1. 解析文本中${…} 占位符，替换它们用对应的属性值。
package org.springframework.util;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
/**
* Helper class for resolving placeholders in texts. Usually applied to file paths.
*
* <p>A text may contain <code>${…}</code> placeholders, to be resolved as
* system properties: e.g. <code>${user.dir}</code>.
*
* @author Juergen Hoeller
* @since 1.2.5
* @see #PLACEHOLDER_PREFIX
* @see #PLACEHOLDER_SUFFIX
* @see System#getProperty(String)
*/
public abstract class SystemPropertyUtils {
/** Prefix for system property placeholders: "${" */
public static final String PLACEHOLDER_PREFIX = "${";
/** Suffix for system property placeholders: "}" */
public static final String PLACEHOLDER_SUFFIX = "}";
private static final Log logger = LogFactory.getLog(SystemPropertyUtils.class);
/**
* Resolve ${…} placeholders in the given text,
* replacing them with corresponding system property values.
* @param text the String to resolve
* @return the resolved String
* @see #PLACEHOLDER_PREFIX
* @see #PLACEHOLDER_SUFFIX
*/
public static String resolvePlaceholders(String text) {
StringBuffer buf = new StringBuffer(text);
// The following code does not use JDK 1.4′s StringBuffer.indexOf(String)
// method to retain JDK 1.3 compatibility. The slight loss in performance
// is not really relevant, as this code will typically just run on startup.
int startIndex = text.indexOf(PLACEHOLDER_PREFIX);
while (startIndex != -1) {
int endIndex = buf.toString().indexOf(PLACEHOLDER_SUFFIX, startIndex + PLACEHOLDER_PREFIX.length());
if (endIndex != -1) {
String placeholder = buf.substring(startIndex + PLACEHOLDER_PREFIX.length(), endIndex);
int nextIndex = endIndex + PLACEHOLDER_SUFFIX.length();
try {
String propVal = System.getProperty(placeholder);
if (propVal == null) {
// Fall back to searching the system environment.
propVal = System.getenv(placeholder);
}
if (propVal != null) {
buf.replace(startIndex, endIndex + PLACEHOLDER_SUFFIX.length(), propVal);
nextIndex = startIndex + propVal.length();
}
else {
if (logger.isWarnEnabled()) {
logger.warn("Could not resolve placeholder ‘" + placeholder + "’ in [" + text +
"] as system property: neither system property nor environment variable found");
}
}
}
catch (Throwable ex) {
if (logger.isWarnEnabled()) {
logger.warn("Could not resolve placeholder ‘" + placeholder + "’ in [" + text +
"] as system property: " + ex);
}
}
startIndex = buf.toString().indexOf(PLACEHOLDER_PREFIX, nextIndex);
}
else {
startIndex = -1;
}
}
return buf.toString();
}
}
2. 使用JSP 2.0 ExpressionEvaluator 或者Jakarta’s JSTL实现的ExpressionEvaluatorManager
package org.springframework.web.util;
import javax.servlet.jsp.JspException;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.el.ELException;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager;
/**
* Convenience methods for easy access to the JSP 2.0 ExpressionEvaluator or
* the ExpressionEvaluatorManager of Jakarta’s JSTL implementation.
*
* <p>Automatically detects JSP 2.0 or Jakarta JSTL; falls back to throwing
* an exception on actual EL expressions if none of the two is available.
*
* <p>The evaluation methods check if the value contains "${"
* before invoking the EL evaluator, treating the value as "normal"
* expression (that is, a conventional String) 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 don’t
* receive actual EL expressions.
*
* @author Juergen Hoeller
* @author Alef Arendsen
* @since 11.07.2003
* @see javax.servlet.jsp.el.ExpressionEvaluator
* @see org.apache.taglibs.standard.lang.support.ExpressionEvaluatorManager
*/
public abstract class ExpressionEvaluationUtils {
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 {
try {
Class.forName(JSP_20_CLASS_NAME);
// JSP 2.0 available
helper = new Jsp20ExpressionEvaluationHelper();
logger.info("Using JSP 2.0 ExpressionEvaluator");
}
catch (ClassNotFoundException ex) {
// JSP 2.0 not available -> try Jakarta JSTL
try {
Class.forName(JAKARTA_JSTL_CLASS_NAME);
// JSP 2.0 available
helper = new JakartaExpressionEvaluationHelper();
logger.info("Using Jakarta JSTL ExpressionEvaluatorManager");
}
catch (ClassNotFoundException ex2) {
// neither JSP 2.0 nor Jakarta JSTL available -> no EL support
helper = new NoExpressionEvaluationHelper();
logger.info("JSP expression evaluation not available");
}
}
}
/**
* Check if the given expression value is an EL expression.
* @param value the expression to check
* @return <code>true</code> if the expression is an EL expression,
* <code>false</code> otherwise
*/
public static boolean isExpressionLanguage(String value) {
return (value != null && value.indexOf("${") != -1);
}
/**
* Evaluate the given expression to an Object, be it EL or a conventional String.
* @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
*/
public static Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
throws JspException {
if (isExpressionLanguage(attrValue)) {
return helper.evaluate(attrName, attrValue, resultClass, pageContext);
}
else {
return attrValue;
}
}
/**
* Evaluate the given expression to a String, be it EL or a conventional 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) helper.evaluate(attrName, attrValue, String.class, pageContext);
}
else {
return attrValue;
}
}
/**
* Evaluate the given expression to an integer, be it EL or a conventional 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 int evaluateInteger(String attrName, String attrValue, PageContext pageContext)
throws JspException {
if (isExpressionLanguage(attrValue)) {
return ((Integer) helper.evaluate(attrName, attrValue, Integer.class, pageContext)).intValue();
}
else {
return Integer.parseInt(attrValue);
}
}
/**
* Evaluate the given expression to a boolean, be it EL or a conventional 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 boolean evaluateBoolean(String attrName, String attrValue, PageContext pageContext)
throws JspException {
if (isExpressionLanguage(attrValue)) {
return ((Boolean) helper.evaluate(attrName, attrValue, Boolean.class, pageContext)).booleanValue();
}
else {
return Boolean.valueOf(attrValue).booleanValue();
}
}
/**
* 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;
}
/**
* 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 {
public Object evaluate(String attrName, String attrValue, Class resultClass, PageContext pageContext)
throws JspException {
try {
return pageContext.getExpressionEvaluator().evaluate(
attrValue, resultClass, pageContext.getVariableResolver(), null);
}
catch (ELException ex) {
throw new JspException("Parsing of JSP EL expression \"" + attrValue + "\" failed", ex);
}
}
}
/**
* 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);
}
}
/**
* 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 + "\"");
}
}
}
