Order

This article mainly studies the use of Java Flight Recorder.

Command

There are mainly five commands, configure, check, start, dump, stop. In order of execution, first start, then dump, and finally stop.

JFR.configure

Command instance

jcmd 5793 JFR.configure
5793:
Current configuration:

Repository path: /private/var/folders/9r/v55wkcr91m5_g8h7lhgjzgr00000gn/T/2018_09_27_16_30_53_5793

Stack depth: 64
Global buffer count: 20
Global buffer size: 512.0 kB
Thread buffer size: 8.0 kB
Memory size: 10.0 MB
Max chunk size: 12.0 MB
Sample threads: true

JFR.start

Command instance

jcmd 5793 JFR.start name=demojfr dumponexit=true
5793:
Started recording 1. No limit specified, using maxsize=250MB as default.

Use jcmd 5793 JFR.dump name=demojfr filename=FILEPATH to copy recording data to file.

JFR.check

Command instance

jcmd 5793 JFR.check
5793:
Recording 1: name=demojfr maxsize=250.0MB (running)

JFR.dump

Command instance

jcmd 5793 JFR.dump name=demojfr filename=/tmp/demo.jfr
5793:
Dumped recording "demojfr", 480.8 kB written to:

/tmp/demo.jfr

JFR.stop

Command instance

jcmd 5793 JFR.stop name=demojfr
5793:
Stopped recording "demojfr".

JMC

The screenshot of JMC opening jfr file instance is as follows:

Read JFR file

    @Test
    public void testReadJfr() throws IOException {
        Path p = Paths.get(getClass().getClassLoader().getResource("demo.jfr").getPath());
        List<RecordedEvent> events = RecordingFile.readAllEvents(p);
        events.stream()
                .forEach(e -> LOGGER.info("eventType:{},startTime:{},endTime:{},fields:{}",e.getEventType().getName(),e.getStartTime(),e.getEndTime(),e.getFields()));
        List<String> eventNames = events.stream()
                .map(e -> e.getEventType().getName())
                .distinct()
                .collect(Collectors.toList());
        System.out.println(eventNames.toString());
    }

• Directly using jdk’s api can parse jfr files and read RecordedEvent
• The eventType type output is as follows:

[jdk.ExceptionStatistics, jdk.NativeMethodSample, jdk.ThreadSleep, jdk.JavaMonitorWait, jdk.CPULoad, jdk.JavaThreadStatistics, jdk.ClassLoadingStatistics, jdk.CompilerStatistics, jdk.ClassLoaderStatistics, jdk.ModuleExport, jdk.CodeCacheStatistics, jdk.CodeSweeperStatistics, jdk.GCConfiguration, jdk.ActiveSetting, jdk.ActiveRecording, jdk.InitialSystemProperty, jdk.InitialEnvironmentVariable, jdk.CPUInformation, jdk.CPUTimeStampCounter, jdk.ThreadAllocationStatistics, jdk.PhysicalMemory, jdk.NativeLibrary, jdk.CompilerConfiguration, jdk.CodeCacheConfiguration, jdk.CodeSweeperConfiguration, jdk.IntFlag, jdk.UnsignedIntFlag, jdk.LongFlag, jdk.UnsignedLongFlag, jdk.DoubleFlag, jdk.BooleanFlag, jdk.StringFlag, jdk.ThreadEnd, jdk.ThreadCPULoad, jdk.NetworkUtilization, jdk.ThreadStart, jdk.ThreadContextSwitchRate, jdk.GCSurvivorConfiguration, jdk.GCTLABConfiguration, jdk.GCHeapConfiguration, jdk.YoungGenerationConfiguration, jdk.SystemProcess, jdk.ThreadDump, jdk.JVMInformation, jdk.OSInformation, jdk.ModuleRequire]

In addition to the system-defined eventType, you can also customize the eventevent.

Custom event

• DemoEvent

@Label("Demo Event")
@Description("Helps the programmer getting started")
public class DemoEvent extends Event {
    @Label("Message")
    private String message;

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

• Publish event

        DemoEvent event = new DemoEvent();
        event.setMessage("hello, world!");
        event.commit();

After parsing the jfr file using the api, you can see a custom event whose name is com.example.jfr.DemoEvent
You can start-xx with the following parameters: startlightrecording = duration = 120s, filename =/tmp/event.jfr, settings = default, name = demoventrecording

Related modules

JDK11 has two modules on jfr, jdk.jfr.jmod and jdk.management.jfr.jmod, which are as follows:

➜  jmods ../bin/jmod describe jdk.jfr.jmod
jdk.jfr@11
exports jdk.jfr
exports jdk.jfr.consumer
requires java.base mandated
qualified exports jdk.jfr.internal.management to jdk.management.jfr
contains jdk.jfr.events
contains jdk.jfr.internal
contains jdk.jfr.internal.cmd
contains jdk.jfr.internal.consumer
contains jdk.jfr.internal.dcmd
contains jdk.jfr.internal.handlers
contains jdk.jfr.internal.instrument
contains jdk.jfr.internal.jfc
contains jdk.jfr.internal.settings
contains jdk.jfr.internal.test
contains jdk.jfr.internal.types
platform macos-amd64

➜  jmods ../bin/jmod describe jdk.management.jfr.jmod
jdk.management.jfr@11
exports jdk.management.jfr
requires java.base mandated
requires java.management transitive
requires jdk.jfr
requires jdk.management
provides sun.management.spi.PlatformMBeanProvider with jdk.management.jfr.internal.FlightRecorderMXBeanProvider
contains jdk.management.jfr.internal
platform macos-amd64

Summary

• Java Flight Recorder is an excellent diagnostic tool for Java applications. It used to be a commercial feature and is now open source in java11. jfr fi les it exports can be analyzed with Java Mission Control.
• JDK11 has built-in API, which can be used to parse jfr files or publish custom events in applications.
• JFR can be started by JVM command, or it can be operated at runtime by using JFR. of jcmd, which is very convenient.

doc

• Using Java Flight Recorder
• Java Flight Recorder Cheat Sheet
• 2 Diagnostic Command Reference
• Clarifying some confusion around Java Flight Recordings
• Java Flight Recorder：The Secret Arrow in Your Quiver