Essential Java Debugging & Performance Tools: jstack, jmap, jhat, and More

A good set of tools can greatly improve Java development efficiency; this article presents several essential Java utilities in a Q&A format.

Q: Which tool checks for memory leaks?

A: jmap generates heap dump files, and jhat visualizes them.

Q: Which tool helps investigate a process that constantly occupies CPU?

A: Use top -Hp <pid> to find the hottest thread, then jstack <pid> to inspect its stack trace.

jstack

Full name: JVM Stack Trace. Purpose: View the stack of a Java process to identify deadlocks, I/O waits, infinite loops, etc.

Command usage: jstack <pid> Example output shows waiting locks and locked resources (see image).

jstat

Purpose: Display memory usage statistics of a JVM process (basic stats only). jstat -class <pid> – shows loaded classes and their space usage. jstat -compiler 10 – shows compiled method count.

Q: How to print memory usage every 2 seconds?

Use jstat -gcutil <pid> 2000. The output shows percentages for Eden (E) and Old (O) generations; sustained 99‑100% indicates OOM risk.

jmap

Full name: JVM Memory Map. Purpose: Generate a heap snapshot of a process.

Command usage: jmap -dump:file=./dumpfile.dump <pid> To view object allocation for a specific class:

jmap -histo:live <pid> | grep ClassName

Q: Example of using jmap to detect memory leaks

Three snapshots were taken at 1‑hour intervals. The object count for ObjectA increased fastest, indicating a potential leak.

jhat

Full name: JVM Heap Analysis Tool. Purpose: Analyze heap dumps generated by jmap; starts a web server for interactive inspection.

jhat -J-Xmx515M dumpfile.dump

jdb

Full name: Java Debugger. Purpose: Debug core files and running Java processes, similar to gdb.

jdb -classpath . Test

jdb -attach 8000 -sourcepath /Users/wefit/Development/study/java/jtest/src/

jcmd

Purpose: Multifunctional tool for heap dumps, thread info, GC, and performance sampling.

List all JVM processes: jcmd -l Print performance counters: jcmd <pid> PerfCounter.print Show available commands: jcmd <PID> help Print thread stack: jcmd <PID> Thread.print Dump heap:

jcmd <PID> GC.heap_dump <FILE_NAME>

jps

Full name: JVM Process Status Tool. Purpose: List all Java processes for the current user. jps -v – show JVM arguments. jps -l – show main class or JAR path. jps -m – show arguments passed to main.

jinfo

Full name: JVM Information. Purpose: Similar to jps but can query a specific PID for JVM and system parameters; does not show memory usage.

javap

Disassembles Java bytecode into readable source.

javac

Java compiler; the compilation process is illustrated in the following diagram.

jconsole

Monitors local or remote JVMs, allowing inspection of threads, memory, and deadlocks.

jvisualvm

Provides remote JVM monitoring, can attach to a running Java process without extra configuration.