JAVA对象内存布局
对象头：
- markword：记录锁的信息、GC信息
- 类型指针class pointer：记录对象属于哪个Class类型的
实例数据instance data：成员变量
对齐 padding：当整个对象的字节数不能被8整除时，补齐剩余字节。因为JDK读内存时按照内存宽度读取，被8整除读取速度较快
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| <!-- 工具：JOL=JAVA Object Layout -->
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.14</version>
</dependency>
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| public static void main(String[] args) {
Object o = new Object();
System.out.println(ClassLayout.parseInstance(o).toPrintable());
}
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| java.lang.Object object internals:
OFFSET(从什么位置开始) SIZE(往后的长度) TYPE DESCRIPTION(类型描述) VALUE
0 4 (object header)[markword] 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
4 4 (object header)[markword] 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header)[class pointer] e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)[padding]
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
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| java -XX:+PrintCommandLineFlags -version
-XX:InitialHeapSize=267312128 -XX:MaxHeapSize=4276994048 -XX:+PrintCommandLineFlags -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:-UseLargePagesIndividualAllocation -XX:+UseParallelGC
java version "1.8.0_221"
Java(TM) SE Runtime Environment (build 1.8.0_221-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.221-b11, mixed mode)
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-XX:+UseCompressedClassPointers：默认开启类型指针class pointer压缩，8个字节压缩成4个字节
-XX:+UseCompressedOops：默认开启实例数据instance data压缩，比方说String类型占用8个字节，压缩成4个字节
数组的话会有一个数组长度length，占用4个字节。
| 类型 |
占用字节 |
占用位数 |
| byte |
1 |
8 |
| short |
2 |
16 |
| int |
4 |
32 |
| long |
8 |
64 |
| float |
4 |
32 |
| double |
8 |
64 |
| char |
2 |
16 |
| boolean |
1 |
8 |
示例：
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| @Data
public class Account {
private long id;
private String username;
private String password;
public static void main(String[] args) {
Object o1 = new Object();
System.out.println("空对象：" + ClassLayout.parseInstance(o1).toPrintable());
Object o2 = new Account();
System.out.println("带属性对象：" + ClassLayout.parseInstance(o2).toPrintable());
Object o3 = new int[1];
System.out.println("数组：" + ClassLayout.parseInstance(o3).toPrintable());
Object o4 = new ArrayList<>();
System.out.println("空List：" + ClassLayout.parseInstance(o4).toPrintable());
}
}
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控制台输出：
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| 空对象：java.lang.Object 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) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
带属性对象：cn.happyloves.example.dto.Account object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 54 c3 00 f8 (01010100 11000011 00000000 11111000) (-134167724)
12 4 java.lang.String Account.username null
16 8 long Account.id 0
24 4 java.lang.String Account.password null
28 4 (loss due to the next object alignment)
Instance size: 32 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
数组：[I 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) 6d 01 00 f8 (01101101 00000001 00000000 11111000) (-134217363)
12 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1)
16 4 int [I.<elements> N/A
20 4 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
空List：java.util.ArrayList object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) 7f 2f 00 f8 (01111111 00101111 00000000 11111000) (-134205569)
12 4 int AbstractList.modCount 0
16 4 int ArrayList.size 0
20 4 java.lang.Object[] ArrayList.elementData []
Instance size: 24 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
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