Java.Multithreading.How to dump a thread?

🧵🛠️ How to Dump Threads in Java

✅ 1. Using Command-Line Tools

🔸 A) jstack — Most Common

jstack <PID>

Dumps all thread stack traces of a running JVM.

You can get the <PID> from jps:

jps
jstack 12345 > thread-dump.txt

✅ Shows:

• Thread states (RUNNABLE, BLOCKED, WAITING)
• Locks held
• Deadlocks (with -l flag)

✅ 2. Using Thread.getAllStackTraces() (Java Code)

If you want to dump threads programmatically:

public class ThreadDump {
    public static void main(String[] args) {
        Map<Thread, StackTraceElement[]> map = Thread.getAllStackTraces();

        for (Map.Entry<Thread, StackTraceElement[]> entry : map.entrySet()) {
            Thread thread = entry.getKey();
            System.out.println("\nThread: " + thread.getName() + " [" + thread.getState() + "]");

            for (StackTraceElement element : entry.getValue()) {
                System.out.println("\tat " + element);
            }
        }
    }
}

✅ 3. Using Visual Tools

• JVisualVM (comes with JDK)
  • Attach to your process
  • Click “Thread Dump” button
• JConsole — basic thread monitoring
• Mission Control (JMC) — advanced profiling

✅ 4. Log a Dump on Ctrl+\ (UNIX/Linux only)

You can use kill -3 <PID>:

kill -3 12345

Dumps thread stack to the standard output (console/logs).

Safe to use anytime — doesn’t kill the process.

Thread t = new Thread(() -> { while(true) {} }, "MyCustomThread");
t.start();

🧠 Use Cases:

• Hanging or unresponsive app? 📛 Dump threads.
• Deadlock? 🔒 Look at BLOCKED threads and owned locks.
• Performance issues? 🔁 Look for RUNNABLE threads stuck in loops.