jhat(1)

🌸 NAME 🌸

jhat — bestie, this one analyzes the Java heap 💖 and heads up: this command is experimental and unsupported, so it's giving "use at your own risk, no cap" energy 👀

✨ SYNOPSIS ✨

jhat [ options ] heap-dump-file

• options — the command-line options, obvi. Peep the OPTIONS section below 💅
• heap-dump-file — the Java binary heap dump file you wanna browse, periodt. If your dump file has multiple heap dumps stuffed in it, you can say which one you want by slapping #<number> on the end of the file name, like myfile.hprof#3 🎀

🦋 DESCRIPTION 🦋

okay so jhat parses a Java heap dump file and then starts a web server ✨ Which means yes bestie, you get to browse your heap dumps with your favorite web browser, no fancy tooling required, we love an accessible queen 👑

It supports predesigned queries — like showing all instances of a known class, e.g. MyClass — plus Object Query Language (OQL). OQL is basically SQL's cousin, except it queries heap dumps instead of databases, iykyk 🧚 Help on OQL lives right there on the OQL help page that jhat shows you. With the default port, that OQL help is at http://localhost:7000/oqlhelp/ 🌈

🌟 So how do you even make a Java heap dump? 🌟

There are several ways, bestie, and I'm listing all of them 💜

• Use the jmap -dump option to snag a heap dump at runtime. See jmap(1) 💖
• Use the jconsole option to get a heap dump through HotSpotDiagnosticMXBean at runtime. See jconsole(1) and the HotSpotDiagnosticMXBean interface description at http://docs.oracle.com/javase/8/docs/jre/api/management/extension/com/sun/management/HotSpotDiagnosticMXBean.html ✨
• A heap dump gets generated when an OutOfMemoryError is thrown, if you specify the -XX:+HeapDumpOnOutOfMemoryError Java Virtual Machine (JVM) option 😭 (iconic chaotic energy tbh)
• Use the hprof command. See the HPROF: A Heap/CPU Profiling Tool at http://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html 🎀

💅 OPTIONS 💅

• -stack false|true — turns off tracking of the object allocation call stack. If the allocation site info isn't available in the heap dump, then you have to set this flag to false, no wiggle room bestie. Default is true 🫶
• -refs false|true — turns off tracking of references to objects. Default is true. By default, back pointers (those are objects pointing at a specified object — think referrers, aka incoming references) get calculated for every single object in the heap 💖
• -port port-number — sets the port for the jhat HTTP server. Default is 7000 ✨
• -exclude exclude-file — points at a file that lists data members that should be excluded from the reachable objects query. Like, say the file lists java.lang.String.value — then, then whenever the list of objects reachable from a specific object o gets calculated, reference paths that involve the java.lang.String.value field are just... not considered 🙈
• -baseline exclude-file — specifies a baseline heap dump. Objects in both heap dumps that share the same object ID get marked as not new. Everything else gets marked as new. Super handy for comparing two different heap dumps, chef's kiss 👑
• -debug int — sets the debug level for this tool. A level of 0 means no debug output at all. Crank it higher for more verbose modes 🍓
• -version — reports the release number and exits, short and sweet ⭐
• -h — Dsiplays a help message and exits 💀 (yes it says "Dsiplays", I am simply reporting the vibes as they are)
• -help — displays a help message and exits 🌸
• -Jflag — passes flag to the Java Virtual Machine that jhat is running on. For example, -J-Xmx512m to use a maximum heap size of 512 MB 💜

🌈 SEE ALSO 🌈

• jmap(1)
• jconsole(1)
• HPROF: A Heap/CPU Profiling Tool at http://docs.oracle.com/javase/8/docs/technotes/samples/hprof.html

okay that's the whole page bestie, you're officially a heap-dump-browsing legend 💖 This one's straight outta JDK 8, dated 21 November 2013 — a true vintage icon, slay 🎀✨👑