How to get a thread and heap dump of a Java process on Windows that's not running in a console

Question

I have a Java application that I run from a console which in turn executes an another Java process. I want to get a thread/heap dump of that child process.

On Unix, I could do a kill -3 <pid> but on Windows AFAIK the only way to get a thread dump is Ctrl-Break in the console. But that only gives me the dump of the parent process, not the child.

Is there another way to get that heap dump?

Solution 1

You can use jmap to get a dump of any process running, assuming you know the pid.

Use Task Manager or Resource Monitor to get the pid. Then

jmap -dump:format=b,file=heap.hprof <pid>

to get the heap for that process.

For systems where bash and pgrep are installed and a single Java process is running, try:

jmap -dump:format=b,file=heap.hprof $(pgrep java)

Solution 2

You are confusing two different java dumps. kill -3 generates a thread dump, not a heap dump.

Thread dump = stack traces for each thread in the JVM output to stdout as text.

Heap dump = memory contents for the JVM process output to a binary file.

To take a thread dump on Windows, CTRL+BREAK if your JVM is the foreground process is the simplest way. If you have a unix-like shell on Windows like Cygwin or MobaXterm, you can use kill -3 {pid} like you can in Unix.

To take a thread dump in Unix, CTRL+C if your JVM is the foreground process or kill -3 {pid} will work as long as you get the right PID for the JVM.

With either platform, Java comes with several utilities that can help. For thread dumps, jstack {pid} is your best bet. http://docs.oracle.com/javase/1.5.0/docs/tooldocs/share/jstack.html

Just to finish the dump question out: Heap dumps are not commonly used because they are difficult to interpret. But, they have a lot of useful information in them if you know where/how to look at them. The most common usage is to locate memory leaks. It is a good practice to set the -D on the java command-line so that the heap dump is generated automatically upon an OutOfMemoryError, -XX:+HeapDumpOnOutOfMemoryError But, you can manually trigger a heap dump, also. The most common way is to use the java utility jmap.

NOTE: this utility is not available on all platforms. As of JDK 1.6, jmap is available on Windows.

An example command-line would look something like

jmap -dump:file=myheap.bin {pid of the JVM}

The output "myheap.bin" is not human readable (for most of us), and you will need a tool to analyze it. My preference is MAT. http://www.eclipse.org/mat/

Solution 3

I think the best way to create .hprof file in Linux process is with jmap command. For example: jmap -dump:format=b,file=filename.hprof {PID}

Solution 4

In addition to using the mentioned jconsole/visualvm, you can use jstack -l <vm-id> on another command line window, and capture that output.

The <vm-id> can be found using the task manager (it is the process id on windows and unix), or using jps.

Both jstack and jps are include in the Sun JDK version 6 and higher.

Solution 5

I recommend the Java VisualVM distributed with the JDK (jvisualvm.exe). It can connect dynamically and access the threads and heap. I have found in invaluable for some problems.

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Comments

[iny]

See java.sun.com/javase/6/docs/technotes/tools/#troubleshoot

[Vanchinathan Chandrasekaran]

These tools are not supported in Java 1.6. Java 1.6 has only jconsole.

[ankon]

You might be mixing up JDK and JRE, I explicitly mentioned JDK. See the documentation for the tools: download.oracle.com/javase/6/docs/technotes/tools/share/… and download.oracle.com/javase/6/docs/technotes/tools/share/…

[lava]

Thanks Daniel. Where is this file created on a windows machine ? Is there a default path ?

[kamczak]

@lava You can set the path through -XX:HeapDumpPath, as described at Oracle's VM Options page.

[nafg]

On my linux Ctrl-C interrupts (terminates) it, I do Ctrl-\

[mjaggard]

This thread has become so popular that I've just heard someone referring to a heap dump as a "cheap.bin"

[ChiefTwoPencils]

Consider this and its general impact on "To take a thread dump on Windows, CTRL+BREAK". It's actually dependent on the engineering decision of the manufacturer. FE, Lenova, IIRC, is cntrl+fn+p.

[Hammad Dar]

That's most of the times not feasible as it has an overhead attached to it and thread dumps are generally retrieved from production machines.

[Jaberino]

original question is about a 'not-runnin' process. It's likely that jvisualvm cannot connect.

[Lawrence Dol]

@Jaberino: No, it's about a currently running Java process, in Windows, with no console associated with it.

[Basil]

Awesome. I wanted to run a test overnight in hopes of showing memory leak but was worried of OOM and crashign while I'm not present. This is perfect.

[xeruf]

best answer by far! Didn't know of this till now and it's really practical!

[Pragalathan M]

In most of the production systems, we only have jre and not jdk. So this helps.

[bitsabhi]

Make sure to use correct user which started the java process. In my case it was tomcat8 ps -C java -o pid sudo -u tomcat8 jmap -dump:format=b,file=<filename> <pid>

[Vadzim]

In latest java releases Java VisualVM was superceded by JMC/JFR. See also What are the differences between JVisualVM and Java Mission Control?

[Vadzim]

But jstack prooved to be more reliable alternative on Windows: stackoverflow.com/a/47723393/603516