io.micronaut.http.reactive.execution.FlowAsMono Maven / Gradle / Ivy
/*
* Copyright 2017-2024 original 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 io.micronaut.http.reactive.execution;
import io.micronaut.core.annotation.Internal;
import io.micronaut.core.execution.ExecutionFlow;
import io.micronaut.core.execution.ImperativeExecutionFlow;
import org.reactivestreams.Subscription;
import reactor.core.CoreSubscriber;
import reactor.core.Fuseable;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Operators;
import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
/**
* {@link Mono} implementation that is based on an {@link ExecutionFlow}.
*
* @param The value type
* @author Jonas Konrad
* @since 4.8.0
*/
@Internal
final class FlowAsMono extends Mono implements Fuseable {
final ExecutionFlow flow;
FlowAsMono(ExecutionFlow flow) {
this.flow = flow;
}
@Override
public void subscribe(CoreSubscriber actual) {
ImperativeExecutionFlow complete = flow.tryComplete();
if (complete != null) {
// these subscriptions support SYNC fusion
if (complete.getError() != null) {
Operators.error(actual, complete.getError());
} else if (complete.getValue() != null) {
actual.onSubscribe(Operators.scalarSubscription(actual, complete.getValue()));
} else {
actual.onSubscribe(Operators.emptySubscription());
}
} else {
// fallback to a normal subscription
new SubscriptionImpl(actual).callOnSubscribe();
}
}
private final class SubscriptionImpl implements QueueSubscription {
@SuppressWarnings("rawtypes")
private static final AtomicIntegerFieldUpdater STATE = AtomicIntegerFieldUpdater.newUpdater(FlowAsMono.SubscriptionImpl.class, "state");
private static final int STATE_WAITING = 0;
private static final int STATE_SUBSCRIBING = 1;
private static final int STATE_DONE = 2;
private final CoreSubscriber actual;
/**
* State flag to avoid reentrancy. We only do two distinct operations on the subscriber:
* {@link CoreSubscriber#onSubscribe} and the completion operations (
* {@link CoreSubscriber#onNext} and then {@link CoreSubscriber#onComplete()}). By the
* reactive spec these must not happen in a reentrant fashion, i.e. we can't complete
* inside the {@link CoreSubscriber#onSubscribe} call. This field acts like a simple lock
* to avoid that.
* Just before {@link CoreSubscriber#onSubscribe}, the state is set to
* {@link #STATE_SUBSCRIBING}. If the subscriber then calls {@link Subscription#request} in
* that method and the request can immediately be fulfilled, the completion handler will
* notice that the state is {@link #STATE_SUBSCRIBING}, switch it to {@link #STATE_DONE}
* and hold back the result for now. Once {@link CoreSubscriber#onSubscribe} finishes,
* {@link #callOnSubscribe()} will notice that the field is {@link #STATE_DONE} and forward
* the actual result.
*
If the request is not immediately fulfilled, {@link #callOnSubscribe()} will
* set the state to {@link #STATE_WAITING}. Once the flow completes, the completion handler
* will see this state and forward the result immediately instead of holding it back.
*/
private volatile int state;
private boolean requested;
private T result;
private Throwable error;
SubscriptionImpl(CoreSubscriber actual) {
this.actual = actual;
}
void callOnSubscribe() {
state = STATE_SUBSCRIBING;
actual.onSubscribe(this);
if (STATE.getAndSet(this, STATE_WAITING) == STATE_DONE) {
// onComplete was already called but result held back to avoid reentrancy, need to
// forward its result
forward(result, error);
}
}
@Override
public void request(long n) {
if (!requested) {
requested = true;
flow.onComplete((v, e) -> {
result = v;
error = e;
if (STATE.getAndSet(this, STATE_DONE) == STATE_WAITING) {
// onSubscribe is already done so we can forward immediately
forward(v, e);
}
});
}
}
private void forward(T v, Throwable e) {
if (v != null) {
actual.onNext(v);
}
if (error == null) {
actual.onComplete();
} else {
actual.onError(e);
}
}
@Override
public void cancel() {
requested = true;
flow.cancel();
}
@Override
public int requestFusion(int requestedMode) {
// while we implement QueueSubscription, we don't actually support it. only the
// short-circuit flows for ImperativeExecutionFlow above do. there's little value in
// supporting it here
return 0;
}
@Override
public T poll() {
throw noFusion();
}
@Override
public int size() {
throw noFusion();
}
@Override
public boolean isEmpty() {
throw noFusion();
}
@Override
public void clear() {
throw noFusion();
}
private static UnsupportedOperationException noFusion() {
return new UnsupportedOperationException("fusion not supported");
}
}
}