李亚楠
6 months ago
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--- |
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title: 使用jolcore库查看Java对象大小 |
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tags: Java技术 |
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categories: 技术随笔 |
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abbrlink: 155251202 |
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date: 2024-03-06 14:22:05 |
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keywords: |
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description: |
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cover: |
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--- |
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|
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> 对象大小可以用jolcore包查看，本篇介绍jolcore的基本使用 |
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|
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|
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# 引入依赖 |
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|
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``` |
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<dependency> |
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<groupId>org.openjdk.jol</groupId> |
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<artifactId>jol-core</artifactId> |
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<version>0.9</version> |
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</dependency> |
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``` |
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# 具体使用 |
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``` |
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ClassLayout layout = ClassLayout.parseInstance(new Object()); |
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System.out.println(layout.toPrintable()); |
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``` |
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|
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# 测试代码 |
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``` |
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import org.openjdk.jol.info.ClassLayout; |
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public class JOLSample { |
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public static void main(String[] args) { |
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ClassLayout layout = ClassLayout.parseInstance(new Object()); |
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System.out.println(layout.toPrintable()); |
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System.out.println(); |
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ClassLayout layout1 = ClassLayout.parseInstance(new int[]{1,2,3,4,5}); |
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System.out.println(layout1.toPrintable()); |
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System.out.println(); |
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ClassLayout layout2 = ClassLayout.parseInstance(new A()); |
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System.out.println(layout2.toPrintable()); |
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} |
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// -XX:+UseCompressedOops 默认开启的压缩所有指针 |
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// -XX:+UseCompressedClassPointers 默认开启的只压缩对象头里的类型指针Klass Pointer |
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// Oops:Ordinary Object Pointers |
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//堆内存大于32G时，指针压缩会失效。 |
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//堆内存小于4G时，不需要启用指针压缩，jvm会直接去除高32位地址。 |
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public static class A { |
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//8B mark word |
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//4B Klass Pointer |
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int id = 6; //4B |
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String name; //4B |
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byte b; //1B |
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Object o; //4B |
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} |
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} |
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``` |
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# 打印结果 |
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|
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``` |
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java.lang.Object object internals: |
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OFFSET SIZE TYPE DESCRIPTION VALUE |
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0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) |
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4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) |
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8 4 (object header) 00 1c 2f c5 (00000000 00011100 00101111 11000101) (-986768384) |
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12 4 (object header) 9a 02 00 00 (10011010 00000010 00000000 00000000) (666) |
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Instance size: 16 bytes |
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Space losses: 0 bytes internal + 0 bytes external = 0 bytes total |
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|
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[I object internals: |
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OFFSET SIZE TYPE DESCRIPTION VALUE |
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0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) |
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4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) |
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8 4 (object header) 68 0b 2f c5 (01101000 00001011 00101111 11000101) (-986772632) |
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12 4 (object header) 9a 02 00 00 (10011010 00000010 00000000 00000000) (666) |
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16 4 (object header) 05 00 00 00 (00000101 00000000 00000000 00000000) (5) |
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20 4 (alignment/padding gap) |
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24 20 int [I.<elements> N/A |
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44 4 (loss due to the next object alignment) |
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Instance size: 48 bytes |
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Space losses: 4 bytes internal + 4 bytes external = 8 bytes total |
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|
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com.locaris.jvm.JOLSample$A object internals: |
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OFFSET SIZE TYPE DESCRIPTION VALUE |
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0 4 (object header) 01 00 00 00 (00000001 00000000 00000000 00000000) (1) |
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4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0) |
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8 4 (object header) 10 f6 9c c5 (00010000 11110110 10011100 11000101) (-979569136) |
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12 4 (object header) 9a 02 00 00 (10011010 00000010 00000000 00000000) (666) |
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16 4 int A.id 6 |
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20 1 byte A.b 0 |
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21 3 (alignment/padding gap) |
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24 8 java.lang.String A.name null |
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32 8 java.lang.Object A.o null |
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Instance size: 40 bytes |
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Space losses: 3 bytes internal + 0 bytes external = 3 bytes total |
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``` |
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