Spring Framework example source code file (ExpressionEvaluationUtils.java)

This example Spring Framework source code file (ExpressionEvaluationUtils.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

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apache, class, class, expressioncachekey, expressionevaluationhelper, http, jsp, jsp, jsp20expressionevaluationhelper, jspexception, jspexception, map, object, pagecontext, pagecontext, servlet, string, string, taglib, util

The Spring Framework ExpressionEvaluationUtils.java source code

/*
 * Copyright 2002-2007 the original author or authors.
 *
 * 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.
 */

package org.springframework.web.util;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletContext;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.el.ELException;
import javax.servlet.jsp.el.Expression;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager;

import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

/**
 * Convenience methods for transparent access to JSP 2.0's built-in
 * {@link javax.servlet.jsp.el.ExpressionEvaluator} or the standalone
 * {@link org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager}
 * of Jakarta's JSTL implementation.
 *
 * <p>Automatically detects JSP 2.0 or Jakarta JSTL, preferring the JSP 2.0
 * mechanism if available. Also detects the JSP 2.0 API being present but
 * the runtime JSP engine not actually supporting JSP 2.0, falling back to
 * the Jakarta JSTL in this case. Throws an exception when encountering actual
 * EL expressions if neither JSP 2.0 nor the Jakarta JSTL is available.
 *
 * <p>In the case of JSP 2.0, this class will by default use standard
 * <code>evaluate calls. If your application server happens to be
 * inefficient in that respect, consider setting Spring's "cacheJspExpressions"
 * context-param in <code>web.xml to "true", which will use
 * <code>parseExpression calls with cached Expression objects instead.
 *
 * <p>The evaluation methods check if the value contains "${" before
 * invoking the EL evaluator, treating the value as "normal" expression
 * (i.e. a literal String value) else.
 *
 * <p>Note: The evaluation methods do not have a runtime dependency on
 * JSP 2.0 or on Jakarta's JSTL implementation, as long as they are not
 * asked to process actual EL expressions. This allows for using EL-aware
 * tags with Java-based JSP expressions instead, for example.
 *
 * @author Juergen Hoeller
 * @author Alef Arendsen
 * @since 11.07.2003
 * @see javax.servlet.jsp.el.ExpressionEvaluator#evaluate
 * @see javax.servlet.jsp.el.ExpressionEvaluator#parseExpression
 * @see org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager
 */
public abstract class ExpressionEvaluationUtils {

	/**
	 * JSP 2.0 expression cache parameter at the servlet context level
	 * (i.e. a context-param in <code>web.xml): "cacheJspExpressions".
	 */
	public static final String EXPRESSION_CACHE_CONTEXT_PARAM = "cacheJspExpressions";

	public static final String EXPRESSION_PREFIX = "${";

	public static final String EXPRESSION_SUFFIX = "}";


	private static final String EXPRESSION_CACHE_FLAG_CONTEXT_ATTR =
			ExpressionEvaluationUtils.class.getName() + ".CACHE_JSP_EXPRESSIONS";

	private static final String EXPRESSION_CACHE_MAP_CONTEXT_ATTR =
			ExpressionEvaluationUtils.class.getName() + ".JSP_EXPRESSION_CACHE";

	private static final String JSP_20_CLASS_NAME =
			"javax.servlet.jsp.el.ExpressionEvaluator";

	private static final String JAKARTA_JSTL_CLASS_NAME =
			"org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager";


	private static final Log logger = LogFactory.getLog(ExpressionEvaluationUtils.class);

	private static ExpressionEvaluationHelper helper;


	static {
		ClassLoader cl = ExpressionEvaluationUtils.class.getClassLoader();
		if (ClassUtils.isPresent(JSP_20_CLASS_NAME, cl)) {
			logger.debug("Found JSP 2.0 ExpressionEvaluator");
			if (ClassUtils.isPresent(JAKARTA_JSTL_CLASS_NAME, cl)) {
				logger.debug("Found Jakarta JSTL ExpressionEvaluatorManager");
				helper = new Jsp20ExpressionEvaluationHelper(new JakartaExpressionEvaluationHelper());
			}
			else {
				helper = new Jsp20ExpressionEvaluationHelper(new NoExpressionEvaluationHelper());
			}
		}
		else if (ClassUtils.isPresent(JAKARTA_JSTL_CLASS_NAME, cl)) {
			logger.debug("Found Jakarta JSTL ExpressionEvaluatorManager");
			helper = new JakartaExpressionEvaluationHelper();
		}
		else {
			logger.debug("JSP expression evaluation not available");
			helper = new NoExpressionEvaluationHelper();
		}
	}


	/**
	 * Check if the given expression value is an EL expression.
	 * @param value the expression to check
	 * @return <code>true if the expression is an EL expression,
	 * <code>false otherwise
	 */
	public static boolean isExpressionLanguage(String value) {
		return (value != null && value.indexOf(EXPRESSION_PREFIX) != -1);
	}

	/**
	 * Evaluate the given expression (be it EL or a literal String value)
	 * to an Object of a given type,
	 * @param attrName name of the attribute (typically a JSP tag attribute)
	 * @param attrValue value of the attribute
	 * @param resultClass class that the result should have (String, Integer, Boolean)
	 * @param pageContext current JSP PageContext
	 * @return the result of the evaluation
	 * @throws JspException in case of parsing errors, also in case of type mismatch
	 * if the passed-in literal value is not an EL expression and not assignable to
	 * the result class
	 */
	public static Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
	    throws JspException {

		if (isExpressionLanguage(attrValue)) {
			return doEvaluate(attrName, attrValue, resultClass, pageContext);
		}
		else if (attrValue != null && resultClass != null && !resultClass.isInstance(attrValue)) {
			throw new JspException("Attribute value \"" + attrValue + "\" is neither a JSP EL expression nor " +
					"assignable to result class [" + resultClass.getName() + "]");
		}
		else {
			return attrValue;
		}
	}

	/**
	 * Evaluate the given expression (be it EL or a literal String value) to an Object.
	 * @param attrName name of the attribute (typically a JSP tag attribute)
	 * @param attrValue value of the attribute
	 * @param pageContext current JSP PageContext
	 * @return the result of the evaluation
	 * @throws JspException in case of parsing errors
	 */
	public static Object evaluate(String attrName, String attrValue, PageContext pageContext)
	    throws JspException {

		if (isExpressionLanguage(attrValue)) {
			return doEvaluate(attrName, attrValue, Object.class, pageContext);
		}
		else {
			return attrValue;
		}
	}

	/**
	 * Evaluate the given expression (be it EL or a literal String value) to a String.
	 * @param attrName name of the attribute (typically a JSP tag attribute)
	 * @param attrValue value of the attribute
	 * @param pageContext current JSP PageContext
	 * @return the result of the evaluation
	 * @throws JspException in case of parsing errors
	 */
	public static String evaluateString(String attrName, String attrValue, PageContext pageContext)
	    throws JspException {

		if (isExpressionLanguage(attrValue)) {
			return (String) doEvaluate(attrName, attrValue, String.class, pageContext);
		}
		else {
			return attrValue;
		}
	}

	/**
	 * Evaluate the given expression (be it EL or a literal String value) to an integer.
	 * @param attrName name of the attribute (typically a JSP tag attribute)
	 * @param attrValue value of the attribute
	 * @param pageContext current JSP PageContext
	 * @return the result of the evaluation
	 * @throws JspException in case of parsing errors
	 */
	public static int evaluateInteger(String attrName, String attrValue, PageContext pageContext)
			throws JspException {

		if (isExpressionLanguage(attrValue)) {
			return ((Integer) doEvaluate(attrName, attrValue, Integer.class, pageContext)).intValue();
		}
		else {
			return Integer.parseInt(attrValue);
		}
	}

	/**
	 * Evaluate the given expression (be it EL or a literal String value) to a boolean.
	 * @param attrName name of the attribute (typically a JSP tag attribute)
	 * @param attrValue value of the attribute
	 * @param pageContext current JSP PageContext
	 * @return the result of the evaluation
	 * @throws JspException in case of parsing errors
	 */
	public static boolean evaluateBoolean(String attrName, String attrValue, PageContext pageContext)
	    throws JspException {

		if (isExpressionLanguage(attrValue)) {
			return ((Boolean) doEvaluate(attrName, attrValue, Boolean.class, pageContext)).booleanValue();
		}
		else {
			return Boolean.valueOf(attrValue).booleanValue();
		}
	}


	/**
	 * Actually evaluate the given expression (be it EL or a literal String value)
	 * to an Object of a given type. Supports concatenated expressions,
	 * for example: "${var1}text${var2}"
	 * @param attrName name of the attribute
	 * @param attrValue value of the attribute
	 * @param resultClass class that the result should have
	 * @param pageContext current JSP PageContext
	 * @return the result of the evaluation
	 * @throws JspException in case of parsing errors
	 */
	private static Object doEvaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
	    throws JspException {

		Assert.notNull(attrValue, "Attribute value must not be null");
		Assert.notNull(resultClass, "Result class must not be null");
		Assert.notNull(pageContext, "PageContext must not be null");

		if (resultClass.isAssignableFrom(String.class)) {
			StringBuffer resultValue = null;
			int exprPrefixIndex = -1;
			int exprSuffixIndex = 0;
			do {
				exprPrefixIndex = attrValue.indexOf(EXPRESSION_PREFIX, exprSuffixIndex);
				if (exprPrefixIndex != -1) {
					int prevExprSuffixIndex = exprSuffixIndex;
					exprSuffixIndex = attrValue.indexOf(EXPRESSION_SUFFIX, exprPrefixIndex + EXPRESSION_PREFIX.length());
					String expr = null;
					if (exprSuffixIndex != -1) {
						exprSuffixIndex += EXPRESSION_SUFFIX.length();
						expr = attrValue.substring(exprPrefixIndex, exprSuffixIndex);
					}
					else {
						expr = attrValue.substring(exprPrefixIndex);
					}
					if (expr.length() == attrValue.length()) {
						// A single expression without static prefix or suffix ->
						// parse it with the specified result class rather than String.
						return helper.evaluate(attrName, attrValue, resultClass, pageContext);
					}
					else {
						// We actually need to concatenate partial expressions into a String.
						if (resultValue == null) {
							resultValue = new StringBuffer();
						}
						resultValue.append(attrValue.substring(prevExprSuffixIndex, exprPrefixIndex));
						resultValue.append(helper.evaluate(attrName, expr, String.class, pageContext));
					}
				}
				else {
					if (resultValue == null) {
						resultValue = new StringBuffer();
					}
					resultValue.append(attrValue.substring(exprSuffixIndex));
				}
			}
			while (exprPrefixIndex != -1 && exprSuffixIndex != -1);
			return resultValue.toString();
		}

		else {
			return helper.evaluate(attrName, attrValue, resultClass, pageContext);
		}
	}

	/**
	 * Determine whether JSP 2.0 expressions are supposed to be cached
	 * and return the corresponding cache Map, or <code>null if
	 * caching is not enabled.
	 * @param pageContext current JSP PageContext
	 * @return the cache Map, or <code>null if caching is disabled
	 */
	private static Map getJspExpressionCache(PageContext pageContext) {
		ServletContext servletContext = pageContext.getServletContext();
		Map cacheMap = (Map) servletContext.getAttribute(EXPRESSION_CACHE_MAP_CONTEXT_ATTR);
		if (cacheMap == null) {
			Boolean cacheFlag = (Boolean) servletContext.getAttribute(EXPRESSION_CACHE_FLAG_CONTEXT_ATTR);
			if (cacheFlag == null) {
				cacheFlag = Boolean.valueOf(servletContext.getInitParameter(EXPRESSION_CACHE_CONTEXT_PARAM));
				servletContext.setAttribute(EXPRESSION_CACHE_FLAG_CONTEXT_ATTR, cacheFlag);
			}
			if (cacheFlag.booleanValue()) {
				cacheMap = Collections.synchronizedMap(new HashMap());
				servletContext.setAttribute(EXPRESSION_CACHE_MAP_CONTEXT_ATTR, cacheMap);
			}
		}
		return cacheMap;
	}


	/**
	 * Internal interface for evaluating a JSP EL expression.
	 */
	private static interface ExpressionEvaluationHelper {

		public Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
				throws JspException;
	}


	/**
	 * Fallback ExpressionEvaluationHelper:
	 * always throws an exception in case of an actual EL expression.
	 */
	private static class NoExpressionEvaluationHelper implements ExpressionEvaluationHelper {

		public Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
				throws JspException {

			throw new JspException(
					"Neither JSP 2.0 nor Jakarta JSTL available - cannot parse JSP EL expression \"" + attrValue + "\"");
		}
	}


	/**
	 * Actual invocation of the Jakarta ExpressionEvaluatorManager.
	 * In separate inner class to avoid runtime dependency on Jakarta's
	 * JSTL implementation, for evaluation of non-EL expressions.
	 */
	private static class JakartaExpressionEvaluationHelper implements ExpressionEvaluationHelper {

		public Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
		    throws JspException {

			return ExpressionEvaluatorManager.evaluate(attrName, attrValue, resultClass, pageContext);
		}
	}


	/**
	 * Actual invocation of the JSP 2.0 ExpressionEvaluator.
	 * In separate inner class to avoid runtime dependency on JSP 2.0,
	 * for evaluation of non-EL expressions.
	 */
	private static class Jsp20ExpressionEvaluationHelper implements ExpressionEvaluationHelper {

		private final ExpressionEvaluationHelper fallback;

		private boolean fallbackNecessary = false;

		public Jsp20ExpressionEvaluationHelper(ExpressionEvaluationHelper fallback) {
			this.fallback = fallback;
		}

		public Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
				throws JspException {

			if (isFallbackNecessary()) {
				return this.fallback.evaluate(attrName, attrValue, resultClass, pageContext);
			}

			try {
				Map expressionCache = getJspExpressionCache(pageContext);
				if (expressionCache != null) {
					// We are supposed to explicitly create and cache JSP Expression objects.
					ExpressionCacheKey cacheKey = new ExpressionCacheKey(attrValue, resultClass);
					Expression expr = (Expression) expressionCache.get(cacheKey);
					if (expr == null) {
						expr = pageContext.getExpressionEvaluator().parseExpression(attrValue, resultClass, null);
						expressionCache.put(cacheKey, expr);
					}
					return expr.evaluate(pageContext.getVariableResolver());
				}
				else {
					// We're simply calling the JSP 2.0 evaluate method straight away.
					return pageContext.getExpressionEvaluator().evaluate(
							attrValue, resultClass, pageContext.getVariableResolver(), null);
				}
			}
			catch (ELException ex) {
				throw new JspException("Parsing of JSP EL expression \"" + attrValue + "\" failed", ex);
			}
			catch (LinkageError err) {
				logger.debug("JSP 2.0 ExpressionEvaluator API present but not implemented - using fallback", err);
				setFallbackNecessary();
				return this.fallback.evaluate(attrName, attrValue, resultClass, pageContext);
			}
		}

		private synchronized boolean isFallbackNecessary() {
			return this.fallbackNecessary;
		}

		private synchronized void setFallbackNecessary() {
			this.fallbackNecessary = true;
		}
	}


	/**
	 * Cache key class for JSP 2.0 Expression objects.
	 */
	private static class ExpressionCacheKey {

		private final String value;
		private final Class resultClass;
		private final int hashCode;

		public ExpressionCacheKey(String value, Class resultClass) {
			this.value = value;
			this.resultClass = resultClass;
			this.hashCode = this.value.hashCode() * 29 + this.resultClass.hashCode();
		}

		public boolean equals(Object obj) {
			if (!(obj instanceof ExpressionCacheKey)) {
				return false;
			}
			ExpressionCacheKey other = (ExpressionCacheKey) obj;
			return (this.value.equals(other.value) && this.resultClass.equals(other.resultClass));
		}

		public int hashCode() {
			return this.hashCode;
		}
	}

}