详细信息参考：Oracle官方文档
环境信息：Windows10
JDK：1.8
Trace跟踪参数
public class Test {
public static void main(String[] args) {
byte[] bytes = null;
for (int i = 0; i < 10; i++) {
bytes = new byte[1 * 1024 * 1024];
}
}
}
以上述代码为例，展示各个参数结果。
- -verbose:gc 或 -XX:+printGC
作用：打印GC的简要信息
参数：
-Xmx10m -Xms5m -verbose:gc-Xmx10m -Xms5m -XX:+printGC
......
[GC (Allocation Failure) 7362K->7217K(8704K), 0.0006685 secs]
[GC (Allocation Failure) 7217K->7257K(9728K), 0.0006430 secs]
[Full GC (Allocation Failure) 7257K->1828K(6656K), 0.0065725 secs]
- -XX:+PrintGCDetails
作用：打印GC详细信息
参数：
-Xmx10m -Xms5m -XX:+PrintGCDetails
[GC (Allocation Failure) [PSYoungGen: 504K->504K(2560K)] 7133K->7133K(9728K), 0.0005841 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[Full GC (Allocation Failure) [PSYoungGen: 504K->0K(2560K)] [ParOldGen: 6629K->1868K(4096K)] 7133K->1868K(6656K), [Metaspace: 3448K->3448K(1056768K)], 0.0046903 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 2560K, used 1044K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 51% used [0x00000000ffd00000,0x00000000ffe05370,0x00000000fff00000)
from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 5632K, used 4940K [0x00000000ff600000, 0x00000000ffb80000, 0x00000000ffd00000)
object space 5632K, 87% used [0x00000000ff600000,0x00000000ffad31a8,0x00000000ffb80000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K- -XX:+PrintGCDateStamps
作用：打印GC发生的时间戳
参数：
-Xmx10m -Xms5m -XX:+PrintGCDetails -XX:+PrintGCDateStamps......
2018-08-14T10:23:15.832+0800: [GC (Allocation Failure) [PSYoungGen: 496K->480K(2560K)] 7158K->7166K(9728K), 0.0004435 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
2018-08-14T10:23:15.832+0800: [Full GC (Allocation Failure) [PSYoungGen: 480K->0K(2560K)] [ParOldGen: 6686K->1828K(4096K)] 7166K->1828K(6656K), [Metaspace: 3448K->3448K(1056768K)], 0.0043986 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 2560K, used 1085K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 53% used [0x00000000ffd00000,0x00000000ffe0f758,0x00000000fff00000)
from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 5632K, used 4900K [0x00000000ff600000, 0x00000000ffb80000, 0x00000000ffd00000)
object space 5632K, 87% used [0x00000000ff600000,0x00000000ffac9050,0x00000000ffb80000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K- -Xloggc:log/gc.log
作用：指定GC log的输出位置，以文件输出（目录需要手动创建，文件会自动创建）
参数：
-Xmx10m -Xms5m -XX:+PrintGCDetails -Xloggc:d:/log/gc.log
- -XX:+PrintHeapAtGC
作用：每一次GC前后，都打印堆的信息
参数：
-Xmx10m -Xms5m -XX:+PrintHeapAtGC......
{Heap before GC invocations=4 (full 0):
PSYoungGen total 1536K, used 504K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 1024K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000ffe00000)
from space 512K, 98% used [0x00000000ffe00000,0x00000000ffe7e010,0x00000000ffe80000)
to space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
ParOldGen total 7168K, used 6617K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
object space 7168K, 92% used [0x00000000ff600000,0x00000000ffc76760,0x00000000ffd00000)
Metaspace used 3447K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 375K, capacity 388K, committed 512K, reserved 1048576K
Heap after GC invocations=4 (full 0):
PSYoungGen total 2560K, used 480K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000fff00000)
from space 512K, 93% used [0x00000000fff80000,0x00000000ffff8030,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 7168K, used 6697K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
object space 7168K, 93% used [0x00000000ff600000,0x00000000ffc8a770,0x00000000ffd00000)
Metaspace used 3447K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 375K, capacity 388K, committed 512K, reserved 1048576K
}
{Heap before GC invocations=5 (full 1):
PSYoungGen total 2560K, used 480K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000fff00000)
from space 512K, 93% used [0x00000000fff80000,0x00000000ffff8030,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 7168K, used 6697K [0x00000000ff600000, 0x00000000ffd00000, 0x00000000ffd00000)
object space 7168K, 93% used [0x00000000ff600000,0x00000000ffc8a770,0x00000000ffd00000)
Metaspace used 3447K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 375K, capacity 388K, committed 512K, reserved 1048576K
Heap after GC invocations=5 (full 1):
PSYoungGen total 2560K, used 0K [0x00000000ffd00000, 0x0000000100000000, 0x0000000100000000)
eden space 2048K, 0% used [0x00000000ffd00000,0x00000000ffd00000,0x00000000fff00000)
from space 512K, 0% used [0x00000000fff80000,0x00000000fff80000,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 4096K, used 1828K [0x00000000ff600000, 0x00000000ffa00000, 0x00000000ffd00000)
object space 4096K, 44% used [0x00000000ff600000,0x00000000ff7c9020,0x00000000ffa00000)
Metaspace used 3447K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 375K, capacity 388K, committed 512K, reserved 1048576K
}- -Xmx10m -Xms5m -XX:+TraceClassLoading
作用：监控类的加载
参数：
-Xmx10m -Xms5m -XX:+TraceClassLoading[Opened D:\Dev_Tools\Java\jdk1.8.0_161\jre\lib\rt.jar]
[Loaded java.lang.Object from D:\Dev_Tools\Java\jdk1.8.0_161\jre\lib\rt.jar]
[Loaded java.io.Serializable from D:\Dev_Tools\Java\jdk1.8.0_161\jre\lib\rt.jar]
......
[Loaded java.lang.Shutdown from D:\Dev_Tools\Java\jdk1.8.0_161\jre\lib\rt.jar]
[Loaded java.lang.Shutdown$Lock from D:\Dev_Tools\Java\jdk1.8.0_161\jre\lib\rt.jar]- -XX:+PrintClassHistogram
作用：打印类的信息，包含实例数量、总大小、类型等
参数：
-Xmx10m -Xms5m -XX:+PrintClassHistogram按下Ctrl+Break后，打印类的信息
num #instances #bytes class name
----------------------------------------------
1: 46 1082640 [B
2: 3013 416608 [C
3: 639 72976 java.lang.Class
4: 2863 68712 java.lang.String
5: 624 36816 [Ljava.lang.Object;
6: 791 31640 java.util.TreeMap$Entry
7: 782 18768 java.util.LinkedList$Node
8: 485 15520 java.util.HashMap$Node
9: 394 12608 java.util.LinkedList
......
堆分配参数
- -Xmx –Xms
作用：指定最大堆和最小堆。在分配堆空间时，最好保持-Xmx –Xms数值一致，以免
参数：-Xmx20m -Xms5m
byte[] bytes = new byte[1 * 1024 * 1024];
// 堆总大小
System.out.print("Xmx=");
System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");
// 当前剩余可用空间
System.out.print("free mem=");
System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");
// 当前总共分配空间
System.out.print("total mem=");
System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");
分配1M空间给数组
Xmx=18.0M
free mem=4.182762145996094M
total mem=6.5M由于Java会尽可能维持在最小堆运行，所以total mem会尽可能保持较小的值。
byte[] bytes = new byte[6 * 1024 * 1024];
// 堆总大小
System.out.print("Xmx=");
System.out.println(Runtime.getRuntime().maxMemory()/1024.0/1024+"M");
// 当前剩余可用空间
System.out.print("free mem=");
System.out.println(Runtime.getRuntime().freeMemory()/1024.0/1024+"M");
// 当前总共分配空间
System.out.print("total mem=");
System.out.println(Runtime.getRuntime().totalMemory()/1024.0/1024+"M");
分配6M空间给数组
Xmx=18.0M
free mem=4.718879699707031M
total mem=12.0M- -Xmn
作用：设置新生代大小。官方推荐新生代占堆的3/8。
- -XX:NewRatio
作用：设置新生代（eden+2*s）和老年代（不包含永久区）的比值。比如-XX:NewRatio=4表示： 新生代:老年代 = 1:4，即新生代占1/5
- -XX:SurvivorRatio
作用：设置两个Survivor区和eden的比。比如-XX:SurvivorRatio=8表示：两个Survivor:eden=2:8，即一个Survivor占新生代的1/10。官方推荐Survivor占新生代的1/10。
- -XX:+HeapDumpOnOutOfMemoryError
作用：OOM时将堆信息导出到文件中
- -XX:+HeapDumpPath
作用：指定OOM时导出的文件路径
参数：-Xmx20m -Xms5m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=d:/a.dump
public class Test {
public static void main(String[] args) {
Vector vector = new Vector();
for (int index = 0; index < 25; index++) {
vector.add(new byte[1 * 1024 * 1024]);
}
}
}
java.lang.OutOfMemoryError: Java heap space
Dumping heap to d:/a.dump ...
Heap dump file created [15379974 bytes in 0.049 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
at com.example.demo.test.Test.main(Test.java:14)
- -XX:OnOutOfMemoryError
作用：在OOM时，执行一个脚本。可以在OOM时发送邮件告警、甚至重启程序。
- -XX:PermSize -XX:MaxPermSize
作用：设置永久区的初始空间和最大空间，与-Xmx –Xms类似。
堆分配实例
public class Test {
public static void main(String[] args) {
byte[] bytes = null;
for (int index = 0; index < 10; index++) {
bytes = new byte[1 * 1024 * 1024];
}
}
}
- -Xmx20m -Xms20m -Xmn1m -XX:+PrintGCDetails
解释说明：堆大小为20M，新生代大小为1M。
[GC (Allocation Failure) [PSYoungGen: 512K->504K(1024K)] 512K->512K(19968K), 0.0008422 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 1016K->504K(1024K)] 1024K->672K(19968K), 0.0009345 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 1016K->504K(1024K)] 1184K->847K(19968K), 0.0007328 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 1016K->504K(1024K)] 1359K->927K(19968K), 0.0005718 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 1024K, used 830K [0x00000000ffe80000, 0x0000000100000000, 0x0000000100000000)
eden space 512K, 63% used [0x00000000ffe80000,0x00000000ffed1888,0x00000000fff00000)
from space 512K, 98% used [0x00000000fff80000,0x00000000ffffe010,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 18944K, used 10663K [0x00000000fec00000, 0x00000000ffe80000, 0x00000000ffe80000)
object space 18944K, 56% used [0x00000000fec00000,0x00000000ff669f88,0x00000000ffe80000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K
Java HotSpot(TM) 64-Bit Server VM warning: NewSize (1536k) is greater than the MaxNewSize (1024k). A new max generation size of 1536k will be used.新生代无法分配（Eden和两个Survivor总大小为1M），对象直接在老年代分配。总共分配对象大小为10M，并未触发Full GC，所以老年代空间使用在10M左右，并没有回收。
注意最后一行，虚拟机提示新生代大小最好设置为1536K。
- -Xmx20m -Xms20m -Xmn1536K -XX:+PrintGCDetails
[GC (Allocation Failure) [PSYoungGen: 512K->504K(1024K)] 512K->544K(19968K), 0.0006121 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 1016K->504K(1024K)] 1056K->672K(19968K), 0.0008765 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 1016K->504K(1024K)] 1184K->819K(19968K), 0.0007404 secs] [Times: user=0.09 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 1016K->504K(1024K)] 1331K->963K(19968K), 0.0007809 secs] [Times: user=0.02 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 1024K, used 823K [0x00000000ffe80000, 0x0000000100000000, 0x0000000100000000)
eden space 512K, 62% used [0x00000000ffe80000,0x00000000ffecfc58,0x00000000fff00000)
from space 512K, 98% used [0x00000000fff80000,0x00000000ffffe010,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 18944K, used 10699K [0x00000000fec00000, 0x00000000ffe80000, 0x00000000ffe80000)
object space 18944K, 56% used [0x00000000fec00000,0x00000000ff672d50,0x00000000ffe80000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K貌似没啥效果！
- -Xmx20m -Xms20m -Xmn15m -XX:+PrintGCDetails
解释说明：堆大小为20M，新生代大小为15M。
[GC (Allocation Failure) [PSYoungGen: 12249K->1520K(13824K)] 12249K->1969K(18944K), 0.0011009 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 13824K, used 2667K [0x00000000ff100000, 0x0000000100000000, 0x0000000100000000)
eden space 12288K, 9% used [0x00000000ff100000,0x00000000ff21ed08,0x00000000ffd00000)
from space 1536K, 98% used [0x00000000ffd00000,0x00000000ffe7c020,0x00000000ffe80000)
to space 1536K, 0% used [0x00000000ffe80000,0x00000000ffe80000,0x0000000100000000)
ParOldGen total 5120K, used 449K [0x00000000fec00000, 0x00000000ff100000, 0x00000000ff100000)
object space 5120K, 8% used [0x00000000fec00000,0x00000000fec706d0,0x00000000ff100000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K对象全部分配在新生代中，老年代基本没有使用。新生代触发GC并回收了一部分内存（Survivor大小为1536K，因此GC时不需要老年代进行担保，即对象不会进入老年代），只有部分对象依然存在于内存中，大部分对象已被回收。
- -Xmx20m -Xms20m -Xmn7m -XX:+PrintGCDetails
解释说明：堆大小为20M，新生代大小为7M。
[GC (Allocation Failure) [PSYoungGen: 5557K->488K(6656K)] 5557K->1909K(19968K), 0.0010793 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 5724K->488K(6656K)] 7146K->2969K(19968K), 0.0012265 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 6656K, used 2708K [0x00000000ff900000, 0x0000000100000000, 0x0000000100000000)
eden space 6144K, 36% used [0x00000000ff900000,0x00000000ffb2b110,0x00000000fff00000)
from space 512K, 95% used [0x00000000fff80000,0x00000000ffffa040,0x0000000100000000)
to space 512K, 0% used [0x00000000fff00000,0x00000000fff00000,0x00000000fff80000)
ParOldGen total 13312K, used 2481K [0x00000000fec00000, 0x00000000ff900000, 0x00000000ff900000)
object space 13312K, 18% used [0x00000000fec00000,0x00000000fee6c6e0,0x00000000ff900000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K对象全部分配在新生代，但是新生代GC时Survivor大小不足1M，部分对象进入老年代，还有一部分对象在GC时被回收。
- -Xmx20m -Xms20m -Xmn7m -XX:SurvivorRatio=2 -XX:+PrintGCDetails
解释说明：堆大小为20M，新生代大小为7M，但是增大Survivor大小为新生代的1/4。
[GC (Allocation Failure) [PSYoungGen: 3457K->1000K(5632K)] 3457K->2032K(18944K), 0.0015567 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 4148K->1528K(5632K)] 5180K->2947K(18944K), 0.0010745 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 4669K->1520K(5632K)] 6088K->3019K(18944K), 0.0006911 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 5632K, used 4747K [0x00000000ff900000, 0x0000000100000000, 0x0000000100000000)
eden space 4096K, 78% used [0x00000000ff900000,0x00000000ffc26cd8,0x00000000ffd00000)
from space 1536K, 98% used [0x00000000ffd00000,0x00000000ffe7c050,0x00000000ffe80000)
to space 1536K, 0% used [0x00000000ffe80000,0x00000000ffe80000,0x0000000100000000)
ParOldGen total 13312K, used 1499K [0x00000000fec00000, 0x00000000ff900000, 0x00000000ff900000)
object space 13312K, 11% used [0x00000000fec00000,0x00000000fed76c80,0x00000000ff900000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K由于扩大了Survivor大小，进入老年代的对象减少。
- -Xmx20m -Xms20m -Xmn7m -XX:SurvivorRatio=1 -XX:+PrintGCDetails
解释说明：堆大小为20M，新生代大小为7M，但是增大Survivor大小为新生代的1/3。
[GC (Allocation Failure) [PSYoungGen: 2441K->904K(5120K)] 2441K->912K(18432K), 0.0018878 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 3007K->1976K(5120K)] 3015K->1984K(18432K), 0.0009336 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 4132K->1960K(5120K)] 4140K->1968K(18432K), 0.0008194 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 4059K->1960K(5120K)] 4067K->1968K(18432K), 0.0006821 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 4062K->1960K(5120K)] 4070K->1968K(18432K), 0.0006000 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 5120K, used 4126K [0x00000000ff900000, 0x0000000100000000, 0x0000000100000000)
eden space 3072K, 70% used [0x00000000ff900000,0x00000000ffb1d810,0x00000000ffc00000)
from space 2048K, 95% used [0x00000000ffc00000,0x00000000ffdea030,0x00000000ffe00000)
to space 2048K, 0% used [0x00000000ffe00000,0x00000000ffe00000,0x0000000100000000)
ParOldGen total 13312K, used 8K [0x00000000fec00000, 0x00000000ff900000, 0x00000000ff900000)
object space 13312K, 0% used [0x00000000fec00000,0x00000000fec02000,0x00000000ff900000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K继续扩大Survivor大小，没有进入老年代的对象。
- -Xmx20m -Xms20m -XX:NewRatio=1 -XX:SurvivorRatio=2 -XX:+PrintGCDetails
解释说明：堆大小为20M，新生代与老年代比例为1:1，即新生代占一半内存空间（10M），并且新生代中两个Survivor:Eden比值为2:2，即每个Survivor占新生代1/4空间（2.5M）。
[GC (Allocation Failure) [PSYoungGen: 4564K->2056K(7680K)] 4564K->2064K(17920K), 0.0012851 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]
[GC (Allocation Failure) [PSYoungGen: 6248K->1976K(7680K)] 6256K->1984K(17920K), 0.0010730 secs] [Times: user=0.03 sys=0.00, real=0.00 secs]
Heap
PSYoungGen total 7680K, used 6315K [0x00000000ff600000, 0x0000000100000000, 0x0000000100000000)
eden space 5120K, 84% used [0x00000000ff600000,0x00000000ffa3ce18,0x00000000ffb00000)
from space 2560K, 77% used [0x00000000ffd80000,0x00000000fff6e040,0x0000000100000000)
to space 2560K, 0% used [0x00000000ffb00000,0x00000000ffb00000,0x00000000ffd80000)
ParOldGen total 10240K, used 8K [0x00000000fec00000, 0x00000000ff600000, 0x00000000ff600000)
object space 10240K, 0% used [0x00000000fec00000,0x00000000fec02000,0x00000000ff600000)
Metaspace used 3454K, capacity 4496K, committed 4864K, reserved 1056768K
class space used 376K, capacity 388K, committed 512K, reserved 1048576K对象全部在新生代分配，且没有对象进入老年代。
栈分配参数
- -Xss
作用：指定栈大小，通常只有几百K，决定了函数调用的深度。
垃圾收集器相关参数
- -XX:+UseSerialGC：在新生代和老年代使用串行收集器
- -XX:+UseParNewGC：在新生代使用并行收集器
- -XX:+UseParallelGC ：在新生代使用并行收集器，吞吐量优先
- -XX:+UseParallelOldGC：在老年代使用并行收集器，吞吐量优先
- -XX:ParallelGCThreads：设置用于并行垃圾回收的线程数，以CPU核数为依据进行设置
- -XX:+UseConcMarkSweepGC：新生代使用并行收集器，老年代使用CMS+串行收集器（CMS为并发收集器，进行回收时程序依然在运行。程序可能在内存使用将满进行垃圾回收时，继续申请内存从而导致回收失败，此时使用备用的串行收集器进行垃圾回收）
- -XX:MaxGCPauseMills：最大停顿时间（毫秒），GC尽力保证每次回收时间不超过设定值
- -XX:GCTimeRatio：垃圾收集时间占总时间的比值。默认为99，即最大允许1%的时间做GC。
- -XX:ParallelCMSThreads：设定CMS的线程数量
- -XX:CMSInitiatingOccupancyFraction：设置CMS收集器在老年代空间被使用多少后触发（防止CMS回收失败）
- -XX:+UseCMSCompactAtFullCollection：设置CMS收集器在完成垃圾收集后是否要进行一次内存碎片的整理（CMS使用标记-清除算法，可能会有碎片存在）
- -XX:CMSFullGCsBeforeCompaction：设定进行多少次CMS垃圾回收后，进行一次内存压缩（即碎片整理）
- -XX:+CMSClassUnloadingEnabled：允许对类元数据进行回收
- -XX:CMSInitiatingPermOccupancyFraction：当永久区占用率达到这一百分比时，启动CMS回收
- -XX:UseCMSInitiatingOccupancyOnly：表示只在到达阀值的时候，才进行CMS回收
-XX:MaxGCPauseMills和-XX:GCTimeRatio两个参数是互相矛盾的，因为停顿时间和吞吐量不可能同时调优。
垃圾回收的工作总量基本不变，如果停顿时间设置较小，势必需要频繁GC（每次GC都需要先标记再回收）才能完成全部回收工作，那么GC所占用的时间会更多（用于响应请求的时间会相对较少，吞吐量相对较低）；如果想要吞吐量较高，则GC占用时间相应较少，势必需要减少GC次数，那么每次GC的停顿时间相对较长。
垃圾回收的工作总量基本不变，如果停顿时间设置较小，势必需要频繁GC（每次GC都需要先标记再回收）才能完成全部回收工作，那么GC所占用的时间会更多（用于响应请求的时间会相对较少，吞吐量相对较低）；如果想要吞吐量较高，则GC占用时间相应较少，势必需要减少GC次数，那么每次GC的停顿时间相对较长。