Struts2与Ognl (转载)
一.struts2的context对象
1.下面的脚本能打印出context的内容(也可通过<s:debug>来查阅)
<%@page language="java" contentType="text/html;charset=GBK" %>
<%@taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
</head>
<body>
<s:set name="v1" value="'0123456789'"/>
<%
Map ctx =((ValueStack)request.getAttribute("struts.valueStack")).getContext();
Set keys = ctx.keySet();
for (Object key:keys) {
out.println("key=" + key + ",");
if (ctx.get(key) != null) {
out.println("value=" + ctx.get(key));
}
out.println("<br/>");
}
%>
</body>
</html>上面设置了一个字符串变量v1,值是'01234567890'.
2.结果:
key=com.opensymphony.xwork2.ActionContext.parameters,value={}
key=last.bean.accessed,
key=parameters,value={}
key=com.opensymphony.xwork2.ActionContext.locale,value=zh_CN
key=current.property.path,
key=session,value={}
key=attr,value=org.apache.struts2.util.AttributeMap@c8570c
key=com.opensymphony.xwork2.util.ValueStack.ValueStack,value=com.opensymphony.xwork2.util.OgnlValueStack@caf0ed
key=com.opensymphony.xwork2.ActionContext.application,value={org.apache.catalina.jsp_classpath=...,...}
key=v1,value=0123456789
key=__component_stack,value=[]
key=com.opensymphony.xwork2.dispatcher.HttpServletRequest,value=org.apache.struts2.dispatcher.StrutsRequestWrapper@717d91
key=com.opensymphony.xwork2.dispatcher.ServletContext,value=org.apache.catalina.core.ApplicationContextFacade@eafb71
key=last.property.accessed,
key=com.opensymphony.xwork2.dispatcher.PageContext,value=org.apache.jasper.runtime.PageContextImpl@121df2a
key=com.opensymphony.xwork2.dispatcher.HttpServletResponse,value=org.apache.catalina.connector.ResponseFacade@c26b16
key=request,value={struts.valueStack=com.opensymphony.xwork2.util.OgnlValueStack@caf0ed}
key=com.opensymphony.xwork2.ActionContext.session,value={}
key=application,value={org.apache.catalina.jsp_classpath=...,...}
二.struts2的root对象
是OgnlValueStack实例,实现了ValueStack接口.
OgnlValueStack实例包含了一个ArrayList容器,提供了push和pop方法来装载对象.
当要我们通过Ognl获取一个属性时,例如<s:propertyvalue="age">,因为不带#号,说明是root对象属性,Struts对逐一顺次查找ArrayList容器中的对象,看它们是否拥有age属性,或者是有key="age"的Map.
1.com.opensymphony.xwork2.util.CompoundRootAccessor的getProperty方法源码:
public Object getProperty(Map context, Object target, Object name) throws OgnlException {
CompoundRoot root = (CompoundRoot) target;
OgnlContext ognlContext = (OgnlContext) context;
if (name instanceof Integer) {
Integer index = (Integer) name;
return root.cutStack(index.intValue());
} else if (name instanceof String) {
if ("top".equals(name)) {
if (root.size() > 0) {
return root.get(0);
} else {
return null;
}
}
for (Iterator iterator = root.iterator(); iterator.hasNext();) {
Object o = iterator.next();
if (o == null) {
continue;
}
try {
if ((OgnlRuntime.hasGetProperty(ognlContext, o, name)) || ((o instanceof Map) && ((Map) o).containsKey(name)))
{
return OgnlRuntime.getProperty(ognlContext, o, name);
}
} catch (OgnlException e) {
if (e.getReason() != null) {
final String msg = "Caught an Ognl exception while getting property " + name;
throw new XWorkException(msg, e);
}
} catch (IntrospectionException e) {
// this is OK if this happens, we'll just keep trying the next
}
}
return null;
} else {
return null;
}
}2.查看ValueStack内容(也可通过<s:debug>来查阅):
<%@page language="java" contentType="text/html;charset=GBK" %>
<%@taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
</head>
<body>
<s:push value="'01234567890'">
<%
ValueStack vs = (ValueStack)request.getAttribute("struts.valueStack");
int objCount = vs.size();
List<Object> l = new ArrayList<Object>();
while(--objCount >= 0) {
Object o = vs.pop();
l.add(o);
if (o != null) {
out.println("class:" + o.getClass().getName() + ":" + o + "<br/>");
} else {
break;
}
}
for (int i = l.size() - 1; i >= 0; i--) {
Object o = l.get(i);
vs.push(o);
}
%>
</s:push>
</body>
</html>3.结果
class:java.lang.String:01234567890
class:com.opensymphony.xwork2.DefaultTextProvider:com.opensymphony.xwork2.DefaultTextProvider@6f8b2b
三.访问
1.访问context中的变量
<%@page import="com.opensymphony.xwork2.util.ValueStack,test.model.*,java.util.*" %>
<%@page language="java" contentType="text/html;charset=GBK" %>
<%@taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
</head>
<body>
<jsp:useBean id="user0" class="test.model.User" scope="request"/>
<jsp:setProperty name="user0" property="name" value="LiSi"/>
<jsp:setProperty name="user0" property="id" value="0001"/>
<s:set name="user1" value="#request['user0']"/>
#user1.name:<s:property value="#user1.name"/><br/>
#user1.id:<s:property value="#user1.id"/><br/>
</body>]</html>语法是<s:propertyvalue="#变量名.属性名"/>
2.访问root中的变量属性
<%@page import="com.opensymphony.xwork2.util.ValueStack,holly.model.*,java.util.*" %>
<%@page language="java" contentType="text/html;charset=GBK" %>
<%@taglib prefix="s" uri="/struts-tags" %>
<html>
<head>
</head>
<body>
<jsp:useBean id="user0" class="holly.model.User" scope="request"/>
<jsp:setProperty name="user0" property="name" value="LiSi"/>
<jsp:setProperty name="user0" property="id" value="0001"/>
<s:push value="#request['user0']">
name:<s:property value="name"/><br/>
id:<s:property value="id"/><br/>
</s:push>
</body>
</html>语法是<s:propertyvalue="属性名"/>,也可以是<s:propertyvalue="[index_number].属性名"/>index_number是元素在ArrayList容器中的位置,从0计起
四.<s:set>
1.属性
name:生成的新变量的名称
value:将赋给变量的值,如果没有指定,则将ValueStack栈顶的值赋予它.
scope:变量放置的范围.接受application,session,request,page,action五个值.不指定时,将默认放在page和context对象中.
2.例子
<s:set value="'012345678'" name="v1" scope="session"/> <s:property value="#session.v1"/><!--等同于#session['v1']--> <s:property value="#attr.v1"/><!--等同于#attr['v1']--> <s:set value="'01234567890'" name="v2"/> <s:property value="#v2"/> <s:property value="#attr.v2"/>
v1放在了session容器中,session容器又是context对象中的一员,所以可以通过#session.v1访问.而v2因为没有指定scope,会同时放在context和page(可通过attr引用到,attr会依次在page,request,session,application中查找)中.
五.<s:push>
开始时将值推入ValueStack栈顶,结束后将值推出栈.
value属性:需要放到ValueStack栈顶的值
六.疑惑与解惑
1.疑惑
在<<Struts2权威指南>>第10.3.2iterator标签小节,看到下面这个例子:
<table border="1" width="100"
<s:iterator value="{'Spring2.0宝典','轻量级J2EE企业应用实践','基于J2EE的Ajax宝典'}" id="name">
<tr>
<td><s:property value="name"/></td>
</tr>
</s:iterator>
</table><s:iterator>的当前值会放入两个地方,一是ValueStack栈顶,二是context容器中.引用的方法也相应有两种:
<s:property/>和<s:propertyvalue="#name"/>.
<s:propertyvalue="name"/>为什么也生效了?它找的是ArrayList容器中某个对象的name属性或'name'键对应的值.ValueStack中并没有这样的对象啊.
2.解惑
从com.opensymphony.xwork2.util.OgnlValueStack中找到了答案.
public Object findValue(String expr) {
try {
if (expr == null) {
return null;
}
if ((overrides != null) && overrides.containsKey(expr)) {
expr = (String) overrides.get(expr);
}
if (defaultType != null) {
return findValue(expr, defaultType);
}
Object value = OgnlUtil.getValue(expr, context, root);
if (value != null) {
return value;
} else {
return findInContext(expr);//程序会运行到此
}
} catch (OgnlException e) {
return findInContext(expr);
} catch (Exception e) {
logLookupFailure(expr, e);
return findInContext(expr);
} finally {
OgnlContextState.clear(context);
}
}
private Object findInContext(String name) {
return getContext().get(name);
}当ValueStack的ArrayList容器中以及Ognl表达式求解都不能获取到对象时,ValueStack直接从context对象中获取对象.而#name之前已经以"name='书名'"存于context中,所以会有值返回.
注意:<s:iterator>结束后,并未清除context中的name变量
3.进一步
是不是其它context中的对象都可以不通过#来访问呢?
<s:propertyvalue="session"/>行不行呢?
答案是不行,会抛出下面的异常:
java.lang.ClassCastException:org.apache.struts2.dispatcher.SessionMap
org.apache.struts2.components.Property.start(Property.java:136)
org.apache.struts2.views.jsp.ComponentTagSupport.doStartTag(ComponentTagSupport.java:54)
看org.apache.struts2.components.Property源码
public boolean start(Writer writer) {
boolean result = super.start(writer);
String actualValue = null;
if (value == null) {
value = "top";
}
else if (altSyntax()) {
// the same logic as with findValue(String)
// if value start with %{ and end with }, just cut it off!
if (value.startsWith("%{") && value.endsWith("}")) {
value = value.substring(2, value.length() - 1);
}
}
// exception: don't call findString(), since we don't want the
// expression parsed in this one case. it really
// doesn't make sense, in fact.
actualValue = (String) getStack().findValue(value, String.class);//这是出错的地方
try {
if (actualValue != null) {
writer.write(prepare(actualValue));
} else if (defaultValue != null) {
writer.write(prepare(defaultValue));
}
} catch (IOException e) {
LOG.info("Could not print out value '" + value + "'", e);
}
return result;
}getStack().findValue(value,String.class)返回的是org.apache.struts2.dispatcher.SessionMap对象,不能被cast成String.而我们之前的变量name本身就是String,所以不会出错.
4.结论
context中的String变量可以不加#来访问.
<s:set name="v1" value="'123'"/> <s:property value="v1"/> <s:property value="#v1"/>为了规范起见, 还是应该加#访问
解析源码是我们学习的最好途径