com.github.triceo.robozonky.api.Refreshable Maven / Gradle / Ivy
/*
* Copyright 2017 Lukáš Petrovický
*
* 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
*
* http://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 com.github.triceo.robozonky.api;
import java.util.Objects;
import java.util.Optional;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Supplier;
import com.github.triceo.robozonky.internal.api.Retriever;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* Represents a resource that can be periodically checked for new results.
*
* The aim of this class is to be scheduled using a {@link ScheduledExecutorService}, while another thread is calling
* {@link #getLatest()} to retrieve the latest version of the resource.
*
* @param Type of the resource.
*/
public abstract class Refreshable implements Runnable {
/**
* Create an instance of this class that will always return empty resource.
*
* @return The returned instance's {@link #getLatest()} will always return {@link Optional#empty()}.
*/
public static Refreshable createImmutable() {
return Refreshable.createImmutable(null);
}
/**
* Create an instance of this class that will never refresh the given resource.
*
* @param toReturn Instance will always return this object.
* @param Type of object to return.
* @return The returned instance will never change it's {@link #getLatest()}.
*/
public static Refreshable createImmutable(final I toReturn) {
return new Refreshable() {
@Override
protected Supplier> getLatestSource() {
return () -> Optional.of("");
}
@Override
protected Optional transform(final String source) {
return Optional.ofNullable(toReturn);
}
};
}
protected final Logger LOGGER = LoggerFactory.getLogger(this.getClass());
private final Semaphore valueIsMissing = new Semaphore(1);
private final AtomicReference latestKnownSource = new AtomicReference<>();
private final AtomicReference cachedResult = new AtomicReference<>();
/**
* Will be used to prevent {@link #getLatest()} from returning before {@link #run()} fetched a value once.
*/
private final CountDownLatch completionAssurance = new CountDownLatch(1);
public Refreshable() {
this.valueIsMissing.acquireUninterruptibly();
}
/**
* Whether or not the refresh of this resource depends on the refresh of another resource. This method exists so
* that any scheduler can properly include all the required {@link Refreshable}s by overriding this one.
* @return Present if this resource needs another resource to be properly refreshed.
*/
public Optional> getDependedOn() {
return Optional.empty();
}
/**
* Result of this method will be used to fetch the latest resource state. While {@link #run()} is being
* executed, if the result of the call no longer {@link #equals(Object)} its value from previous call,
* {@link #transform(String)} will be called, resulting in {@link #getLatest()} changing its return value.
*
* The result of this method will be treated as a blocking operation, assuming it contains I/O calls.
*
* @return Method to retrieve identifier for the content. If empty, {@link #transform(String)} will not be called
* and {@link #getLatest()} will become empty.
*/
protected abstract Supplier> getLatestSource();
/**
* Transform resource source into a new version of the resource. This method will be called when a fresh resource
* is being requested.
*
* @param source The source to use when creating fresh instance of the resource.
* @return The fresh version of the resource. Empty if source could not be parsed.
*/
protected abstract Optional transform(final String source);
/**
* Will block until {@link #getLatest()} is able to return a non-empty result.
* @return The return of {@link #getLatest()}'s optional.
*/
public T getLatestBlocking() {
try {
valueIsMissing.acquireUninterruptibly();
return cachedResult.get();
} finally {
valueIsMissing.release();
}
}
/**
* Latest version of the resource. Will block until {@link #run()} has finished at least once.
*
* @return Empty if the source could not be parsed or if the wait operation was interrupted.
*/
public Optional getLatest() {
try {
completionAssurance.await();
return Optional.ofNullable(cachedResult.get());
} catch (final InterruptedException e) {
return Optional.empty();
}
}
private void storeResult(final T result) {
final T previous = cachedResult.getAndSet(result);
if (Objects.equals(previous, result)) {
return;
}
if (previous == null && result != null) { // value newly available
LOGGER.debug("New value.");
valueIsMissing.release();
} else if (previous != null && result == null) { // value lost
LOGGER.debug("No value.");
valueIsMissing.acquireUninterruptibly();
}
}
private void runLocked() {
final Optional maybeNewSource = Retriever.retrieve(this.getLatestSource());
if (maybeNewSource.isPresent()) {
final String newSource = maybeNewSource.get();
if (Objects.equals(newSource, latestKnownSource.get())) {
return;
}
// source changed, result needs to be refreshed
latestKnownSource.set(newSource);
final Optional maybeNewResult = transform(newSource);
if (maybeNewResult.isPresent()) {
final T newResult = maybeNewResult.get();
LOGGER.debug("Source successfully transformed to {}.", newResult.getClass());
storeResult(newResult);
} else {
LOGGER.debug("Source not transformed.");
storeResult(null);
}
} else {
this.latestKnownSource.set(null);
storeResult(null);
}
}
/**
* Update the value of {@link #getLatest()}, based on whether {@link #getLatestSource()} indicates there were any
* changes in the resource.
*/
@Override
public void run() {
try {
runLocked();
} finally {
completionAssurance.countDown();
}
}
}