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 arrives 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

      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

      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:

    • 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 arrives 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:

    • 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

      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:

    • 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: