Struts OGNL

OGNL stands for Object Graph Navigation Language.

It’s an expression language for getting/setting properties of Java objects and other extra functions. It works as a binding between GUI elements or UI components to model objects.

This post is a summary after studying Apache Struts 2 Documentation - ONGL.

Import tag lib

Before using ONGL, we need to import it in JSP page:

<%@ taglib prefix="s" uri="/struts-tags" %>

Control Tags

if/elseif/else
iterator
append
merge
generator
subset
sort

if/elseif/else

if and elseif must have test attribute in which should be a boolean expression.

// Set a parameter called 'age' with value 30
<s:set name="age" value="30"/>
<s:if test="#age>60">    
	old  
</s:if>    
<s:elseif test="#age>35">    
    adult   
</s:elseif>    
<s:else>    
    young  
</s:else>

iterator

Tag to iterate a collection (List, set, array or map). It has three attributes:

Name	Required	Default	Description
begin	X	0	Index to start iteration
end	X	size of collection	Index to end iteration
status	X		Instance of IteratorStatus which will be pushed into stack upon each iteration
step	X	1	Iteration index will be increased by this value on each iteration
value	X		Collection to iterate
var	X		Name used to reference the value pushed into the Value Stack

Instance of IteratorStatus also has following functions:

if status="test",

#test.count returns number of element already iterated. (equals to int getCount())
#test.index returns index of current element. (equals to int getIndex())
#test.even returns wether current element is even. (equals to boolean isEven())
#test.odd returns wether current element is odd. (equals to boolean isOdd())
#test.first returns wether current element is the first one. (equals to boolean isFirst())
#test.last returns wether current element is the last one. (equals to boolean isLast())

If now we have a list called employeeList which is List<Employee>. Employee has two attributes name and age with getter/setter:

<table>
	<s:iterator value="employeeList" status="list">
		<tr>
			<td><s:property value="#list.index" /></td>
			<td><s:property value="name" /></td>
			<td><s:property value="age" /></td>
		</tr>   
	</s:iterator>  
</table>

If it’s List<String>, use s:property without value to present a string element in the list:

<table>
	<s:iterator value="numberList" status="list2">
		<tr>
			<td><s:property value="#list2.index" /></td>
			<td><s:property/></td>
		</tr>   
	</s:iterator>  
</table>

If it’s Map:

<table>
	<s:iterator value="keyValueMap" status="map">
		<tr>
			<td><s:property value="#map.index" /></td>
			<td><s:property value="key" /></td>
			<td><s:property value="value" /></td>   
		</tr>   
	</s:iterator>  
</table>

append

It appends several collections to form a new collection.

The new collection has the following order (different from tag `merge):

First Entry of the First Iterator
Second Entry of the First Iterator
Third Entry of the First Iterator
First Entry of the Second Iterator
Second Entry of the Second Iterator
Third Entry of the Second Iterator

The following example shows three lists of type List<String> to be appended together. These three lists are passed by some Action.

<s:append var="newList">
     <s:param value="list1" />
     <s:param value="list2" />
     <s:param value="list3" />
</s:append>
<s:iterator value="#newList">
     <s:property />
</s:iterator>

merge

The same as append but in different order:

First Entry of the First Iterator
First Entry of the Second Iterator
Second Entry of the First Iterator
Second Entry of the Second Iterator
Third Entry of the First Iterator
Third Entry of the Second Iterator

generator

Generate an iterator based on give string. If var not set, it will only exist in Value Stack which means you can not use it outside the tag. (Not in page context)

Name	Required	Description
separator	O	The separator to be used in separating the val into entries of the iterator
val	O	The source to be parsed into an iterator
var	X	The name to store the resultant iterator into page context
count	X	Max number of entries to be in the iterator
converter	X	Convert the String entry parsed from val into an object

A string separated by “,”:

<s:generator val="%{'aaa,bbb,ccc,ddd,eee'}" count="3" separator=",">
	<s:iterator>
    	<s:property />
	</s:iterator>
</s:generator>

Generate an iterator with comparator attribute:

<s:generator val="%{'aaa,bbb,ccc,ddd,eee'}" separator="," converter="myConverter">
	<s:iterator>
     return<s:property /><br/>
	</s:iterator>
</s:generator>

public class GeneratorTagAction extends ActionSupport {
    ...
    // getMyConverter function name -> converter="myConverter"
    public Converter getMyConverter() {
        return new Converter() {
            public Object convert(String value) throws Exception {
                return "converter-"+value;
            }
        };
    }
    ...
}

This will generate an iterator with each entries decided by the converter supplied. With this converter, it simply add “converter-“ to each entries.

subset

Outputs a subset of an iterator.

Name	Required	Default	Description
count	X		Indicate the number of entries to be in the resulting subset iterator
decider	X		Extension to plug-in a decider to determine if that particular entry is to be included in the resulting subset iterator
var	X		The name to store the resultant iterator into page context
source	X		Source to do subset
start	X	0	Indicate the startig index

Simply subset:

<!-- myList is a List<String> given by action -->
<s:subset source="myList">
    <s:iterator>
		<s:property />
    </s:iterator>
</s:subset>

With decider:

public class MySubsetTagAction extends ActionSupport {
	
	...
    public Decider getMyDecider() {
    	return new Decider() {
        	public boolean decide(Object element) throws Exception {
            	int i = ((Integer)element).intValue();
            	return (((i % 2) == 0)?true:false);
         	}
    	};
    }
}

<!-- myList is a List<String> given by action -->
<s:subset source="myList" decider="myDecider">
    <s:iterator>
 		<s:property />
    </s:iterator>
</s:subset>

Sort

It sorts a List using a Comparator

Name	Required	Description
comparator	O	The comparator to use (java.util.Comparator)
var	X	The name to store the resultant iterator into page context
source	X	Source to sort

Example

Sort by string length:

<s:set name="test" value="{'aefi','abc','world!','kkkkk'}" />
<s:bean id="sort" name="action.SortComparator"></s:bean>
<s:sort source="test" comparator="sort">
    <s:iterator>
        <s:property />
    </s:iterator>
</s:sort>

public class SortComparator implements Comparator<Object> {
    public int compare(Object arg0, Object arg1) {
    	// Compare by length
        return arg0.toString().length() - arg1.toString().length();
    }
}

Data Tags

action

It could call actions directly from a JSP page by specifying the action name and an optional namespace.

Any result processor defined for this action in struts.xml will be ignored, unless the executeResult parameter is specified.

Name	Required	Default	Description
executeResult	x	false	Whether the result of this action (probably a view) should be executed/rendered
var	X		Name used to reference the value pushed into the Value Stack
name	O		Name of the action to be executed (without the extension suffix eg. .action)
namespace	X	Namespace from where tag is used	Namespace of the action
ignoreContextParams	X	false	Whether the request parameters are to be passed to the action
rethrowException	X	false	Whether an exception should be rethrown, if the target action throws an exception

Example

public class ActionTagAction extends ActionSupport {
 
	public String execute() throws Exception {
     	return SUCCESS;
 	}
 
 	public String test() throws Exception {
    	ServletActionContext.getRequest().setAttribute("variable", "A variable");
     	return SUCCESS;
 	}
}

<action name="actionA" class="com.dong.test">
    <result name="success">test.jsp</result>
</action>
<action name="actionB" class="com.dong.test" method="test">
    <result name="success">test.jsp</result>
</action>

When we acess the test page with a request parameter - localhost:8000/index?author=Dong:

<!-- success.jsp will be included in this page -->
<!-- And show "test - Dong"-->
<s:action name="actionA" executeResult="true" />

<!-- test.jsp will be included in this page-->
<!-- No request parameter is allowed, so could only show "test -"-->
<s:action name="actionB" executeResult="true" ignoreContextParams="true" />

<!-- test.jsp will NOT be included in this page-->
<!-- But put a String in request scope under an id "variable" which will be retrieved using property tag -->
<s:action name="actionB" executeResult="false" />
<s:property value="#attr.variable" /><!-- #attr is to access to PageContext if available , we use value="variable" directly -->

Ref

Tag reference

Struts OGNL

Import tag lib

Control Tags

if/elseif/else

iterator

append

merge

generator

subset

Sort

Example

Data Tags

action

Example

Ref

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