最近在看周志明所著的《深入理解Java虚拟机》,有几个问题不太明白,希望对虚拟机有研究的哥们儿帮我解答一下。
先说一下我进行试验的环境:
操作系统:Mac OS X 10.11.6 EI Capitan Java环境: java version "1.8.0_92" Java(TM) SE Runtime Environment (build 1.8.0_92-b14) Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)
1. 我发现在年轻代中,即使我只是一个空类,没有进行任何的内存分配工作,Eden区域内存还是会被占用一部分,例子如下:
public class BlankedClassSize { public static void main(String[] args) { // TODO Auto-generated method stub } }
虚拟机参数设置:
-verbose:gc -XX:+UseSerialGC -XX:+PrintGCDetails -Xmx20M -Xms20M -Xmn10M
虚拟机GC情况如下:
Heap def new generation total 9216K, used1164K [0x00000007bf000000, 0x00000007bfa00000, 0x00000007bfa00000) eden space 8192K, 14% used [0x00000007bf000000, 0x00000007bf1231d0, 0x00000007bf800000) from space 1024K, 0% used [0x00000007bf800000, 0x00000007bf800000, 0x00000007bf900000) to space 1024K, 0% used [0x00000007bf900000, 0x00000007bf900000, 0x00000007bfa00000) tenured generation total 6144K, used 0K [0x00000007bfa00000, 0x00000007c0000000, 0x00000007c0000000) the space 6144K, 0% used [0x00000007bfa00000, 0x00000007bfa00000, 0x00000007bfa00200, 0x00000007c0000000) Metaspace used 2666K, capacity 4486K, committed 4864K, reserved 1056768K class space used 289K, capacity 386K, committed 512K, reserved 1048576K
问题1:这里是什么占用了内存?
2.当年轻代进行一次GC后,因为年轻代GC是采用复制算法收集内存的,那么正常情况下进行一次Minor GC后,Eden区域占存为什么不是零?是什么占用着内存?例子如下:
public class MinorGC { private static final int_1MB = 1024*1024; public static void main(String[] args) { // TODO Auto-generated method stub testEdenGC(); } public static void testEdenGC(){ byte[] allocation1,allocation2,allocation3,allocation4,allocation5,allocation6; allocation1=newbyte[_1MB]; allocation2=newbyte[_1MB]; allocation3=newbyte[_1MB]; allocation4=newbyte[_1MB]; allocation5=newbyte[_1MB]; allocation6=newbyte[_1MB]; } }
虚拟机参数设置:
-verbose:gc -XX:TenuringThreshold=1 -Xms20M -Xmx20M -Xmn10M -XX:SurvivorRation=3 -XX:+PrintGCDetails -XX:+UseSerialGC
虚拟机GC情况:
[GC (Allocation Failure) DefNew: 6029K->1354K(8192K), 0.0032985 secs] 6029K->5450K(18432K), 0.0033216 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
def new generation total 8192K, used 2440K [0x00000007bec00000, 0x00000007bf600000, 0x00000007bf600000)
eden space 6144K, 17% used [0x00000007bec00000, 0x00000007bed0f748, 0x00000007bf200000)
from space 2048K, 66% used [0x00000007bf400000, 0x00000007bf5529b0, 0x00000007bf600000)
to space 2048K, 0% used [0x00000007bf200000, 0x00000007bf200000, 0x00000007bf400000)
tenured generation total 10240K, used 4096K [0x00000007bf600000, 0x00000007c0000000, 0x00000007c0000000)
the space 10240K, 40% used [0x00000007bf600000, 0x00000007bfa00040, 0x00000007bfa00200, 0x00000007c0000000)
Metaspace used 2666K, capacity 4486K, committed 4864K, reserved 1056768K
class space used 289K, capacity 386K, committed 512K, reserved 1048576K
问题2:根据6026K->1354K(8192K)可知,进行GC后,allocation1转入survivor1,allocation2~4转入老年区,此时eden中还有1354K内存被占用,是什么占用的?根据6029K->5450K(18432K),知道GC后,Java堆内存减少了579K,减少的这部分是谁的内存(明显新建的几个对象都可达,不至于被回收)?
3. 当给新对象分配内存的时候,因为年轻代空间不足,会发生一次minor GC,将年轻代中依然存活的对象复制到老年区,给新对象腾出空间,根据我的实验,此时若老年区内存不够将年轻代对象复制过去但是够新对象的存储,(此时会发生担保失败)则会直接将新对象放进老年代,例子如下:
public class MinorGC { private static final int_1MB = 1024*1024; public static void main(String[] args) { // TODO Auto-generated method stub testAllocation(); } public static void testAllocation(){ byte[] allocation1,allocation2,allocation3,allocation4; allocation1 = newbyte[2*_1MB]; allocation2 = newbyte[2*_1MB]; allocation3 = newbyte[2*_1MB]; allocation4 = newbyte[4*_1MB]; } }
虚拟机参数设置如下:
-verbose:gc -Xms16M -Xmx16M -Xmn10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+HeapDumpOnOutOfMemoryError -XX:MaxTenuringThreshold=15 -XX:+UseSerialGC
GC情况如下:
[GC (Allocation Failure) [DefNew (promotion failed) : 7144K->7474K(9216K), 0.0028055 secs][Tenured: 4096K->4096K(6144K), 0.0015627 secs] 7144K->6473K(15360K), [Metaspace: 2660K->2660K(1056768K)], 0.0044048 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] Heap def new generation total 9216K, used 6555K [0x00000007bf000000, 0x00000007bfa00000, 0x00000007bfa00000) eden space 8192K, 80% used [0x00000007bf000000, 0x00000007bf666ed0, 0x00000007bf800000) from space 1024K, 0% used [0x00000007bf900000, 0x00000007bf900000, 0x00000007bfa00000) to space 1024K, 0% used [0x00000007bf800000, 0x00000007bf800000, 0x00000007bf900000) tenured generation total 6144K, used 4096K[0x00000007bfa00000, 0x00000007c0000000, 0x00000007c0000000) the space 6144K, 66% used [0x00000007bfa00000, 0x00000007bfe00020, 0x00000007bfe00200, 0x00000007c0000000) Metaspace used 2666K, capacity 4486K, committed 4864K, reserved 1056768K class space used 289K, capacity 386K, committed 512K, reserved 1048576K
根据上面输出结果,可以看出,GC过程中,年轻代7144K->;7144K(9216K),并没有将年轻代中的对象转移走,而老年代中4096K->4096K(6144K),tenured generation total 6144K, used 4096K,说明allocation4直接被转移到老年代中了。
问题3:难道这种情况下虚拟机会自动设置大对象阈值??将新对象直接放入老年去?
以上三个问题让我这个虚拟机小白百思不得其解,还希望大家帮我解释一下,在此谢过。