RegexBasedInterpolator.java

package org.codehaus.plexus.interpolation;

/*
 * Copyright 2001-2008 Codehaus Foundation.
 *
 * Licensed 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.
 */

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

import org.codehaus.plexus.interpolation.util.StringUtils;

/**
 * Expansion of the original RegexBasedInterpolator, found in plexus-utils, this
 * interpolator provides options for setting custom prefix/suffix regex parts,
 * and includes a {@link RecursionInterceptor} parameter in its interpolate(..)
 * call, to allow the detection of cyclical expression references.
 *
 */
public class RegexBasedInterpolator implements Interpolator {

    private String startRegex;

    private String endRegex;

    private Map existingAnswers = new HashMap();

    private List<ValueSource> valueSources = new ArrayList<ValueSource>();

    private List<InterpolationPostProcessor> postProcessors = new ArrayList<InterpolationPostProcessor>();

    private boolean reusePatterns = false;

    private boolean cacheAnswers = false;

    public static final String DEFAULT_REGEXP = "\\$\\{(.+?)\\}";

    /**
     * the key is the regex the value is the Pattern
     * At the class construction time the Map will contains the default Pattern
     */
    private Map<String, Pattern> compiledPatterns = new WeakHashMap<String, Pattern>();

    /**
     * Setup a basic interpolator.
     * <p><b>NOTE:</b> You will have to call</p>
     * {@link RegexBasedInterpolator#addValueSource(ValueSource)} at least once
     * if you use this constructor!
     */
    public RegexBasedInterpolator() {
        compiledPatterns.put(DEFAULT_REGEXP, Pattern.compile(DEFAULT_REGEXP));
    }

    /**
     *
     * @param reusePatterns already compiled patterns will be reused
     */
    public RegexBasedInterpolator(boolean reusePatterns) {
        this();
        this.reusePatterns = reusePatterns;
    }

    /**
     * Setup an interpolator with no value sources, and the specified regex pattern
     * prefix and suffix in place of the default one.
     * <p><b>NOTE:</b> You will have to call
     * {@link RegexBasedInterpolator#addValueSource(ValueSource)} at least once
     * if you use this constructor!</p>
     *
     * @param startRegex start of the regular expression to use
     * @param endRegex end of the regular expression to use
     */
    public RegexBasedInterpolator(String startRegex, String endRegex) {
        this();
        this.startRegex = startRegex;
        this.endRegex = endRegex;
    }

    /**
     * Setup a basic interpolator with the specified list of value sources.
     *
     * @param valueSources The list of value sources to use
     */
    public RegexBasedInterpolator(List valueSources) {
        this();
        this.valueSources.addAll(valueSources);
    }

    /**
     * Setup an interpolator with the specified value sources, and the specified
     * regex pattern prefix and suffix in place of the default one.
     *
     * @param startRegex start of the regular expression to use
     * @param endRegex end of the regular expression to use
     * @param valueSources The list of value sources to use
     */
    public RegexBasedInterpolator(String startRegex, String endRegex, List valueSources) {
        this();
        this.startRegex = startRegex;
        this.endRegex = endRegex;
        this.valueSources.addAll(valueSources);
    }

    /**
     * {@inheritDoc}
     */
    public void addValueSource(ValueSource valueSource) {
        valueSources.add(valueSource);
    }

    /**
     * {@inheritDoc}
     */
    public void removeValuesSource(ValueSource valueSource) {
        valueSources.remove(valueSource);
    }

    /**
     * {@inheritDoc}
     */
    public void addPostProcessor(InterpolationPostProcessor postProcessor) {
        postProcessors.add(postProcessor);
    }

    /**
     * {@inheritDoc}
     */
    public void removePostProcessor(InterpolationPostProcessor postProcessor) {
        postProcessors.remove(postProcessor);
    }

    /**
     * Attempt to resolve all expressions in the given input string, using the
     * given pattern to first trim an optional prefix from each expression. The
     * supplied recursion interceptor will provide protection from expression
     * cycles, ensuring that the input can be resolved or an exception is
     * thrown.
     *
     * @param input The input string to interpolate
     *
     * @param thisPrefixPattern An optional pattern that should be trimmed from
     *                          the start of any expressions found in the input.
     *
     * @param recursionInterceptor Used to protect the interpolation process
     *                             from expression cycles, and throw an
     *                             exception if one is detected.
     */
    public String interpolate(String input, String thisPrefixPattern, RecursionInterceptor recursionInterceptor)
            throws InterpolationException {
        if (input == null) {
            // return empty String to prevent NPE too
            return "";
        }
        if (recursionInterceptor == null) {
            recursionInterceptor = new SimpleRecursionInterceptor();
        }

        if (thisPrefixPattern != null && thisPrefixPattern.length() == 0) {
            thisPrefixPattern = null;
        }

        int realExprGroup = 2;
        Pattern expressionPattern;
        final String expressionDelimiterStart;
        final String expressionDelimiterEnd;
        if (startRegex != null || endRegex != null) {
            if (thisPrefixPattern == null) {
                expressionPattern = getPattern(startRegex + endRegex);
                realExprGroup = 1;
            } else {
                expressionPattern = getPattern(startRegex + thisPrefixPattern + endRegex);
            }
            expressionDelimiterStart = startRegex;
            expressionDelimiterEnd = endRegex;

        } else {
            expressionDelimiterStart = "${";
            expressionDelimiterEnd = "}";
            if (thisPrefixPattern != null) {
                expressionPattern = getPattern("\\$\\{(" + thisPrefixPattern + ")?(.+?)\\}");
            } else {
                expressionPattern = getPattern(DEFAULT_REGEXP);
                realExprGroup = 1;
            }
        }
        try {
            return interpolate(
                    input,
                    recursionInterceptor,
                    expressionPattern,
                    expressionDelimiterStart,
                    expressionDelimiterEnd,
                    realExprGroup);
        } finally {
            if (!cacheAnswers) {
                clearAnswers();
            }
        }
    }

    private Pattern getPattern(String regExp) {
        if (!reusePatterns) {
            return Pattern.compile(regExp);
        }

        Pattern pattern;
        synchronized (this) {
            pattern = compiledPatterns.get(regExp);

            if (pattern != null) {
                return pattern;
            }

            pattern = Pattern.compile(regExp);
            compiledPatterns.put(regExp, pattern);
        }

        return pattern;
    }

    /**
     * Entry point for recursive resolution of an expression and all of its
     * nested expressions.
     *
     * @todo Ensure unresolvable expressions don't trigger infinite recursion.
     */
    private String interpolate(
            String input,
            RecursionInterceptor recursionInterceptor,
            Pattern expressionPattern,
            String expressionDelimiterStart,
            String expressionDelimiterEnd,
            int realExprGroup)
            throws InterpolationException {
        if (input == null) {
            // return empty String to prevent NPE too
            return "";
        }
        String result = input;

        Matcher matcher = expressionPattern.matcher(result);

        while (matcher.find()) {
            String wholeExpr = matcher.group(0);
            String realExpr = matcher.group(realExprGroup);

            if (realExpr.startsWith(".")) {
                realExpr = realExpr.substring(1);
            }

            if (recursionInterceptor.hasRecursiveExpression(realExpr)) {
                throw new InterpolationCycleException(recursionInterceptor, realExpr, wholeExpr);
            }

            recursionInterceptor.expressionResolutionStarted(realExpr);
            try {
                Object value = existingAnswers.get(realExpr);
                for (ValueSource vs : valueSources) {
                    if (value != null) break;

                    value = vs.getValue(realExpr, expressionDelimiterStart, expressionDelimiterEnd);
                }

                if (value != null) {
                    value = interpolate(
                            String.valueOf(value),
                            recursionInterceptor,
                            expressionPattern,
                            expressionDelimiterStart,
                            expressionDelimiterEnd,
                            realExprGroup);

                    if (postProcessors != null && !postProcessors.isEmpty()) {
                        for (InterpolationPostProcessor postProcessor : postProcessors) {
                            Object newVal = postProcessor.execute(realExpr, value);
                            if (newVal != null) {
                                value = newVal;
                                break;
                            }
                        }
                    }

                    // could use:
                    // result = matcher.replaceFirst( stringValue );
                    // but this could result in multiple lookups of stringValue, and replaceAll is not correct behaviour
                    result = StringUtils.replace(result, wholeExpr, String.valueOf(value));

                    matcher.reset(result);
                }
            } finally {
                recursionInterceptor.expressionResolutionFinished(realExpr);
            }
        }

        return result;
    }

    /**
     * Return any feedback messages and errors that were generated - but
     * suppressed - during the interpolation process. Since unresolvable
     * expressions will be left in the source string as-is, this feedback is
     * optional, and will only be useful for debugging interpolation problems.
     *
     * @return a {@link List} that may be interspersed with {@link String} and
     * {@link Throwable} instances.
     */
    public List getFeedback() {
        List messages = new ArrayList();
        for (Object valueSource : valueSources) {
            ValueSource vs = (ValueSource) valueSource;
            List feedback = vs.getFeedback();
            if (feedback != null && !feedback.isEmpty()) {
                messages.addAll(feedback);
            }
        }

        return messages;
    }

    /**
     * Clear the feedback messages from previous interpolate(..) calls.
     */
    public void clearFeedback() {
        for (Object valueSource : valueSources) {
            ValueSource vs = (ValueSource) valueSource;
            vs.clearFeedback();
        }
    }

    /**
     * See {@link RegexBasedInterpolator#interpolate(String, String, RecursionInterceptor)}.
     * <p>
     * This method triggers the use of a {@link SimpleRecursionInterceptor}
     * instance for protection against expression cycles.</p>
     *
     * @param input The input string to interpolate
     *
     * @param thisPrefixPattern An optional pattern that should be trimmed from
     *                          the start of any expressions found in the input.
     */
    public String interpolate(String input, String thisPrefixPattern) throws InterpolationException {
        return interpolate(input, thisPrefixPattern, null);
    }

    /**
     * See {@link RegexBasedInterpolator#interpolate(String, String, RecursionInterceptor)}.
     * <p>
     * This method triggers the use of a {@link SimpleRecursionInterceptor}
     * instance for protection against expression cycles. It also leaves empty the
     * expression prefix which would otherwise be trimmed from expressions. The
     * result is that any detected expression will be resolved as-is.</p>
     *
     * @param input The input string to interpolate
     */
    public String interpolate(String input) throws InterpolationException {
        return interpolate(input, null, null);
    }

    /**
     * See {@link RegexBasedInterpolator#interpolate(String, String, RecursionInterceptor)}.
     * <p>
     * This method leaves empty the expression prefix which would otherwise be
     * trimmed from expressions. The result is that any detected expression will
     * be resolved as-is.</p>
     *
     * @param input The input string to interpolate
     *
     * @param recursionInterceptor Used to protect the interpolation process
     *                             from expression cycles, and throw an
     *                             exception if one is detected.
     */
    public String interpolate(String input, RecursionInterceptor recursionInterceptor) throws InterpolationException {
        return interpolate(input, null, recursionInterceptor);
    }

    public boolean isReusePatterns() {
        return reusePatterns;
    }

    public void setReusePatterns(boolean reusePatterns) {
        this.reusePatterns = reusePatterns;
    }

    public boolean isCacheAnswers() {
        return cacheAnswers;
    }

    public void setCacheAnswers(boolean cacheAnswers) {
        this.cacheAnswers = cacheAnswers;
    }

    public void clearAnswers() {
        existingAnswers.clear();
    }
}