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/*
 * 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.clerezza.sparql;

import org.apache.clerezza.sparql.query.Query;
import org.osgi.service.component.annotations.Component;

import java.io.StringReader;

/**
 * This class implements an OSGi service to provide a method to parse a
 * SPARQL query and generate a {@link Query} object.
 *
 * @author hasan
 */

@Component(service = QueryParser.class)
public class QueryParser {

    private static volatile QueryParser instance;
    public QueryParser() {
        QueryParser.instance = this;
    }

    /**
     * Returns an instance of this class.
     * This method is provided due to backward compatibility.
     */
    public static QueryParser getInstance() {
        if (instance == null) {
            synchronized (QueryParser.class) {
                if (instance == null) {
                    new QueryParser();
                }
            }
        }
        return instance;
    }

    /**
     * Parses a SPARQL query string into a {@link Query} object.
     *
     * @param queryString
     *        SPARQL query string
     * @return
     *        {@link Query} object corresponding to the specified query string
     *
     * @throws org.apache.clerezza.sparql.ParseException
     */
    public Query parse(final String queryString) throws ParseException {
        JavaCCGeneratedQueryParser parser = new JavaCCGeneratedQueryParser(
                new StringReader(queryString));
        return parser.parse();
    }
}