Java Object Layout(jol)

http://bboniao.com/openjdk/2014-06/java-object-layoutjol.htm

http://openjdk.java.net/projects/code-tools/jol/

hg clone http://hg.openjdk.java.net/code-tools/jol/ jol
cd jol
mvn clean install

pom.xml添加依赖

<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>

代码

public static void main(String[] args) {
    int size = 10;
    List<Integer> list = new ArrayList<Integer>(size);
    for (int i = 0; i < size; i++) {
        list.add(i);
    }
    //虚拟机信息
    out.println(VMSupport.vmDetails());
    //打印类内部的占用
    out.println(ClassLayout.parseClass(ArrayList.class).toPrintable());
    //打印实例内部的占用
    out.println(ClassLayout.parseClass(ArrayList.class).toPrintable(list));
    //打印实例外部的占用
    out.println(GraphLayout.parseInstance(list).toPrintable());
    //打印实例各个依赖的占用,并汇总
    out.println(GraphLayout.parseInstance(list).toFootprint());
}

输出

Running 64-bit HotSpot VM.

Using compressed references 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.util.ArrayList object internals:

OFFSET SIZE TYPE DESCRIPTION VALUE

0 12 (object header) N/A
12 4 int AbstractList.modCount N/A
16 4 int ArrayList.size N/A
20 4 Object[] ArrayList.elementData N/A
Instance size: 24 bytes (estimated, the sample instance is not available)
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
java.util.ArrayList object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 19 00 00 00 (0001 1001 0000 0000 0000 0000 0000 0000)
4 4 (object header) 00 00 00 00 (0000 0000 0000 0000 0000 0000 0000 0000)
8 4 (object header) 31 32 00 f8 (0011 0001 0011 0010 0000 0000 1111 1000)
12 4 int AbstractList.modCount 10
16 4 int ArrayList.size 10
20 4 Object[] ArrayList.elementData [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Instance size: 24 bytes (estimated, add this JAR via -javaagent: to get accurate result)
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total

java.util.ArrayList object externals:

ADDRESS SIZE TYPE PATH VALUE

74002ca98 16 java.lang.Integer .elementData[9] 9

74002caa8 16 java.lang.Integer .elementData[8] 8

74002cab8 16 java.lang.Integer .elementData[7] 7

74002cac8 16 java.lang.Integer .elementData[6] 6

74002cad8 16 java.lang.Integer .elementData[5] 5

74002cae8 16 java.lang.Integer .elementData[4] 4

74002caf8 16 java.lang.Integer .elementData[3] 3

74002cb08 16 java.lang.Integer .elementData[2] 2

74002cb18 16 java.lang.Integer .elementData[1] 1

74002cb28 16 java.lang.Integer .elementData[0] 0

74002cb38 491560 (something else) (somewhere else) (something else)
7400a4b60 56 [Ljava.lang.Object; .elementData [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
7400a4b98 1432458504 (something else) (somewhere else) (something else)
7956be0a0 24 java.util.ArrayList (object)

java.util.ArrayList instance footprint:

COUNT AVG SUM DESCRIPTION

1 56 56 [Ljava.lang.Object;

10 16 160 java.lang.Integer

1 24 24 java.util.ArrayList

12 240 (total)

维护记录

时间 修改内容 原因
2026-10-09 修复本地 Hugo 构建的 Raw HTML 警告：代码/配置放入代码块，正文中的尖括号占位符改为行内代码；文件重命名为 jol.md；url 改为 jol；旧 url 加入 aliases；categories 改为 Java 正文中的 HTML/XML 片段被当作原始 HTML 丢弃；文件名/URL/标题按规范调整