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Java garbage collection (GC) logs help you determine whether the Java heap is retaining more objects over time, how often collections run, and what pauses they cause. A high process or container memory reading alone does not prove a heap leak: first establish the JVM and collector, then compare collection trends and, if needed, inspect object-level evidence.
Start by identifying the JVM and collector
GC log syntax and diagnostic commands vary by Java version and runtime implementation. Before interpreting a log, capture the exact Java version, vendor or distribution, startup arguments, heap limits, and active garbage collector. Oracle recommends recording the Java version and JVM flags during troubleshooting: Oracle Java SE 26 Troubleshooting Guide.
- Run
java -versionin the same runtime environment as the application. - Record the JVM startup flags and configured heap limits.
- Identify the active collector from the runtime’s available diagnostic information or startup configuration.
The logging example below is documented for Oracle Java SE 24. Check the equivalent syntax and available commands for the runtime actually in use; do not assume every JVM vendor or release behaves identically.
Enable and preserve GC logs
Oracle Java SE 24 example
Oracle documents this unified logging option:
-Xlog:gc*,gc+phases=debug:gc.log
In this example, gc* enables GC-tagged messages at info level, while the exact gc,phases tags are logged at debug level. Output is written to gc.log. Oracle notes that writing to a discrete file makes logs easier to read and preserves them across restarts; log rotation can limit how much history is retained. Adapt the output location and rotation policy to the deployed JDK and operational requirements. See Oracle Java SE 24 java command documentation.
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Keep enough history to compare multiple collections and, when diagnosing a trend, multiple periods of application activity. A single collection rarely answers whether memory is accumulating.
Read patterns across collections, not one high reading
Heap occupancy normally rises as the application allocates objects and falls when a collection reclaims garbage. The useful signal is what happens over repeated collections, especially after old-generation collections—not whether the heap was high at one moment.
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- Post-collection live set: compare the amount still used after old collections. A steadily rising live set is more concerning than ordinary growth between collections.
- Reclamation: note whether collections free meaningful space or repeatedly recover very little.
- Collection type and frequency: distinguish ordinary collections from repeated full or old-generation collections and note how often they occur.
- Pause time: track pauses alongside occupancy. Frequent or long pauses can be operationally significant even if they do not establish a leak.
- Other pools: where the log reports them, check old-generation and metaspace occupancy and reclamation as well as heap totals.
Oracle’s Java SE 26 troubleshooting guide advises: “Watch for a steadily increasing heap size over time that could indicate a memory leak.” A rising post-old-collection live set or repeated full collections that recover little space is a reason to investigate, not proof of a leak. See Oracle’s memory leak troubleshooting guidance.
Use object-level evidence to find what is growing
GC logs show collection behavior, but they do not identify which code is retaining objects. If the post-collection live set keeps rising, compare class histograms at different times. Oracle describes histograms as listing classes in descending size; a sequence can reveal which instance counts or sizes are increasing.
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Compare class histograms
Use the JDK’s jcmd utility:
jcmd <pid> GC.class_histogram
Replace <pid> with the application process ID. Compare snapshots taken under reasonably comparable workload conditions, looking for class instance counts and sizes that grow persistently. Oracle recommends jcmd over jmap for enhanced diagnostics and reduced performance overhead, but histogram impact can still be high depending on heap size and contents. Consult Oracle’s jcmd command reference.
Capture a heap dump when a histogram is not enough
A heap dump provides an object graph that can be examined for retained objects and reference chains:
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jcmd <pid> GC.heap_dump filename=heapdump.hprof
Oracle’s jcmd specification says heap dump generation has high impact and may request a full GC. Plan when to run it, confirm that the destination has enough space, and restrict access: heap dumps can contain sensitive application data. The same reference documents -XX:+HeapDumpOnOutOfMemoryError, which configures a dump when an OutOfMemoryError occurs. That option is a fallback for an error event, not a substitute for monitoring and planning dump storage.
Observe growth over a recording window
Java Flight Recorder (JFR) with heap statistics enabled can show object types and top growers over time. Oracle notes that heap statistics trigger an old collection at the beginning and end of the recording, allowing comparison of live-set behavior. Account for those collections when choosing the recording window and interpreting its effects. See Oracle’s Java SE 26 guidance on memory leak diagnosis.
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When process memory is high but the heap does not explain it
GC logs describe garbage collection and Java heap behavior; they do not account for every kind of process memory. If process RSS or container memory is high while heap trends do not show corresponding growth, investigate memory outside the Java heap. Potential categories include HotSpot internal native memory, direct or native-library allocations, thread stacks, mapped files, and operating-system accounting.
HotSpot Native Memory Tracking (NMT) can help examine internal JVM memory, but Oracle explicitly notes that NMT does not track allocations made by non-JVM code. Native libraries may therefore require operating-system-supported tools suited to the runtime and platform. The right procedure depends on the operating system and where the application runs; do not treat NMT as a complete accounting of process RSS. See Oracle’s memory troubleshooting guide.
Choose the next diagnostic by the question you need to answer
| Method | What it shows | Limit or operational consideration |
|---|---|---|
| GC log | Collection cadence, pauses, and heap occupancy trends | Does not identify object retainers by itself |
| Repeated class histograms | Class instance counts and sizes at snapshots; comparison can expose growing types | Impact can be high on large heaps |
| Heap dump | Detailed object graph and retention evidence | High impact; may request a full GC and create a large, sensitive file |
| JFR with heap statistics | Time-based JVM evidence and object types that grow during a recording | Heap statistics trigger old collections at recording start and end |
| NMT and operating-system tools | Native memory evidence when Java heap trends do not explain process memory | NMT covers HotSpot internal memory, not allocations by non-JVM code; OS tools depend on platform |
For command details and diagnostic cautions, see Oracle’s jcmd reference and memory troubleshooting guide.
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