在Struts中对用户输入信息的校验一般在FromBean中进行（除非需要访问数据库进行诸如登录信息的校验，因为这是Action的工作），本文将阐述如何在Struts中实现可配置的信息校验。
一、在FormBean中手工实现
最简单的方法是直接在FormBean中重写ActionForm类的validate方法，validate方法签名如下：
public ActionErrors validate(ActionMapping mapping, HttpServletRequest req)
比如需要校验age字段必须填写数字：
1public ActionErrors validate(ActionMapping mapping, HttpServletRequest req){
2 ActionErrors errors = new ActionErrors();
3
4 String age = this.getAge();
5 if(!this.isNumber(age)){ // isNumber() is not implemented
6 errors.add( , );
7 }
8
9 return errors;
10}
在form提交后，容器会调用validate方法对表单数据进行校验，如果返回的ActionErrors为空（即校验通过），则将FormBean提交Action，否则重定向到提交form的页面。
这种方法实现简单，直观，容易测试、调试，但不可避免地存在以下缺点：
1、很难重用，导致重复开发
有很多校验逻辑在整个网站中是相同的，比如上述的数字校验，还有email校验、长度校验等等，而通过覆盖validate方法很难对这些校验过程进行重用，除非定义一些helper类封装校验方法（比如上述的isNumber()）。而当需要为另一个FormBean加入相同的校验逻辑时必须重复地覆盖validate方法
2、难于扩展
当要对一个表单增、删、改校验逻辑时必须修改validate方法，重新打包、部署
3、不可配置
因为校验逻辑硬编码于class文件中，运行时不可能做到灵活地配置校验逻辑
因此，Struts中加入了另一种更灵活的校验机制：
二、使用Validator
Validator提供了一种基于xml配置文件的校验模型，要使用这一模型必须做如下实现：
1、FormBean继承org.apache.struts.validator.ValidatorForm而不是ActionForm
2、不覆盖validate方法
3、创建validator-rules.xml及validation.xml文件
validator-rules.xml定义了可用来配置的校验逻辑，如：
1<form-validation>
2 <global>
3 <validator name="required"
4 classname="org.apache.struts.validator.FieldChecks"
5 method="validateRequired"
6 methodParams="java.lang.Object,
7 org.apache.commons.validator.ValidatorAction,
8 org.apache.commons.validator.Field,
9 org.apache.struts.action.ActionMessages,
10 org.apache.commons.validator.Validator,
11 javax.servlet.http.HttpServletRequest"
12 msg="errors.required"/>
13 <validator name="requiredif"
14 classname="org.apache.struts.validator.FieldChecks"
15 method="validateRequiredIf"
16 methodParams="java.lang.Object,
17 org.apache.commons.validator.ValidatorAction,
18 org.apache.commons.validator.Field,
19 org.apache.struts.action.ActionMessages,
20 org.apache.commons.validator.Validator,
21 javax.servlet.http.HttpServletRequest"
22 msg="errors.required"/>
23 <validator name="validwhen"
24 msg="errors.required"
25 classname="org.apache.struts.validator.validwhen.ValidWhen"
26 method="validateValidWhen"
27 methodParams="java.lang.Object,
28 org.apache.commons.validator.ValidatorAction,
29 org.apache.commons.validator.Field,
30 org.apache.struts.action.ActionMessages,
31 org.apache.commons.validator.Validator,
32 javax.servlet.http.HttpServletRequest"/>
33 <validator name="minlength"
34 classname="org.apache.struts.validator.FieldChecks"
35 method="validateMinLength"
36 methodParams="java.lang.Object,
37 org.apache.commons.validator.ValidatorAction,
38 org.apache.commons.validator.Field,
39 org.apache.struts.action.ActionMessages,
40 org.apache.commons.validator.Validator,
41 javax.servlet.http.HttpServletRequest"
42 depends=""
43 msg="errors.minlength"
44 jsFunction="org.apache.commons.validator.javascript.validateMinLength"/>
45 <validator name="maxlength"
46 classname="org.apache.struts.validator.FieldChecks"
47 method="validateMaxLength"
48 methodParams="java.lang.Object,
49 org.apache.commons.validator.ValidatorAction,
50 org.apache.commons.validator.Field,
51 org.apache.struts.action.ActionMessages,
52 org.apache.commons.validator.Validator,
53 javax.servlet.http.HttpServletRequest"
54 depends=""
55 msg="errors.maxlength"
56 jsFunction="org.apache.commons.validator.javascript.validateMaxLength"/>
57 <validator name="mask"
58 classname="org.apache.struts.validator.FieldChecks"
59 method="validateMask"
60 methodParams="java.lang.Object,
61 org.apache.commons.validator.ValidatorAction,
62 org.apache.commons.validator.Field,
63 org.apache.struts.action.ActionMessages,
64 org.apache.commons.validator.Validator,
65 javax.servlet.http.HttpServletRequest"
66 depends=""
67 msg="errors.invalid"/>
68 <validator name="byte"
69 classname="org.apache.struts.validator.FieldChecks"
70 method="validateByte"
71 methodParams="java.lang.Object,
72 org.apache.commons.validator.ValidatorAction,
73 org.apache.commons.validator.Field,
74 org.apache.struts.action.ActionMessages,
75 org.apache.commons.validator.Validator,
76 javax.servlet.http.HttpServletRequest"
77 depends=""
78 msg="errors.byte"
79 jsFunctionName="ByteValidations"/>
80 <validator name="short"
81 classname="org.apache.struts.validator.FieldChecks"
82 method="validateShort"
83 methodParams="java.lang.Object,
84 org.apache.commons.validator.ValidatorAction,
85 org.apache.commons.validator.Field,
86 org.apache.struts.action.ActionMessages,
87 org.apache.commons.validator.Validator,
88 javax.servlet.http.HttpServletRequest"
89 depends=""
90 msg="errors.short"
91 jsFunctionName="ShortValidations"/>
92 <validator name="integer"
93 classname="org.apache.struts.validator.FieldChecks"
94 method="validateInteger"
95 methodParams="java.lang.Object,
96 org.apache.commons.validator.ValidatorAction,
97 org.apache.commons.validator.Field,
98 org.apache.struts.action.ActionMessages,
99 org.apache.commons.validator.Validator,
100 javax.servlet.http.HttpServletRequest"
101 depends=""
102 msg="errors.integer"
103 jsFunctionName="IntegerValidations"/>
104 <validator name="long"
105 classname="org.apache.struts.validator.FieldChecks"
106 method="validateLong"
107 methodParams="java.lang.Object,
108 org.apache.commons.validator.ValidatorAction,
109 org.apache.commons.validator.Field,
110 org.apache.struts.action.ActionMessages,
111 org.apache.commons.validator.Validator,
112 javax.servlet.http.HttpServletRequest"
113 depends=""
114 msg="errors.long"/>
115 <validator name="float"
116 classname="org.apache.struts.validator.FieldChecks"
117 method="validateFloat"
118 methodParams="java.lang.Object,
119 org.apache.commons.validator.ValidatorAction,
120 org.apache.commons.validator.Field,
121 org.apache.struts.action.ActionMessages,
122 org.apache.commons.validator.Validator,
123 javax.servlet.http.HttpServletRequest"
124 depends=""
125 msg="errors.float"
126 jsFunctionName="FloatValidations"/>
127 <validator name="double"
128 classname="org.apache.struts.validator.FieldChecks"
129 method="validateDouble"
130 methodParams="java.lang.Object,
131 org.apache.commons.validator.ValidatorAction,
132 org.apache.commons.validator.Field,
133 org.apache.struts.action.ActionMessages,
134 org.apache.commons.validator.Validator,
135 javax.servlet.http.HttpServletRequest"
136 depends=""
137 msg="errors.double"/>
138 <validator name="date"
139 classname="org.apache.struts.validator.FieldChecks"
140 method="validateDate"
141 methodParams="java.lang.Object,
142 org.apache.commons.validator.ValidatorAction,
143 org.apache.commons.validator.Field,
144 org.apache.struts.action.ActionMessages,
145 org.apache.commons.validator.Validator,
146 javax.servlet.http.HttpServletRequest"
147 depends=""
148 msg="errors.date"
149 jsFunctionName="DateValidations"/>
150 <validator name="intRange"
151 classname="org.apache.struts.validator.FieldChecks"
152 method="validateIntRange"
153 methodParams="java.lang.Object,
154 org.apache.commons.validator.ValidatorAction,
155 org.apache.commons.validator.Field,
156 org.apache.struts.action.ActionMessages,
157 org.apache.commons.validator.Validator,
158 javax.servlet.http.HttpServletRequest"
159 depends="integer"
160 msg="errors.range"/>
161 <validator name="floatRange"
162 classname="org.apache.struts.validator.FieldChecks"
163 method="validateFloatRange"
164 methodParams="java.lang.Object,
165 org.apache.commons.validator.ValidatorAction,
166 org.apache.commons.validator.Field,
167 org.apache.struts.action.ActionMessages,
168 org.apache.commons.validator.Validator,
169 javax.servlet.http.HttpServletRequest"
170 depends="float"
171 msg="errors.range"/>
172 <validator name="doubleRange"
173 classname="org.apache.struts.validator.FieldChecks"
174 method="validateDoubleRange"
175 methodParams="java.lang.Object,
176 org.apache.commons.validator.ValidatorAction,
177 org.apache.commons.validator.Field,
178 org.apache.struts.action.ActionMessages,
179 org.apache.commons.validator.Validator,
180 javax.servlet.http.HttpServletRequest"
181 depends="double"
182 msg="errors.range"/>
183 <validator name="creditCard"
184 classname="org.apache.struts.validator.FieldChecks"
185 method="validateCreditCard"
186 methodParams="java.lang.Object,
187 org.apache.commons.validator.ValidatorAction,
188 org.apache.commons.validator.Field,
189 org.apache.struts.action.ActionMessages,
190 org.apache.commons.validator.Validator,
191 javax.servlet.http.HttpServletRequest"
192 depends=""
193 msg="errors.creditcard"/>
194 <validator name="email"
195 classname="org.apache.struts.validator.FieldChecks"
196 method="validateEmail"
197 methodParams="java.lang.Object,
198 org.apache.commons.validator.ValidatorAction,
199 org.apache.commons.validator.Field,
200 org.apache.struts.action.ActionMessages,
201 org.apache.commons.validator.Validator,
202 javax.servlet.http.HttpServletRequest"
203 depends=""
204 msg="errors.email"/>
205 <validator name="url"
206 classname="org.apache.struts.validator.FieldChecks"
207 method="validateUrl"
208 methodParams="java.lang.Object,
209 org.apache.commons.validator.ValidatorAction,
210 org.apache.commons.validator.Field,
211 org.apache.struts.action.ActionMessages,
212 org.apache.commons.validator.Validator,
213 javax.servlet.http.HttpServletRequest"
214 depends=""
215 msg="errors.url"/>
216 <!--
217 This simply allows struts to include the validateUtilities into a page, it should
218 not be used as a validation rule.
219 -->
220 <validator name="includeJavaScriptUtilities"
221 classname=""
222 method=""
223 methodParams=""
224 depends=""
225 msg=""
226 jsFunction="org.apache.commons.validator.javascript.validateUtilities"/>
227 <validator name="codeinput"
228 classname="consultII.web.utils.MyValidator"
229 method="validateCodeInput"
230 methodParams="java.lang.Object,
231 org.apache.commons.validator.ValidatorAction,
232 org.apache.commons.validator.Field,
233 org.apache.struts.action.ActionMessages,
234 javax.servlet.http.HttpServletRequest"
235 msg="errors.code"/>
236 <validator name="userinfo"
237 classname="consultII.web.utils.MyValidator"
238 method="validateUserInfo"
239 methodParams="java.lang.Object,
240 org.apache.commons.validator.ValidatorAction,
241 org.apache.commons.validator.Field,
242 org.apache.struts.action.ActionMessages,
243 javax.servlet.http.HttpServletRequest"
244 msg="errors.info"/>
245 </global>
246</form-validation>
该配置文件的具体格式请查阅Struts的官方文档。可以看出其中对很多常用的校验逻辑进行了封装，比如数字校验（92行到103行），长度校验（33行到56行）等。
而validation.xml则是Validator的核心，其中可对各个FormBean中需要校验字段及校验逻辑进行灵活的配置：
1<form-validation>
2 <formset>
3 <form name="login">
4 <field property="username" depends="required"/>
5 <field property="password" depends="required"/>
6 <field property="input" depends="required,codeinput"/>
7 </form>
8 <form name="search">
9 <field property="keyword" depends="required"/>
10 </form>
11 <form name="signin">
12 <field property="username" depends="required"/>
13 <field property="password" depends="required"/>
14 <field property="passagain" depends="required"/>
15 <field property="question" depends="required,userinfo"/>
16 <field property="answer" depends="required,userinfo"/>
17 <field property="age" depends="required,integer"/>
18 <field property="input" depends="required,codeinput"/>
19 <field property="email" depends="email"/>
20 </form>
21 <form name="ask">
22 <field property="content" depends="required"/>
23 </form>
24 </formset>
25</form-validation>
26
具体格式可查阅官方文档，如上例，3到5行定义了需要对名为“loging”的FormBean的“username”、“password”分别进行“required”校验（即必填字段）。校验逻辑之间可以组合，如第6行的“required,codeinput”，定义了先进行required校验，通过了再进行codeinput校验。
与在validate方法内进行校验相比，使用Validator的优势就很明显了，可重用（从上述xml文件可以看出对很多字段都进行了required校验）；可灵活配置、扩展，只需修改validation.xml文件就可以改变校验逻辑。