Module jdk.jfr

# Package jdk.jfr

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

Event setting names and their purpose.
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`.

Since:
  9

- Related Packages
  Package
  Description
  jdk.jfr.consumer
  This package contains classes for consuming Flight Recorder data.
- Class
  Description
  AnnotationElement
  Describes event metadata, such as labels, descriptions and units.
  BooleanFlag
  Event field annotation, specifies that the value is a boolean flag, a `true` or `false` value.
  Category
  Event annotation, to associate the event type with a category, in the format of a human-readable path.
  Configuration
  A collection of settings and metadata describing the configuration.
  ContentType
  Meta annotation, specifies that an annotation represents a content type, such as a time span or a frequency.
  DataAmount
  Event field annotation, specifies that a value represents an amount of data (for example, bytes).
  Description
  Annotation that describes an element by using a sentence or two.
  Enabled
  Event annotation, determines if an event should be enabled by default.
  Event
  Base class for events, to be subclassed in order to define events and their fields.
  EventFactory
  Class for defining an event at runtime.
  EventSettings
  Convenience class for applying event settings to a recording.
  EventType
  Describes an event, its fields, settings and annotations.
  Experimental
  Annotation that specifies that an element is experimental and may change without notice.
  FlightRecorder
  Class for accessing, controlling, and managing Flight Recorder.
  FlightRecorderListener
  Callback interface to monitor Flight Recorder's life cycle.
  FlightRecorderPermission
  Permission for controlling access to Flight Recorder.
  Frequency
  Event field annotation, specifies that the value is a frequency, measured in Hz.
  Label
  Annotation that sets a human-readable name for an element (for example, `"Maximum Throughput"`).
  MemoryAddress
  Event field annotation, specifies that the value is a memory address.
  MetadataDefinition
  Meta annotation for defining new types of event metadata.
  Name
  Annotation that sets the default name for an element.
  Percentage
  Event field annotation to use on fractions, typically between `0.0` and `1.0`, to specify that the value is a percentage.
  Period
  Event annotation, specifies the default setting value for a periodic event.
  Recording
  Provides means to configure, start, stop and dump recording data to disk.
  RecordingState
  Indicates a state in the life cycle of a recording.
  Registered
  Event annotation, for programmatic event registration.
  Relational
  Meta annotation for relational annotations, to be used on an annotation.
  SettingControl
  Base class to extend to create setting controls.
  SettingDefinition
  Annotation that specifies that a method in an event class should be used to filter out events.
  SettingDescriptor
  Describes an event setting.
  StackTrace
  Event annotation, determines whether an event by default has a stack trace or not.
  Threshold
  Event annotation, specifies the default duration below which an event is not recorded (for example, `"20 ms"`).
  Timespan
  Event field annotation, specifies that the value is a duration.
  Timestamp
  Event field annotation, specifies that the value is a point in time.
  TransitionFrom
  Event field annotation, specifies that the event transitioned from a thread.
  TransitionTo
  Event field annotation, specifies that the event will soon transition to a thread.
  Unsigned
  Event field annotation, specifies that the value is of an unsigned data type.
  ValueDescriptor
  Describes the event fields and annotation elements.