S0、S1 分別代表了 Survivor0 和 Survivor1，E 代表 Eden 區，O 代表老年區， P 代表永久代。YGC 代表 Young GC 的次數，YGCT 代表時間，后面一樣解釋。
Usage:
jinfo [option] <pid>
(to connect to running process)
jinfo [option] <executable <core>
(to connect to a core file)
jinfo [option] [server_id@]<remote server IP or hostname>
(to connect to remote debug server)
where <option> is one of:
-flag <name> to print the value of the named VM flag
-flag [+|-]<name> to enable or disable the named VM flag
-flag <name>=<value> to set the named VM flag to the given value
-flags to print VM flags
-sysprops to print Java system properties
<no option> to print both of the above
-h | -help to print this help message</pre>
jmap 生產 java 內存 dump
jmap 除了可以生成 dump 文件外，還可以查詢 finalize 執行隊列，java 堆和永久代的詳細信息，如空間使用率和當前用的是哪種收集器等。
具體的 jamp 如何操作部多介紹，看下面提供的說明，比較簡單：
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
-permstat to print permanent generation 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</pre>
jhat 虛擬機堆快照分析工具
我們可以使用 jhat 來分析 jmap 生成的 dump 文件
>jhat tmp.dump
Reading from tmp.dump...
Dump file created Sat May 09 17:10:52 CST 2015
Snapshot read, resolving...
Resolving 0 objects...
WARNING: hprof file does not include java.lang.Class!
WARNING: hprof file does not include java.lang.String!
WARNING: hprof file does not include java.lang.ClassLoader!
Chasing references, expect 0 dots
Eliminating duplicate references
Snapshot resolved.
Started HTTP server on port 7000
Server is ready.
默認會開 7000 端口進行 web 訪問。一般不使用這個命令來分析，會使用專業的工具來分析 dump 文件，如 eclipse memory analyzer 等。
jstack 分析 java 堆棧
jstack 用來生成當前時刻線程快照。
使用方式如下：
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</pre>
【參考資料】
- 深入理解 JAVA 虛擬機
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