com.amazonaws.services.kinesis.multilang.GetNextMessageTask Maven / Gradle / Ivy

/*
 * Copyright 2014 Amazon.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Amazon Software License (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 * http://aws.amazon.com/asl/
 *
 * or in the "license" file accompanying this file. This file 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 com.amazonaws.services.kinesis.multilang;

import java.io.BufferedReader;
import java.io.IOException;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import com.amazonaws.services.kinesis.multilang.messages.Message;

import com.fasterxml.jackson.databind.ObjectMapper;

/**
 * Gets the next message off the STDOUT of the child process. Throws an exception if a message is not found before the
 * end of the input stream is reached.
 */
class GetNextMessageTask extends LineReaderTask {

    private static final Log LOG = LogFactory.getLog(GetNextMessageTask.class);

    private ObjectMapper objectMapper;

    private static final String EMPTY_LINE = "";

    /**
     * Constructor.
     * 
     * @param objectMapper An object mapper for decoding json messages from the input stream.
     */
    GetNextMessageTask(ObjectMapper objectMapper) {
        this.objectMapper = objectMapper;
    }

    /**
     * Checks if a line is an empty line.
     * 
     * @param line A string
     * @return True if the line is an empty string, i.e. "", false otherwise.
     */
    static boolean isEmptyLine(String line) {
        return EMPTY_LINE.equals(line);
    }

    @Override
    protected HandleLineResult handleLine(String line) {
        try {
            /*
             * If the line is an empty line we don't bother logging anything because we expect the child process to be
             * nesting its messages between new lines, e.g. "\n\n". If there are no other entities writing
             * to the child process's STDOUT then this behavior will result in patterns like
             * "...\n\n\n\n..." which contains empty lines.
             */
            if (!isEmptyLine(line)) {
                return new HandleLineResult(objectMapper.readValue(line, Message.class));
            }
        } catch (IOException e) {
            LOG.info("Skipping unexpected line on STDOUT for shard " + getShardId() + ": " + line);
        }
        return new HandleLineResult();
    }

    @Override
    protected Message returnAfterException(Exception e) {
        throw new RuntimeException("Encountered an error while reading a line from STDIN for shard " + getShardId()
                + " so won't be able to return a message.", e);
    }

    @Override
    protected Message returnAfterEndOfInput() {
        throw new RuntimeException("Reached end of STDIN of child process for shard " + getShardId()
                + " so won't be able to return a message.");
    }

    public LineReaderTask initialize(BufferedReader reader, String shardId) {
        return initialize(reader, shardId, "Reading next message from STDIN for " + shardId);
    }
}