查看普通java对象的内部布局工具JOL(JAVA OBJECT LAYOUT)
使用此工具可以查看new出来的一个java对象的内部布局,以及一个普通的java对象占用多少字节。
导入依赖或者jar包
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<dependency>
<groupId>org.openjdk.jol</groupId>
<artifactId>jol-core</artifactId>
<version>0.9</version>
<scope>provided</scope>
</dependency>
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使用
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public class JOL {
public static void main(String[] args) {
Object o = new Object();
System.out.println(ClassLayout.parseInstance(o).toPrintable());
synchronized (o){
System.out.println(ClassLayout.parseInstance(o).toPrintable());
}
}
}
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打印：
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| 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) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
java.lang.Object object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 4 (object header) 28 f2 5a 03 (00101000 11110010 01011010 00000011) (56291880)
4 4 (object header) 00 00 00 00 (00000000 00000000 00000000 00000000) (0)
8 4 (object header) e5 01 00 f8 (11100101 00000001 00000000 11111000) (-134217243)
12 4 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total
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前面两行是java对象头的Mark Word共占8个字节,第三行是java对象头的Klass Pointer类指针,本来也是占8个字节,因JDK默认开启类指针压缩所以占4个字节,总共12个字节,又因12不能被8整除,所有最后又加了四个空字节.所以java中new一个空对象总共占16个字节.
涉及内容：jVM对象的内存布局，请看博客：
