Java Object Layout(jol)

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

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

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());
}

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)
 http://bboniao.com/openjdk/2014-06/java-object-layoutjol.html[/code]