java-profiling

name	java-profiling
description	JVM performance profiling with Java Flight Recorder (JFR), jcmd, and GC analysis. Use for identifying bottlenecks and memory issues. USE WHEN: user mentions "Java profiling", "JFR", "JVM performance", asks about "Java Flight Recorder", "jcmd", "heap dump", "GC tuning", "thread dump", "Java memory leak" DO NOT USE FOR: Node.js/Python profiling - use respective skills instead
allowed-tools	Read, Grep, Glob, Bash, mcp__performance_profiler__*

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