Comparing jstack and jcmd with FusionReactor – see how much more you get

In one of our previous blog post, we compared Java VisualVM with FusionReactor. Today we will compare features that jcmd and jstack provide against FusionReactor.

What is the difference between jcmd and jstack?

What is jstack?

Let’s start with jstack. According to Oracle’s documentation:

The jstack command-line utility attaches to the specified process or core file and prints the stack traces of all threads that are attached to the virtual machine, including Java threads and VM internal threads, and optionally native stack frames. The utility also performs deadlock detection. First, we need to mention that Java VisuaVM provides all the features available by jstack


To make it simple, jstack is a terminal-based utility that prints the stacktraces of the given JVM process. For example, in order to see stacktraces of the Java program with PID 1302, you need to run the following command in the terminal jstack 1302. Here is the sample output that you can get from a running Spring-Boot application with embedded Tomcat

"http-nio-8080-exec-1" #23 daemon prio=5 os_prio=0 cpu=0.24ms elapsed=12.29s tid=0x00007f153cb0a800 nid=0x156fd waiting on condition  [0x00007f14a60b1000]
   java.lang.Thread.State: WAITING (parking)
        at jdk.internal.misc.Unsafe.park(java.base@11.0.8/Native Method)
        - parking to wait for  <0x000000008e3e0e10> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
        at java.util.concurrent.locks.LockSupport.park(java.base@11.0.8/
        at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(java.base@11.0.8/
        at java.util.concurrent.LinkedBlockingQueue.take(java.base@11.0.8/
        at org.apache.tomcat.util.threads.TaskQueue.take(
        at org.apache.tomcat.util.threads.TaskQueue.take(
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.getTask(
        at org.apache.tomcat.util.threads.ThreadPoolExecutor.runWorker(
        at org.apache.tomcat.util.threads.ThreadPoolExecutor$
        at org.apache.tomcat.util.threads.TaskThread$

As you can see, jstack shows us one of Tomcat’s threads which is waiting for new requests to come.

jstack is available as a part of Oracle’s JDK since JDK 6. However, we have to mention that since JDK 8, Oracle wants to deprecate this utility. According to jstack official documentation

The release of JDK 8 introduced Java Mission Control, Java Flight Recorder, and jcmd utility for diagnosing problems with JVM and Java applications. It is suggested to use the latest utility, jcmd instead Which means that for java versions starting from 8 and higher the jcmd should be used

What is jcmd?

Now let’s cover jcmd. According to the official documentation

The jcmd utility is used to send diagnostic command requests to the JVM, where these requests are useful for controlling Java Flight Recordings, troubleshoot, and diagnose JVM and Java Applications. jcmd is available as part of the JDK starting from version 8. It can do a lot more than jstack and is used as a general console based performance tracker tool for JVM. It is used in conjuction with events you are interested in. For example , in order to see the same stacktraces as we got with jstack above , the following command must be used jcmd fusion-reactor.jar Thread.print. From now on , we will compare jcmd with FusionReactor because all jstack features are already available in jcmd

Comparing jstack and jcmd with FusionReactor

FusionReactor is a full-blown APM, but let’s keep the comparison fair

Before comparing both tools, we want to emphasize that jcmd can only take statistics snapshots of the running JVM process which were available at the time of execution. On the other hand, FusionReactor is a complete monitoring tool that supports a huge range of monitoring options including Garbage collector, Heap allocations, JDBC, Web performance, Live debugging, and a lot more. To be fair, we will only compare features that are available in both tools.

Console vs Web

The first big difference comes with the way both tools are launched. FusionReactor offers a web interface to monitor your JVM processes. The interface can be launched locally or through the Cloud in order to monitor remote processes in a production environment. On the other hand, jcmd can only be used within a terminal. Most people who get used to a great user experience offered by web or desktop apps won’t feel comfortable opening the terminal to monitor the performance of the underlying application. Moreover, jcmd can only get snapshots of the JVM processes that run on the same host which means that jcmd does not support remote processes. In order to use jcmd in production, the developer will have to have ssh access to the remote machine.  However, with FusionReactor it’s a matter of opening your browser.

Troubleshooting Threads

As was shown above, jcmd can print the stack traces of all threads running in the JVM with the following command jcmd <process id/main class> Thread.print. For regular Servlet-based backends with a few hundred threads serving user requests, the output will be unreadable. Developer must rely on other utilities such as grep or awk in order to get the stack trace of the Servlet Thread, they are interested in. On the other hand, FusionReactor offers a separate page that covers all Thread-related information.

Comparing jstack and jcmd with FusionReactor

The interface offers a list of available threads with corresponding states (E.g RUNNABLE or WAITING). Moreover, FusionReactor doesn’t show the stack trace as a plain text, all stack traces are shown in a prettified format that helps developers to better understand what the thread is actually doing

Comparing jstack and jcmd with FusionReactor

Apart from that, the dynamic nature of FusionReactor allows you to stop or pause the running thread.

Heap troubleshooting

Heap is one of the main places to look at when you have production-level JVM problems such as long GC pauses or unreasonably big memory usage. jcmd supports heap statistics through the heap dumps.

A heap dump is a snapshot of all the objects in the Java Virtual Machine (JVM) heap at a certain point in time To get a heapdump of the running process you can use the following command jcmd <pid> GC.heap_dump filename=Myheapdump which will create a heap snapshot in the file named Myheapdump. After that you will have to use a separate software that can read heapdumps, the official one recommended by Oracle is Java VisualVM. These two steps are really cumbersome and take a lot of time. Moreover jcmd can only show heap insides on the time of the execution, previous heap statistics are not available.

With FusionReactor you have a memory profiler page that shows all memory spaces with corresponding graphs

paring jstack and jcmd with FusionReactor

You don’t have to create heap dumps, FusionReactor will do it for you. Moreover, all memory metrics are recorded and persisted for future viewing.

Troubleshooting using Java Flight Recorder

Java 8 introduced a new monitoring tool called Java Flight Recorder (JFR) which collects events from the running JVM process. Let’s say you want to understand why calling a specific REST endpoint eats up a lot of CPU time. In this case, the jcmd command can record events from the process and save them into the file. Here is the command to use jcmd <pid> JFR.start name=MyRecording settings=profile duration=20s filename=myrecording.jfr. You can run this command which will collect events for 20 seconds, then call the endpoint and then he jfr file will have all the events for this period of time. You will however need another tool to work with jfr files. The default one that comes with a JDK is a Java VisualVM

Comparing jstack and jcmd with FusionReactor

In the screen above you can see that JFR makes a great job gathering a lot of metrics about the Java process (Threads, Exceptions, GC, Locks, etc). One big drawback here is that as with the heap, jcmd itself can only create a file which then has to be opened by a different program. Another problem is that you must run a flight recorder to get the data from JVM, if a problem occurs without a running flight recorder you won’t be able to understand what went wrong.

With FusionReactor, the application is constantly monitored, you don’t have to create jfr files and analyze them. If you got a CPU-related issue, FusionReactor will show you the thread that took up the most CPU time along with corresponding Stracktraces and information on each stack and how much time it took

Comparing jstack and jcmd with FusionReactor

Shell scripting

As jcmd is a command-line utility it can be used as part of a shell script. A developer might want to write a script that periodically runs jcmd to create heap dumps or make a jfr snapshots. Along with the flexibility to use a shell ecosystem it also brings a burden for the development team to support these scripts and in most cases to integrate them with Cloud providers. With FusionReactor all the monitoring use cases are already implemented and those cases which are not covered can easily be integrated into FusionReactor with an awesome plugin system. Most importantly, FusionReactor supports integration with the most popular cloud providers (such as AWS)

Comparing jstack and jcmd with FusionReactor, is FusionReactor right for you?

jcmd is a great tool for simple Java profiling that only covers the current state of the JVM. If you need a whole monitoring system that supports all the features offered in  jcmd and more (Web performance, Live Debugging, System Monitoring) then consider trying FusionReactor.

Posts by admin

June 2022