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This class provides static methods for parsing and interpreting OGNL expressions.

The simplest use of the Ognl class is to get the value of an expression from an object, without extra context or pre-parsing.

 import ognl.Ognl;
 import ognl.OgnlException;
 try {
 result = Ognl.getValue(expression, root);
 } catch (OgnlException ex) {
 // Report error or recover
 }
 

This will parse the expression given and evaluate it against the root object given, returning the result. If there is an error in the expression, such as the property is not found, the exception is encapsulated into an ognl.OgnlException OgnlException .

Other more sophisticated uses of Ognl can pre-parse expressions. This provides two advantages: in the case of user-supplied expressions it allows you to catch parse errors before evaluation and it allows you to cache parsed expressions into an AST for better speed during repeated use. The pre-parsed expression is always returned as an Object to simplify use for programs that just wish to store the value for repeated use and do not care that it is an AST. If it does care it can always safely cast the value to an AST type.

The Ognl class also takes a context map as one of the parameters to the set and get methods. This allows you to put your own variables into the available namespace for OGNL expressions. The default context contains only the #root and #context keys, which are required to be present. The addDefaultContext(Object, Map) method will alter an existing Map to put the defaults in. Here is an example that shows how to extract the documentName property out of the root object and append a string with the current user name in parens:

 private Map	context = new HashMap();
 public void setUserName(String value)
 {
 context.put("userName", value);
 }
 try {
 // get value using our own custom context map
 result = Ognl.getValue("documentName + \" (\" + ((#userName == null) ? \"<nobody>\" : #userName) + \")\"", context, root);
 } catch (OgnlException ex) {
 // Report error or recover
 }