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JSR 356: Java API for WebSocket
There is a newer version: 1.1
/*
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 *
 * Copyright (c) 2012-2013 Oracle and/or its affiliates. All rights reserved.
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 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  You
 * may not use this file except in compliance with the License.  You can
 * obtain a copy of the License at
 * http://glassfish.java.net/public/CDDL+GPL_1_1.html
 * or packager/legal/LICENSE.txt.  See the License for the specific
 * language governing permissions and limitations under the License.
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 * file and include the License file at packager/legal/LICENSE.txt.
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 */
package javax.websocket;

import java.io.IOException;
import java.io.OutputStream;
import java.io.Writer;
import java.nio.ByteBuffer;

/**
 * The Encoder interface defines how developers can provide a way to convert
 * their custom objects into web socket messages. The Encoder interface contains
 * subinterfaces that allow encoding algorithms to encode custom objects to: text,
 * binary data, character
 * stream and write to an output stream.
 * The websocket implementation creates a new instance of the encoder per endpoint instance per connection.
 * This means that each encoder instance has at most one calling thread at a time.
 *
 * @author dannycoward
 * @since DRAFT 002
 */
public interface Encoder {

    /**
     * This interface defines how to provide a way to convert a custom
     * object into a text message.
     *
     * @param  The type of the custom developer object that this Encoder can encode into a String.
     */
    interface Text extends Encoder {
        /**
         * Encode the given object into a String.
         *
         * @param object the object being encoded.
         * @return the encoded object as a string.
         */
        String encode(T object) throws EncodeException;
    }

    /**
     * This interface may be implemented by encoding algorithms
     * that want to write the encoded object to a character stream.
     *
     * @param  the type of the object this encoder can encode to a CharacterStream.
     * @since DRAFT 006 / EDR
     */
    interface TextStream extends Encoder {
        /**
         * Encode the given object to a character stream writing it
         * to the supplied Writer. Implementations of this method may use the EncodeException
         * to indicate a failure to convert the supplied object to an encoded form, and may
         * use the IOException to indicate a failure to write the data to the supplied
         * stream.
         *
         * @param object the object to be encoded.
         * @param writer the writer provided by the web socket runtime to write the encoded data.
         * @throws EncodeException if there was an error encoding the object due to its state.
         * @throws IOException     if there was an exception writing to the writer.
         */
        void encode(T object, Writer writer) throws EncodeException, IOException;
    }

    /**
     * This interface defines how to provide a way to convert a custom
     * object into a binary message.
     *
     * @param  The type of the custom object that this Encoder can encoder to a ByteBuffer.
     */
    interface Binary extends Encoder {
        /**
         * Encode the given object into a byte array.
         *
         * @param object the object being encoded.
         * @return the binary data.
         */
        ByteBuffer encode(T object) throws EncodeException;
    }

    /**
     * This interface may be implemented by encoding algorithms
     * that want to write the encoded object to a binary stream.
     *
     * @param  the type of the object this encoder can encode.
     * @since DRAFT 006 / EDR
     */
    interface BinaryStream extends Encoder {
        /**
         * Encode the given object into a binary stream written to the
         * implementation provided OutputStream.
         *
         * @param object the object being encoded.
         * @param os     the output stream where the encoded data is written.
         */
        void encode(T object, OutputStream os) throws EncodeException, IOException;
    }
}