src/main/java/org/apache/log4j/rule/ExpressionRule.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.log4j.rule;

import java.util.Map;
import java.util.Stack;
import org.apache.log4j.chainsaw.logevents.ChainsawLoggingEvent;

/**
 * A Rule class supporting both infix and postfix expressions,
 * accepting any rule which
 * is supported by the <code>RuleFactory</code>.
 *
 * NOTE: parsing is supported through the use of
 * <code>StringTokenizer</code>, which
 * implies two limitations:
 * 1: all tokens in the expression must be separated by spaces,
 * including parenthesis
 * 2: operands which contain spaces MUST be wrapped in single quotes.
 *    For example, the expression:
 *      msg == 'some msg'
 *    is a valid expression.
 * 3: To group expressions, use parentheses.
 *    For example, the expression:
 *      level >= INFO || ( msg == 'some msg' || logger == 'test' )
 *    is a valid expression.
 * See org.apache.log4j.rule.InFixToPostFix for a
 * description of supported operators.
 * See org.apache.log4j.spi.LoggingEventFieldResolver for field keywords.
 *
 * @author Scott Deboy (sdeboy@apache.org)
 */
public class ExpressionRule extends AbstractRule {
    /**
     * Serialization ID.
     */
    static final long serialVersionUID = 5809121703146893729L;
    /**
     * Converter.
     */
    private static final InFixToPostFix CONVERTER = new InFixToPostFix();
    /**
     * Compiler.
     */
    private static final PostFixExpressionCompiler COMPILER = new PostFixExpressionCompiler();
    /**
     * Rule.
     */
    private final Rule rule;

    /**
     * Create new instance.
     * @param r rule
     */
    private ExpressionRule(final Rule r) {
        super();
        this.rule = r;
    }

    /**
     * Get rule.
     * @param expression expression.
     * @return rule.
     */
    public static Rule getRule(final String expression) {
        return getRule(expression, false);
    }

    /**
     * Get rule.
     * @param expression expression.
     * @param isPostFix If post-fix.
     * @return rule
     */
    public static Rule getRule(final String expression, final boolean isPostFix) {
        String postFix = expression;
        if (!isPostFix) {
            postFix = CONVERTER.convert(expression);
        }

        return new ExpressionRule(COMPILER.compileExpression(postFix));
    }

    /**
     * {@inheritDoc}
     */
    public boolean evaluate(final ChainsawLoggingEvent event, Map matches) {
        return rule.evaluate(event, matches);
    }

    /**
     * {@inheritDoc}
     */
    public String toString() {
        return rule.toString();
    }

    /**
     * Evaluate a boolean postfix expression.
     *
     */
    static final class PostFixExpressionCompiler {
        /**
         * Compile expression.
         * @param expression expression.
         * @return rule.
         */
        public Rule compileExpression(final String expression) {
            RuleFactory factory = RuleFactory.getInstance();

            Stack stack = new Stack();
            InFixToPostFix.CustomTokenizer tokenizer = new InFixToPostFix.CustomTokenizer(expression);

            while (tokenizer.hasMoreTokens()) {
                // examine each token
                String token = tokenizer.nextToken();
                if (token.startsWith("'") || token.startsWith("\"")) {
                    String quoteChar = token.substring(0, 1);
                    token = token.substring(1);
                    while (!token.endsWith(quoteChar) && tokenizer.hasMoreTokens()) {
                        token = token + " " + tokenizer.nextToken();
                    }
                    if (token.length() > 0) {
                        token = token.substring(0, token.length() - 1);
                    }
                } else {
                    // if a symbol is found, pop 2 off the stack,
                    // evaluate and push the result
                    if (factory.isRule(token)) {
                        Rule r = factory.getRule(token, stack);
                        stack.push(r);
                        // null out the token so we don't try to push it below
                        token = null;
                    }
                }
                // variables or constants are pushed onto the stack
                if (token != null && token.length() > 0) {
                    stack.push(token);
                }
            }

            if ((stack.size() == 1) && (!(stack.peek() instanceof Rule))) {
                // while this may be an attempt at creating an expression,
                // for ease of use, convert this single entry to a partial-text
                // match on the MSG field
                Object o = stack.pop();
                stack.push("MSG");
                stack.push(o);
                return factory.getRule("~=", stack);
            }

            // stack should contain a single rule if the expression is valid
            if ((stack.size() != 1) || (!(stack.peek() instanceof Rule))) {
                throw new IllegalArgumentException("invalid expression: " + expression);
            } else {
                return (Rule) stack.pop();
            }
        }
    }
}