introduce
In the production environment, various performance problems are often encountered, so it is necessary to master the most basic JVM command-line monitoring tools
| name | main function |
|---|---|
| jps | View running Java processes |
| jstack | print thread snapshot |
| jmap | Export heap memory image file |
| jstat | View jvm statistics |
| jinfo | View and modify jvm configuration parameters in real time |
| jhat | Used to analyze heapdump files |
jps: View running Java processes
jps can list the running Java processes and display the name of the virtual machine execution main class (Main Class, the class where the main() function is located) and the process id
If you want to see a command, add the -help parameter after it
[root@VM-0-14-centos ~]# jps -help
usage: jps [-help]
jps [-q] [-mlvV] [<hostid>]
Definitions:
<hostid>: <hostname>[:<port>]
It can be seen that remote services can be monitored, but based on security considerations, it is generally not used
Common options are as follows
| Options | effect |
|---|---|
| -q | output only process id |
| -m | Output the parameters passed to the main class main function |
| -l | Output the full class name of the main class, if the process executes a Jar package, output the name of the jar package |
| -v | JVM parameters specified at program startup |
cis@mt002:~$ jps
70208 KmpService
183525 LinkAnalysisServer
25160 MipSerachServer
cis@mt002:~$ jps -l
70208 com.st.kmp.main.KmpService
183525 com.st.cis.main.LinkAnalysisServer
25160 com.st.cis.main.MipSerachServer
jstack: print thread snapshot
View the status of all threads in a Java process. It is generally used to locate the reason for the long pause of the thread, such as the occurrence of an infinite loop, deadlock, requesting external resources to wait for a long time, etc.!
public class DeadLockDemo {
private static Object lockA = new Object();
private static Object lockB = new Object();
public static void main(String[] args) {
Thread threadA = new Thread(() -> {
synchronized (lockA) {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("get lockA");
synchronized (lockB) {
System.out.println("threadA run finish");
}
}
});
Thread threadB = new Thread(() -> {
synchronized (lockB) {
try {
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("get lockB");
synchronized (lockA) {
System.out.println("threadB run finish");
}
}
});
threadA.setName("myThreadA");
threadB.setName("myThreadB");
threadA.start();
threadB.start();
}
}
I wrote an example of deadlock, execute jps after startup to find the process id is 19457
peng@pengdeMacBook-Pro ~ % jps
19457 DeadLockDemo
19458 Launcher
2658
19459 Jps
Then execute the following command to list the execution status of each thread of this process
jstack 19457
It can be seen from the figure that there are many threads, GC threads, myThreadA, myThreadB, etc. The thread status of myThreadA and myThreadB is BLOCKED, and the location of the deadlock is indicated at the end
jmap: export heap memory image file
jmap is mainly used to export heap memory image files to see if memory leaks occur.
In the production environment, we generally configure the following parameters to allow the virtual machine to automatically generate a dump file after an OOM exception occurs.
-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/Users/peng
Execute the following command to manually obtain the dump file
jmap -dump:file=文件名.dump 进程id
There are many tools for analyzing heap memory, such as Java VisualVM, jhat, etc. But personally I think the best one is Eclipse Memory Analyzer, none of them
jstat: View jvm statistics
jstat can display running data such as class loading, memory, garbage collection, JIT compilation, etc. in local or remote virtual machine processes
Use jstat to view class loading information. I personally rarely use this command. It is not as convenient to read garbage collection information on the command line as to see the graphical interface, so I won't introduce it further.
[root@VM-0-14-centos ~]# jstat -class 19402
Loaded Bytes Unloaded Bytes Time
10229 19679.1 52 76.0 5.33
The meaning is as follows
| Loaded | explain |
|---|---|
| Loaded | number of loaded classes |
| Bytes | The number of bytes to load the class |
| Unloaded | Number of uninstalled classes |
| Bytes | Bytes of unloaded classes |
| Time | time spent |
jinfo: View and modify jvm configuration parameters in real time
The role of jinfo is to view and modify various parameters of the virtual machine in real time.
Use the -v parameter of the jps command to view the list of parameters explicitly specified when the virtual machine starts , but if you want to know the system default values of parameters that are not explicitly specified, you can only use jinfo's -flag in addition to looking for information The option is queried (if it is limited to JDK 1.6 or above, it is also a good choice to use java -XX:+PrintFlagsFinal to view the default value of the parameter)
jinfo flags pid can only be used normally in high version, I can't use it normally in jdk1.8 version
The application sets the following parameters
-Xmx10m -Xms10m
Check the maximum heap memory and whether to print the GC log. You can see that the GC log is not printed.
Then set to print the GC log, check it again, and print the GC log.
peng@pengdeMacBook-Pro ~ % jinfo -flag MaxHeapSize 20253
-XX:MaxHeapSize=10485760
peng@pengdeMacBook-Pro ~ % jinfo -flag PrintGCDetails 20253
-XX:-PrintGCDetails
peng@pengdeMacBook-Pro ~ % jinfo -flag +PrintGCDetails 20253
peng@pengdeMacBook-Pro ~ % jinfo -flag PrintGCDetails 20253
-XX:+PrintGCDetails
Xmx is an alias for MaxHeapSize
Reference blog
[1] https://www.cnblogs.com/yueshutong/p/9812464.html