原文链接：www.iteye.com/blog/josh-persistence-2161848
现实企业级Java应用开发、维护中，有时候我们会碰到下面这些问题：- OutOfMemoryError，内存不足
- 内存泄露
- 线程死锁
- 锁争用(Lock Contention)
- Java进程消耗CPU过高
- ......
jps [options] [hostid]如果不指定hostid就默认为当前主机或服务器。
命令行参数选项说明如下：
-q 不输出类名、Jar名和传入main方法的参数
-m 输出传入main方法的参数
-l 输出main类或Jar的全限名
-v 输出传入JVM的参数比如下面：
root@ubuntu:/# jps -m -l
2458 org.artifactory.standalone.main.Main /usr/local/artifactory-2.2.5/etc/jetty.xml
29920 com.sun.tools.hat.Main -port 9998 /tmp/dump.dat
3149 org.apache.catalina.startup.Bootstrap start
30972 sun.tools.jps.Jps -m -l
8247 org.apache.catalina.startup.Bootstrap start
25687 com.sun.tools.hat.Main -port 9999 dump.dat
21711 mrf-center.jar二、 jstack
jstack主要用来查看某个Java进程内的线程堆栈信息。语法格式如下：
jstack [option] pid
jstack [option] executable core
jstack [option] [server-id@]remote-hostname-or-ip命令行参数选项说明如下：
-l long listings，会打印出额外的锁信息，在发生死锁时可以用jstack -l pid来观察锁持有情况-m mixed mode，不仅会输出Java堆栈信息，还会输出C/C++堆栈信息(比如Native方法)root@ubuntu:/# ps -ef | grep mrf-center | grep -v grep
root 21711 1 1 14:47 pts/3 00:02:10 java -jar mrf-center.jar
printf "%x
" 21742root@ubuntu:/# jstack 21711 | grep 54ee
"PollIntervalRetrySchedulerThread" prio=10 tid=0x00007f950043e000 nid=0x54ee in Object.wait() [0x00007f94c6eda000]// Idle wait
getLog().info("Thread [" + getName() + "] is idle waiting...");
schedulerThreadState = PollTaskSchedulerThreadState.IdleWaiting;
long now = System.currentTimeMillis();
long waitTime = now + getIdleWaitTime();
long timeUntilContinue = waitTime - now;
synchronized(sigLock) {try {
if(!halted.get()) {
sigLock.wait(timeUntilContinue);
}
} catch (InterruptedException ignore) {
}
}jmap [option] pid
jmap [option] executable core
jmap [option] [server-id@]remote-hostname-or-ip如果运行在64位JVM上，可能需要指定-J-d64命令选项参数。
jmap -permstat pid
root@ubuntu:/# jmap -heap 21711
Attaching to process ID 21711, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 20.10-b01
using thread-local object allocation.
Parallel GC with 4 thread(s)
Heap Configuration:
MinHeapFreeRatio = 40
MaxHeapFreeRatio = 70
MaxHeapSize = 2067791872 (1972.0MB)
NewSize = 1310720 (1.25MB)
MaxNewSize = 17592186044415 MB
OldSize = 5439488 (5.1875MB)
NewRatio = 2
SurvivorRatio = 8
PermSize = 21757952 (20.75MB)
MaxPermSize = 85983232 (82.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 6422528 (6.125MB)
used = 5445552 (5.1932830810546875MB)
free = 976976 (0.9317169189453125MB)
84.78829520089286% used
From Space:
capacity = 131072 (0.125MB)
used = 98304 (0.09375MB)
free = 32768 (0.03125MB)
75.0% used
To Space:
capacity = 131072 (0.125MB)
used = 0 (0.0MB)
free = 131072 (0.125MB)
0.0% used
PS Old Generation
capacity = 35258368 (33.625MB)
used = 4119544 (3.9287033081054688MB)
free = 31138824 (29.69629669189453MB)
11.683876009235595% used
PS Perm Generation
capacity = 52428800 (50.0MB)
used = 26075168 (24.867218017578125MB)
free = 26353632 (25.132781982421875MB)
49.73443603515625% used
....root@ubuntu:/# jmap -histo:live 21711 | more
num #instances #bytes class name----------------------------------------------
1: 38445 5597736 2: 38445 5237288 3: 3500 3749504 4: 60858 3242600 5: 3500 2715264 6: 2796 2131424 7: 5543 1317400 [I8: 13714 1010768 [C9: 4752 1003344 [B10: 1225 639656 11: 14194 454208 java.lang.String12: 3809 396136 java.lang.Class13: 4979 311952 [S14: 5598 287064 [[I15: 3028 266464 java.lang.reflect.Method16: 280 163520 17: 4355 139360 java.util.HashMap$Entry18: 1869 138568 [Ljava.util.HashMap$Entry;19: 2443 97720 java.util.LinkedHashMap$Entry20: 2072 82880 java.lang.ref.SoftReference21: 1807 71528 [Ljava.lang.Object;22: 2206 70592 java.lang.ref.WeakReference23: 934 52304 java.util.LinkedHashMap24: 871 48776 java.beans.MethodDescriptor25: 1442 46144 java.util.concurrent.ConcurrentHashMap$HashEntry26: 804 38592 java.util.HashMap27: 948 37920 java.util.concurrent.ConcurrentHashMap$Segment28: 1621 35696 [Ljava.lang.Class;29: 1313 34880 [Ljava.lang.String;30: 1396 33504 java.util.LinkedList$Entry31: 462 33264 java.lang.reflect.Field32: 1024 32768 java.util.Hashtable$Entry33: 948 31440 [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;class name是对象类型，说明如下：
B byte
C char
D double
F float
I int
J long
Z boolean
[ 数组，如[I表示int[]
[L+类名 其他对象jmap -dump:format=b,file=dumpFileName pid我一样地对上面进程ID为21711进行Dump：
root@ubuntu:/# jmap -dump:format=b,file=/tmp/dump.dat 21711
Dumping heap to /tmp/dump.dat ...
Heap dump file createddump出来的文件可以用MAT、VisualVM等工具查看，这里用jhat查看：
root@ubuntu:/# jhat -port 9998 /tmp/dump.dat
Reading from /tmp/dump.dat...
Dump file created Tue Jan 28 17:46:14 CST 2014Snapshot read, resolving...
Resolving 132207 objects...
Chasing references, expect 26 dots..........................
Eliminating duplicate references..........................
Snapshot resolved.
Started HTTP server on port 9998Server is ready.
jstat [ generalOption | outputOptions vmid [interval[s|ms] [count]] ]root@ubuntu:/# jstat -gc 21711 250 4
S0C S1C S0U S1U EC EU OC OU PC PU YGC YGCT FGC FGCT GCT
192.0 192.0 64.0 0.0 6144.0 1854.9 32000.0 4111.6 55296.0 25472.7 702 0.431 3 0.218 0.649
192.0 192.0 64.0 0.0 6144.0 1972.2 32000.0 4111.6 55296.0 25472.7 702 0.431 3 0.218 0.649
192.0 192.0 64.0 0.0 6144.0 1972.2 32000.0 4111.6 55296.0 25472.7 702 0.431 3 0.218 0.649
192.0 192.0 64.0 0.0 6144.0 2109.7 32000.0 4111.6 55296.0 25472.7 702 0.431 3 0.218 0.649要明白上面各列的意义，先看JVM堆内存布局：
可以看出：
堆内存 = 年轻代 + 年老代 + 永久代
年轻代 = Eden区 + 两个Survivor区(From和To)现在来解释各列含义：
S0C、S1C、S0U、S1U：Survivor 0/1区容量(Capacity)和使用量(Used)
EC、EU：Eden区容量和使用量
OC、OU：年老代容量和使用量
PC、PU：永久代容量和使用量
YGC、YGT：年轻代GC次数和GC耗时
FGC、FGCT：Full GC次数和Full GC耗时
GCT：GC总耗时五、hprof(Heap/CPU Profiling Tool)：
hprof能够展现CPU使用率，统计堆内存使用情况。
语法格式如下：
java -agentlib:hprof[=options] ToBeProfiledClass
java -Xrunprof[:options] ToBeProfiledClass
javac -J-agentlib:hprof[=options] ToBeProfiledClass完整的命令选项如下：
Option Name and Value Description Default
--------------------- ----------- -------
heap=dump|sites|all heap profiling all
cpu=samples|times|old CPU usage off
monitor=y|n monitor contention n
format=a|b text(txt) or binary output a
file= write data to file java.hprof[.txt]
net=: send data over a socket off
depth=stack trace depth 4
interval= sample interval in ms 10
cutoff= output cutoff point 0.0001
lineno=y|n line number in traces? y
thread=y|n thread in traces? n
doe=y|n dump on exit? y
msa=y|n Solaris micro state accounting n
force=y|n force output to y
verbose=y|n print messages about dumps y来几个官方指南上的实例。
CPU Usage Sampling Profiling(cpu=samples)的例子：
java -agentlib:hprof=cpu=samples,interval=20,depth=3 Hellojavac -J-agentlib:hprof=cpu=times Hello.javaHeap Allocation Profiling(heap=sites)的例子：
javac -J-agentlib:hprof=heap=sites Hello.javajavac -J-agentlib:hprof=heap=dump Hello.java
