https://github.com/spring-projects/spring-framework: Spring Web (Spring IO)
  • Juergen Hoeller 
/*
 * Copyright 2002-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http.server.reactive;

import java.io.IOException;
import java.util.concurrent.atomic.AtomicLongFieldUpdater;
import java.util.concurrent.atomic.AtomicReference;

import org.apache.commons.logging.Log;
import org.reactivestreams.Publisher;
import org.reactivestreams.Subscriber;
import org.reactivestreams.Subscription;
import reactor.core.publisher.Operators;

import org.springframework.core.log.LogDelegateFactory;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;


Abstract base class for Publisher implementations that bridge between event-listener read APIs and Reactive Streams. 
Specifically a base class for reading from the HTTP request body with
Servlet 3.1 non-blocking I/O and Undertow XNIO as well as handling incoming
WebSocket messages with standard Java WebSocket (JSR-356), Jetty, and
Undertow.

Author:Arjen Poutsma, Violeta Georgieva, Rossen Stoyanchev
Type parameters:
  • <T> – the type of element signaled
Since:5.0
/**
 * Abstract base class for {@code Publisher} implementations that bridge between
 * event-listener read APIs and Reactive Streams.
 *
 * <p>Specifically a base class for reading from the HTTP request body with
 * Servlet 3.1 non-blocking I/O and Undertow XNIO as well as handling incoming
 * WebSocket messages with standard Java WebSocket (JSR-356), Jetty, and
 * Undertow.
 *
 * @author Arjen Poutsma
 * @author Violeta Georgieva
 * @author Rossen Stoyanchev
 * @since 5.0
 * @param <T> the type of element signaled
 */
public abstract class AbstractListenerReadPublisher<T> implements Publisher<T> {

	
Special logger for debugging Reactive Streams signals.

See Also:
/**
	 * Special logger for debugging Reactive Streams signals.
	 * @see LogDelegateFactory#getHiddenLog(Class)
	 * @see AbstractListenerWriteProcessor#rsWriteLogger
	 * @see AbstractListenerWriteFlushProcessor#rsWriteFlushLogger
	 * @see WriteResultPublisher#rsWriteResultLogger
	 */
	protected static Log rsReadLogger = LogDelegateFactory.getHiddenLog(AbstractListenerReadPublisher.class);


	private final AtomicReference<State> state = new AtomicReference<>(State.UNSUBSCRIBED);

	private volatile long demand;

	@SuppressWarnings("rawtypes")
	private static final AtomicLongFieldUpdater<AbstractListenerReadPublisher> DEMAND_FIELD_UPDATER =
			AtomicLongFieldUpdater.newUpdater(AbstractListenerReadPublisher.class, "demand");

	@Nullable
	private volatile Subscriber<? super T> subscriber;

	private volatile boolean completionBeforeDemand;

	@Nullable
	private volatile Throwable errorBeforeDemand;

	private final String logPrefix;


	public AbstractListenerReadPublisher() {
		this("");
	}

	
Create an instance with the given log prefix.

Since:5.1
/**
	 * Create an instance with the given log prefix.
	 * @since 5.1
	 */
	public AbstractListenerReadPublisher(String logPrefix) {
		this.logPrefix = logPrefix;
	}


	
Return the configured log message prefix.

Since:5.1
/**
	 * Return the configured log message prefix.
	 * @since 5.1
	 */
	public String getLogPrefix() {
		return this.logPrefix;
	}


	// Publisher implementation...

	@Override
	public void subscribe(Subscriber<? super T> subscriber) {
		this.state.get().subscribe(this, subscriber);
	}


	// Async I/O notification methods...

	
Invoked when reading is possible, either in the same thread after a check via checkOnDataAvailable(), or as a callback from the underlying container. 
/**
	 * Invoked when reading is possible, either in the same thread after a check
	 * via {@link #checkOnDataAvailable()}, or as a callback from the underlying
	 * container.
	 */
	public final void onDataAvailable() {
		rsReadLogger.trace(getLogPrefix() + "onDataAvailable");
		this.state.get().onDataAvailable(this);
	}

	
Sub-classes can call this method to delegate a contain notification when
all data has been read.

/**
	 * Sub-classes can call this method to delegate a contain notification when
	 * all data has been read.
	 */
	public void onAllDataRead() {
		rsReadLogger.trace(getLogPrefix() + "onAllDataRead");
		this.state.get().onAllDataRead(this);
	}

	
Sub-classes can call this to delegate container error notifications.

/**
	 * Sub-classes can call this to delegate container error notifications.
	 */
	public final void onError(Throwable ex) {
		if (rsReadLogger.isTraceEnabled()) {
			rsReadLogger.trace(getLogPrefix() + "Connection error: " + ex);
		}
		this.state.get().onError(this, ex);
	}


	// Read API methods to be implemented or template methods to override...

	
Check if data is available and either call onDataAvailable() immediately or schedule a notification. 
/**
	 * Check if data is available and either call {@link #onDataAvailable()}
	 * immediately or schedule a notification.
	 */
	protected abstract void checkOnDataAvailable();

	
Read once from the input, if possible.

Returns:the item that was read; or null
/**
	 * Read once from the input, if possible.
	 * @return the item that was read; or {@code null}
	 */
	@Nullable
	protected abstract T read() throws IOException;

	
Invoked when reading is paused due to a lack of demand.

Note: This method is guaranteed not to compete with checkOnDataAvailable() so it can be used to safely suspend reading, if the underlying API supports it, i.e. without competing with an implicit call to resume via checkOnDataAvailable(). 
Since:5.0.2
/**
	 * Invoked when reading is paused due to a lack of demand.
	 * <p><strong>Note:</strong> This method is guaranteed not to compete with
	 * {@link #checkOnDataAvailable()} so it can be used to safely suspend
	 * reading, if the underlying API supports it, i.e. without competing with
	 * an implicit call to resume via {@code checkOnDataAvailable()}.
	 * @since 5.0.2
	 */
	protected abstract void readingPaused();

	
Invoked after an I/O read error from the underlying server or after a
cancellation signal from the downstream consumer to allow sub-classes
to discard any current cached data they might have.

Since:5.0.11
/**
	 * Invoked after an I/O read error from the underlying server or after a
	 * cancellation signal from the downstream consumer to allow sub-classes
	 * to discard any current cached data they might have.
	 * @since 5.0.11
	 */
	protected abstract void discardData();


	// Private methods for use in State...

	
Read and publish data one at a time until there is no more data, no more
demand, or perhaps we completed in the mean time.

Returns:true if there is more demand; false if there is no more demand or we have completed.
/**
	 * Read and publish data one at a time until there is no more data, no more
	 * demand, or perhaps we completed in the mean time.
	 * @return {@code true} if there is more demand; {@code false} if there is
	 * no more demand or we have completed.
	 */
	private boolean readAndPublish() throws IOException {
		long r;
		while ((r = this.demand) > 0 && !this.state.get().equals(State.COMPLETED)) {
			T data = read();
			if (data != null) {
				if (r != Long.MAX_VALUE) {
					DEMAND_FIELD_UPDATER.addAndGet(this, -1L);
				}
				Subscriber<? super T> subscriber = this.subscriber;
				Assert.state(subscriber != null, "No subscriber");
				if (rsReadLogger.isTraceEnabled()) {
					rsReadLogger.trace(getLogPrefix() + "Publishing data read");
				}
				subscriber.onNext(data);
			}
			else {
				if (rsReadLogger.isTraceEnabled()) {
					rsReadLogger.trace(getLogPrefix() + "No more data to read");
				}
				return true;
			}
		}
		return false;
	}

	private boolean changeState(State oldState, State newState) {
		boolean result = this.state.compareAndSet(oldState, newState);
		if (result && rsReadLogger.isTraceEnabled()) {
			rsReadLogger.trace(getLogPrefix() + oldState + " -> " + newState);
		}
		return result;
	}

	private void changeToDemandState(State oldState) {
		if (changeState(oldState, State.DEMAND)) {
			// Protect from infinite recursion in Undertow, where we can't check if data
			// is available, so all we can do is to try to read.
			// Generally, no need to check if we just came out of readAndPublish()...
			if (!oldState.equals(State.READING)) {
				checkOnDataAvailable();
			}
		}
	}

	private void handleCompletionOrErrorBeforeDemand() {
		State state = this.state.get();
		if (!state.equals(State.UNSUBSCRIBED) && !state.equals(State.SUBSCRIBING)) {
			if (this.completionBeforeDemand) {
				rsReadLogger.trace(getLogPrefix() + "Completed before demand");
				this.state.get().onAllDataRead(this);
			}
			Throwable ex = this.errorBeforeDemand;
			if (ex != null) {
				if (rsReadLogger.isTraceEnabled()) {
					rsReadLogger.trace(getLogPrefix() + "Completed with error before demand: " + ex);
				}
				this.state.get().onError(this, ex);
			}
		}
	}

	private Subscription createSubscription() {
		return new ReadSubscription();
	}


	
Subscription that delegates signals to State.

/**
	 * Subscription that delegates signals to State.
	 */
	private final class ReadSubscription implements Subscription {


		@Override
		public final void request(long n) {
			if (rsReadLogger.isTraceEnabled()) {
				rsReadLogger.trace(getLogPrefix() + n + " requested");
			}
			state.get().request(AbstractListenerReadPublisher.this, n);
		}

		@Override
		public final void cancel() {
			if (rsReadLogger.isTraceEnabled()) {
				rsReadLogger.trace(getLogPrefix() + "Cancellation");
			}
			state.get().cancel(AbstractListenerReadPublisher.this);
		}
	}


	
Represents a state for the Publisher to be in. 
       UNSUBSCRIBED
            |
            v
       SUBSCRIBING
            |
            v
   +---- NO_DEMAND ---------------> DEMAND ---+
   |        ^                         ^       |
   |        |                         |       |
   |        +------- READING <--------+       |
   |                    |                     |
   |                    v                     |
   +--------------> COMPLETED <---------------+



/**
	 * Represents a state for the {@link Publisher} to be in.
	 * <p><pre>
	 *        UNSUBSCRIBED
	 *             |
	 *             v
	 *        SUBSCRIBING
	 *             |
	 *             v
	 *    +---- NO_DEMAND ---------------> DEMAND ---+
	 *    |        ^                         ^       |
	 *    |        |                         |       |
	 *    |        +------- READING <--------+       |
	 *    |                    |                     |
	 *    |                    v                     |
	 *    +--------------> COMPLETED <---------------+
	 * </pre>
	 */
	private enum State {

		UNSUBSCRIBED {
			@Override
			<T> void subscribe(AbstractListenerReadPublisher<T> publisher, Subscriber<? super T> subscriber) {
				Assert.notNull(publisher, "Publisher must not be null");
				Assert.notNull(subscriber, "Subscriber must not be null");
				if (publisher.changeState(this, SUBSCRIBING)) {
					Subscription subscription = publisher.createSubscription();
					publisher.subscriber = subscriber;
					subscriber.onSubscribe(subscription);
					publisher.changeState(SUBSCRIBING, NO_DEMAND);
					publisher.handleCompletionOrErrorBeforeDemand();
				}
				else {
					throw new IllegalStateException("Failed to transition to SUBSCRIBING, " +
							"subscriber: " + subscriber);
				}
			}

			@Override
			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
				publisher.completionBeforeDemand = true;
				publisher.handleCompletionOrErrorBeforeDemand();
			}

			@Override
			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable ex) {
				publisher.errorBeforeDemand = ex;
				publisher.handleCompletionOrErrorBeforeDemand();
			}
		},

		
Very brief state where we know we have a Subscriber but must not
send onComplete and onError until we after onSubscribe.

/**
		 * Very brief state where we know we have a Subscriber but must not
		 * send onComplete and onError until we after onSubscribe.
		 */
		SUBSCRIBING {
			@Override
			<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
				if (Operators.validate(n)) {
					Operators.addCap(DEMAND_FIELD_UPDATER, publisher, n);
					publisher.changeToDemandState(this);
				}
			}

			@Override
			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
				publisher.completionBeforeDemand = true;
				publisher.handleCompletionOrErrorBeforeDemand();
			}

			@Override
			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable ex) {
				publisher.errorBeforeDemand = ex;
				publisher.handleCompletionOrErrorBeforeDemand();
			}
		},

		NO_DEMAND {
			@Override
			<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
				if (Operators.validate(n)) {
					Operators.addCap(DEMAND_FIELD_UPDATER, publisher, n);
					publisher.changeToDemandState(this);
				}
			}
		},

		DEMAND {
			@Override
			<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
				if (Operators.validate(n)) {
					Operators.addCap(DEMAND_FIELD_UPDATER, publisher, n);
					// Did a concurrent read transition to NO_DEMAND just before us?
					publisher.changeToDemandState(NO_DEMAND);
				}
			}

			@Override
			<T> void onDataAvailable(AbstractListenerReadPublisher<T> publisher) {
				if (publisher.changeState(this, READING)) {
					try {
						boolean demandAvailable = publisher.readAndPublish();
						if (demandAvailable) {
							publisher.changeToDemandState(READING);
						}
						else {
							publisher.readingPaused();
							if (publisher.changeState(READING, NO_DEMAND)) {
								// Demand may have arrived since readAndPublish returned
								long r = publisher.demand;
								if (r > 0) {
									publisher.changeToDemandState(NO_DEMAND);
								}
							}
						}
					}
					catch (IOException ex) {
						publisher.onError(ex);
					}
				}
				// Else, either competing onDataAvailable (request vs container), or concurrent completion
			}
		},

		READING {
			@Override
			<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
				if (Operators.validate(n)) {
					Operators.addCap(DEMAND_FIELD_UPDATER, publisher, n);
					// Did a concurrent read transition to NO_DEMAND just before us?
					publisher.changeToDemandState(NO_DEMAND);
				}
			}
		},

		COMPLETED {
			@Override
			<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
				// ignore
			}
			@Override
			<T> void cancel(AbstractListenerReadPublisher<T> publisher) {
				// ignore
			}
			@Override
			<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
				// ignore
			}
			@Override
			<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable t) {
				// ignore
			}
		};

		<T> void subscribe(AbstractListenerReadPublisher<T> publisher, Subscriber<? super T> subscriber) {
			throw new IllegalStateException(toString());
		}

		<T> void request(AbstractListenerReadPublisher<T> publisher, long n) {
			throw new IllegalStateException(toString());
		}

		<T> void cancel(AbstractListenerReadPublisher<T> publisher) {
			if (publisher.changeState(this, COMPLETED)) {
				publisher.discardData();
			}
			else {
				publisher.state.get().cancel(publisher);
			}
		}

		<T> void onDataAvailable(AbstractListenerReadPublisher<T> publisher) {
			// ignore
		}

		<T> void onAllDataRead(AbstractListenerReadPublisher<T> publisher) {
			if (publisher.changeState(this, COMPLETED)) {
				Subscriber<? super T> s = publisher.subscriber;
				if (s != null) {
					s.onComplete();
				}
			}
			else {
				publisher.state.get().onAllDataRead(publisher);
			}
		}

		<T> void onError(AbstractListenerReadPublisher<T> publisher, Throwable t) {
			if (publisher.changeState(this, COMPLETED)) {
				publisher.discardData();
				Subscriber<? super T> s = publisher.subscriber;
				if (s != null) {
					s.onError(t);
				}
			}
			else {
				publisher.state.get().onError(publisher, t);
			}
		}
	}

}