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