本篇文章基于V0.16 JOLSample_01_Basic
这个例子演示 了最基本的对象字段布局：
- 对象头消耗多少内存
- 对象属性是如何布局的
- 对象大小是如何对齐的
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| import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.vm.VM;
import static java.lang.System.out;
public class JOLSample_01_Basic {
public static void main(String[] args) throws Exception {
out.println(VM.current().details());
out.println(ClassLayout.parseClass(A.class).toPrintable());
}
public static class A {
boolean f;
}
}
|
运行结果
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| # Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass 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]
testjol.JOLSample_01_Basic$A object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 1 boolean A.f N/A
13 3 (loss due to the next object alignment)
Instance size: 16 bytes
Space losses: 0 bytes internal + 3 bytes external = 3 bytes total
|
对象头占用了12个byte, A.f属性占用一个byte, 使用3个byte进行对象大小对齐. 实例大小是 16bytes
