SSH答疑解惑系列（三）——Struts2的异常处理
Struts2的异常采用声明式异常捕捉，具体通过拦截器来实现。
在项目中，我们可以在Action中直接抛出异常，剩下的就交给Struts2的拦截器来处理了。当然，我们需要进行相关配置。
Struts2配置了默认的拦截器，来处理异常。在struts-default.xml中，可以找到配置。有了这个配置以后，我们在项目中就方便多了。
<interceptor name="exception" class="com.opensymphony.xwork2.interceptor.ExceptionMappingInterceptor"/>在项目中，只需要使用exception-mapping标签来配置：
1）全局异常
适用对象：所有Action
<global-results >
<!-- 跳转到全局异常的页面 -->
<result name="global-error">/global-error.jsp</result>
</global-results>
<global-exception-mappings>
<exception-mapping result="global-error" exception="com.struts2.ApplicationException"></exception-mapping>
</global-exception-mappings>2）局部异常
使用对象：异常所在的Action
<!-- 局部异常，LoginAction -->
<action name="login" class="com.struts2.LoginAction">
<exception-mapping result="error" exception="com.struts2.ApplicationException"></exception-mapping>
<result>/success.jsp</result>
<result name="error">/error.jsp</result>
</action>ApplicationException
Throwable是所有异常的根
Error是错误
Exception是异常，RuntimeException是运行中的异常。
除了Struts2自带的拦截器，我们还可以自定义拦截器，覆盖底层的拦截器，使得异常的捕捉更加灵活。
自定义拦截器的配置也十分方便。
<interceptors>
<!-- 声明拦截器 -->
<interceptor name="errorInterceptor" class="cn.itcast.util.ErrorInterceptor" />
<!-- 配置拦截器栈 -->
<interceptor-stack name="myErrorInterceptor">
<interceptor-ref name="defaultStack" />
<interceptor-ref name="errorInterceptor" />
</interceptor-stack>
</interceptors>
<!-- 覆盖底层的拦截器栈 对包中的所有action都有效 -->
<default-interceptor-ref name="myErrorInterceptor"/>
<global-results>
<result name="errorMsg">/errorMsg.jsp</result>
</global-results>
<global-exception-mappings>
<exception-mapping result="errorMsg" exception="java.lang.Exception"></exception-mapping>
</global-exception-mappings>
Struts2的“错误页”
经常使用ssh框架开发，会发现Struts2中各种报错页面的布局和显示信息很类似。这是因为Struts2提供了一个它自己的“错误页”，在org.apache.struts2.dispatcher包下。
<html>
<head>
<title>Struts Problem Report</title>
<style>
pre {
margin: 0;
padding: 0;
}
</style>
</head>
<body>
<h2>Struts Problem Report</h2>
<p>
Struts has detected an unhandled exception:
</p>
<#assign msgs = [] />
<#list chain as ex>
<#if ex.message??>
<#assign msgs = [ex.message] + msgs/>
</#if>
</#list>
<#assign rootex = exception/>
<#list chain as ex>
<#if (ex.location?? && (ex.location != unknown))>
<#assign rootloc = ex.location/>
<#assign rootex = ex/>
<#else>
<#assign tmploc = locator.getLocation(ex) />
<#if (tmploc != unknown)>
<#assign rootloc = tmploc/>
<#assign rootex = ex/>
</#if>
</#if>
</#list>
<div id="exception-info">
<table>
<tr>
<td><strong>Messages</strong>:</td>
<td>
<#if (msgs?size > 1)>
<ol>
<#list msgs as msg>
<#if (msg?is_method)>
<li>${msg[0]}</li>
<#else>
<li>${msg}</li>
</#if>
</#list>
</ol>
<#elseif (msgs?size == 1)>
<#if (msgs[0]?is_method)>
<li>${msgs[0][0]}</li>
<#else>
<li>${msgs[0]}</li>
</#if>
</#if>
</td>
</tr>
<#if rootloc??>
<tr>
<td><strong>File</strong>:</td>
<td>${rootloc.URI}</td>
</tr>
<tr>
<td><strong>Line number</strong>:</td>
<td>${rootloc.lineNumber}</td>
</tr>
<#if (rootloc.columnNumber >= 0)>
<tr>
<td><strong>Column number</strong>:</td>
<td>${rootloc.columnNumber}</td>
</tr>
</#if>
</#if>
</table>
</div>
<#if rootloc??>
<#assign snippet = rootloc.getSnippet(2) />
<#if (snippet?size > 0)>
<div id="snippet">
<hr />
<#list snippet as line>
<#if (line_index == 2)>
<#if (rootloc.columnNumber >= 3)>
<pre style="background:yellow">${(line[0..(rootloc.columnNumber-3)]?html)}<span style="background:red">${(line[(rootloc.columnNumber-2)]?html)}</span><#if ((rootloc.columnNumber)<line.length())>${(line[(rootloc.columnNumber-1)..]?html)}</#if></pre>
<#else>
<pre style="background:yellow">${line?html}</pre>
</#if>
<#else>
<pre>${line?html}</pre>
</#if>
</#list>
</div>
</#if>
</#if>
<div id="stacktraces">
<hr />
<h3>Stacktraces</h3>
<#list chain as ex>
<div class="stacktrace" style="padding-left: ${ex_index * 2}em">
<strong>${ex}</strong>
<div>
<pre>
<#list ex.stackTrace as frame>
${frame}
</#list>
</pre>
</div>
</div>
</#list>
</div>
<div class="footer">
<hr />
<p>
You are seeing this page because development mode is enabled. Development mode, or devMode, enables extra
debugging behaviors and reports to assist developers. To disable this mode, set:
<pre>
struts.devMode=false
</pre>
in your <code>WEB-INF/classes/struts.properties</code> file.
</p>
</div>
</body>
</html>
