jcmd、jstack、jstat、jmap的應用

背景

說明

java堆棧信息應用

jcmd: Java cmd

功能： 導出堆、查看Java進程、導出線程信息、執行GC

我一般用來查看機器上的jvm進程pid, 和jps是一樣的

jcmd
jps

jstack: Java stack

功能：分析java應用的線程堆棧信息

在代碼設計層面、code review基本就杜絕了,所以我在實際中沒有使用過該工具

Usage:
    jstack [-l] <pid>
        (to connect to running process)
    jstack -F [-m] [-l] <pid>
        (to connect to a hung process)
    jstack [-m] [-l] <executable> <core>
        (to connect to a core file)
    jstack [-m] [-l] [server_id@]<remote server IP or hostname>
        (to connect to a remote debug server)

Options:
    -F  to force a thread dump. Use when jstack <pid> does not respond (process is hung)
    -m  to print both java and native frames (mixed mode)
    -l  long listing. Prints additional information about locks
    -h or -help to print this help message

jstat

功能：用於查看堆內存各年代的使用量，加載類的使用量

當發生堆溢出導致項目無法使用的時候，用jstat分析各內存使用情況，分析gc情況，使用爲 jstat -gcold pid

Usage: jstat -help|-options
       jstat -<option> [-t] [-h<lines>] <vmid> [<interval> [<count>]]

Definitions:
  <option>      An option reported by the -options option
  <vmid>        Virtual Machine Identifier. A vmid takes the following form:
                     <lvmid>[@<hostname>[:<port>]]
                Where <lvmid> is the local vm identifier for the target
                Java virtual machine, typically a process id; <hostname> is
                the name of the host running the target Java virtual machine;
                and <port> is the port number for the rmiregistry on the
                target host. See the jvmstat documentation for a more complete
                description of the Virtual Machine Identifier.
  <lines>       Number of samples between header lines.
  <interval>    Sampling interval. The following forms are allowed:
                    <n>["ms"|"s"]
                Where <n> is an integer and the suffix specifies the units as 
                milliseconds("ms") or seconds("s"). The default units are "ms".
  <count>       Number of samples to take before terminating.
  -J<flag>      Pass <flag> directly to the runtime system.



--options:

-class              類加載器
-compiler           JIT
-gc                 gc堆狀態
-gccapacity         各區大小
-gccause            最近一次Gc信息統計
-gcmetacapacity     
-gcnew
-gcnewcapacity
-gcold
-gcoldcapacity
-gcutil             Gc統計彙總
-printcompilation   HotSpot編譯統計

jmap

功能：查看堆內對象的統計信息，ClassLoader 的信息以及 finalizer 隊列。可以生成java程序的dump程序

我一般只應用 jmap histo pid 查看對象的統計信息

Usage:
    jmap [option] <pid>
        (to connect to running process)
    jmap [option] <executable <core>
        (to connect to a core file)
    jmap [option] [server_id@]<remote server IP or hostname>
        (to connect to remote debug server)

where <option> is one of:
    <none>               to print same info as Solaris pmap
    -heap                to print java heap summary
    -histo[:live]        to print histogram of java object heap; if the "live"
                         suboption is specified, only count live objects
    -clstats             to print class loader statistics
    -finalizerinfo       to print information on objects awaiting finalization
    -dump:<dump-options> to dump java heap in hprof binary format
                         dump-options:
                           live         dump only live objects; if not specified,
                                        all objects in the heap are dumped.
                           format=b     binary format
                           file=<file>  dump heap to <file>
                         Example: jmap -dump:live,format=b,file=heap.bin <pid>
    -F                   force. Use with -dump:<dump-options> <pid> or -histo
                         to force a heap dump or histogram when <pid> does not
                         respond. The "live" suboption is not supported
                         in this mode.
    -h | -help           to print this help message
    -J<flag>             to pass <flag> directly to the runtime system

堆棧溢出的可能性

堆

1. poi非流式導出大量數據
2. 全局容器存儲對象的引用導致對象無法Gc
3. new 對象太多
4. 做業務計算時使用的對象太多 select * from table;

棧 程序所要求的棧深度過大

1. 遞歸導致
2. 死循環