Java Object Layout(jol)

下载源代码:

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)

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