jmap
Prints shared object memory maps or heap memory details for a process, core file, or remote debug server. This command is experimental and unsupported.
为Java进程、core文件或远程debug服务器打印共享对象映射或堆内存细节。
用法
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
参数
<no option>
将会打印目标虚拟机中加载的每个共享对象的起始地址、映射大小以及共享对象文件的路径全称。这与Solaris的pmap工具比较相似。
-heap
打印一个堆的概要的信息。
举个例子大致解释下：
>jmap -heap 5232
Attaching to process ID 5232, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.101-b13 //版本
using parallel threads in the new generation. //说明新生代使用parnew
using thread-local object allocation.
Concurrent Mark-Sweep GC //说明老年代使用CMS垃圾收集器
Heap Configuration: //配置信息
MinHeapFreeRatio = 40 //-XX:MinHeapFreeRatio， minimum allowed percentage of free heap space (0 to 100) after a GC event. 默认值是40
MaxHeapFreeRatio = 70 //-XX:MaxHeapFreeRatio， the maximum allowed percentage of free heap space (0 to 100) after a GC event. 默认值是70
MaxHeapSize = 786432000 (750.0MB) //-XX:MaxHeapSize=size, 最大heap size
NewSize = 44695552 (42.625MB) //-XX:NewSize=szie, 新生代大小
MaxNewSize = 262144000 (250.0MB) //-XX:MaxNewSize=size，最大的新生代大小
OldSize = 89522176 (85.375MB) //老年代大小，没有对应的java启动参数
NewRatio = 2 //-XX:NewRatio=ratio, 新生代和老生代的大小比例
SurvivorRatio = 8 //-XX:SurvivorRatio=ratio，eden区和其中一个survivor区的比例（注意，是与其中一个survivor区，因为两个survivor区必是等大小）
MetaspaceSize = 21807104 (20.796875MB)//-XX:MaxMetaspaceSize=size，metaspace大小
CompressedClassSpaceSize = 1073741824 (1024.0MB) //-XX:CompressedClassSpaceSize，默认1G，这个参数主要是设置Klass Metaspace的大小，不过这个参数设置了也不一定起作用，前提是能开启压缩指针，假如-Xmx超过了32G，压缩指针是开启不来的。如果有Klass Metaspace，那这块内存是和Heap连着的。更详细的解读请参考[你假笨 | JVM源码分析之Metaspace解密](http://lovestblog.cn/blog/2016/10/29/metaspace/)
MaxMetaspaceSize = 17592186044415 MB //-XX:MaxMetaspaceSize=size,the maximum amount of native memory that can be allocated for class metadata.默认是不限制大小的
G1HeapRegionSize = 0 (0.0MB) //-XX:G1HeapRegionSize=size，如果使用g1垃圾收集器，那么这里显示的是设定的region大小，默认region是1m到32m。
Heap Usage: //Heap使用
New Generation (Eden + 1 Survivor Space):
capacity = 83427328 (79.5625MB)
used = 39811536 (37.96723937988281MB)
free = 43615792 (41.59526062011719MB)
47.72001807369403% used
Eden Space:
capacity = 74186752 (70.75MB)
used = 35999512 (34.331809997558594MB)
free = 38187240 (36.418190002441406MB)
48.52552649831603% used
From Space:
capacity = 9240576 (8.8125MB)
used = 3812024 (3.6354293823242188MB)
free = 5428552 (5.177070617675781MB)
41.25309937389184% used
To Space:
capacity = 9240576 (8.8125MB)
used = 0 (0.0MB)
free = 9240576 (8.8125MB)
0.0% used
concurrent mark-sweep generation: //老年代
capacity = 185057280 (176.484375MB)
used = 109427528 (104.35822296142578MB)
free = 75629752 (72.12615203857422MB)
59.13170668022355% used
-histo[:live]
Prints a histogram of the heap.For each Java class, the number of objects, memory size in bytes, and the fully qualified class names are printed.
打印一个条形图(histogram)，里面包含对应每个Java类的对象数、内存占用大小(byte)、类全称的数据。(不懂为何叫histogram，在我看来只是个破表格。。)。
当加上:live时，只打印存活对象。
-clstats
输出各Classloader的数据统计。
-finalizerinfo
打印等待终结的对象信息。
-dump:[live,]format=b,file=<filename>
生成hprof格式的java heap dump文件。可使用jhat或一些profiler工具分析。
-F
强制输出选项。当使用jmap -dump或jmap -histo选项没响应时可在原命令中添加-F再运行。注：此option不支持live子选项。
-J<flag>
指定传递给运行jmap的java进程的JVM参数。