Master JDK Command-Line Tools for Java Troubleshooting

JDK Command-Line Tools Overview

Environment: JDK 1.8.0_92. The JDK (Java Development Kit) includes a set of command-line tools that help developers troubleshoot Java applications.

Commonly Used Tools

jps – Displays JVM process IDs (PIDs) and can show full package and class names with the -l option or launch arguments with -m . Useful for locating running Java processes.

jstat – Monitors runtime JVM information such as class loading, memory usage, and garbage collection. Options include -gc , -class , -compiler , and many others to view specific metrics.

jstack – Retrieves thread stack traces of a JVM process, aiding in diagnosing hangs or crashes. Options like -F , -l , and -m provide additional details.

jmap – Generates heap dump files for memory analysis. The -dump option creates a snapshot, while -heap and -histo display heap details and object statistics.

Additional tools such as jconsole and jvisualvm are also available for troubleshooting.

Tool Location

All tools reside in the %JAVA_HOME%\bin directory. Their executable size is typically around 17 KB because they are thin wrappers around classes in tools.jar .

Tool Details

1. jps

Displays all currently running Java virtual machine processes.

Options:

-l : Output full package and class name, or JAR path if applicable.

-m : Show arguments passed to the main method.

Example program:

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

Running with parameters -v displays JVM launch arguments.

2. jstat

Monitors various JVM runtime information such as class loading, memory, and garbage collection.

Usage:

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

Key options:

-class : Shows class loading/unloading counts and times.

-compiler : Displays JIT compilation statistics.

-gc : Monitors heap usage and GC times.

-gcutil : Shows used space as a percentage of total.

Example to report GC every 3 seconds:

<code>jstat -gc 16480 3s</code>

3. jinfo

Shows and modifies JVM parameters in real time.

<code>jinfo option pid</code>

Example: display all JVM information for a process.

4. jmap

Generates heap dump snapshots and provides heap statistics.

<code>jmap -dump:live,format=b,file=heap.bin &lt;pid&gt;</code>

Other useful commands:

-heap : Detailed heap information.

-histo : Object histogram.

5. jhat

Analyzes heap dump files generated by jmap .

<code>jhat &lt;file&gt;</code>

Starts a web service (default port 7000) to explore the heap.

6. jstack

Creates a snapshot of all threads in a JVM process.

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

Options:

-F : Force thread dump if the JVM is unresponsive.

-l : Include lock information.

-m : Show native stack for C/C++ frames.

End of guide.