com.px100systems.util.RuleEngine.java Source code

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Here is the source code for com.px100systems.util.RuleEngine.java

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/*
 * This file is part of Px100 Data.
 *
 * Px100 Data is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
    
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see http://www.gnu.org/licenses/
 */
package com.px100systems.util;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import org.springframework.beans.factory.annotation.Required;
import org.springframework.expression.Expression;
import org.springframework.expression.spel.standard.SpelExpressionParser;

/**
 * Generic if/then rule.
 * 
 * @version 0.3 <br>Copyright (c) 2015 Px100 Systems. All Rights Reserved.<br>
 * @author Alex Rogachevsky
 */
public class RuleEngine {
    private List<Expression> rules; // returns first not-null result

    public static class Context {
        private Map<String, Object> args;

        public Context(Map<String, Object> args) {
            this.args = args;
        }

        public Map<String, Object> getArgs() {
            return args;
        }
    }

    /**
     * Set the list of rules. teh first one returning non-null, wins.
     * @param rules a list of Spring EL expressions typically "condition ? result : laternateResult" ones
     */
    @Required
    public void setRules(List<String> rules) {
        this.rules = new ArrayList<Expression>();

        SpelExpressionParser parser = new SpelExpressionParser();
        for (String s : rules)
            this.rules.add(parser.parseExpression(s));
    }

    /**
     * Evaluate the rules on the given args
     * @param args arguments, accessed normally as "xxx" in expressions
     * @param defaultValue default value if all expressions returned null
     * @return the first not null expression result or defaultValue
     */
    public Object eval(Map<String, Object> args, Object defaultValue) {
        SpringELCtx ctx = new SpringELCtx(new Context(args));

        for (Expression rule : rules) {
            Object result = rule.getValue(ctx);
            if (result != null)
                return result;
        }

        return defaultValue;
    }
}