2.x: Expand the JavaDocs of the Scheduler API (ReactiveX#5843)

* 2.x: Expand the JavaDocs of the Scheduler API

* Fix typo and grammar.

Author: akarnokd

src/main/java/io/reactivex/Scheduler.java

@@ -27,14 +27,72 @@
 
 /**
  * A {@code Scheduler} is an object that specifies an API for scheduling
- * units of work with or without delays or periodically.
- * You can get common instances of this class in {@link io.reactivex.schedulers.Schedulers}.
+ * units of work provided in the form of {@link Runnable}s to be
+ * executed without delay (effectively as soon as possible), after a specified time delay or periodically
+ * and represents an abstraction over an asynchronous boundary that ensures
+ * these units of work get executed by some underlying task-execution scheme
+ * (such as custom Threads, event loop, {@link java.util.concurrent.Executor Executor} or Actor system)
+ * with some uniform properties and guarantees regardless of the particular underlying
+ * scheme.
+ * <p>
+ * You can get various standard, RxJava-specific instances of this class via
+ * the static methods of the {@link io.reactivex.schedulers.Schedulers} utility class.
+ * <p>
+ * The so-called {@link Worker}s of a {@code Scheduler} can be created via the {@link #createWorker()} method which allow the scheduling
+ * of multiple {@link Runnable} tasks in an isolated manner. {@code Runnable} tasks scheduled on a {@code Worker} are guaranteed to be
+ * executed sequentially and in a non-overlapping fashion. Non-delayed {@code Runnable} tasks are guaranteed to execute in a
+ * First-In-First-Out order but their execution may be interleaved with delayed tasks.
+ * In addition, outstanding or running tasks can be cancelled together via
+ * {@link Worker#dispose()} without affecting any other {@code Worker} instances of the same {@code Scheduler}.
+ * <p>
+ * Implementations of the {@link #scheduleDirect} and {@link Worker#schedule} methods are encouraged to call the {@link io.reactivex.plugins.RxJavaPlugins#onSchedule(Runnable)}
+ * method to allow a scheduler hook to manipulate (wrap or replace) the original {@code Runnable} task before it is submitted to the
+ * underlying task-execution scheme.
+ * <p>
+ * The default implementations of the {@code scheduleDirect} methods provided by this abstract class
+ * delegate to the respective {@code schedule} methods in the {@link Worker} instance created via {@link #createWorker()}
+ * for each individual {@link Runnable} task submitted. Implementors of this class are encouraged to provide
+ * a more efficient direct scheduling implementation to avoid the time and memory overhead of creating such {@code Worker}s
+ * for every task.
+ * This delegation is done via special wrapper instances around the original {@code Runnable} before calling the respective
+ * {@code Worker.schedule} method. Note that this can lead to multiple {@code RxJavaPlugins.onSchedule} calls and potentially
+ * multiple hooks applied. Therefore, the default implementations of {@code scheduleDirect} (and the {@link Worker#schedulePeriodically(Runnable, long, long, TimeUnit)})
+ * wrap the incoming {@code Runnable} into a class that implements the {@link io.reactivex.schedulers.SchedulerRunnableIntrospection}
+ * interface which can grant access to the original or hooked {@code Runnable}, thus, a repeated {@code RxJavaPlugins.onSchedule}
+ * can detect the earlier hook and not apply a new one over again.
+ * <p>
+ * The default implementation of {@link #now(TimeUnit)} and {@link Worker#now(TimeUnit)} methods to return current
+ * {@link System#currentTimeMillis()} value in the desired time unit. Custom {@code Scheduler} implementations can override this
+ * to provide specialized time accounting (such as virtual time to be advanced programmatically).
+ * Note that operators requiring a {@code Scheduler} may rely on either of the {@code now()} calls provided by
+ * {@code Scheduler} or {@code Worker} respectively, therefore, it is recommended they represent a logically
+ * consistent source of the current time.
+ * <p>
+ * The default implementation of the {@link Worker#schedulePeriodically(Runnable, long, long, TimeUnit)} method uses
+ * the {@link Worker#schedule(Runnable, long, TimeUnit)} for scheduling the {@code Runnable} task periodically.
+ * The algorithm calculates the next absolute time when the task should run again and schedules this execution
+ * based on the relative time between it and {@link Worker#now(TimeUnit)}. However, drifts or changes in the
+ * system clock could affect this calculation either by scheduling subsequent runs too frequently or too far apart.
+ * Therefore, the default implementation uses the {@link #clockDriftTolerance()} value (set via
+ * {@code rx2.scheduler.drift-tolerance} in minutes) to detect a drift in {@link Worker#now(TimeUnit)} and
+ * re-adjust the absolute/relative time calculation accordingly.
+ * <p>
+ * The default implementations of {@link #start()} and {@link #shutdown()} do nothing and should be overridden if the
+ * underlying task-execution scheme supports stopping and restarting itself.
+ * <p>
+ * If the {@code Scheduler} is shut down or a {@code Worker} is disposed, the {@code schedule} methods
+ * should return the {@link io.reactivex.disposables.Disposables#disposed()} singleton instance indicating the shut down/disposed
+ * state to the caller. Since the shutdown or dispose can happen from any thread, the {@code schedule} implementations
+ * should make best effort to cancel tasks immediately after those tasks have been submitted to the
+ * underlying task-execution scheme if the shutdown/dispose was detected after this submission.
+ * <p>
+ * All methods on the {@code Scheduler} and {@code Worker} classes should be thread safe.
  */
 public abstract class Scheduler {
     /**
      * The tolerance for a clock drift in nanoseconds where the periodic scheduler will rebase.
      * <p>
-     * The associated system parameter, {@code rx.scheduler.drift-tolerance}, expects its value in minutes.
+     * The associated system parameter, {@code rx2.scheduler.drift-tolerance}, expects its value in minutes.
      */
     static final long CLOCK_DRIFT_TOLERANCE_NANOSECONDS;
     static {
@@ -44,7 +102,7 @@ public abstract class Scheduler {
 
     /**
      * Returns the clock drift tolerance in nanoseconds.
-     * <p>Related system property: {@code rx2.scheduler.drift-tolerance} in minutes
+     * <p>Related system property: {@code rx2.scheduler.drift-tolerance} in minutes.
      * @return the tolerance in nanoseconds
      * @since 2.0
      */
@@ -54,11 +112,13 @@ public static long clockDriftTolerance() {
 
 
     /**
-     * Retrieves or creates a new {@link Scheduler.Worker} that represents serial execution of actions.
+     * Retrieves or creates a new {@link Scheduler.Worker} that represents sequential execution of actions.
      * <p>
-     * When work is completed it should be unsubscribed using {@link Scheduler.Worker#dispose()}.
+     * When work is completed, the {@code Worker} instance should be released
+     * by calling {@link Scheduler.Worker#dispose()} to avoid potential resource leaks in the
+     * underlying task-execution scheme.
      * <p>
-     * Work on a {@link Scheduler.Worker} is guaranteed to be sequential.
+     * Work on a {@link Scheduler.Worker} is guaranteed to be sequential and non-overlapping.
      *
      * @return a Worker representing a serial queue of actions to be executed
      */
@@ -78,29 +138,37 @@ public long now(@NonNull TimeUnit unit) {
     /**
      * Allows the Scheduler instance to start threads
      * and accept tasks on them.
-     * <p>Implementations should make sure the call is idempotent and thread-safe.
+     * <p>
+     * Implementations should make sure the call is idempotent, thread-safe and
+     * should not throw any {@code RuntimeException} if it doesn't support this
+     * functionality.
+     *
      * @since 2.0
      */
     public void start() {
 
     }
 
     /**
-     * Instructs the Scheduler instance to stop threads
-     * and stop accepting tasks on any outstanding Workers.
-     * <p>Implementations should make sure the call is idempotent and thread-safe.
+     * Instructs the Scheduler instance to stop threads,
+     * stop accepting tasks on any outstanding {@link Worker} instances
+     * and clean up any associated resources with this Scheduler.
+     * <p>
+     * Implementations should make sure the call is idempotent, thread-safe and
+     * should not throw any {@code RuntimeException} if it doesn't support this
+     * functionality.
      * @since 2.0
      */
     public void shutdown() {
 
     }
 
     /**
-     * Schedules the given task on this scheduler non-delayed execution.
+     * Schedules the given task on this Scheduler without any time delay.
      *
      * <p>
      * This method is safe to be called from multiple threads but there are no
-     * ordering guarantees between tasks.
+     * ordering or non-overlapping guarantees between tasks.
      *
      * @param run the task to execute
      *
@@ -113,7 +181,7 @@ public Disposable scheduleDirect(@NonNull Runnable run) {
     }
 
     /**
-     * Schedules the execution of the given task with the given delay amount.
+     * Schedules the execution of the given task with the given time delay.
      *
      * <p>
      * This method is safe to be called from multiple threads but there are no
@@ -139,15 +207,16 @@ public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull Tim
     }
 
     /**
-     * Schedules a periodic execution of the given task with the given initial delay and period.
+     * Schedules a periodic execution of the given task with the given initial time delay and repeat period.
      *
      * <p>
      * This method is safe to be called from multiple threads but there are no
      * ordering guarantees between tasks.
      *
      * <p>
-     * The periodic execution is at a fixed rate, that is, the first execution will be after the initial
-     * delay, the second after initialDelay + period, the third after initialDelay + 2 * period, and so on.
+     * The periodic execution is at a fixed rate, that is, the first execution will be after the
+     * {@code initialDelay}, the second after {@code initialDelay + period}, the third after
+     * {@code initialDelay + 2 * period}, and so on.
      *
      * @param run the task to schedule
      * @param initialDelay the initial delay amount, non-positive values indicate non-delayed scheduling
@@ -254,13 +323,43 @@ public <S extends Scheduler & Disposable> S when(@NonNull Function<Flowable<Flow
     }
 
     /**
-     * Sequential Scheduler for executing actions on a single thread or event loop.
+     * Represents an isolated, sequential worker of a parent Scheduler for executing {@code Runnable} tasks on
+     * an underlying task-execution scheme (such as custom Threads, event loop, {@link java.util.concurrent.Executor Executor} or Actor system).
+     * <p>
+     * Disposing the {@link Worker} should cancel all outstanding work and allows resource cleanup.
      * <p>
-     * Disposing the {@link Worker} cancels all outstanding work and allows resource cleanup.
+     * The default implementations of {@link #schedule(Runnable)} and {@link #schedulePeriodically(Runnable, long, long, TimeUnit)}
+     * delegate to the abstract {@link #schedule(Runnable, long, TimeUnit)} method. Its implementation is encouraged to
+     * track the individual {@code Runnable} tasks while they are waiting to be executed (with or without delay) so that
+     * {@link #dispose()} can prevent their execution or potentially interrupt them if they are currently running.
+     * <p>
+     * The default implementation of the {@link #now(TimeUnit)} method returns current
+     * {@link System#currentTimeMillis()} value in the desired time unit. Custom {@code Worker} implementations can override this
+     * to provide specialized time accounting (such as virtual time to be advanced programmatically).
+     * Note that operators requiring a scheduler may rely on either of the {@code now()} calls provided by
+     * {@code Scheduler} or {@code Worker} respectively, therefore, it is recommended they represent a logically
+     * consistent source of the current time.
+     * <p>
+     * The default implementation of the {@link #schedulePeriodically(Runnable, long, long, TimeUnit)} method uses
+     * the {@link #schedule(Runnable, long, TimeUnit)} for scheduling the {@code Runnable} task periodically.
+     * The algorithm calculates the next absolute time when the task should run again and schedules this execution
+     * based on the relative time between it and {@link #now(TimeUnit)}. However, drifts or changes in the
+     * system clock would affect this calculation either by scheduling subsequent runs too frequently or too far apart.
+     * Therefore, the default implementation uses the {@link #clockDriftTolerance()} value (set via
+     * {@code rx2.scheduler.drift-tolerance} in minutes) to detect a drift in {@link #now(TimeUnit)} and
+     * re-adjust the absolute/relative time calculation accordingly.
+     * <p>
+     * If the {@code Worker} is disposed, the {@code schedule} methods
+     * should return the {@link io.reactivex.disposables.Disposables#disposed()} singleton instance indicating the disposed
+     * state to the caller. Since the {@link #dispose()} call can happen on any thread, the {@code schedule} implementations
+     * should make best effort to cancel tasks immediately after those tasks have been submitted to the
+     * underlying task-execution scheme if the dispose was detected after this submission.
+     * <p>
+     * All methods on the {@code Worker} class should be thread safe.
      */
     public abstract static class Worker implements Disposable {
         /**
-         * Schedules a Runnable for execution without delay.
+         * Schedules a Runnable for execution without any time delay.
          *
          * <p>The default implementation delegates to {@link #schedule(Runnable, long, TimeUnit)}.
          *
@@ -274,15 +373,16 @@ public Disposable schedule(@NonNull Runnable run) {
         }
 
         /**
-         * Schedules an Runnable for execution at some point in the future.
+         * Schedules an Runnable for execution at some point in the future specified by a time delay
+         * relative to the current time.
          * <p>
          * Note to implementors: non-positive {@code delayTime} should be regarded as non-delayed schedule, i.e.,
          * as if the {@link #schedule(Runnable)} was called.
          *
          * @param run
          *            the Runnable to schedule
          * @param delay
-         *            time to wait before executing the action; non-positive values indicate an non-delayed
+         *            time to "wait" before executing the action; non-positive values indicate an non-delayed
          *            schedule
          * @param unit
          *            the time unit of {@code delayTime}
@@ -292,12 +392,20 @@ public Disposable schedule(@NonNull Runnable run) {
         public abstract Disposable schedule(@NonNull Runnable run, long delay, @NonNull TimeUnit unit);
 
         /**
-         * Schedules a cancelable action to be executed periodically. This default implementation schedules
-         * recursively and waits for actions to complete (instead of potentially executing long-running actions
-         * concurrently). Each scheduler that can do periodic scheduling in a better way should override this.
+         * Schedules a periodic execution of the given task with the given initial time delay and repeat period.
+         * <p>
+         * The default implementation schedules and reschedules the {@code Runnable} task via the
+         * {@link #schedule(Runnable, long, TimeUnit)}
+         * method over and over and at a fixed rate, that is, the first execution will be after the
+         * {@code initialDelay}, the second after {@code initialDelay + period}, the third after
+         * {@code initialDelay + 2 * period}, and so on.
          * <p>
          * Note to implementors: non-positive {@code initialTime} and {@code period} should be regarded as
          * non-delayed scheduling of the first and any subsequent executions.
+         * In addition, a more specific {@code Worker} implementation should override this method
+         * if it can perform the periodic task execution with less overhead (such as by avoiding the
+         * creation of the wrapper and tracker objects upon each periodic invocation of the
+         * common {@link #schedule(Runnable, long, TimeUnit)} method).
          *
          * @param run
          *            the Runnable to execute periodically

src/main/java/io/reactivex/schedulers/SchedulerRunnableIntrospection.java

@@ -15,10 +15,15 @@
 import io.reactivex.plugins.RxJavaPlugins;
 
 /**
- * Interface to wrap an action inside internal scheduler's task.
- *
- * You can check if runnable implements this interface and unwrap original runnable.
- * For example inside of the {@link RxJavaPlugins#setScheduleHandler(Function)}
+ * Interface to indicate the implementor class wraps a {@code Runnable} that can
+ * be accessed via {@link #getWrappedRunnable()}.
+ * <p>
+ * You can check if a {@link Runnable} task submitted to a {@link io.reactivex.Scheduler Scheduler} (or its
+ * {@link io.reactivex.Scheduler.Worker Scheduler.Worker}) implements this interface and unwrap the
+ * original {@code Runnable} instance. This could help to avoid hooking the same underlying {@code Runnable}
+ * task in a custom {@link RxJavaPlugins#onSchedule(Runnable)} hook set via
+ * the {@link RxJavaPlugins#setScheduleHandler(Function)} method multiple times due to internal delegation
+ * of the default {@code Scheduler.scheduleDirect} or {@code Scheduler.Worker.schedule} methods.
  *
  * @since 2.1.7 - experimental
  */