Java tutorial
/* * Copyright (C) 2009 The Guava 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 * * 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.google.common.util.concurrent; import static com.google.common.base.Preconditions.checkArgument; import static com.google.common.base.Preconditions.checkNotNull; import static com.google.common.base.Preconditions.checkState; import static com.google.common.util.concurrent.Service.State.FAILED; import static com.google.common.util.concurrent.Service.State.NEW; import static com.google.common.util.concurrent.Service.State.RUNNING; import static com.google.common.util.concurrent.Service.State.STARTING; import static com.google.common.util.concurrent.Service.State.STOPPING; import static com.google.common.util.concurrent.Service.State.TERMINATED; import com.google.common.annotations.Beta; import com.google.common.util.concurrent.ListenerCallQueue.Callback; import com.google.common.util.concurrent.Monitor.Guard; import com.google.common.util.concurrent.Service.State; // javadoc needs this import com.google.j2objc.annotations.WeakOuter; import java.util.ArrayList; import java.util.Collections; import java.util.List; import java.util.concurrent.Executor; import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeoutException; import javax.annotation.Nullable; import javax.annotation.concurrent.GuardedBy; import javax.annotation.concurrent.Immutable; /** * Base class for implementing services that can handle {@link #doStart} and {@link #doStop} * requests, responding to them with {@link #notifyStarted()} and {@link #notifyStopped()} * callbacks. Its subclasses must manage threads manually; consider {@link * AbstractExecutionThreadService} if you need only a single execution thread. * * @author Jesse Wilson * @author Luke Sandberg * @since 1.0 */ @Beta public abstract class AbstractService implements Service { private static final Callback<Listener> STARTING_CALLBACK = new Callback<Listener>("starting()") { @Override void call(Listener listener) { listener.starting(); } }; private static final Callback<Listener> RUNNING_CALLBACK = new Callback<Listener>("running()") { @Override void call(Listener listener) { listener.running(); } }; private static final Callback<Listener> STOPPING_FROM_STARTING_CALLBACK = stoppingCallback(STARTING); private static final Callback<Listener> STOPPING_FROM_RUNNING_CALLBACK = stoppingCallback(RUNNING); private static final Callback<Listener> TERMINATED_FROM_NEW_CALLBACK = terminatedCallback(NEW); private static final Callback<Listener> TERMINATED_FROM_RUNNING_CALLBACK = terminatedCallback(RUNNING); private static final Callback<Listener> TERMINATED_FROM_STOPPING_CALLBACK = terminatedCallback(STOPPING); private static Callback<Listener> terminatedCallback(final State from) { return new Callback<Listener>("terminated({from = " + from + "})") { @Override void call(Listener listener) { listener.terminated(from); } }; } private static Callback<Listener> stoppingCallback(final State from) { return new Callback<Listener>("stopping({from = " + from + "})") { @Override void call(Listener listener) { listener.stopping(from); } }; } private final Monitor monitor = new Monitor(); private final Guard isStartable = new IsStartableGuard(); @WeakOuter private final class IsStartableGuard extends Guard { IsStartableGuard() { super(AbstractService.this.monitor); } @Override public boolean isSatisfied() { return state() == NEW; } } private final Guard isStoppable = new IsStoppableGuard(); @WeakOuter private final class IsStoppableGuard extends Guard { IsStoppableGuard() { super(AbstractService.this.monitor); } @Override public boolean isSatisfied() { return state().compareTo(RUNNING) <= 0; } } private final Guard hasReachedRunning = new HasReachedRunningGuard(); @WeakOuter private final class HasReachedRunningGuard extends Guard { HasReachedRunningGuard() { super(AbstractService.this.monitor); } @Override public boolean isSatisfied() { return state().compareTo(RUNNING) >= 0; } } private final Guard isStopped = new IsStoppedGuard(); @WeakOuter private final class IsStoppedGuard extends Guard { IsStoppedGuard() { super(AbstractService.this.monitor); } @Override public boolean isSatisfied() { return state().isTerminal(); } } /** * The listeners to notify during a state transition. */ @GuardedBy("monitor") private final List<ListenerCallQueue<Listener>> listeners = Collections .synchronizedList(new ArrayList<ListenerCallQueue<Listener>>()); /** * The current state of the service. This should be written with the lock held but can be read * without it because it is an immutable object in a volatile field. This is desirable so that * methods like {@link #state}, {@link #failureCause} and notably {@link #toString} can be run * without grabbing the lock. * * <p>To update this field correctly the lock must be held to guarantee that the state is * consistent. */ @GuardedBy("monitor") private volatile StateSnapshot snapshot = new StateSnapshot(NEW); /** Constructor for use by subclasses. */ protected AbstractService() { } /** * This method is called by {@link #startAsync} to initiate service startup. The invocation of * this method should cause a call to {@link #notifyStarted()}, either during this method's run, * or after it has returned. If startup fails, the invocation should cause a call to {@link * #notifyFailed(Throwable)} instead. * * <p>This method should return promptly; prefer to do work on a different thread where it is * convenient. It is invoked exactly once on service startup, even when {@link #startAsync} is * called multiple times. */ protected abstract void doStart(); /** * This method should be used to initiate service shutdown. The invocation of this method should * cause a call to {@link #notifyStopped()}, either during this method's run, or after it has * returned. If shutdown fails, the invocation should cause a call to {@link * #notifyFailed(Throwable)} instead. * * <p>This method should return promptly; prefer to do work on a different thread where it is * convenient. It is invoked exactly once on service shutdown, even when {@link #stopAsync} is * called multiple times. */ protected abstract void doStop(); @Override public final Service startAsync() { if (monitor.enterIf(isStartable)) { try { snapshot = new StateSnapshot(STARTING); starting(); doStart(); } catch (Throwable startupFailure) { notifyFailed(startupFailure); } finally { monitor.leave(); executeListeners(); } } else { throw new IllegalStateException("Service " + this + " has already been started"); } return this; } @Override public final Service stopAsync() { if (monitor.enterIf(isStoppable)) { try { State previous = state(); switch (previous) { case NEW: snapshot = new StateSnapshot(TERMINATED); terminated(NEW); break; case STARTING: snapshot = new StateSnapshot(STARTING, true, null); stopping(STARTING); break; case RUNNING: snapshot = new StateSnapshot(STOPPING); stopping(RUNNING); doStop(); break; case STOPPING: case TERMINATED: case FAILED: // These cases are impossible due to the if statement above. throw new AssertionError("isStoppable is incorrectly implemented, saw: " + previous); default: throw new AssertionError("Unexpected state: " + previous); } } catch (Throwable shutdownFailure) { notifyFailed(shutdownFailure); } finally { monitor.leave(); executeListeners(); } } return this; } @Override public final void awaitRunning() { monitor.enterWhenUninterruptibly(hasReachedRunning); try { checkCurrentState(RUNNING); } finally { monitor.leave(); } } @Override public final void awaitRunning(long timeout, TimeUnit unit) throws TimeoutException { if (monitor.enterWhenUninterruptibly(hasReachedRunning, timeout, unit)) { try { checkCurrentState(RUNNING); } finally { monitor.leave(); } } else { // It is possible due to races the we are currently in the expected state even though we // timed out. e.g. if we weren't event able to grab the lock within the timeout we would never // even check the guard. I don't think we care too much about this use case but it could lead // to a confusing error message. throw new TimeoutException("Timed out waiting for " + this + " to reach the RUNNING state."); } } @Override public final void awaitTerminated() { monitor.enterWhenUninterruptibly(isStopped); try { checkCurrentState(TERMINATED); } finally { monitor.leave(); } } @Override public final void awaitTerminated(long timeout, TimeUnit unit) throws TimeoutException { if (monitor.enterWhenUninterruptibly(isStopped, timeout, unit)) { try { checkCurrentState(TERMINATED); } finally { monitor.leave(); } } else { // It is possible due to races the we are currently in the expected state even though we // timed out. e.g. if we weren't event able to grab the lock within the timeout we would never // even check the guard. I don't think we care too much about this use case but it could lead // to a confusing error message. throw new TimeoutException( "Timed out waiting for " + this + " to reach a terminal state. " + "Current state: " + state()); } } /** Checks that the current state is equal to the expected state. */ @GuardedBy("monitor") private void checkCurrentState(State expected) { State actual = state(); if (actual != expected) { if (actual == FAILED) { // Handle this specially so that we can include the failureCause, if there is one. throw new IllegalStateException( "Expected the service to be " + expected + ", but the service has FAILED", failureCause()); } throw new IllegalStateException("Expected the service to be " + expected + ", but was " + actual); } } /** * Implementing classes should invoke this method once their service has started. It will cause * the service to transition from {@link State#STARTING} to {@link State#RUNNING}. * * @throws IllegalStateException if the service is not {@link State#STARTING}. */ protected final void notifyStarted() { monitor.enter(); try { // We have to examine the internal state of the snapshot here to properly handle the stop // while starting case. if (snapshot.state != STARTING) { IllegalStateException failure = new IllegalStateException( "Cannot notifyStarted() when the service is " + snapshot.state); notifyFailed(failure); throw failure; } if (snapshot.shutdownWhenStartupFinishes) { snapshot = new StateSnapshot(STOPPING); // We don't call listeners here because we already did that when we set the // shutdownWhenStartupFinishes flag. doStop(); } else { snapshot = new StateSnapshot(RUNNING); running(); } } finally { monitor.leave(); executeListeners(); } } /** * Implementing classes should invoke this method once their service has stopped. It will cause * the service to transition from {@link State#STOPPING} to {@link State#TERMINATED}. * * @throws IllegalStateException if the service is neither {@link State#STOPPING} nor {@link * State#RUNNING}. */ protected final void notifyStopped() { monitor.enter(); try { // We check the internal state of the snapshot instead of state() directly so we don't allow // notifyStopped() to be called while STARTING, even if stop() has already been called. State previous = snapshot.state; if (previous != STOPPING && previous != RUNNING) { IllegalStateException failure = new IllegalStateException( "Cannot notifyStopped() when the service is " + previous); notifyFailed(failure); throw failure; } snapshot = new StateSnapshot(TERMINATED); terminated(previous); } finally { monitor.leave(); executeListeners(); } } /** * Invoke this method to transition the service to the {@link State#FAILED}. The service will * <b>not be stopped</b> if it is running. Invoke this method when a service has failed critically * or otherwise cannot be started nor stopped. */ protected final void notifyFailed(Throwable cause) { checkNotNull(cause); monitor.enter(); try { State previous = state(); switch (previous) { case NEW: case TERMINATED: throw new IllegalStateException("Failed while in state:" + previous, cause); case RUNNING: case STARTING: case STOPPING: snapshot = new StateSnapshot(FAILED, false, cause); failed(previous, cause); break; case FAILED: // Do nothing break; default: throw new AssertionError("Unexpected state: " + previous); } } finally { monitor.leave(); executeListeners(); } } @Override public final boolean isRunning() { return state() == RUNNING; } @Override public final State state() { return snapshot.externalState(); } /** * @since 14.0 */ @Override public final Throwable failureCause() { return snapshot.failureCause(); } /** * @since 13.0 */ @Override public final void addListener(Listener listener, Executor executor) { checkNotNull(listener, "listener"); checkNotNull(executor, "executor"); monitor.enter(); try { if (!state().isTerminal()) { listeners.add(new ListenerCallQueue<Listener>(listener, executor)); } } finally { monitor.leave(); } } @Override public String toString() { return getClass().getSimpleName() + " [" + state() + "]"; } /** * Attempts to execute all the listeners in {@link #listeners} while not holding the * {@link #monitor}. */ private void executeListeners() { if (!monitor.isOccupiedByCurrentThread()) { // iterate by index to avoid concurrent modification exceptions for (int i = 0; i < listeners.size(); i++) { listeners.get(i).execute(); } } } @GuardedBy("monitor") private void starting() { STARTING_CALLBACK.enqueueOn(listeners); } @GuardedBy("monitor") private void running() { RUNNING_CALLBACK.enqueueOn(listeners); } @GuardedBy("monitor") private void stopping(final State from) { if (from == State.STARTING) { STOPPING_FROM_STARTING_CALLBACK.enqueueOn(listeners); } else if (from == State.RUNNING) { STOPPING_FROM_RUNNING_CALLBACK.enqueueOn(listeners); } else { throw new AssertionError(); } } @GuardedBy("monitor") private void terminated(final State from) { switch (from) { case NEW: TERMINATED_FROM_NEW_CALLBACK.enqueueOn(listeners); break; case RUNNING: TERMINATED_FROM_RUNNING_CALLBACK.enqueueOn(listeners); break; case STOPPING: TERMINATED_FROM_STOPPING_CALLBACK.enqueueOn(listeners); break; case STARTING: case TERMINATED: case FAILED: default: throw new AssertionError(); } } @GuardedBy("monitor") private void failed(final State from, final Throwable cause) { // can't memoize this one due to the exception new Callback<Listener>("failed({from = " + from + ", cause = " + cause + "})") { @Override void call(Listener listener) { listener.failed(from, cause); } }.enqueueOn(listeners); } /** * An immutable snapshot of the current state of the service. This class represents a consistent * snapshot of the state and therefore it can be used to answer simple queries without needing to * grab a lock. */ @Immutable private static final class StateSnapshot { /** * The internal state, which equals external state unless * shutdownWhenStartupFinishes is true. */ final State state; /** * If true, the user requested a shutdown while the service was still starting * up. */ final boolean shutdownWhenStartupFinishes; /** * The exception that caused this service to fail. This will be {@code null} * unless the service has failed. */ @Nullable final Throwable failure; StateSnapshot(State internalState) { this(internalState, false, null); } StateSnapshot(State internalState, boolean shutdownWhenStartupFinishes, @Nullable Throwable failure) { checkArgument(!shutdownWhenStartupFinishes || internalState == STARTING, "shudownWhenStartupFinishes can only be set if state is STARTING. Got %s instead.", internalState); checkArgument(!(failure != null ^ internalState == FAILED), "A failure cause should be set if and only if the state is failed. Got %s and %s " + "instead.", internalState, failure); this.state = internalState; this.shutdownWhenStartupFinishes = shutdownWhenStartupFinishes; this.failure = failure; } /** @see Service#state() */ State externalState() { if (shutdownWhenStartupFinishes && state == STARTING) { return STOPPING; } else { return state; } } /** @see Service#failureCause() */ Throwable failureCause() { checkState(state == FAILED, "failureCause() is only valid if the service has failed, service is %s", state); return failure; } } }