Java/JVM Performance Profiling

Java/JVM 性能剖析

When NOT to Use This Skill

不适用本技能的场景

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.

Node.js/JavaScript 性能剖析：请使用
```
nodejs-profiling
```
技能进行V8剖析器和堆分析
Python 性能剖析：请使用
```
python-profiling
```
技能进行cProfile和tracemalloc分析
应用层级优化：本技能仅适用于JVM层级剖析，不涉及算法优化
数据库查询调优：请使用数据库专属的剖析工具
前端性能优化：请使用浏览器DevTools进行客户端性能剖析

深度知识：调用
mcp__documentation__fetch_docs
并指定技术为
java
，可获取完整的JFR配置、GC调优和JVM诊断相关文档。

Java Flight Recorder (JFR)

Starting JFR

启动JFR

bash

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bash

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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

undefined

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

undefined

JFR Configuration

JFR配置

undefined

undefined

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

分析JFR文件

bash

undefined

bash

undefined

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

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

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jcmd Diagnostics

jcmd诊断

Process Information

进程信息

bash

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bash

undefined

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

undefined

jcmd <pid> GC.class_histogram

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Memory Analysis

内存分析

bash

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bash

undefined

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

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jcmd <pid> GC.run

undefined

GC Tuning

GC调优

GC Selection

GC选择

bash

undefined

bash

undefined

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

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java -XX:+UseParallelGC -jar app.jar

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GC Logging

GC日志

bash

undefined

bash

undefined

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

undefined

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

undefined

G1GC Tuning

G1GC调优

bash

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

bash

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

内存优化

Heap Sizing

堆大小配置

bash

undefined

bash

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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

undefined

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

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Memory Leak Detection

内存泄漏检测

java

// 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();
    }
}

java

// 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

CPU剖析

async-profiler (recommended)

async-profiler（推荐使用）

bash

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bash

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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>

undefined

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

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JMC (Java Mission Control)

bash

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bash

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Open JFR recording in JMC

jmc recording.jfr

undefined

jmc recording.jfr

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Common Bottleneck Patterns

常见性能瓶颈模式

Synchronization Issues

同步问题

java

// ❌ 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(); }
}

java

// ❌ 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

字符串操作

java

// ❌ 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);

java

// ❌ 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

集合优化

java

// ❌ 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);

java

// ❌ 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

装箱/拆箱

java

// ❌ 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();
}

java

// ❌ 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

JIT优化

Warm-up

预热

java

// 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;
}

java

// 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

JIT日志

bash

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bash

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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

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

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Profiling Checklist

剖析检查清单

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`

检查项	工具	命令
CPU热点	JFR	`jcmd <pid> JFR.start`
内存使用情况	jcmd	`jcmd <pid> GC.heap_info`
GC行为	GC日志	`-Xlog:gc*`
线程竞争	JFR	`jdk.JavaMonitorWait` events
内存泄漏	堆转储	`jcmd <pid> GC.heap_dump`
类加载情况	jcmd	`jcmd <pid> VM.classloaders`

Production Flags

生产环境启动参数

bash

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

bash

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

反模式

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

反模式	问题原因	正确方案
热点路径中使用 `System.out.println()`	同步IO性能极差	使用SLF4J搭配异步追加器
创建大量短生命周期对象	增加GC压力和分配开销	复用对象，使用对象池
热点路径使用 `synchronized`	引发线程竞争，扩展性差	使用 `ConcurrentHashMap` 、 `ReentrantLock` 或无锁结构
循环中用 `+` 拼接字符串	生成大量中间字符串	使用 `StringBuilder`
循环中自动装箱/拆箱	生成大量包装类对象	使用基本数据类型
不为集合预设容量	频繁扩容导致内存颠簸	初始化时指定容量 `new ArrayList<>(expectedSize)`
用 `finalize()` 做资源清理	执行不可预测，已被废弃	使用try-with-resources或显式清理逻辑
忽略GC日志	无法及时发现性能退化	生产环境务必开启GC日志
堆大小配置一刀切	GC停顿不符合业务负载要求	根据应用行为调优堆配置
不使用连接池	连接创建开销大	使用HikariCP等连接池组件

Quick Troubleshooting

快速问题排查

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

问题	诊断方法	解决方案
GC停顿过长	堆过大或GC选型错误	使用ZGC/Shenandoah，或调优G1GC停顿目标
`OutOfMemoryError: Java heap space`	内存泄漏或堆容量不足	分析堆转储，必要时调大 `-Xmx`
`OutOfMemoryError: Metaspace`	加载类过多	调大 `-XX:MaxMetaspaceSize` 或修复类加载器泄漏
CPU使用率过高	热点循环、算法效率低	用JFR或async-profiler做CPU剖析
线程竞争	锁竞争	分析线程转储，缩小锁的作用范围
启动缓慢	类加载、初始化耗时	使用AppCDS、懒加载逻辑
内存泄漏	资源未关闭、静态集合无限增长	对比堆转储，定位持续增长的对象
GC开销超出限制	GC占用超过98%的CPU时间	修复内存泄漏或调大堆容量
Full GC过于频繁	老年代快速占满	调优堆比例，修复对象晋升问题
应用无响应	死锁或GC停顿过长	线程转储排查死锁，查看GC日志

java-profiling

Original

Translation

Java/JVM Performance Profiling

Java/JVM 性能剖析

When NOT to Use This Skill

不适用本技能的场景

Java Flight Recorder (JFR)

Java Flight Recorder (JFR)

Starting JFR

启动JFR

Start recording with application

Start recording with application

Start recording on running JVM

Start recording on running JVM

Continuous recording (always-on)

Continuous recording (always-on)

Dump current recording

Dump current recording

JFR Configuration

JFR配置

custom.jfc

custom.jfc

Analyzing JFR Files

分析JFR文件

Print JFR summary

Print JFR summary

Print specific events

Print specific events

Export to JSON

Export to JSON

jcmd Diagnostics

jcmd诊断

Process Information

进程信息

List all Java processes

List all Java processes

VM info

VM info

Thread dump

Thread dump

Heap info

Heap info

Class histogram

Class histogram

Memory Analysis

内存分析

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

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

Heap dump

Heap dump

Force GC

Force GC

GC Tuning

GC调优

GC Selection

GC选择

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

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

ZGC (low latency, JDK 15+)

ZGC (low latency, JDK 15+)

Shenandoah (low latency, OpenJDK)

Shenandoah (low latency, OpenJDK)

Parallel GC (throughput)

Parallel GC (throughput)

GC Logging

GC日志

JDK 9+ unified logging

JDK 9+ unified logging

Common GC flags

Common GC flags

G1GC Tuning

G1GC调优

Memory Optimization

内存优化

Heap Sizing

堆大小配置

Set heap size

Set heap size

Metaspace sizing