Understanding JVM Metrics via Spring Boot Actuator

Products

• VMware Tanzu Application Service

Issue/Introduction

Spring Boot Actuator exposes several JVM memory–related metrics (for example,
/actuator/metrics/jvm.memory.used) that provide visibility into heap, non-heap, and off-heap memory usage as well as Garbage Collection (GC) activity of a Java application.

This article explains some of the most important JVM metrics available via Actuator and what each metric represents.

Environment

 

• Spring Boot applications with Spring Boot Actuator

• VMware Tanzu Application Service (TAS) / Elastic Application Runtime

 

Resolution

JVM Memory Metrics

 

1. jvm.memory.used

Endpoint

/actuator/metrics/jvm.memory.used

 

Description
Returns the current amount of memory in use, in bytes.

This metric is tagged, allowing it to be filtered by memory area and memory pool.

Tags

• area: heap | nonheap

• id: specific memory pool (for example, Eden Space, Tenured Gen, Metaspace)

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2. Heap memory metrics

Total heap used

 

/actuator/metrics/jvm.memory.used?tag=area:heap

 

Represents all heap usage combined, across:

• Eden Space

• Survivor Space

• Tenured (Old) Generation

This value fluctuates depending on GC activity.

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Eden Space

/actuator/metrics/jvm.memory.used?tag=id:Eden%20Space

 

 

• Short-lived object allocation area

• High fluctuation is expected

• Frequent growth does not indicate a memory leak

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Survivor Space

/actuator/metrics/jvm.memory.used?tag=id:Survivor%20Space

 

 

• Objects that survived at least one GC

• Usually small

• Helps explain object aging behavior

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Tenured Gen (Old Gen)

 

 

/actuator/metrics/jvm.memory.used?tag=id:Tenured%20Gen

Most important metric for leak detection

• Represents long-lived objects

• If this value steadily increases over time after full GCs, it may indicate a heap leak

• If stable, heap is healthy

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3. Non-heap memory metrics

Total non-heap used

 

 

/actuator/metrics/jvm.memory.used?tag=area:nonheap

Includes:

• Metaspace

• Compressed Class Space

• Code Cache

This memory does not belong to the Java heap but is still part of the JVM process RSS.

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Metaspace

 

 

/actuator/metrics/jvm.memory.used?tag=id:Metaspace

Stores:

• Class metadata

• Reflection data

• Dynamic proxies

Steady growth may indicate:

• Ongoing class loading

• Classloader retention

• Dynamic bytecode generation

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Compressed Class Space

 

 

/actuator/metrics/jvm.memory.used?tag=id:Compressed%20Class%20Space

• Holds compressed class pointers

• Typically stable

• Grows with number of loaded classes

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Code Cache

 

 

/actuator/metrics/jvm.memory.used?tag=id:Code%20Cache

Stores JIT-compiled native machine code.

• Gradual growth is normal during JVM warm-up

• May increase over long uptimes

• Rarely a leak unless it grows without bound

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4. Memory capacity metrics

Heap committed

 

 

/actuator/metrics/jvm.memory.committed?tag=area:heap

• Memory currently reserved by the JVM

• Often equals -Xmx once the heap is fully expanded

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Heap max

 

 

/actuator/metrics/jvm.memory.max?tag=area:heap

• Maximum heap size

• Typically equal to -Xmx

• Fixed unless JVM is restarted

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5. GC metrics

GC pause time

 

 

/actuator/metrics/jvm.gc.pause

Provides:

• COUNT: number of GC pauses

• TOTAL_TIME: total time spent in GC

• MAX: longest pause observed

Useful for:

• Correlating GC behavior with memory pressure

• Verifying GC health (many short pauses are usually fine)

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6. Direct / off-heap buffers

Buffer memory

 

 

/actuator/metrics/jvm.buffer.memory.used

Reports usage of:

• Direct buffers

• Mapped buffers

Usually small unless application uses:

• Netty

• NIO direct buffers

• Memory-mapped files