Module jdk.jfr
Package jdk.jfr

Class Event

  • public abstract class Event
extends Object
    Base class for events, to be subclassed in order to define events and their fields.

    The following example shows how to implement an Event class. 

     
 import jdk.jfr.Event;
 import jdk.jfr.Description;
 import jdk.jfr.Label;

 public class Example {

   @Label("Hello World")
   @Description("Helps programmer getting started")
   static class HelloWorld extends Event {
       @Label("Message")
       String message;
   }

   public static void main(String... args) {
       HelloWorld event = new HelloWorld();
       event.message = "hello, world!";
       event.commit();
   }
 }

    After an event is allocated and its field members are populated, it can be written to the Flight Recorder system by using the #commit() method.

    By default, an event is enabled. To disable an event annotate the Event class with @Enabled(false).

    Supported field types are the Java primitives: boolean, char, byte, short, int, long, float, and double. Supported reference types are: String, Thread and Class. Arrays, enums, and other reference types are silently ignored and not included. Fields that are of the supported types can be excluded by using the transient modifier. Static fields, even of the supported types, are not included.

    Tools can visualize data in a meaningful way when annotations are used (for example, Label, Description, and Timespan). Annotations applied to an Event class or its fields are included if they are present (indirectly, directly, or associated), have the MetadataDefinition annotation, and they do not contain enums, arrays, or classes.

    Gathering data to store in an event can be expensive. The shouldCommit() method can be used to verify whether an event instance would actually be written to the system when the Event#commit()commit method is invoked. If shouldCommit() returns false, then those operations can be avoided.

    Since:
    9

    • Constructor Summary

      Constructors 
      Modifier Constructor Description
      protected Event()
      Sole constructor, for invocation by subclass constructors, typically implicit.

    • Method Summary

      All Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      void begin()
      Starts the timing of this event.
      void commit()
      Writes the field values, time stamp, and event duration to the Flight Recorder system.
      void end()
      Ends the timing of this event.
      boolean isEnabled()
      Returns true if at least one recording is running, and the enabled setting for this event is set to true, otherwise false is returned.
      void set​(int index, Object value)
      Sets a field value.
      boolean shouldCommit()
      Returns true if the enabled setting for this event is set to true and if the duration is within the threshold for the event, false otherwise.

    • Constructor Detail

      • Event

        protected Event()
        Sole constructor, for invocation by subclass constructors, typically implicit.

    • Method Detail

      • begin

        public final void begin()
        Starts the timing of this event.

      • end

        public final void end()
        Ends the timing of this event. The end method must be invoked after the begin method.

      • commit

        public final void commit()
        Writes the field values, time stamp, and event duration to the Flight Recorder system.

        If the event starts with an invocation of the begin method, but does not end with an explicit invocation of the end method, then the event ends when the commit method is invoked.

      • isEnabled

        public final boolean isEnabled()
        Returns true if at least one recording is running, and the enabled setting for this event is set to true, otherwise false is returned.
        Returns:
        true if event is enabled, false otherwise

      • shouldCommit

        public final boolean shouldCommit()
        Returns true if the enabled setting for this event is set to true and if the duration is within the threshold for the event, false otherwise. The threshold is the minimum threshold for all running recordings.
        Returns:
        true if the event can be written to the Flight Recorder system, false otherwise

      • set

        public final void set​(int index,
                      Object value)
        Sets a field value.

        Applicable only if the event is dynamically defined using the EventFactory class.

        The supplied index corresponds to the index of the ValueDescriptor object passed to the factory method of the EventFactory class.

        Parameters:
        index - the index of the field that is passed to EventFactory#create(String, java.util.List, java.util.List)
        value - value to set, can be null
        Throws:
        UnsupportedOperationException - if it's not a dynamically generated event
        IndexOutOfBoundsException - if index is less than 0 or greater than or equal to the number of fields specified for the event
        See Also:
        EventType.getFields(), EventFactory