一、方法一

打印虛擬機所有參數

[root@localhost ~]# java -XX:+PrintFlagsFinal -version | grep :

uintx InitialHeapSize := 258689024 {product}

uintx MaxHeapSize := 4139778048 {product}

bool PrintFlagsFinal := true {product}

bool UseCompressedOops := true {lp64_product}

bool UseParallelGC := true {product}

二、方法二

[root@localhost ~]# java -XX:+PrintCommandLineFlags -version

-XX:InitialHeapSize=258689024 -XX:MaxHeapSize=4139024384 -XX:+PrintCommandLineFlags -XX:+UseCompressedOops-XX:+UseParallelGC

參數解讀：

針對上述的-XX：UseParallelGC，這邊我們引用《深入理解Java虛擬機：JVM高級特性與最佳實踐》的介紹：

也就是說，打開此開關，使用的垃圾收集器是：新生代(Parallel Scavenge)，老年代(Ps MarkSweep)組合。

jdk1.7 默認垃圾收集器Parallel Scavenge(新生代)+Parallel Old(老年代)

jdk1.8 默認垃圾收集器Parallel Scavenge(新生代)+Parallel Old(老年代)

jdk1.9 默認垃圾收集器G1

三、方法三:

[root@iZ2zeapchxZ tomcat7]# jmap -heap 13297

Attaching to process ID 13297, please wait...

Debugger attached successfully.

Server compiler detected.

JVM version is 25.77-b03

using parallel threads in the new generation.

using thread-local object allocation.

Concurrent Mark-Sweep GC

Heap Configuration:

MinHeapFreeRatio = 40

MaxHeapFreeRatio = 70

MaxHeapSize = 1073741824 (1024.0MB)

NewSize = 87228416 (83.1875MB)

MaxNewSize = 87228416 (83.1875MB)

OldSize = 986513408 (940.8125MB)

NewRatio = 2

SurvivorRatio = 8

MetaspaceSize = 21807104 (20.796875MB)

CompressedClassSpaceSize = 1073741824 (1024.0MB)

MaxMetaspaceSize = 17592186044415 MB

G1HeapRegionSize = 0 (0.0MB)

Heap Usage:

New Generation (Eden + 1 Survivor Space):

capacity = 78512128 (74.875MB)

used = 17016048 (16.227767944335938MB)

free = 61496080 (58.64723205566406MB)

21.673145835507096% used

Eden Space:

capacity = 69795840 (66.5625MB)

used = 10923608 (10.417564392089844MB)

free = 58872232 (56.144935607910156MB)

15.650800964642018% used

From Space:

capacity = 8716288 (8.3125MB)

used = 6092440 (5.810203552246094MB)

free = 2623848 (2.5022964477539062MB)

69.89718559093045% used

To Space:

capacity = 8716288 (8.3125MB)

used = 0 (0.0MB)

free = 8716288 (8.3125MB)

0.0% used

concurrent mark-sweep generation:

capacity = 986513408 (940.8125MB)

used = 41311272 (39.397499084472656MB)

free = 945202136 (901.4150009155273MB)

4.187603702594583% used

19916 interned Strings occupying 2422360 bytes.

Concurrent Mark-Sweep GC:CMS回收器

Mark Sweep Compact GC:串行GC(Serial GC)

Parallel GC with 2 thread(s):并行GC(ParNew)

如何修改GC回收器:設置 JAVA_OPTS

總結

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