camel-core/src/main/java/org/apache/camel/processor/Pipeline.java

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF 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.apache.camel.processor;

import java.util.Collection;
import java.util.Iterator;
import java.util.List;

import org.apache.camel.Exchange;
import org.apache.camel.Message;
import org.apache.camel.Processor;
import org.apache.camel.util.ExchangeHelper;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Creates a Pipeline pattern where the output of the previous step is sent as
 * input to the next step, reusing the same message exchanges
 *
 * @version $Revision$
 */
public class Pipeline extends MulticastProcessor implements Processor {
    private static final transient Log LOG = LogFactory.getLog(Pipeline.class);

    public Pipeline(Collection<Processor> processors) {
        super(processors);
    }

    public static Processor newInstance(List<Processor> processors) {
        if (processors.isEmpty()) {
            return null;
        } else if (processors.size() == 1) {
            return processors.get(0);
        }
        return new Pipeline(processors);
    }

    public void process(Exchange exchange) throws Exception {
        Iterator<Processor> processors = getProcessors().iterator();
        Exchange nextExchange = exchange;
        boolean first = true;

        while (continueRouting(processors, nextExchange)) {
            if (first) {
                first = false;
            } else {
                // prepare for next run
                nextExchange = createNextExchange(nextExchange);
            }

            // get the next processor
            Processor processor = processors.next();

            // process the next exchange
            try {
                if (LOG.isTraceEnabled()) {
                    // this does the actual processing so log at trace level
                    LOG.trace("Processing exchangeId: " + nextExchange.getExchangeId() + " >>> " + nextExchange);
                }
                processor.process(nextExchange);
            } catch (Exception e) {
                nextExchange.setException(e);
            }

            // check for error if so we should break out
            boolean exceptionHandled = hasExceptionBeenHandled(nextExchange);
            if (nextExchange.isFailed() || exceptionHandled) {
                // The Exchange.EXCEPTION_HANDLED property is only set if satisfactory handling was done
                // by the error handler. It's still an exception, the exchange still failed.
                if (LOG.isDebugEnabled()) {
                    LOG.debug("Message exchange has failed so breaking out of pipeline: " + nextExchange
                              + " exception: " + nextExchange.getException() + " fault: "
                              + (nextExchange.hasFault() ? nextExchange.getFault() : null)
                              + (exceptionHandled ? " handled by the error handler" : ""));
                }
                break;
            }
        }

        if (LOG.isTraceEnabled()) {
            // logging nextExchange as it contains the exchange that might have altered the payload and since
            // we are logging the completion if will be confusing if we log the original instead
            // we could also consider logging the original and the nextExchange then we have *before* and *after* snapshots
            LOG.trace("Processing compelete for exchangeId: " + exchange.getExchangeId() + " >>> " + nextExchange);
        }

        // copy results back to the original exchange
        ExchangeHelper.copyResults(exchange, nextExchange);
    }

    private static boolean hasExceptionBeenHandled(Exchange nextExchange) {
        return Boolean.TRUE.equals(nextExchange.getProperty(Exchange.EXCEPTION_HANDLED));
    }

    /**
     * Strategy method to create the next exchange from the previous exchange.
     * <p/>
     * Remember to copy the original exchange id otherwise correlation of ids in the log is a problem
     *
     * @param previousExchange the previous exchange
     * @return a new exchange
     */
    protected Exchange createNextExchange(Exchange previousExchange) {
        Exchange answer = previousExchange.newInstance();
        // we must use the same id as this is a snapshot strategy where Camel copies a snapshot
        // before processing the next step in the pipeline, so we have a snapshot of the exchange
        // just before. This snapshot is used if Camel should do redeliveries (re try) using
        // DeadLetterChannel. That is why it's important the id is the same, as it is the *same*
        // exchange being routed.
        answer.setExchangeId(previousExchange.getExchangeId());

        answer.getProperties().putAll(previousExchange.getProperties());

        // now lets set the input of the next exchange to the output of the
        // previous message if it is not null
        Message in = answer.getIn();
        if (previousExchange.hasOut()) {
            in.copyFrom(previousExchange.getOut());
        } else {
            in.copyFrom(previousExchange.getIn());
        }
        return answer;
    }

    protected boolean continueRouting(Iterator<Processor> it, Exchange exchange) {
        Object stop = exchange.getProperty(Exchange.ROUTE_STOP);
        if (stop != null) {
            boolean doStop = exchange.getContext().getTypeConverter().convertTo(Boolean.class, stop);
            if (doStop) {
                if (LOG.isDebugEnabled()) {
                    LOG.debug("Exchange is marked to stop routing: " + exchange);
                }
                return false;
            } else {
                return true;
            }
        } else {
            return it.hasNext();
        }
    }

    @Override
    public String toString() {
        return "Pipeline" + getProcessors();
    }

}