javadoc

spring-webflow/src/main/java/org/springframework/webflow/core/DefaultExpressionParserFactory.java

@@ -21,12 +21,10 @@ import org.springframework.binding.expression.ParserException;
 import org.springframework.binding.expression.SettableExpression;
 
 /**
- * Static helper factory that creates instances of the default expression parser
- * used by Spring Web Flow when requested. Marked final with a private
- * constructor to prevent subclassing.
+ * Static helper factory that creates instances of the default expression parser used by Spring Web Flow when requested.
+ * Marked final with a private constructor to prevent subclassing.
  * <p>
- * The default is an OGNL based expression parser. Also asserts that OGNL is in
- * the classpath when this class is loaded.
+ * The default is an OGNL based expression parser. Also asserts that OGNL is in the classpath the first time the parser is used.
  * 
  * @author Keith Donald
  * @author Erwin Vervaet
@@ -43,28 +41,29 @@ public final class DefaultExpressionParserFactory {
 	}
 
 	/**
-	 * Returns the default expression parser. The returned expression parser is
-	 * a thread-safe object.
+	 * Returns the default expression parser. The returned expression parser is a thread-safe object.
 	 * @return the expression parser
 	 */
 	public static synchronized ExpressionParser getExpressionParser() {
 		// return a wrapper that will lazily load the default expression parser
+		// this prevents the default OGNL-based parser from being intialized until it is actually used
+		// which allows OGNL to be an optional dependency if the expression parser wrapper is replaced and never used
 		return new ExpressionParser() {
 			public boolean isDelimitedExpression(String expressionString) {
 				return getDefaultExpressionParser().isDelimitedExpression(expressionString);
 			}
-			
+
 			public Expression parseExpression(String expressionString) throws ParserException {
 				return getDefaultExpressionParser().parseExpression(expressionString);
 			}
-			
-			public SettableExpression parseSettableExpression(String expressionString)
-					throws ParserException, UnsupportedOperationException {
+
+			public SettableExpression parseSettableExpression(String expressionString) throws ParserException,
+					UnsupportedOperationException {
 				return getDefaultExpressionParser().parseSettableExpression(expressionString);
 			}
 		};
 	}
-	
+
 	/**
 	 * Returns the default expression parser, creating it if necessary.
 	 * @return the default expression parser
@@ -88,14 +87,14 @@ public final class DefaultExpressionParserFactory {
 		catch (ClassNotFoundException e) {
 			throw new IllegalStateException(
 					"Unable to load the default expression parser: OGNL could not be found in the classpath.  "
-					+ "Please add OGNL 2.x to your classpath or set the default ExpressionParser instance to something that is in the classpath.  "
-					+ "Details: " + e.getMessage());
+							+ "Please add OGNL 2.x to your classpath or set the default ExpressionParser instance to something that is in the classpath.  "
+							+ "Details: " + e.getMessage());
 		}
 		catch (NoClassDefFoundError e) {
 			throw new IllegalStateException(
 					"Unable to construct the default expression parser: ognl.Ognl could not be instantiated.  "
-					+ "Please add OGNL 2.x to your classpath or set the default ExpressionParser instance to something that is in the classpath.  "
-					+ "Details: " + e);
+							+ "Please add OGNL 2.x to your classpath or set the default ExpressionParser instance to something that is in the classpath.  "
+							+ "Details: " + e);
 		}
 	}
 }