MultipleSubjectMessageHandler.java

/*
 * Copyright 2015 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
 *
 * 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.springframework.xd.rxjava;

import java.lang.reflect.Method;
import java.util.Hashtable;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import org.springframework.beans.factory.DisposableBean;
import org.springframework.core.ResolvableType;
import org.springframework.expression.EvaluationContext;
import org.springframework.expression.Expression;
import org.springframework.expression.spel.standard.SpelExpressionParser;
import org.springframework.expression.spel.support.StandardEvaluationContext;
import org.springframework.integration.context.IntegrationContextUtils;
import org.springframework.integration.handler.AbstractMessageProducingHandler;
import org.springframework.messaging.Message;
import org.springframework.messaging.MessageHandlingException;
import org.springframework.messaging.support.MessageBuilder;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.ReflectionUtils;

import rx.Observable;
import rx.Subscription;
import rx.functions.Action0;
import rx.functions.Action1;
import rx.subjects.PublishSubject;
import rx.subjects.Subject;

/**
 * Adapts the item at a time delivery of a {@link org.springframework.messaging.MessageHandler}
 * by delegating processing to an Observable based on a partitionExpression.
 * <p/>
 * The specific Observable that the message is delegated is determined by the partitionExpression value.
 * Unless you change the scheduling of the inputStream in your processor, you should ensure that the
 * partitionExpression does not map messages delivered on different message bus dispatcher threads to the same
 * observable.  This is due to the underlying use of a <code>PublishSubject</code>.
 * <p/>
 * For example, using the expression <code>T(java.lang.Thread).currentThread().getId()</code> would map the current
 * dispatcher thread id to an instance of a RxJava Observable.  If you wanted to have an Observable per
 * Kafka partition, the expression header['kafka_partition_id'] since the MessageBus dispatcher thread will be
 * the same for each partition.
 * <p/>
 * If the Observable mapped to the partitionExpression value has an error or completes, it will be recreated when the
 * next message consumed maps to the same partitionExpression value.
 * <p/>
 * All error handling is the responsibility of the processor implementation.
 *
 * @author Mark Pollack
 * @author Gary Russell
 */
@SuppressWarnings({ "unchecked", "rawtypes" })
public class MultipleSubjectMessageHandler extends AbstractMessageProducingHandler implements DisposableBean {

    protected final Logger logger = LoggerFactory.getLogger(getClass());

    private final ConcurrentMap<Object, PublishSubject<Object>> subjectMap =
            new ConcurrentHashMap<Object, PublishSubject<Object>>();

    private final Map<Object, Subscription> subscriptionMap = new Hashtable<Object, Subscription>();

    @SuppressWarnings("rawtypes")
    private final Processor processor;

    private final ResolvableType inputType;

    private final Expression partitionExpression;

    private final SpelExpressionParser spelExpressionParser = new SpelExpressionParser();

    private EvaluationContext evaluationContext = new StandardEvaluationContext();

    private boolean evaluationContextSet;

    @SuppressWarnings({ "rawtypes" })
    public MultipleSubjectMessageHandler(Processor processor, String partitionExpression) {
        Assert.notNull(processor, "processor cannot be null.");
        Assert.notNull(partitionExpression, "Partition expression can not be null");
        this.processor = processor;
        this.partitionExpression = spelExpressionParser.parseExpression(partitionExpression);
        Method method = ReflectionUtils.findMethod(this.processor.getClass(), "process", Observable.class);
        this.inputType = ResolvableType.forMethodParameter(method, 0).getNested(2);
    }

    public void setIntegrationEvaluationContext(EvaluationContext evaluationContext) {
        this.evaluationContext = evaluationContext;
        this.evaluationContextSet = true;
    }

    @Override
    protected void onInit() throws Exception {
        super.onInit();
        if (!this.evaluationContextSet) {
            this.evaluationContext = IntegrationContextUtils.getEvaluationContext(getBeanFactory());
        }
    }

    @Override
    protected void handleMessageInternal(Message<?> message) throws Exception {
        Subject subjectToUse = getSubject(message);
        if (ClassUtils.isAssignable(inputType.getRawClass(), message.getClass())) {
            subjectToUse.onNext(message);
        }
        else if (ClassUtils.isAssignable(inputType.getRawClass(), message.getPayload().getClass())) {
            subjectToUse.onNext(message.getPayload());
        }
        else {
            throw new MessageHandlingException(message, "Processor signature does not match [" + message.getClass()
                    + "] or [" + message.getPayload().getClass() + "]");
        }
    }

    private Subject getSubject(Message message) {
        final Object idToUse = partitionExpression.getValue(evaluationContext, message, Object.class);
        if (logger.isDebugEnabled()) {
            logger.debug("Partition Expression evaluated to " + idToUse);
        }
        Subject subject = subjectMap.get(idToUse);
        if (subject == null) {
            Subject existingSubject = subjectMap.putIfAbsent(idToUse, PublishSubject.create());
            if (existingSubject == null) {
                subject = subjectMap.get(idToUse);
                //user defined stream processing
                Observable<?> outputStream = processor.process(subject);

                final Subscription subscription = outputStream.subscribe(new Action1<Object>() {

                    @Override
                    public void call(Object outputObject) {
                        if (ClassUtils.isAssignable(Message.class, outputObject.getClass())) {
                            getOutputChannel().send((Message) outputObject);
                        }
                        else {
                            getOutputChannel().send(MessageBuilder.withPayload(outputObject).build());
                        }
                    }
                }, new Action1<Throwable>() {

                    @Override
                    public void call(Throwable throwable) {
                        logger.error(throwable.getMessage(), throwable);
                        subjectMap.remove(idToUse);
                    }
                });

                outputStream.doOnCompleted(new Action0() {

                    @Override
                    public void call() {
                        logger.error("Subscription close for [" + subscription + "]");
                        subjectMap.remove(idToUse);
                    }
                });

                subscriptionMap.put(idToUse, subscription);
            }
            else {
                subject = existingSubject;
            }
        }
        return subject;

    }

    @Override
    public void destroy() throws Exception {
        for (Subscription subscription : subscriptionMap.values()) {
            subscription.unsubscribe();
        }
    }

}