关于struts2验证框架中的fieldexpression验证，非常灵活。

例如判断输入的密码和重复输入的密码是否相同：

<field-validator type="fieldexpression">
??????????? <param name="expression">
??????????????? (user.confirmPassword.equals(user.password))
??????????? </param>

</field-validator>

?

fieldexpression方法的源代码在
com.opensymphony.xwork2.validator.validators.FieldExpressionValidator.class

?

public class FieldExpressionValidator extends FieldValidatorSupport {    private String expression;    public void setExpression(String expression) {        this.expression = expression;    }    public String getExpression() {        return expression;    }    public void validate(Object object) throws ValidationException {        String fieldName = getFieldName();        Boolean answer = Boolean.FALSE;        Object obj = null;        try {            obj = getFieldValue(expression, object);        } catch (ValidationException e) {            throw e;        } catch (Exception e) {            // let this pass, but it will be logged right below        }        if ((obj != null) && (obj instanceof Boolean)) {            answer = (Boolean) obj;        } else {            log.warn("Got result of " + obj + " when trying to get Boolean.");        }        if (!answer.booleanValue()) {            addFieldError(fieldName, object);        }    }}

?可以看出参数expression是一段代码的字符串，然后判断返回值boolean，true的话通过验证，否则不通过。