先看一個例子:jmap -heap pid該命令用於:展示pid的整體堆信息,運行結果如下
zhengchao1991deMacBook-Pro:~ zhengchao1991$ jmap -heap 33628
Attaching to process ID 33628, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 24.79-b02
using thread-local object allocation.
Parallel GC with 8 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 536870912 (512.0MB)
NewSize = 1310720 (1.25MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 21757952 (20.75MB)
MaxPermSize = 268435456 (256.0MB)
G1HeapRegionSize = 0 (0.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 145227776 (138.5MB)
used = 3721808 (3.5493927001953125MB)
free = 141505968 (134.9506072998047MB)
2.5627384116933665% used
From Space:
capacity = 7864320 (7.5MB)
used = 0 (0.0MB)
free = 7864320 (7.5MB)
0.0% used
To Space:
capacity = 7864320 (7.5MB)
used = 0 (0.0MB)
free = 7864320 (7.5MB)
0.0% used
PS Old Generation
capacity = 157810688 (150.5MB)
used = 111650416 (106.47813415527344MB)
free = 46160272 (44.02186584472656MB)
70.74959080084614% used
PS Perm Generation
capacity = 129499136 (123.5MB)
used = 129255872 (123.26800537109375MB)
free = 243264 (0.23199462890625MB)
99.81215009805162% used
45138 interned Strings occupying 4690064 bytes.
這些參數有什麼用呢,我們看如下幾行:
PS Young Generation
Eden Space:#Eden區內存分佈
capacity = 87883776 (83.8125MB)
used = 31053080 (29.614524841308594MB)
free = 56830696 (54.197975158691406MB)
35.33425782706469% used
From Space:#其中一個Survivor區的內存分佈
capacity = 13828096 (13.1875MB)
used = 196608 (0.1875MB)
free = 13631488 (13.0MB)
1.4218009478672986% used
To Space:#另一個Survivor區的內存分佈
capacity = 16384000 (15.625MB)
used = 0 (0.0MB)
free = 16384000 (15.625MB)
0.0% used
PS Old Generation#當前的Old區內存分佈
capacity = 156172288 (148.9375MB)
used = 27098208 (25.842864990234375MB)
free = 129074080 (123.09463500976562MB)
17.35148299805917% used
我們可以看出:年輕代三個區域(eden、From Space、To Space)的大小、年老代的大小,以及已經使用、剩餘的大小;
根據這些結果、再結合:
jstat -gcnew pid:查詢new對象的信息
jstat -gcold pid:查詢old對象的信息
這兩條命令中的gc頻率,我們可以得出設置的各個代的大小是否合理,若不合理,如何調整。調整的策略必須以實際應用爲主,比如說:full gc的時間停頓讓客戶感覺明顯,我們分析原因得知是old區域設置的過大,導致一次full gc需要的時間過久;如果我們直接縮小old區域的大小,又回發現full gc過於頻繁;這個時候也許我們需要更換gc的算法........
我們現在開始看jmap的定義:
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
我們先只看幾個簡單的:
-heap:打印jvm heap的情況
-histo:打印jvm heap的直方圖。其輸出信息包括類名,對象數量,對象佔用大小。
-heap上面已經給出過例子了,下面我們用-histo來分析內存溢出;
首先寫一個能導致內存溢出的程序:
package deadlock;
import java.math.BigDecimal;
public class Apple {
public String name;
public BigDecimal price;
public Apple(String name, BigDecimal price) {
this.name = name;
this.price = price;
System.out.println(name);
}
}
package deadlock;
import java.math.BigDecimal;
public class TestJmap {
public static void main(String[] args){
int i=0;
while(true){
new Apple("apple"+i++,BigDecimal.ONE);
}
}
}
運行後,執行如下命令:
zhengchao1991deMacBook-Pro:~ zhengchao1991$ jmap -histo 38876
num #instances #bytes class name
----------------------------------------------
1: 293271 14077008 java.nio.HeapCharBuffer
2: 147692 5957408 [C
3: 147672 3544128 java.lang.String
4: 146636 3519264 deadlock.Apple
5: 116 2852184 [I
6: 6162 794048 <methodKlass>
7: 6162 718552 <constMethodKlass>
8: 403 488912 <constantPoolKlass>
9: 367 300000 <constantPoolCacheKlass>
10: 403 278312 <instanceKlassKlass>
11: 565 94464 [B
12: 466 45816 java.lang.Class
13: 688 45360 [[I
14: 613 38072 [S
15: 46 25024 <objArrayKlassKlass>
16: 73 24952 <meth
我們看這幾行:num #instances #bytes class name
----------------------------------------------
1: 293271 14077008 java.nio.HeapCharBuffer
2: 147692 5957408 [C
3: 147672 3544128 java.lang.String
4: 146636 3519264 deadlock.Apple
發現產生了140000多個Apple實例,140000多個String對象,並且在不斷的增加,這是肯定有問題的,至於怎麼修改,由於我們寫的程序沒有太大的意義,就不做說明了,根據實際情況來修改,此處只是爲了展示。
總結:給的例子並不好,想說的是,根據命令的輸出結果去發現異常信息最重要。
分析問題流程:
1.如果程序內存不足或者頻繁GC,很有可能存在內存泄露情況,這時候就要藉助Java堆Dump查看對象的情況。
2.要製作堆Dump可以直接使用jvm自帶的jmap命令
3.可以先使用jmap -heap命令查看堆的使用情況,看一下各個堆空間的佔用情況。
4.使用jmap -histo:[live]查看堆內存中的對象的情況。如果有大量對象在持續被引用,並沒有被釋放掉,那就產生了內存泄露,就要結合代碼,把不用的對象釋放掉。
5.也可以使用 jmap -dump:format=b,file=<fileName>命令將堆信息保存到一個文件中,再借助jhat命令查看詳細內容
6.在內存出現泄露、溢出或者其它前提條件下,建議多dump幾次內存,把內存文件進行編號歸檔,便於後續內存整理分析。