Master Java’s Built‑In Diagnostic Tools: jps, jstat, jinfo, jmap, jhat & jstack

Environment: JDK 1.8.0_92. The command‑line tools are located in %JAVA_HOME%\bin.

Most tools are about 17 KB because the executable files are thin wrappers around classes in %JAVA_HOME%\tools.jar.

Below is an overview of commonly used tools.

jps Displays all currently running HotSpot JVM processes. Options:

-l : prints the full package and class name, or the jar file path if the process runs a jar.

-m : prints the arguments passed to the JVM at startup.

-v : prints the JVM arguments used to start the process.

-q : prints only the process ID.

Example program:

<code>public class JpsMain {
    public static void main(String[] args) throws Exception {
        System.out.println(Arrays.toString(args));
        System.in.read();
    }
}
</code>

Running the program with three arguments (a b c) and then executing jps -l -m -v -q shows the corresponding output.

jstat Monitors various JVM runtime statistics such as class loading, memory usage, and garbage‑collection activity. Usage:

<code>jstat -&lt;option&gt; [-t] [-h&lt;lines&gt;] &lt;vmid&gt; [&lt;interval&gt; [&lt;count&gt;]]</code>

where vmid is the JVM process ID, interval is the query interval (s or ms), and count is the number of queries. Common options:

-class : class loading/unloading statistics.

-compiler : JIT compilation statistics.

-gc : heap usage and GC timing.

-gcutil : same as

-gc

but shows used‑space percentages.

-gcnew : young‑generation GC information.

-gcold : old‑generation GC information.

Example: jstat -gc 16480 3s reports GC data every three seconds.

jinfo Displays and modifies JVM flags of a running process. Usage:

<code>jinfo &lt;option&gt; &lt;pid&gt;</code>

Example: jinfo -flag CICompilerCount 16480 shows the value of the CICompilerCount flag.

jmap Generates heap‑dump snapshots and provides heap‑related information. Usage:

<code>jmap [option] &lt;pid&gt;</code>

Key options:

-dump : creates a heap dump (e.g.,

jmap -dump:live,format=b,file=heap.bin <pid>

).

-heap : prints detailed heap configuration.

-histo : shows a histogram of objects in the heap.

jhat Analyzes a heap‑dump file. Usage: jhat <file> After generating a dump, run jhat heap.bin and open the provided HTTP service (default port 7000) in a browser.

jstack Produces a thread dump of a running JVM. Usage:

<code>jstack [option] &lt;pid&gt;</code>

Options:

-F : forces a thread dump when the JVM does not respond.

-l : includes lock information.

-m : shows native stack frames for C/C++ methods.