小是8的倍数，如果不是8的倍数，jvm在创建对象分配内存时会填充字节用以对齐
JUC lock与synchronized 差异
线程状态
// Thread State
// ReentrantLock.lock -> java.lang.Thread.State: WAITING (parking)
waiting on condition
// synchronized -> java.lang.Thread.State: BLOCKED (on object monitor) waiting for monitor entry
// get lock on sleep -> java.lang.Thread.State: TIMED_WAITING (sleeping)