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JFR and jcmd

Learn to profile with Java Flight Recorder and jcmd.

Profiling with JFR

JFR (Java Flight Recorder) collects low-overhead event records from a running application. Method sampling, allocation, garbage collection and lock contention events are recorded. It can run continuously in production.

Recording starts with jcmd PID JFR.start duration=60s filename=rec.jfr. The resulting .jfr file is inspected with jfr print or JDK Mission Control.

The jcmd tool

jcmd is a versatile tool that sends commands to a running JVM. Besides JFR recording it supports heap dumps, thread dumps, GC control and diagnostic commands. jcmd PID help lists the available commands.

After the JFR recording finishes jcmd PID JFR.dump filename=rec.jfr dumps the data. JFR is the first tool to reach for in production issue analysis.

Practice Write the JfrRecorder class

Define class JfrRecorder with a static String start(int pid, int seconds) method that returns "jcmd " + pid + " JFR.start duration=" + seconds + "s". In main print System.out.println(start(1234, 60));.

Starter code

Main.java
// Define the JfrRecorder class: start method builds the jcmd command

// main: print the command for PID 1234 and 60 seconds

Sample solution

Main.java
class JfrRecorder {
    static String start(int pid, int seconds) {
        return "jcmd " + pid + " JFR.start duration=" + seconds + "s";
    }
    public static void main(String[] args) {
        System.out.println(start(1234, 60));
    }
}

Expected output

jcmd 1234 JFR.start duration=60s

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the JFR start command

Question 
jcmd ____ duration=60s filename=rec.jfr
Answer 
jcmd JFR.start duration=60s filename=rec.jfr

Explanation: The jcmd JFR.start command starts recording.

2 What is the distinctive advantage of JFR?

  • Low overhead Correct answer
  • It compiles classes
  • It starts threads
  • It deletes files

Explanation: JFR can run in production with low overhead.

3 Match the JFR tools

Term Meaning
JFR.start Starts recording
jcmd Control tool
JMC Analysis tool
duration Recording length

All lessons in Expert

1 JVM internals

  1. JVM architecture
  2. Class loaders
  3. Bytecode
  4. JIT compiler
  5. Runtime data areas
  6. Summary - JVM internals

2 Memory model

  1. Heap and stack
  2. happens-before
  3. volatile
  4. final field guarantees
  5. Metaspace
  6. Summary - Memory model

3 NIO

  1. Buffer
  2. Channel
  3. Selector
  4. Charset
  5. Path and Files (NIO.2)
  6. Summary - NIO

4 Reactive

  1. Flow API
  2. Publisher and Subscriber
  3. SubmissionPublisher
  4. Backpressure
  5. Project Reactor Mono
  6. Summary - Reactive

5 JNI

  1. native methods
  2. javah and headers
  3. JNIEnv
  4. C/C++ implementation
  5. Foreign Function & Memory API
  6. Summary - JNI

6 Performance and GC

  1. JMH
  2. JFR and jcmd
  3. GC tuning
  4. Heap dump
  5. JIT tuning
  6. Summary - Performance and GC

7 Bytecode (ASM)

  1. Project design
  2. ClassReader
  3. ClassVisitor
  4. Bytecode transformation
  5. Putting it together

8 Expert final

  1. Project design
  2. Reactive layer
  3. Performance layer
  4. Microservice basics
  5. Putting it together