10 Examples of HotSpot JVM Options in Java

There are hundreds of JVM parameters or JVM Options exists inside sun JDK and its virtually impossible to keep track of every single JVM option and based on my experience we don't even use most of JVM flags except a couple of important JVM option related to java heap size, java options for printing garbage collection details and most likely JVM switches for setting up remote debugging in Java. but there are many other useful categories of JVM parameters that you at least like to be familiar with even if not intending to use them more frequently.

In this article, we will see examples of 10 different categories of JVM parameters that I found useful and use more frequently than others. I would recommend getting a full knowledge of what does a particular JVM option does by referring to the official list of JVM options.

JVM parameters in Java

On the basis of how we specify the JVM option it can be divided into two parts, JVM Options which starts with –X and those which starts with -XX:

1)    JVM Options that begin with -X are non-standard (they are not guaranteed to be supported on all JVM implementations) and are subject to change without notice in subsequent releases of the JDK.

2)    JVM Options or parameters that are specified with -XX are not stable and are not recommended for casual use. These options are subject to change without notice also.

I was thinking about writing a post on JVM options when I completed my post on Java Heap Size and Java Garbage Collection because these are two main areas where we see usages of various JVM flags. 

But it didn’t happen even after I covered the OutOfMemoryError post which has some JVM options to solve OutOfMemoryError in Java. Now I am happy that I have completed this piece of information and it's ready to be published. 

As always I look for your feedback, suggestions and any other JVM flags which I have missed and you guys find useful to share. 

Good knowledge of JVM options especially related to GC tuning is important for time-critical applications e.g. high volume low latency electronic trading platform where every microseconds matter. though getting the right combination requires a lot of profiling and trial and error and depends heavily on the nature of the trading application.

Important Points about JVM Options:

1)    Boolean JVM options can be  turned on with -XX:+ and can be turned off with -XX:-.

2)    Numeric JVM Options can be set with -XX:=. Numbers can include 'm' or 'M' for megabytes, 'k' or 'K' for kilobytes, and 'g' or 'G' for gigabytes (for example, 32k is the same as 32768).

3)    String JVM options can be set by using -XX:=, and usually used to specify a file, a path, or a list of commands.

The command java -help lists the standard options (standard across different JVM implementations) for the Java application launcher. The command java -X can be used to see the Java application launcher's non-standard (X for extension specific to that JVM) arguments.

The -X options are non-standard and subject to change without notice. If you wish to detect which JVM arguments your currently running Java application is using, you can use the 

ManagementFactory.getRuntimeMXBean().getInputArguments()

Now here is my list of important JVM flags, switches, options or parameters which is most commonly used while running Java applications:

1) JVM memory options related to the java heap size

The following three JVM options are used to specify initial and max heap size and thread stack size while running Java programs.

-Xms        set initial Java heap size
 -Xmx        set maximum Java heap size
 -Xss>         set java thread stack size

2) JVM option to print gc details

-verbose:gc logs garbage collector runs and how long they're taking. I generally use this as my first tool to investigate if GC is a bottleneck for a given application.

-XX:+PrintGCDetails includes the data from -verbose:gc but also adds information about the size of the new generation and more accurate timings.

-XX:-PrintGCTimeStamps  Print timestamps at garbage collection.

You can further see these Java Performance and JVM internal courses on Udemy to learn more bout the performance tuning of Java applications. It's one of the advanced courses for Java programmers to learn more about Performance and Memory management including troubleshooting memory leaks in Java.

3. JVM parameters to specify Java Garbage collector

-XX:+UseParallelGC      Use parallel garbage collection for scavenges

-XX:-UseConcMarkSweepGC Use concurrent mark-sweep collection for the old generation. (Introduced in 1.4.1)

-XX:-UseSerialGC        Use serial garbage collection. (Introduced in 5.0.)

beware when you use GC Parameters if you are working on a time-critical application e.g. high-frequency trading application. GC is a time-consuming operation and it desired to create a balance.

4. JVM debug options JVM options for remote debugging

-Xdebug -Xnoagent -Xrunjdwp:transport=dt_socket,server=y,suspend=n,address=8000
to read more about remote debugging check How to Setup Java remote debugging in Eclipse and 10 Java debugging tips in Eclipse 

5. JVM options related to profiling

-Xprof

-Xrunhprof

6. JVM options related to java classpath

Xbootclasspath specifies classpath entries you want to be loaded without verification. The JVM verifies all classes it loads to ensure they don't try to dereference an object with an int, pop extra entries off the stack or push too many, and so on. 

This verification is part of the reason why the JVM is very stable, but it's also rather costly and responsible for a large part of startup delay. Putting classes on the bootclasspath skips this cost, but should only be used when you know the classes have been verified many times before. 

In JRuby, this reduced startup time by half or more for a simple script. The -Xbootclasspath option can be used to either prepend (/p) or append (/a) resources to the bootstrap classpath. You Can read more about Java Classpath in my articles How Classpath works in Java and How to Solve ClassNotFoundException in Java

7. JVM options to change  Perm Gen Size

These JVM options are quite useful to solve java.lang.OutOfMemoryError:Perm Gen Space.

-XX:PermSize and MaxPermSize
-XX:NewRatio=2  Ratio of new/old generation sizes.
-XX:MaxPermSize=64m     Size of the Permanent Generation.

8. JVM parameters to trace classloading and unloading

-XX:+TraceClassLoading and -XX:+TraceClassUnloading is two JVM options that we use to print logging information whenever classes load into JVM or unload from JVM. These JVM flags are extremely useful if you have any memory leak related to classloader and or suspecting that classes are not unloading or garbage collected.

9. JVM switches related to logging

-XX:+TraceClassLoading and -XX:+TraceClassUnloading print information class loads and unloads. Useful for investigating if you have a class leak or if old classes (like JITed Ruby methods in JRuby) are getting collected or not. You can read more about logging in to Java on my post 10 Tips while logging in Java

-XX:+PrintCompilation prints out the name of each Java method Hotspot decides to JIT compile. The list will usually show a bunch of core Java class methods initially and then turn to methods in your application. In JRuby, it eventually starts to show Ruby methods as well

10. JVM Switches for debugging purposes

-XX:HeapDumpPath=./java_pid.hprof  Path to directory or file name for heap dump.

-XX:-PrintConcurrentLocks       Print java.util.concurrent locks in the Ctrl-Break thread dump.

-XX:-PrintCommandLineFlags   Print flags that appeared on the command line.

That’s all on JVM Options, I understand it's not possible to remember all JVM flags but at least having an idea of what kind of JVM flags are available is a good asset. The image for JVM parameters is from Java tuning and Nutshell.  For full list of JVM options, you can refer to these links from the Oracle Java site: Java Hotspot VM Options


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