Java Diagnostic Lab

This project is designed for practicing JVM diagnostics, including memory leaks, heap dumps, and garbage collection analysis. It simulates common JVM problems to train working with tools like VisualVM, jcmd, and Eclipse MAT.

Project Structure

src/main/java/dev/abykov/pets/
├── diagnostic_lab
│ ├── MainApp.java
│ ├── deadlock
│ │ └── DeadlockSimulator.java
│ ├── gc
│ │ └── GcPressureSimulator.java
│ └── leak
│   └── MemoryLeakSimulator.java

Memory Leak scenario (OutOfMemoryError)

Run:

java -Xms256m -Xmx256m \
-XX:+HeapDumpOnOutOfMemoryError \
-XX:HeapDumpPath=./diagnostics/oomdump.hprof \
-cp target/diagnostic-lab-1.0-SNAPSHOT.jar \
dev.abykov.pets.diagnostic_lab.demo.MainApp memory

Expected:

• Continuous allocation of byte[1MB] arrays.
• JVM OutOfMemoryError.
• Heap dump automatically saved.

Tools:

• VisualVM: Monitor heap usage, take snapshots.
• Eclipse MAT: Analyze .hprof, check Retained Size and GC roots.

Sample finding:

OutOfMemoryError caused by ArrayList holding ~266MB of byte[] arrays.

Deadlock scenario (Thread Dump)

Run:

java -cp target/diagnostic-lab-1.0-SNAPSHOT.jar \
dev.abykov.pets.diagnostic_lab.demo.MainApp deadlock

Expected: two threads will block each other on synchronized locks.

Tools:

• VisualVM: Take Thread Dump.
• jcmd <pid> Thread.print: Analyze blocked threads and deadlock.

GC Pressure scenario

Run:

java -Xms256m -Xmx256m -XX:+UseG1GC -Xloggc:./diagnostics/gc.log \
-cp target/diagnostic-lab-1.0-SNAPSHOT.jar \
dev.abykov.pets.diagnostic_lab.demo.MainApp gc

Expected:

• High GC activity.
• GC logs show Full GC frequency.

Diagnostics Artifacts

diagnostics/
├── oommemory.hprof         # Heap dump after OOM
├── threads.txt             # Thread dump example
├── gc.log                  # GC logs with pauses
├── memory-sampling.csv     # VisualVM sampling export
├── analysis-report.txt     # Manual summary

Recommended Tools

• VisualVM
• Eclipse MAT (Memory Analyzer Tool)
• jcmd, jstack
• GC Easy for logs: https://gceasy.io/

License

MIT License