15.7 Flight Recorder Explained

Java Flight Recorder (JFR) is a powerful tool introduced in Java SE 11 that allows developers to collect detailed diagnostic and profiling data from a running Java application. It is designed to be low-overhead, making it suitable for use in production environments.

Key Concepts

1. Flight Recorder

Flight Recorder is a monitoring tool that captures events from a Java application. These events include CPU usage, memory allocation, thread activity, and more. The data collected can be analyzed to diagnose performance issues and understand application behavior.

Example

        jcmd <pid> JFR.start name=MyRecording settings=profile duration=60s filename=recording.jfr
    

2. Events

Events are the core data points captured by Flight Recorder. They represent significant occurrences within the application, such as method calls, memory allocations, and I/O operations. Events are categorized and can be customized based on the needs of the application.

Example

        jcmd <pid> JFR.dump name=MyRecording filename=recording.jfr
    

3. Recording

A recording is a collection of events captured over a period of time. Recordings can be started, stopped, and dumped to a file for later analysis. The duration and settings of a recording can be configured to capture the necessary data.

Example

        jcmd <pid> JFR.start name=MyRecording settings=profile duration=60s filename=recording.jfr
    

4. Event Types

Flight Recorder supports various event types, including:

• Allocation events: Track memory allocation patterns.
• GC events: Monitor garbage collection activities.
• Thread events: Capture thread start, stop, and park actions.
• I/O events: Record file and socket I/O operations.

Example

        jcmd <pid> JFR.check
    

5. Low Overhead

One of the key features of Flight Recorder is its low overhead. It is designed to have minimal impact on the performance of the running application, making it suitable for use in production environments without significant performance degradation.

Example

        jcmd <pid> JFR.start name=MyRecording settings=profile duration=60s filename=recording.jfr
    

Examples and Analogies

Think of Flight Recorder as a black box in an airplane. Just as a black box records flight data to help understand what happened during a flight, Flight Recorder captures runtime data to help diagnose issues in a Java application. The events captured are like the various parameters recorded by the black box, such as altitude, speed, and engine performance.

For instance, if you are running a high-traffic web application, you can use Flight Recorder to capture events related to memory usage and garbage collection. This data can help you identify memory leaks or inefficient garbage collection patterns, allowing you to optimize the application for better performance.

Example

        jcmd <pid> JFR.start name=MyRecording settings=profile duration=60s filename=recording.jfr
        jcmd <pid> JFR.dump name=MyRecording filename=recording.jfr
    

By mastering Flight Recorder, you can gain deep insights into the runtime behavior of your Java applications, enabling you to diagnose and resolve performance issues more effectively.