One of the small but welcome features introduced with Oracle’s HotSpot implementation of Java 8 is the addition of many common HotSpot Java launcher (java) options/flags to the launcher’s documentation. In the past, a developer interested in even some fairly common HotSpot JVM options has had to potentially look in multiple different sources to find “official” documentation on these flags. These sources included the aforementioned Java launcher documentation as we all other resources such as Troubleshooting Guide for Java SE 6 with HotSpot VM, Troubleshooting Guide for HotSpot VM (7), and Java 8 Troubleshooting Guide. There have also been numerous “unofficial” sources documenting these options such as in books and The most complete list of -XX options for Java JVM.
One of the risks to be aware of when reading about HotSpot JVM options from disparate sources is that it is easy to start reading about a certain JVM option not realizing that the documentation is explicitly for JVMs other than HotSpot such as JRockit or IBM’s JVM. JRockit’s documentation can be particularly misleading if the developer is not paying close attention because it is available with an Oracle URL and with Oracle logos.
An example of JRockit JVM option documentation being confused as HotSpot JVM option documentation is a feedback comment on the blog post How to Fix java.lang.OufOfMemoryError: Direct Buffer Memory that “corrects” the author’s statement about the default JVM maximum direct buffer limit by referencing JRockit documentation (but JRockit’s default of 0 leads to very different behavior than HotSpot’s default also of 0). In a different but related example, a poster on the StackOverflow thread Default for XX:MaxDirectMemorySize makes the same mistake and also references the JRockit documentation despite the question being, “What is the default value for XX:MaxDirectMemorySize for SUN JVM 1.6?” Having common HotSpot options documented in the main Java launcher documentation should help reduce these frequent causes of confusion about HotSpot VM options’ behavior.
Two examples of HotSpot JVM options now documented directly in the Java launcher (the executable java to which the options apply) documentation are -XX:+HeapDumpOnOutOfMemory and -XX:MaxDirectMemorySize. The documentation for the Oracle HotSpot 8 Java launcher describes these two example options as shown next:
These and several other HotSpot JVM options that were available before Java 8 are not described in prior versions of the Java launcher documentation such as for Java 7 and Java 6. This is a small but nonetheless welcome addition that is just one small part of a long list of reasons for moving to Java 8 from older versions.
| Reference: | Improved Documentation of HotSpot Options in Java 8 from our JCG partner Dustin Marx at the Inspired by Actual Events blog. |
