Java/JVM Performance Profiling

When NOT to Use This Skill

• Node.js/JavaScript profiling - Use the nodejs-profiling skill for V8 profiler and heap analysis

• Python profiling - Use the python-profiling skill for cProfile and tracemalloc

• Application-level optimization - This is for JVM-level profiling, not algorithm optimization

• Database query tuning - Use database-specific profiling tools

• Frontend performance - Use browser DevTools for client-side profiling

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: java for comprehensive JFR configuration, GC tuning, and JVM diagnostics.

Java Flight Recorder (JFR)

Starting JFR

Start recording with application

java -XX:+FlightRecorder
-XX:StartFlightRecording=duration=60s,filename=recording.jfr
-jar app.jar

Start recording on running JVM

jcmd <pid> JFR.start duration=60s filename=recording.jfr

Continuous recording (always-on)

java -XX:+FlightRecorder
-XX:FlightRecorderOptions=stackdepth=256
-XX:StartFlightRecording=disk=true,maxsize=500m,maxage=1d
-jar app.jar

Dump current recording

jcmd <pid> JFR.dump filename=dump.jfr

JFR Configuration

custom.jfc

<?xml version="1.0" encoding="UTF-8"?> <configuration version="2.0"> <event name="jdk.CPULoad"> <setting name="enabled">true</setting> <setting name="period">1 s</setting> </event> <event name="jdk.GCHeapSummary"> <setting name="enabled">true</setting> </event> <event name="jdk.ObjectAllocationInNewTLAB"> <setting name="enabled">true</setting> <setting name="stackTrace">true</setting> </event> </configuration>

Analyzing JFR Files

Print JFR summary

jfr summary recording.jfr

Print specific events

jfr print --events jdk.CPULoad recording.jfr jfr print --events jdk.ExecutionSample --json recording.jfr

Export to JSON

jfr print --json recording.jfr > recording.json

jcmd Diagnostics

Process Information

List all Java processes

jcmd

VM info

jcmd <pid> VM.version jcmd <pid> VM.flags jcmd <pid> VM.system_properties jcmd <pid> VM.command_line

Thread dump

jcmd <pid> Thread.print

Heap info

jcmd <pid> GC.heap_info

Class histogram

jcmd <pid> GC.class_histogram

Memory Analysis

Native memory tracking (requires -XX:NativeMemoryTracking=summary)

jcmd <pid> VM.native_memory summary

Heap dump

jcmd <pid> GC.heap_dump /path/to/dump.hprof

Force GC

jcmd <pid> GC.run

GC Tuning

GC Selection

G1GC (default in JDK 9+, recommended for heap > 4GB)

java -XX:+UseG1GC -jar app.jar

ZGC (low latency, JDK 15+)

java -XX:+UseZGC -jar app.jar

Shenandoah (low latency, OpenJDK)

java -XX:+UseShenandoahGC -jar app.jar

Parallel GC (throughput)

java -XX:+UseParallelGC -jar app.jar

GC Logging

JDK 9+ unified logging

java -Xlog:gc*:file=gc.log:time,uptime,level,tags:filecount=5,filesize=10m
-jar app.jar

Common GC flags

java -XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintTenuringDistribution
-Xloggc:gc.log
-jar app.jar

G1GC Tuning

java -XX:+UseG1GC
-XX:MaxGCPauseMillis=200 \ # Target pause time -XX:G1HeapRegionSize=16m \ # Region size -XX:InitiatingHeapOccupancyPercent=45 \ # Start marking at 45% -XX:G1ReservePercent=10 \ # Reserve for promotions -XX:ConcGCThreads=4 \ # Concurrent GC threads -XX:ParallelGCThreads=8 \ # Parallel GC threads -jar app.jar

Memory Optimization

Heap Sizing

Set heap size

java -Xms4g -Xmx4g -jar app.jar # Fixed heap (recommended for production)

Metaspace sizing

java -XX:MetaspaceSize=256m -XX:MaxMetaspaceSize=512m -jar app.jar

Direct memory

java -XX:MaxDirectMemorySize=256m -jar app.jar

Memory Leak Detection

// Common leak patterns

// ❌ Bad: Static collections that grow private static List<Object> cache = new ArrayList<>(); public void process(Object obj) { cache.add(obj); // Never removed }

// ✅ Good: Bounded cache with eviction private static final Cache<String, Object> cache = Caffeine.newBuilder() .maximumSize(10_000) .expireAfterWrite(Duration.ofMinutes(10)) .build();

// ❌ Bad: Unclosed resources public void readFile(String path) { InputStream is = new FileInputStream(path); // Missing is.close() }

// ✅ Good: Try-with-resources public void readFile(String path) { try (InputStream is = new FileInputStream(path)) { // Process } }

// ❌ Bad: ThreadLocal not cleaned private static ThreadLocal<Connection> connHolder = new ThreadLocal<>(); public void process() { connHolder.set(getConnection()); // Missing connHolder.remove() }

// ✅ Good: Always clean ThreadLocal public void process() { try { connHolder.set(getConnection()); // Process } finally { connHolder.remove(); } }

CPU Profiling

async-profiler (recommended)

Profile CPU

./profiler.sh -d 30 -f profile.html <pid>

Profile allocations

./profiler.sh -d 30 -e alloc -f alloc.html <pid>

Profile locks

./profiler.sh -d 30 -e lock -f lock.html <pid>

Flame graph output

./profiler.sh -d 30 -f flamegraph.html -o flamegraph <pid>

JMC (Java Mission Control)

Open JFR recording in JMC

jmc recording.jfr

Common Bottleneck Patterns

Synchronization Issues

// ❌ Bad: Coarse-grained locking public synchronized void process(String key, Object value) { cache.put(key, value); compute(value); }

// ✅ Good: Fine-grained locking private final ConcurrentHashMap<String, Object> cache = new ConcurrentHashMap<>(); public void process(String key, Object value) { cache.put(key, value); // Lock-free for different keys compute(value); }

// ✅ Good: Read-write lock private final ReadWriteLock lock = new ReentrantReadWriteLock(); public Object get(String key) { lock.readLock().lock(); try { return cache.get(key); } finally { lock.readLock().unlock(); } }

String Operations

// ❌ Bad: String concatenation in loop String result = ""; for (String s : list) { result += s; // Creates new String each iteration }

// ✅ Good: StringBuilder StringBuilder sb = new StringBuilder(); for (String s : list) { sb.append(s); } String result = sb.toString();

// ✅ Good: String.join for simple cases String result = String.join("", list);

Collection Optimization

// ❌ Bad: ArrayList when size known List<String> list = new ArrayList<>(); for (int i = 0; i < 10000; i++) { list.add(getData(i)); // Multiple resizes }

// ✅ Good: Pre-size collections List<String> list = new ArrayList<>(10000);

// ✅ Good: Use primitive collections for performance // Use Eclipse Collections, Trove, or fastutil IntList list = new IntArrayList(10000);

Boxing/Unboxing

// ❌ Bad: Autoboxing in hot path public long sum(List<Long> numbers) { long sum = 0; for (Long n : numbers) { sum += n; // Unboxing each iteration } return sum; }

// ✅ Good: Use primitive streams public long sum(List<Long> numbers) { return numbers.stream().mapToLong(Long::longValue).sum(); }

// ✅ Good: Primitive arrays when possible public long sum(long[] numbers) { return Arrays.stream(numbers).sum(); }

JIT Optimization

Warm-up

// Warm-up critical paths before measuring public static void main(String[] args) { // Warm-up phase for (int i = 0; i < 10_000; i++) { criticalMethod(i); }

// Measurement phase
long start = System.nanoTime();
for (int i = 0; i < 100_000; i++) {
    criticalMethod(i);
}
long duration = System.nanoTime() - start;

}

JIT Logging

Print JIT compilation

java -XX:+PrintCompilation -jar app.jar

Print inlining decisions

java -XX:+UnlockDiagnosticVMOptions -XX:+PrintInlining -jar app.jar

Disable specific optimizations for debugging

java -XX:+UnlockDiagnosticVMOptions -XX:DisableIntrinsic=_hashCode -jar app.jar

Profiling Checklist

Check Tool Command

CPU hotspots JFR jcmd <pid> JFR.start

Memory usage jcmd jcmd <pid> GC.heap_info

GC behavior GC logs -Xlog:gc*

Thread contention JFR jdk.JavaMonitorWait events

Memory leaks Heap dump jcmd <pid> GC.heap_dump

Class loading jcmd jcmd <pid> VM.classloaders

Production Flags

java
-server
-Xms4g -Xmx4g
-XX:+UseG1GC
-XX:MaxGCPauseMillis=200
-XX:+UseStringDeduplication
-XX:+AlwaysPreTouch
-XX:+DisableExplicitGC
-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=/var/log/heap.hprof
-Xlog:gc*:file=/var/log/gc.log:time,uptime:filecount=5,filesize=10m
-jar app.jar

Anti-Patterns

Anti-Pattern Why It's Wrong Correct Approach

System.out.println() in hot paths Extremely slow, synchronous I/O Use SLF4J with async appenders

Creating many short-lived objects GC pressure, allocation overhead Reuse objects, use object pools

synchronized on hot paths Thread contention, poor scalability Use ConcurrentHashMap , ReentrantLock , or lock-free structures

String concatenation with + in loops Creates many intermediate strings Use StringBuilder

Autoboxing in loops Creates wrapper objects Use primitive types

Not sizing collections Frequent resizing, memory churn Pre-size with new ArrayList<>(expectedSize)

finalize() for cleanup Unpredictable, deprecated Use try-with-resources or explicit cleanup

Ignoring GC logs Miss performance degradation Always enable GC logging in production

One-size-fits-all heap Wrong GC pauses for workload Tune heap based on app behavior

Not using connection pooling Connection creation overhead Use HikariCP or similar

Quick Troubleshooting

Issue Diagnosis Solution

Long GC pauses Heap too large or wrong GC Use ZGC/Shenandoah or tune G1GC pause targets

OutOfMemoryError: Java heap space

Memory leak or undersized heap Analyze heap dump, increase -Xmx if needed

OutOfMemoryError: Metaspace

Too many classes loaded Increase -XX:MaxMetaspaceSize or fix classloader leak

High CPU usage Hot loop, inefficient algorithm CPU profile with JFR or async-profiler

Thread contention Lock competition Thread dump analysis, reduce lock scope

Slow startup Class loading, initialization Use AppCDS, lazy initialization

Memory leak Unclosed resources, static collections Heap dump comparison, find growing objects

GC overhead limit exceeded GC taking > 98% of time Fix memory leak or increase heap

Full GC too frequent Old gen filling up Tune heap ratio, fix object tenure issues

Application unresponsive Deadlock or long GC Thread dump to find deadlock, GC logs

Related Skills

• Spring Boot

• Performance Profiling MCP