本篇文章基于V0.16 JOLSample_03_Packing
VM 会将字段组合在一起，以便最小化的内存占用。
在下面的例子中，我们看到对象属性的对齐方式是 按照大小的 8->4->2->1 的顺序来对齐的。我们先在前面使用属性教大的，然后在后面使用属性较小的来填充属性较大的留下的空缺。
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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_03_Packing {
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 bo1, bo2;
byte b1, b2;
char c1, c2;
double d1, d2;
float f1, f2;
int i1, i2;
long l1, l2;
short s1, s2;
}
}
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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_03_Packing$A object internals:
OFFSET SIZE TYPE DESCRIPTION VALUE
0 12 (object header) N/A
12 4 float A.f1 N/A
16 8 double A.d1 N/A
24 8 double A.d2 N/A
32 8 long A.l1 N/A
40 8 long A.l2 N/A
48 4 float A.f2 N/A
52 4 int A.i1 N/A
56 4 int A.i2 N/A
60 2 char A.c1 N/A
62 2 char A.c2 N/A
64 2 short A.s1 N/A
66 2 short A.s2 N/A
68 1 boolean A.bo1 N/A
69 1 boolean A.bo2 N/A
70 1 byte A.b1 N/A
71 1 byte A.b2 N/A
Instance size: 72 bytes
Space losses: 0 bytes internal + 0 bytes external = 0 bytes total
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