How to Monitor Heap Memory Usage with jstat in Java

What is the method to check heap memory usage in Java using jstat?

jstat -gcutil <pid>

The 'jstat' command is a specialized tool used for monitoring the Java Virtual Machine (JVM) performance, specifically memory usage, garbage collection, and class loading. It allows developers to analyze the heap memory utilization in real-time, which is essential for optimizing Java applications and ensuring they run efficiently.

jstat -gcutil <pid>

// Example output:
// S0C    S1C    EC     OC     PC     CC    YGC    YGCT   FGC    FGCT
// 0.00  0.00  15360.00  1728000.00  661504.00  0.00   142   0.180   3  0.204

// Here:
// EC - Eden Space Capacity
// OC - Old Space Capacity
// YGC - Young Generation Garbage Collections
// YGCT - Young Generation Garbage Collection Time.

Causes

• JVM heap memory is used to store objects created by the application.
• Garbage collection is triggered when the heap space is full or at specific intervals, impacting the reported memory usage.

Solutions

• Use the jstat command to observe real-time statistics of heap memory usage.
• For example, to monitor the heap usage, run: `jstat -gcutil <pid>` where `<pid>` is the process ID of the Java application.
• The command outputs statistics including the sizes of different memory regions (Eden Space, Survivor Space, and Tenured Generation).

Mistake: Forgetting to specify the correct process ID (PID) with jstat.

Solution: Ensure you fetch the accurate PID of the Java application you want to monitor using `jps` command.

Mistake: Not having appropriate permissions to run jstat on a JVM.

Solution: Run jstat with the required permissions, or execute it as a user that has access to the JVM you are monitoring.

Mistake: Assuming jstat outputs are always accurate without context.

Solution: Cross-reference jstat output with other monitoring tools or logs to get a complete view of memory usage.