/*
 * Copyright 2002-2020 the original author or 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
 *
 *      https://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.springframework.web.socket.adapter.standard;

import java.nio.ByteBuffer;

import javax.websocket.DecodeException;
import javax.websocket.Decoder;
import javax.websocket.EncodeException;
import javax.websocket.Encoder;
import javax.websocket.EndpointConfig;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.GenericTypeResolver;
import org.springframework.core.convert.ConversionException;
import org.springframework.core.convert.ConversionService;
import org.springframework.core.convert.TypeDescriptor;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.web.context.ContextLoader;

/**
 * Base class that can be used to implement a standard {@link javax.websocket.Encoder}
 * and/or {@link javax.websocket.Decoder}. It provides encode and decode method
 * implementations that delegate to a Spring {@link ConversionService}.
 *
 * <p>By default, this class looks up a {@link ConversionService} registered in the
 * {@link #getApplicationContext() active ApplicationContext} under
 * the name {@code 'webSocketConversionService'}. This works fine for both client
 * and server endpoints, in a Servlet container environment. If not running in a
 * Servlet container, subclasses will need to override the
 * {@link #getConversionService()} method to provide an alternative lookup strategy.
 *
 * <p>Subclasses can extend this class and should also implement one or
 * both of {@link javax.websocket.Encoder} and {@link javax.websocket.Decoder}.
 * For convenience {@link ConvertingEncoderDecoderSupport.BinaryEncoder},
 * {@link ConvertingEncoderDecoderSupport.BinaryDecoder},
 * {@link ConvertingEncoderDecoderSupport.TextEncoder} and
 * {@link ConvertingEncoderDecoderSupport.TextDecoder} subclasses are provided.
 *
 * <p>Since JSR-356 only allows Encoder/Decoder to be registered by type, instances
 * of this class are therefore managed by the WebSocket runtime, and do not need to
 * be registered as Spring Beans. They can, however, by injected with Spring-managed
 * dependencies via {@link Autowired @Autowire}.
 *
 * <p>Converters to convert between the {@link #getType() type} and {@code String} or
 * {@code ByteBuffer} should be registered.
 *
 * @author Phillip Webb
 * @since 4.0
 * @param <T> the type being converted to (for Encoder) or from (for Decoder)
 * @param <M> the WebSocket message type ({@link String} or {@link ByteBuffer})
 * @see ConvertingEncoderDecoderSupport.BinaryEncoder
 * @see ConvertingEncoderDecoderSupport.BinaryDecoder
 * @see ConvertingEncoderDecoderSupport.TextEncoder
 * @see ConvertingEncoderDecoderSupport.TextDecoder
 */
public abstract class ConvertingEncoderDecoderSupport<T, M> {

	private static final String CONVERSION_SERVICE_BEAN_NAME = "webSocketConversionService";


	/**
	 * Called to initialize the encoder/decoder.
	 * @see javax.websocket.Encoder#init(EndpointConfig)
	 * @see javax.websocket.Decoder#init(EndpointConfig)
	 */
	public void init(EndpointConfig config) {
		ApplicationContext applicationContext = getApplicationContext();
		if (applicationContext instanceof ConfigurableApplicationContext) {
			ConfigurableListableBeanFactory beanFactory =
					((ConfigurableApplicationContext) applicationContext).getBeanFactory();
			beanFactory.autowireBean(this);
		}
	}

	/**
	 * Called to destroy the encoder/decoder.
	 * @see javax.websocket.Encoder#destroy()
	 * @see javax.websocket.Decoder#destroy()
	 */
	public void destroy() {
	}

	/**
	 * Strategy method used to obtain the {@link ConversionService}. By default this
	 * method expects a bean named {@code 'webSocketConversionService'} in the
	 * {@link #getApplicationContext() active ApplicationContext}.
	 * @return the {@link ConversionService} (never null)
	 */
	protected ConversionService getConversionService() {
		ApplicationContext applicationContext = getApplicationContext();
		Assert.state(applicationContext != null, "Unable to locate the Spring ApplicationContext");
		try {
			return applicationContext.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class);
		}
		catch (BeansException ex) {
			throw new IllegalStateException("Unable to find ConversionService: please configure a '" +
					CONVERSION_SERVICE_BEAN_NAME + "' or override the getConversionService() method", ex);
		}
	}

	/**
	 * Returns the active {@link ApplicationContext}. Be default this method obtains
	 * the context via {@link ContextLoader#getCurrentWebApplicationContext()}, which
	 * finds the ApplicationContext loaded via {@link ContextLoader} typically in a
	 * Servlet container environment. When not running in a Servlet container and
	 * not using {@link ContextLoader}, this method should be overridden.
	 * @return the {@link ApplicationContext} or {@code null}
	 */
	@Nullable
	protected ApplicationContext getApplicationContext() {
		return ContextLoader.getCurrentWebApplicationContext();
	}

	/**
	 * Returns the type being converted. By default the type is resolved using
	 * the generic arguments of the class.
	 */
	protected TypeDescriptor getType() {
		return TypeDescriptor.valueOf(resolveTypeArguments()[0]);
	}

	/**
	 * Returns the websocket message type. By default the type is resolved using
	 * the generic arguments of the class.
	 */
	protected TypeDescriptor getMessageType() {
		return TypeDescriptor.valueOf(resolveTypeArguments()[1]);
	}

	private Class<?>[] resolveTypeArguments() {
		Class<?>[] resolved = GenericTypeResolver.resolveTypeArguments(getClass(), ConvertingEncoderDecoderSupport.class);
		if (resolved == null) {
			throw new IllegalStateException("ConvertingEncoderDecoderSupport's generic types T and M " +
					"need to be substituted in subclass: " + getClass());
		}
		return resolved;
	}

	/**
	 * Encode an object to a message.
	 * @see javax.websocket.Encoder.Text#encode(Object)
	 * @see javax.websocket.Encoder.Binary#encode(Object)
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	public M encode(T object) throws EncodeException {
		try {
			return (M) getConversionService().convert(object, getType(), getMessageType());
		}
		catch (ConversionException ex) {
			throw new EncodeException(object, "Unable to encode websocket message using ConversionService", ex);
		}
	}

	/**
	 * Determine if a given message can be decoded.
	 * @see #decode(Object)
	 * @see javax.websocket.Decoder.Text#willDecode(String)
	 * @see javax.websocket.Decoder.Binary#willDecode(ByteBuffer)
	 */
	public boolean willDecode(M bytes) {
		return getConversionService().canConvert(getType(), getMessageType());
	}

	/**
	 * Decode the a message into an object.
	 * @see javax.websocket.Decoder.Text#decode(String)
	 * @see javax.websocket.Decoder.Binary#decode(ByteBuffer)
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	public T decode(M message) throws DecodeException {
		try {
			return (T) getConversionService().convert(message, getMessageType(), getType());
		}
		catch (ConversionException ex) {
			if (message instanceof String) {
				throw new DecodeException((String) message,
						"Unable to decode websocket message using ConversionService", ex);
			}
			if (message instanceof ByteBuffer) {
				throw new DecodeException((ByteBuffer) message,
						"Unable to decode websocket message using ConversionService", ex);
			}
			throw ex;
		}
	}


	/**
	 * A binary {@link javax.websocket.Encoder.Binary javax.websocket.Encoder} that delegates
	 * to Spring's conversion service. See {@link ConvertingEncoderDecoderSupport} for details.
	 * @param <T> the type that this Encoder can convert to
	 */
	public abstract static class BinaryEncoder<T> extends ConvertingEncoderDecoderSupport<T, ByteBuffer>
			implements Encoder.Binary<T> {
	}


	/**
	 * A binary {@link javax.websocket.Encoder.Binary javax.websocket.Encoder} that delegates
	 * to Spring's conversion service. See {@link ConvertingEncoderDecoderSupport} for details.
	 * @param <T> the type that this Decoder can convert from
	 */
	public abstract static class BinaryDecoder<T> extends ConvertingEncoderDecoderSupport<T, ByteBuffer>
			implements Decoder.Binary<T> {
	}


	/**
	 * A text {@link javax.websocket.Encoder.Text javax.websocket.Encoder} that delegates
	 * to Spring's conversion service. See {@link ConvertingEncoderDecoderSupport} for
	 * details.
	 * @param <T> the type that this Encoder can convert to
	 */
	public abstract static class TextEncoder<T> extends ConvertingEncoderDecoderSupport<T, String>
			implements Encoder.Text<T> {
	}


	/**
	 * A Text {@link javax.websocket.Encoder.Text javax.websocket.Encoder} that delegates
	 * to Spring's conversion service. See {@link ConvertingEncoderDecoderSupport} for details.
	 * @param <T> the type that this Decoder can convert from
	 */
	public abstract static class TextDecoder<T> extends ConvertingEncoderDecoderSupport<T, String>
			implements Decoder.Text<T> {
	}

}
