Thread Dump

About

A thread dump is a snapshot of all the threads running in a Java Virtual Machine (JVM) at a specific point in time. It provides details about each thread, including:

• Thread state (e.g., RUNNABLE, BLOCKED, WAITING, TIMED_WAITING)

• Stack traces of what the threads are currently executing

• Synchronization locks held by threads (if any)

Why is it Useful?

• Performance bottlenecks: Helps in diagnosing high CPU usage or thread contention issues.

• Deadlocks: Identifies if any threads are stuck waiting for locks.

• Thread states: Understand the distribution of thread states and their current activities.

When to Capture a Thread Dump?

• When the application is not responding (e.g., "hanging").

• During high CPU usage.

• When investigating concurrent programming issues like deadlocks.

How to Generate a Thread Dump?

Using JDK Tools:

• jstack <pid>: Captures a thread dump of the application.

• jcmd <pid> Thread.print: Another way to generate a thread dump.

Using IDEs or Tools:

• Tools like VisualVM, JConsole, or IntelliJ IDEA can capture thread dumps.

Command Line Signal (Linux/Unix):

• kill -3 <pid>: Sends a SIGQUIT signal to generate a thread dump, which will appear in the standard output (e.g., logs).

Example Output (Snippet):

"main" #1 prio=5 os_prio=0 tid=0x00007f93c4008000 nid=0x1 waiting on condition [0x00007f93c4a4f000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        at java.lang.Thread.join(Thread.java:1252)
        - locked <0x00000000c1020000> (a java.lang.Thread)

How to Analyze a Thread Dump?

• Look for threads in BLOCKED state: These are waiting for locks and might indicate contention.

• Look for deadlocks: Deadlocks will be explicitly identified in most thread dumps.

• Analyze thread states: A high number of threads in WAITING or BLOCKED states may indicate issues.