com.twitter.common.base.Closures.java Source code

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Here is the source code for com.twitter.common.base.Closures.java

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// =================================================================================================
// Copyright 2011 Twitter, Inc.
// -------------------------------------------------------------------------------------------------
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this work except in compliance with the License.
// You may obtain a copy of the License in the LICENSE file, or 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.twitter.common.base;

import com.google.common.base.Function;
import com.google.common.base.Predicate;
import com.google.common.base.Predicates;
import com.google.common.base.Throwables;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterables;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;

/**
 * Utilities for dealing with Closures.
 *
 * @author John Sirois
 */
public final class Closures {

    private static final Closure<?> NOOP = new Closure<Object>() {
        @Override
        public void execute(Object item) {
            // noop
        }
    };

    private Closures() {
        // utility
    }

    /**
     * Converts a closure into a function returning {@code null}.
     */
    public static <T> Function<T, Void> asFunction(final ExceptionalClosure<T, ?> closure) {
        checkNotNull(closure);

        // CHECKSTYLE:OFF IllegalCatch
        return new Function<T, Void>() {
            @Override
            public Void apply(T item) {
                try {
                    closure.execute(item);
                } catch (Exception e) {
                    Throwables.propagate(e);
                }
                return null;
            }
        };
        // CHECKSTYLE:ON IllegalCatch
    }

    /**
     * Varargs equivalent of {@link #combine(Iterable)}.
     *
     * @param closures Closures to combine.
     * @param <T> Type accepted by the closures.
     * @return A single closure that will fan out all calls to {@link Closure#execute(Object)} to
     *    the wrapped closures.
     */
    public static <T> Closure<T> combine(Closure<T>... closures) {
        return combine(ImmutableList.copyOf(closures));
    }

    /**
     * Combines multiple closures into a single closure, whose calls are replicated sequentially
     * in the order that they were provided.
     * If an exception is encountered from a closure it propagates to the top-level closure and the
     * remaining closures are not executed.
     *
     * @param closures Closures to combine.
     * @param <T> Type accepted by the closures.
     * @return A single closure that will fan out all calls to {@link Closure#execute(Object)} to
     *    the wrapped closures.
     */
    public static <T> Closure<T> combine(Iterable<Closure<T>> closures) {
        checkNotNull(closures);
        checkArgument(Iterables.all(closures, Predicates.notNull()));

        final Iterable<Closure<T>> closuresCopy = ImmutableList.copyOf(closures);

        return new Closure<T>() {
            @Override
            public void execute(T item) {
                for (Closure<T> closure : closuresCopy) {
                    closure.execute(item);
                }
            }
        };
    }

    /**
     * Applies a filter to a closure, such that the closure will only be called when the filter is
     * satisfied (returns {@code true}}.
     *
     * @param filter Filter to determine when {@code closure} is called.
     * @param closure Closure to filter.
     * @param <T> Type handled by the filter and the closure.
     * @return A filtered closure.
     */
    public static <T> Closure<T> filter(final Predicate<T> filter, final Closure<T> closure) {
        checkNotNull(filter);
        checkNotNull(closure);

        return new Closure<T>() {
            @Override
            public void execute(T item) {
                if (filter.apply(item)) {
                    closure.execute(item);
                }
            }
        };
    }

    /**
     * Returns a closure that will do nothing.
     *
     * @param <T> The closure argument type.
     * @return A closure that does nothing.
     */
    @SuppressWarnings("unchecked")
    public static <T> Closure<T> noop() {
        return (Closure<T>) NOOP;
    }
}