JDK14/Java14源码在线阅读

/*
 * Copyright (c) 2016, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package jdk.jfr;

/**
 * Base class for events, to be subclassed in order to define events and their
 * fields.
 * <p>
 * The following example shows how to implement an {@code Event} class.
 *
 * <pre>
 * <code>
 * import jdk.jfr.Event;
 * import jdk.jfr.Description;
 * import jdk.jfr.Label;
 *
 * public class Example {
 *
 *   &#064;Label("Hello World")
 *   &#064;Description("Helps programmer getting started")
 *   static class HelloWorld extends Event {
 *       &#064;Label("Message")
 *       String message;
 *   }
 *
 *   public static void main(String... args) {
 *       HelloWorld event = new HelloWorld();
 *       event.message = "hello, world!";
 *       event.commit();
 *   }
 * }
 * </code>
 * </pre>
 * <p>
 * After an event is allocated and its field members are populated, it can be
 * written to the Flight Recorder system by using the {@code #commit()} method.
 * <p>
 * By default, an event is enabled. To disable an event annotate the
 * {@link Event} class with {@code @Enabled(false)}.
 * <p>
 * Supported field types are the Java primitives: {@code boolean}, {@code char},
 * {@code byte}, {@code short}, {@code int}, {@code long}, {@code float}, and
 * {@code double}. Supported reference types are: {@code String}, {@code Thread}
 * and {@code 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.
 * <p>
 * Tools can visualize data in a meaningful way when annotations are used (for
 * example, {@code Label}, {@code Description}, and {@code Timespan}).
 * Annotations applied to an {@link Event} class or its fields are included if
 * they are present (indirectly, directly, or associated), have the
 * {@code MetadataDefinition} annotation, and they do not contain enums, arrays,
 * or classes.
 * <p>
 * Gathering data to store in an event can be expensive. The
 * {@link Event#shouldCommit()} method can be used to verify whether an event
 * instance would actually be written to the system when the
 * {@code Event#commit()commit} method is invoked. If
 * {@link Event#shouldCommit()} returns false, then those operations can be
 * avoided.
 *
 * @since 9
 */
@Enabled(true)
@StackTrace(true)
@Registered(true)
abstract public class Event extends jdk.internal.event.Event {
    /**
     * Sole constructor, for invocation by subclass constructors, typically
     * implicit.
     */
    protected Event() {
    }

    /**
     * Starts the timing of this event.
     */
    final public void begin() {
    }

    /**
     * Ends the timing of this event.
     *
     * The {@code end} method must be invoked after the {@code begin} method.
     */
    final public void end() {
    }

    /**
     * Writes the field values, time stamp, and event duration to the Flight
     * Recorder system.
     * <p>
     * If the event starts with an invocation of the {@code begin} method, but does
     * not end with an explicit invocation of the {@code end} method, then the event
     * ends when the {@code commit} method is invoked.
     */
    final public void commit() {
    }

    /**
     * Returns {@code true} if at least one recording is running, and the
     * enabled setting for this event is set to {@code true}, otherwise
     * {@code false} is returned.
     *
     * @return {@code true} if event is enabled, {@code false} otherwise
     */
    final public boolean isEnabled() {
        return false;
    }

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

    /**
     * Sets a field value.
     * <p>
     * Applicable only if the event is dynamically defined using the
     * {@code EventFactory} class.
     * <p>
     * The supplied {@code index} corresponds to the index of the
     * {@link ValueDescriptor} object passed to the factory method of the

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