JOL的全称是Java Object Layout。是一个用来分析JVM中Object布局的小工具。包括Object在内存中的占用情况，实例对象的引用情况等等。
引入依赖：
<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.10</version>
</dependency>
相关API方法：
private static void testObj() {
Object obj = new Object();
System.out.println(ClassLayout.parseInstance(obj).toPrintable());
}
1.object对象占用几个字节
答案：16字节。
其中对象头占用12字节，对齐填充4的字节。
2.String对象占用几个字节
答案JDK11版本中，占用24字节。
其中对象头占用12字节，byte[]数组占用4字节，int类型的hash属性占用4字节，byte类型的coder占用1字节，对齐填充3个字节。
java.lang.String 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) 08 18 00 00 (00001000 00011000 00000000 00000000) (6152)
12 4 byte[] String.value [97, 97, 97]
16 4 int String.hash 0
20 1 byte String.coder 0
21 3 (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 3 bytes external = 3 bytes total
3.Integer类型占用几个字节（基础类型的装箱）
答案：占用16字节。
其中对象头12字节，实例数据int占用4字节；
java.lang.Integer 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) e8 39 01 00 (11101000 00111001 00000001 00000000) (80360)
12 4 int Integer.value 100000
Instance size: 16 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
4.Long占用几个字节
答案：占用24个字节。
其中对象头12字节，实例数据long占用8字节，对齐填充4字节（必须是8的整数倍）。
java.lang.Long 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) 38 3c 01 00 (00111000 00111100 00000001 00000000) (80952)
12 4 (alignment/padding gap)
16 8 long Long.value 100000
Instance size: 24 bytes
Space losses: 4 bytes internal + 0 bytes external = 4 bytes total
5.Object[100]占用几个字节
答案：416字节。
其中对象头16字节，对齐填充100*4=400字节。
[Ljava.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) 88 3e 01 00 (10001000 00111110 00000001 00000000) (81544)
12 4 (object header) 64 00 00 00 (01100100 00000000 00000000 00000000) (100)
16 400 java.lang.Object Object;.<elements> N/A
Instance size: 416 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
5.String[100]占用几个字节
答案：416字节。
其中对象头16字节，String的实例数据+对齐填充占用400字节。
[Ljava.lang.String; 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) 58 55 02 00 (01011000 01010101 00000010 00000000) (152920)
12 4 (object header) 64 00 00 00 (01100100 00000000 00000000 00000000) (100)
16 400 java.lang.String String;.<elements> N/A
Instance size: 416 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
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