core/src/main/java/org/elasticsearch/common/io/stream/Writeable.java


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/*
 * Licensed to Elasticsearch under one or more contributor
 * license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright
 * ownership. Elasticsearch licenses this file to you 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 org.elasticsearch.common.io.stream;

import java.io.IOException;

/**
 * Implementers can be written to a {@linkplain StreamOutput} and read from a {@linkplain StreamInput}. This allows them to be "thrown
 * across the wire" using Elasticsearch's internal protocol. If the implementer also implements equals and hashCode then a copy made by
 * serializing and deserializing must be equal and have the same hashCode. It isn't required that such a copy be entirely unchanged.
 *
 * Prefer implementing this interface over implementing {@link Streamable} where possible. Lots of code depends on {@linkplain Streamable}
 * so this isn't always possible.
 */
public interface Writeable {
    /**
     * Write this into the {@linkplain StreamOutput}.
     */
    void writeTo(StreamOutput out) throws IOException;

    /**
     * Reference to a method that can read some object from a stream. By convention this is a constructor that takes
     * {@linkplain StreamInput} as an argument for most classes and a static method for things like enums. Returning null from one of these
     * is always wrong - for that we use methods like {@link StreamInput#readOptionalWriteable(Reader)}.
     */
    @FunctionalInterface
    interface Reader<R> {
        /**
         * Read R from a stream.
         */
        R read(StreamInput in) throws IOException;
    }
}