Package jdk.jfr

This package provides classes to create events and control Flight Recorder.

Defining events

Flight Recorder collects data as events. An event has a time stamp, duration and usually an application-specific payload, useful for diagnosing the running application up to the failure or crash.

To define a Flight Recorder event, extend Event and add fields that matches the data types of the payload. Metadata about fields, such as labels, descriptions and units, can be added by using the annotations available in the jdk.jfr package, or by using a user-defined annotation that has the MetadataDefinition annotation.

After an event class is defined, instances can be created (event objects). Data is stored in the event by assigning data to fields. Event timing can be explicitly controlled by using the begin and end methods available in the Event class.

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

Sometimes the field layout of an event is not known at compile time. In that case, an event can be dynamically defined. However, dynamic events might not have the same level of performance as statically defined ones and tools might not be able to identify and visualize the data without knowing the layout.

To dynamically define an event, use the EventFactory class and define fields by using the ValueDescriptor class, and define annotations by using the AnnotationElement class. Use the factory to allocate an event and the Event.set(int, Object) method to populate it.

Controlling Flight Recorder

Flight Recorder can be controlled locally by using the jcmd command line tool or remotely by using the FlightRecorderMXBean interface, registered in the platform MBeanServer. When direct programmatic access is needed, a Flight Recorder instance can be obtained by invoking FlightRecorder.getFlightRecorder() and a recording created by using Recording class, from which the amount of data to record is configured.

Settings and configuration

A setting consists of a name/value pair, where name specifies the event and setting to configure, and the value specifies what to set it to.

The name can be formed in the following ways:

<event-name> + "#" + <setting-name>

or

<event-id> + "#" + <setting-name>

For example, to set the sample interval of the CPU Load event to once every second, use the name "jdk.CPULoad#period" and the value "1 s". If multiple events use the same name, for example if an event class is loaded in multiple class loaders, and differentiation is needed between them, then the name is "56#period". The ID for an event is obtained by invoking EventType.getId() method and is valid for the Java Virtual Machine instance that the event is registered in.

A list of available event names is retrieved by invoking FlightRecorder.getEventTypes() and EventType.getName(). A list of available settings for an event type is obtained by invoking EventType.getSettingDescriptors() and ValueDescriptor.getName().

Predefined settings

Name	Description	Default value	Format	Example values
enabled	Specifies whether the event is recorded	"true"	String representation of a Boolean ("true" or "false")	"true" "false"
threshold	Specifies the duration below which an event is not recorded	"0" (no limit)	"0" if no threshold is used, otherwise a string representation of a positive Long followed by a space and one of the following units: "ns" (nanoseconds) "us" (microseconds) "ms" (milliseconds) "s" (seconds) "m" (minutes) "h" (hours) "d" (days)	"0" "10 ms" "1 s"
period	Specifies the interval at which the event is emitted, if it is periodic	"everyChunk"	"everyChunk", if a periodic event should be emitted with every file rotation, otherwise a string representation of a positive Long value followed by an empty space and one of the following units: "ns" (nanoseconds) "us" (microseconds) "ms" (milliseconds) "s" (seconds) "m" (minutes) "h" (hours) "d" (days)	"20 ms" "1 s" "everyChunk"
stackTrace	Specifies whether the stack trace from the Event#commit() method is recorded	"true"	String representation of a Boolean ("true" or "false")	"true", "false"

Null-handling

All methods define whether they accept or return null in the Javadoc. Typically this is expressed as "not null". If a null parameter is used where it is not allowed, a java.lang.NullPointerException is thrown. If a null parameters is passed to a method that throws other exceptions, such as java.io.IOException, the java.lang.NullPointerException takes precedence, unless the Javadoc for the method explicitly states how null is handled, i.e. by throwing java.lang.IllegalArgumentException.