摘要：系列垃圾回收基礎的編譯策略的三大基礎算法的三大高級算法策略的評價指標信息查看通用日志解讀的數據結構類初始化順序對象結構及大小計算的類加載機制對象分配簡要流程年老代過大有什么影響空間溢出實例關于線程與序本文主要講述如何查看應用的信息。

jvm系列

垃圾回收基礎

JVM的編譯策略

GC的三大基礎算法

GC的三大高級算法

GC策略的評價指標

JVM信息查看

GC通用日志解讀

jvm的card table數據結構

Java類初始化順序

Java對象結構及大小計算

Java的類加載機制

Java對象分配簡要流程

年老代過大有什么影響

Survivor空間溢出實例

關于Object=null

Java線程與Xss

本文主要講述如何查看java應用的jvm信息。

Java -XX:+PrintCommandLineFlags 這個參數的作用是顯示出VM初始化完畢后所有跟最初的默認值不同的參數及它們的值。 （JDK5以上支持）

-XX:+PrintFlagsFinal顯示所有可設置的參數及它們的值（ JDK 6 update 21開始才可以用），默認是不包括diagnostic或experimental系的。要在-XX:+PrintFlagsFinal的輸出里看到這兩種參數的信息，分別需要顯式指定-XX:+UnlockDiagnosticVMOptions / -XX:+UnlockExperimentalVMOptions

-XX:+PrintFlagsInitial看下所有XX參數的默認值，也可以通過-XX:+PrintFlagsFinal進行對比出修改過的參數，當然通過-XX:+PrintCommandLineFlags就可以自動顯示出修改過的值

jinfo -flags pid

[xixicat@cloud01 ~]$ jps 
9275 Jps
22855 start.jar
22869 XmlConfiguration
20967 Application
[xixicat@cloud01 ~]$ jinfo -flags 22869
Attaching to process ID 22869, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 23.21-b01
-javaagent:/opt/xixicat/apps/lib/jolokia-jvm-1.1.0-agent.jar=port=23061 -Xloggc:/var/xixicat/logs/app.gc.log.201504301631 -XX:ErrorFile=/var/xixicat/logs/app.vmerr.log.201504301631 -XX:HeapDumpPath=/var/xixicat/logs/app.heaperr.log.201504301631 -Xmx5g -Xms5g -XX:MaxPermSize=256m -XX:+HeapDumpOnOutOfMemoryError -XX:+DisableExplicitGC -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -XX:+PrintGCApplicationStoppedTime -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+PrintCommandLineFlags -XX:+PrintAdaptiveSizePolicy -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC -XX:MaxGCPauseMillis=100 -XX:StringTableSize=49999 -Djetty.home=/opt/xixicat/apps/server/jetty-distribution-8.1.9.v20130131 -Dapp.port=8061 -Dmedis_environment=online -Dcore.step=app -DSTOP.PORT=38061 -Djetty.port=8061 -Dcom.sun.management.jmxremote.authenticate=false -Dapp.logdir=/var/xixicat/logs -DLog4jContextSelector=org.apache.logging.log4j.core.async.AsyncLoggerContextSelector -Dapp.ip=10.64.28.207 -Dapp.context -Dcom.sun.management.jmxremote.ssl=false -Djetty.context -Djava.io.tmpdir=/tmp -Djetty.host=cloud01 -Djava.net.preferIPv6Addresses=false -DSTOP.KEY=stop_app -Dcore.zookeeper=127.0.0.1:9331 -Djetty.logs=/var/xixicat/logs -Dapp.workdir=/opt/xixicat/apps/app/work -Dapp.key=app -Dfile.encoding=UTF-8 -Dapp.host=cloud01 -Duser.timezone=GMT+08 -Djetty.appkey=app -DAsyncLogger.WaitStrategy=Block -Dcom.sun.management.jmxremote.port=8199 -Djetty.home=/opt/xixicat/apps/server/jetty-distribution-8.1.9.v20130131 -Denvironment=online

java -XX:+PrintFlagsFinal -version |grep manageable
通過選項-XX:+PrintFlagsFinal可以列出所有的JVM flag，而其中的標注為manageable 的flag則是值得我們關注的部分。這些flag可通過JDK management interface（-XX:+PrintFlagsFinal）動態(tài)修改。

-XX:+PrintGCDetails

jinfo -flag -PrintGCDetails 12278

[xixicat@cloud01 ~]$ jmap -heap 22869
Attaching to process ID 22869, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 23.21-b01
using thread-local object allocation.
Garbage-First (G1) GC with 4 thread(s)
Heap Configuration:
   MinHeapFreeRatio = 40
   MaxHeapFreeRatio = 70
   MaxHeapSize      = 5368709120 (5120.0MB)
   NewSize          = 1363144 (1.2999954223632812MB)
   MaxNewSize       = 17592186044415 MB
   OldSize          = 5452592 (5.1999969482421875MB)
   NewRatio         = 2
   SurvivorRatio    = 8
   PermSize         = 20971520 (20.0MB)
   MaxPermSize      = 268435456 (256.0MB)
   G1HeapRegionSize = 2097152 (2.0MB)
Heap Usage:
G1 Heap:
   regions  = 2560
   capacity = 5368709120 (5120.0MB)
   used     = 3826721792 (3649.4462890625MB)
   free     = 1541987328 (1470.5537109375MB)
   71.27824783325195% used
G1 Young Generation:
Eden Space:
   regions  = 1068
   capacity = 2808086528 (2678.0MB)
   used     = 2239758336 (2136.0MB)
   free     = 568328192 (542.0MB)
   79.76101568334578% used
Survivor Space:
   regions  = 29
   capacity = 60817408 (58.0MB)
   used     = 60817408 (58.0MB)
   free     = 0 (0.0MB)
   100.0% used
G1 Old Generation:
   regions  = 1000
   capacity = 2499805184 (2384.0MB)
   used     = 1524048896 (1453.4462890625MB)
   free     = 975756288 (930.5537109375MB)
   60.96670675597735% used
Perm Generation:
   capacity = 171966464 (164.0MB)
   used     = 170752872 (162.8426284790039MB)
   free     = 1213592 (1.1573715209960938MB)
   99.29428565792921% used
48213 interned Strings occupying 5246936 bytes.

查看對象分布

[xixicat@cloud01 ~]$ jcmd 15308 VM.flags
15308:
-XX:+DisableExplicitGC -XX:ErrorFile=/var/xixicat/logs/app.vmerr.log.201505071655 -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/xixicat/logs/app.heaperr.log.201505071655 -XX:InitialHeapSize=5368709120 -XX:+ManagementServer -XX:MaxGCPauseMillis=100 -XX:MaxHeapSize=5368709120 -XX:MaxPermSize=268435456 -XX:+PrintAdaptiveSizePolicy -XX:+PrintCommandLineFlags -XX:+PrintGC -XX:+PrintGCApplicationStoppedTime -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:StringTableSize=49999 -XX:+UnlockExperimentalVMOptions -XX:+UseCompressedOops -XX:+UseG1GC