本文继续介绍Java自带的性能监测工具，本文使用jmap工具来玩~

jmap (Java Memory Map) 命令可以生成Java应用程序的堆快照和对象统计信息，对生成的堆快照进行分析，可以分析堆中对象所占用内存的情况，检查大对象等~

先使用jmap -help查看一下jmap命令的基本语法以及选择项(option)说明~

[root@dev18 ~]# jmap -help

Usage:

jmap [option]

(to connect to running process)

jmap [option]

(to connect to a core file)

jmap [option] [server_id@]

(to connect to remote debug server)

where is one of:

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: 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= dump heap to

Example: jmap -dump:live,format=b,file=heap.bin

-F force. Use with -dump: or -histo

to force a heap dump or histogram when does not

respond. The "live" suboption is not supported

in this mode.

-h | -help to print this help message

-J to pass directly to the runtime system

[root@dev18 ~]#

jmap基本语法：

jmap [option]

jmap需要pid，所以需要和jps配合使用，也就是先使用jps获取pid信息，然后使用jmap来处理~

[root@dev18 ~]# jps -l

1656 org.elasticsearch.bootstrap.Elasticsearch

12905 org.apache.zookeeper.server.quorum.QuorumPeerMain

4308 /srv/activemq/apache-activemq-5.14.3//bin/activemq.jar

27265 sun.tools.jps.Jps

[root@dev18 ~]#

接下来，本文使用Zookeeper的服务进程为例，其pid为12905

使用 jmap -heap 12905打印java heap的情况，如

[root@dev18 ~]# jmap -heap 12905

Attaching to process ID 12905, please wait...

Debugger attached successfully.

Server compiler detected.

JVM version is 24.71-b01

using thread-local object allocation.

Parallel GC with 2 thread(s)

Heap Configuration:

MinHeapFreeRatio = 0

MaxHeapFreeRatio = 100

MaxHeapSize = 1004535808 (958.0MB)

NewSize = 1310720 (1.25MB)

MaxNewSize = 17592186044415 MB

OldSize = 5439488 (5.1875MB)

NewRatio = 2

SurvivorRatio = 8

PermSize = 21757952 (20.75MB)

MaxPermSize = 85983232 (82.0MB)

G1HeapRegionSize = 0 (0.0MB)

Heap Usage:

PS Young Generation

Eden Space:

capacity = 15204352 (14.5MB)

used = 12229848 (11.663291931152344MB)

free = 2974504 (2.8367080688476562MB)

80.43649607691272% used

From Space:

capacity = 524288 (0.5MB)

used = 32768 (0.03125MB)

free = 491520 (0.46875MB)

6.25% used

To Space:

capacity = 524288 (0.5MB)

used = 0 (0.0MB)

free = 524288 (0.5MB)

0.0% used

PS Old Generation

capacity = 41418752 (39.5MB)

used = 3584192 (3.41815185546875MB)

free = 37834560 (36.08184814453125MB)

8.65354900118671% used

PS Perm Generation

capacity = 22020096 (21.0MB)

used = 8938800 (8.524703979492188MB)

free = 13081296 (12.475296020507812MB)

40.59382847377232% used

2776 interned Strings occupying 230000 bytes.

[root@dev18 ~]#

使用jmap -histo打印jvm heap的直方图。其输出信息包括类名，对象数量，对象占用大小。

下面的例子使用jmap生成pid为12905的Java程序的对象统计信息(直方图的形式)，并将生成的统计信息输出到时/usr/local目录下的12905.histo文件中，如：

[root@dev18 ~]# jmap -histo 12905 >/usr/local/12905.histo

打开12905.histo文件，查看详细的信息如：

[root@dev18 ~]# vim /usr/local/12905.histo

num #instances #bytes class name

----------------------------------------------

1: 30204 5384536 [B

2: 17521 2252160

3: 17521 2186552

4: 1515 1644272

5: 39568 1582720 java.util.HashMap$KeyIterator

6: 10809 1457680 [C

7: 1515 1040840

8: 1368 1032128

9: 40162 919944 [Ljava.lang.Object;

10: 36461 875064 java.util.ArrayList

11: 35427 850248 java.lang.Long

12: 15953 765744 java.nio.HeapByteBuffer

13: 5384 387648 org.apache.zookeeper.server.Request

14: 11669 373408 java.util.HashMap$Entry

15: 3771 301296 [Ljava.util.HashMap$Entry;

16: 612 296376 [I

17: 5687 272976 java.util.HashMap

18: 3805 254448 [S

19: 7029 224928 java.util.concurrent.locks.AbstractQueuedSynchronizer$Node

20: 453 203136

21: 7138 171312 java.util.concurrent.LinkedBlockingQueue$Node

22: 7008 168192 java.lang.String

23: 1671 161696 java.lang.Class

24: 2507 133624 [[I

25: 5223 125352 java.util.Collections$UnmodifiableRandomAccessList

26: 3513 112416 java.io.DataInputStream

27: 3513 112416 java.io.DataOutputStream

28: 3511 112352 org.apache.zookeeper.proto.ReplyHeader

29: 1872 89856 org.apache.zookeeper.txn.TxnHeader

30: 5486 87776 java.util.HashSet

31: 3513 84312 org.apache.jute.BinaryOutputArchive

32: 3512 84288 java.io.ByteArrayOutputStream

33: 139 75616

34: 1506 72288 org.apache.zookeeper.KeeperException$NodeExistsException

35: 1871 59872 org.apache.zookeeper.server.quorum.QuorumPacket

36: 3561 56976 java.util.HashMap$KeySet

37: 3513 56208 org.apache.zookeeper.server.ByteBufferInputStream

38: 3513 56208 org.apache.jute.BinaryInputArchive

39: 3512 56192 org.apache.zookeeper.server.SessionTrackerImpl$SessionSet

40: 1873 44952 java.util.LinkedList$Node

41: 1871 44904 org.apache.zookeeper.server.quorum.Leader$Proposal

42: 516 41280 java.lang.reflect.Method

43: 1045 33440 java.util.concurrent.ConcurrentHashMap$HashEntry

44: 1694 27104 java.lang.Object

45: 1506 24096 org.apache.zookeeper.txn.ErrorTxn

46: 106 16960 org.apache.zookeeper.server.NIOServerCnxn

47: 426 13632 java.util.concurrent.locks.ReentrantLock$NonfairSync

48: 230 12704 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;

#instances, #bytes以及class name列说明：

instances列：表示当前类有多少个实例。

bytes列：说明当前类的实例总共占用了多少个字节

class name列：表示的就是当前类的名称，class name 解读：

B代表byte

C代表char

D代表double

F代表float

I代表int

J代表long

Z代表boolean

前边有[代表数组，[I 就相当于int[]

对象用[L+类名表示

也可以不生成文件，直接使用jmap -histo 12905查看，因为内容比较多，可以采用添加| more来限定~

[root@dev18 ~]# jmap -histo 12905 | more

num #instances #bytes class name

----------------------------------------------

1: 30330 5407864 [B

2: 17521 2252160

3: 17521 2186552

4: 1515 1644272

5: 39770 1590800 java.util.HashMap$KeyIterator

6: 10827 1460848 [C

7: 1515 1040840

8: 1368 1032128

9: 40346 923032 [Ljava.lang.Object;

10: 36645 879480 java.util.ArrayList

11: 35608 854592 java.lang.Long

12: 16025 769200 java.nio.HeapByteBuffer

13: 5402 388944 org.apache.zookeeper.server.Request

14: 11723 375136 java.util.HashMap$Entry

15: 613 357464 [I

16: 3789 302736 [Ljava.util.HashMap$Entry;

17: 5705 273840 java.util.HashMap

18: 3805 254448 [S

19: 7065 226080 java.util.concurrent.locks.AbstractQueuedSynchronizer$Node

20: 453 203136

21: 7174 172176 java.util.concurrent.LinkedBlockingQueue$Node

22: 7008 168192 java.lang.String

23: 1671 161696 java.lang.Class

24: 2507 133624 [[I

25: 5241 125784 java.util.Collections$UnmodifiableRandomAccessList

26: 3531 112992 java.io.DataInputStream

27: 3531 112992 java.io.DataOutputStream

28: 3529 112928 org.apache.zookeeper.proto.ReplyHeader

29: 1872 89856 org.apache.zookeeper.txn.TxnHeader

30: 5504 88064 java.util.HashSet

31: 3531 84744 org.apache.jute.BinaryOutputArchive

32: 3530 84720 java.io.ByteArrayOutputStream

33: 139 75616

34: 1506 72288 org.apache.zookeeper.KeeperException$NodeExistsException

35: 1871 59872 org.apache.zookeeper.server.quorum.QuorumPacket

36: 3579 57264 java.util.HashMap$KeySet

37: 3531 56496 org.apache.zookeeper.server.ByteBufferInputStream

38: 3531 56496 org.apache.jute.BinaryInputArchive

39: 3530 56480 org.apache.zookeeper.server.SessionTrackerImpl$SessionSet

40: 1873 44952 java.util.LinkedList$Node

41: 1871 44904 org.apache.zookeeper.server.quorum.Leader$Proposal

42: 516 41280 java.lang.reflect.Method

43: 1045 33440 java.util.concurrent.ConcurrentHashMap$HashEntry

44: 1694 27104 java.lang.Object

45: 1506 24096 org.apache.zookeeper.txn.ErrorTxn

46: 106 16960 org.apache.zookeeper.server.NIOServerCnxn

47: 426 13632 java.util.concurrent.locks.ReentrantLock$NonfairSync

48: 230 12704 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;

49: 308 12320 java.lang.ref.SoftReference

使用jmap -permstat 12905打印permanent generation heap情况

[root@dev18 ~]# jmap -permstat 12905

Attaching to process ID 12905, please wait...

Debugger attached successfully.

Server compiler detected.

JVM version is 24.71-b01

finding class loader instances ..done.

computing per loader stat ..done.

please wait.. computing liveness.liveness analysis may be inaccurate ...

class_loaderclassesbytesparent_loaderalive?type

12677343336 null live

0x00000000c429213013048 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c420b21000 null deadsun/misc/Launcher$ExtClassLoader@0x00000000bf1c0120

0x00000000c429217011888 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c42921b011888 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c4292358232144 null deadjavax/management/remote/rmi/NoCallStackClassLoader@0x00000000bf546b68

0x00000000c42921f011888 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c420b1c040126572560x00000000c420b210deadsun/misc/Launcher$AppClassLoader@0x00000000bf21a9c8

0x00000000c42920e0413776 null deadjavax/management/remote/rmi/NoCallStackClassLoader@0x00000000bf546b68

0x00000000c429231813048 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c42922d811888 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c429229813048 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

0x00000000c4312bc8000x00000000c420b1c0deadjava/util/ResourceBundle$RBClassLoader@0x00000000bf6dce70

0x00000000c429225813056 null deadsun/reflect/DelegatingClassLoader@0x00000000bf04ffc8

total = 14168210066264 N/A alive=1, dead=13 N/A

[root@dev18 ~]#

jmap还有一个功能是可以手工生成指定程序的堆快照文件~ 使用-dump选项~

如：

jmap -dump:format=b,file=/usr/local/12905.hprof 12905

该示例为pid为12905的程序生成一个二进制的dump文件，文件放置在/usr/local目录下，

名字为12905.hprof~

[root@dev18 ~]# jmap -dump:format=b,file=/usr/local/12905.hprof 12905

Dumping heap to /usr/local/12905.hprof ...

Heap dump file created

[root@dev18 ~]#

针对这种产生的dump文件，可以使用JDK自带jhat(Java Heap Analyse Tool)小工具来查看，可以将对中的对象以html的形式展示，包括对象的数量、大小等信息，并支持对象查询语言 (OQL)，jhat将会在下一篇文章中介绍~