EventStream (Java SE 26 & JDK 26)
Description: declaration: module: jdk.jfr, package: jdk.jfr.consumer, interface: EventStream
Keywords: jdk.jfr.consumer.EventStream interface

# Interface EventStream

All Superinterfaces:
  `AutoCloseable`

All Known Implementing Classes:
  `RecordingStream, RemoteRecordingStream`

public interface EventStream extends AutoCloseable
Represents a stream of events.

A stream is a sequence of events and the way to interact with a stream is to register actions. The `EventStream` interface is not to be implemented and future versions of the JDK may prevent this completely.

To receive a notification when an event arrives, register an action using the `onEvent(Consumer)` method. To filter the stream for an event with a specific name, use `onEvent(String, Consumer)` method.

By default, the same `RecordedEvent` object can be used to represent two or more distinct events. That object can be delivered multiple times to the same action as well as to other actions. To use an event object after the action is completed, the `setReuse(boolean)` method should be set to `false` so a new object is allocated for each event.

Events are delivered in batches. To receive a notification when a batch is complete, register an action using the `onFlush(Runnable)` method. This is an opportunity to aggregate or push data to external systems while the Java Virtual Machine (JVM) is preparing the next batch.

Events within a batch are sorted chronologically by their end time. Well-ordering of events is only maintained for events available to the JVM at the point of flush, i.e. for the set of events delivered as a unit in a single batch. Events delivered in a batch could therefore be out-of-order compared to events delivered in a previous batch, but never out-of-order with events within the same batch. If ordering is not a concern, sorting can be disabled using the `setOrdered(boolean)` method.

To dispatch events to registered actions, the stream must be started. To start processing in the current thread, invoke the `start()` method. To process actions asynchronously in a separate thread, invoke the `startAsync()` method. To await completion of the stream, use the awaitTermination `awaitTermination()` or the `awaitTermination(Duration)` method.

When a stream ends it is automatically closed. To manually stop processing of events, close the stream by invoking the `close()` method. A stream can also be automatically closed in exceptional circumstances, for example if the JVM that is being monitored exits. To receive a notification in any of these occasions, use the `onClose(Runnable)` method to register an action.

If an unexpected exception occurs in an action, it is possible to catch the exception in an error handler. An error handler can be registered using the `onError(Consumer)` method. If no error handler is registered, the default behavior is to print the exception and its backtrace to the standard error stream.

The following example shows how an `EventStream` can be used to listen to events on a JVM running Flight Recorder

    try (var es = EventStream.openRepository()) {
        es.onEvent("jdk.CPULoad", event -> {
            System.out.println("CPU Load " + event.getEndTime());
            System.out.println(" Machine total: " + 100 * event.getFloat("machineTotal") + "%");
            System.out.println(" JVM User: " + 100 * event.getFloat("jvmUser") + "%");
            System.out.println(" JVM System: " + 100 * event.getFloat("jvmSystem") + "%");
            System.out.println();
        });
        es.onEvent("jdk.GarbageCollection", event -> {
            System.out.println("Garbage collection: " + event.getLong("gcId"));
            System.out.println(" Cause: " + event.getString("cause"));
            System.out.println(" Total pause: " + event.getDuration("sumOfPauses"));
            System.out.println(" Longest pause: " + event.getDuration("longestPause"));
            System.out.println();
        });
        es.start();
    }

To start recording together with the stream, see `RecordingStream`.

Since:
  14

- ## Method Summary

  Modifier and Type
  Method
  Description
  `void`
  `awaitTermination()`
  Blocks until all actions are completed, or the stream is closed, or the current thread is interrupted, whichever happens first.
  `void`
  `awaitTermination(Duration timeout)`
  Blocks until all actions are completed, or the stream is closed, or the timeout occurs, or the current thread is interrupted, whichever happens first.
  `void`
  `close()`
  Releases all resources associated with this stream.
  `void`
  `onClose(Runnable action)`
  Registers an action to perform when the stream is closed.
  `void`
  `onError(Consumer<Throwable> action)`
  Registers an action to perform if an exception occurs.
  `void`
  `onEvent(String eventName, Consumer<RecordedEvent> action)`
  Registers an action to perform on all events matching a name.
  `void`
  `onEvent(Consumer<RecordedEvent> action)`
  Registers an action to perform on all events in the stream.
  `void`
  `onFlush(Runnable action)`
  Registers an action to perform after the stream has been flushed.
  `default void`
  `onMetadata(Consumer<MetadataEvent> action)`
  Registers an action to perform when new metadata arrives in the stream.
  `static EventStream`
  `openFile(Path file)`
  Creates an event stream from a file.
  `static EventStream`
  `openRepository()`
  Creates a stream from the repository of the current Java Virtual Machine (JVM).
  `static EventStream`
  `openRepository(Path directory)`
  Creates an event stream from a disk repository.
  `boolean`
  `remove(Object action)`
  Unregisters an action.
  `void`
  `setEndTime(Instant endTime)`
  Specifies the end time of the stream.
  `void`
  `setOrdered(boolean ordered)`
  Specifies that events arrive in chronological order, sorted by the time they were committed to the stream.
  `void`
  `setReuse(boolean reuse)`
  Specifies that the event object in an `onEvent(Consumer)` action can be reused.
  `void`
  `setStartTime(Instant startTime)`
  Specifies the start time of the stream.
  `void`
  `start()`
  Starts processing of actions.
  `void`
  `startAsync()`
  Starts asynchronous processing of actions.

- ## Method Details

  - ### openRepository

    static EventStream openRepository() throws IOException
    Creates a stream from the repository of the current Java Virtual Machine (JVM).

    By default, the stream starts with the next event flushed by Flight Recorder.

    Returns:
      an event stream, not `null`
    Throws:
      `IOException` - if a stream can't be opened, or an I/O error occurs when trying to access the repository

  - ### openRepository

    static EventStream openRepository(Path directory) throws IOException
    Creates an event stream from a disk repository.

    By default, the stream starts with the next event flushed by Flight Recorder.

    Only trusted disk repositories should be opened.

    Parameters:
      `directory` - location of the disk repository, not `null`
    Returns:
      an event stream, not `null`
    Throws:
      `IOException` - if a stream can't be opened, or an I/O error occurs when trying to access the repository

  - ### openFile

    static EventStream openFile(Path file) throws IOException
    Creates an event stream from a file.

    By default, the stream starts with the first event in the file.

    Only recording files from trusted sources should be opened.

    Parameters:
      `file` - location of the file, not `null`
    Returns:
      an event stream, not `null`
    Throws:
      `IOException` - if the file can't be opened, or an I/O error occurs during reading

  - ### onMetadata

    default void onMetadata(Consumer<MetadataEvent> action)
    Registers an action to perform when new metadata arrives in the stream. The event type of an event always arrives sometime before the actual event. The action must be registered before the stream is started.

    The following example shows how to listen to new event types, register an action if the event type name matches a regular expression and increase a counter if a matching event is found. A benefit of using an action per event type, instead of the generic `onEvent(Consumer)` method, is that a stream implementation can avoid reading events that are of no interest.

        static long count = 0;
        public static void main(String... args) throws IOException {
            Path file = Path.of(args[0]);
            String regExp = args[1];
            var pr = Pattern.compile(regExp).asMatchPredicate();
            try (var s = EventStream.openFile(file)) {
                s.setOrdered(false);
                s.onMetadata(metadata -> metadata.getAddedEventTypes()
                 .stream().map(EventType::getName).filter(pr)
                 .forEach(eventName -> s.onEvent(eventName, event -> count++)));
                s.start();
                System.out.println(count + " events matches " + regExp);
            }
        }

    Implementation Requirements:
      The default implementation of this method is empty.
    Parameters:
      `action` - to perform, not `null`
    Throws:
      `IllegalStateException` - if an action is added after the stream has started
    Since:
      16

  - ### onEvent

    void onEvent(Consumer<RecordedEvent> action)
    Registers an action to perform on all events in the stream.

    To perform an action on a subset of event types, consider using `onEvent(String, Consumer)` and `onMetadata(Consumer)` as it is likely more performant than any selection or filtering mechanism implemented in a generic action.

    Parameters:
      `action` - an action to perform on each `RecordedEvent`, not `null`
    See Also:
      - `onEvent(Consumer)`
      - `onMetadata(Consumer)`

  - ### onEvent

    void onEvent(String eventName, Consumer<RecordedEvent> action)
    Registers an action to perform on all events matching a name.
    Parameters:
      `eventName` - the name of the event, not `null`
      `action` - an action to perform on each `RecordedEvent` matching the event name, not `null`

  - ### onFlush

    void onFlush(Runnable action)
    Registers an action to perform after the stream has been flushed.
    Parameters:
      `action` - an action to perform after the stream has been flushed, not `null`

  - ### onError

    void onError(Consumer<Throwable> action)
    Registers an action to perform if an exception occurs.

    If an action is not registered, an exception stack trace is printed to standard error.

    Registering an action overrides the default behavior. If multiple actions have been registered, they are performed in the order of registration.

    If this method itself throws an exception, resulting behavior is undefined.

    Parameters:
      `action` - an action to perform if an exception occurs, not `null`

  - ### onClose

    void onClose(Runnable action)
    Registers an action to perform when the stream is closed.

    If the stream is already closed, the action will be performed immediately in the current thread.

    Parameters:
      `action` - an action to perform after the stream is closed, not `null`
    See Also:
      - `close()`

  - ### close

    void close()
    Releases all resources associated with this stream.

    If a stream is started, asynchronously or synchronously, it is stopped immediately or after the next flush. This method does NOT guarantee that all registered actions are completed before return.

    Closing a previously closed stream has no effect.

    Specified by:
      `close` in interface `AutoCloseable`

  - ### remove

    boolean remove(Object action)
    Unregisters an action.

    If the action has been registered multiple times, all instances are unregistered.

    Parameters:
      `action` - the action to unregister, not `null`
    Returns:
      `true` if the action was unregistered, `false` otherwise
    See Also:
      - `onEvent(Consumer)`
      - `onEvent(String, Consumer)`
      - `onFlush(Runnable)`
      - `onClose(Runnable)`
      - `onError(Consumer)`

  - ### setReuse

    void setReuse(boolean reuse)
    Specifies that the event object in an `onEvent(Consumer)` action can be reused.

    If reuse is set to `true`, an action should not keep a reference to the event object after the action has completed.

    Parameters:
      `reuse` - `true` if an event object can be reused, `false` otherwise

  - ### setOrdered

    void setOrdered(boolean ordered)
    Specifies that events arrive in chronological order, sorted by the time they were committed to the stream.
    Parameters:
      `ordered` - if event objects arrive in chronological order to `onEvent(Consumer)`

  - ### setStartTime

    void setStartTime(Instant startTime)
    Specifies the start time of the stream.

    The start time must be set before starting the stream

    Parameters:
      `startTime` - the start time, not `null`
    Throws:
      `IllegalStateException` - if the stream is already started
    See Also:
      - `start()`
      - `startAsync()`

  - ### setEndTime

    void setEndTime(Instant endTime)
    Specifies the end time of the stream.

    The end time must be set before starting the stream.

    At end time, the stream is closed.

    Parameters:
      `endTime` - the end time, not `null`
    Throws:
      `IllegalStateException` - if the stream is already started
    See Also:
      - `start()`
      - `startAsync()`

  - ### start

    void start()
    Starts processing of actions.

    Actions are performed in the current thread.

    To stop the stream, use the `close()` method.

    Throws:
      `IllegalStateException` - if the stream is already started or closed

  - ### startAsync

    void startAsync()
    Starts asynchronous processing of actions.

    Actions are performed in a single separate thread.

    To stop the stream, use the `close()` method.

    Throws:
      `IllegalStateException` - if the stream is already started or closed

  - ### awaitTermination

    void awaitTermination(Duration timeout) throws InterruptedException
    Blocks until all actions are completed, or the stream is closed, or the timeout occurs, or the current thread is interrupted, whichever happens first.
    Parameters:
      `timeout` - the maximum time to wait, not `null`
    Throws:
      `IllegalArgumentException` - if timeout is negative
      `InterruptedException` - if interrupted while waiting
    See Also:
      - `start()`
      - `startAsync()`
      - `Thread.interrupt()`

  - ### awaitTermination

    void awaitTermination() throws InterruptedException
    Blocks until all actions are completed, or the stream is closed, or the current thread is interrupted, whichever happens first.
    Throws:
      `InterruptedException` - if interrupted while waiting
    See Also:
      - `start()`
      - `startAsync()`
      - `Thread.interrupt()`