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Wildcards

As an application grows in size, so will the number of action mappings. Wildcards can be used to combine similar mappings into one more generic mapping.

The best way to explain wildcards is to show an example and walk through how it works. This example modifies a conventional mapping to use wildcards to match all pages that start with /edit:

The "*" in the name attribute allows the mapping to match the request URIs /editSubscription, editRegistration, or any other URI that starts with /edit, however /editSubscription/add would not be matched. The part of the URI matched by the wildcard will then be substituted into various attributes of the action mapping and its action results replacing {1}. For the rest of the request, the framework will see the action mapping and its action results containing the new values.

Mappings are matched against the request in the order they appear in the framework's configuration file. If more than one pattern matches the last one wins, so less specific patterns must appear before more specific ones. However, if the request URL can be matched against a path without any wildcards in it, no wildcard matching is performed and order is not important. Also, note that wildcards are not greedy, meaning they only match until the first occurrence of the following string pattern. For example, consider the following mapping:

This mapping would work correctly for the URI ListAccounts but not ListSponsors, because the latter would turn into this configuration:

Wildcard patterns can contain one or more of the following special tokens:

*

Matches zero or more characters excluding the slash ('/') character.

**

Matches zero or more characters including the slash ('/') character.

\character

The backslash character is used as an escape sequence. Thus

'\*'

matches the character asterisk ('*'), and

'\\'

matches the character backslash ('\').

Patterns can optionally be matched "loosely". When the end of the pattern matches *[^*]*$ (wildcard, no wildcard, wildcard), if the pattern fails, it is also matched as if the last two characters didn't exist. The goal is to support the legacy "*!*" syntax, where the "!*" is optional.

In the action mapping and action results, the wildcard-matched values can be accessed with the token {N} where N is a number from 1 to 9 indicating which wildcard-matched value to substitute. The whole request URI can be accessed with the {0} token.

Also, the action mapping and action result properties will accept wildcard-matched strings in their value attribute, like:

(lightbulb) See also Wildcard Method

Parameters in namespaces

From Struts 2.1+ namespace patterns can be extracted as request parameters and bound to the action. To enable this feature, set the following constant in struts.xml:

With that in place, namespace definitions can contain {PARAM_NAME} patterns which will be evaluated against the request URL and extracted as parameters, for example:

If the request URL is /users/10/detail, then the DetailsAction will be executed and its userID field will be set to 10.

Only one PatternMatcher implementation can be used at a time. The two implementations included with Struts 2 are mutually exclusive. You cannot use Wildcards and Named Variable patterns at the same application (if that were required, you'd need to create a custom PatternMatcher implementation).

Some tags tags not are 100% compatible with variables in the namespace. For instance, they may write the literal namespace into the HTML (eg /{user}/2w) instead of the path used in the request (ie. /brett/24). This usually affects attributes that attempt to guess the namespace of an action (eg. Form tag, Action tag, action=). This problem can be avoided by using HTML tags directly with relative paths or explicit URLs.

Similar functionality can also be implemented using a custom ActionMapper. The ActionMapper will need to parse the namespace and request itself to set parameters on the matched action. The default ActonMapper is responsible for invoking the PatternMatcher.

Parameters after the action name

To use parameters in the URL, after the action name, make sure this is set:

Then the action mapping will look like:

When a URL like /edit/person/123 is requested, EditAction will be called, and its "id" field will be set to 123.

Advanced Wildcards

From 2.1.9+ regular expressions can be defined defined in the action name. To use this form of wild card, the following constants must be set:

The regular expressions can be in two forms, the simplest one is {FIELD_NAME}, in which case the field with the FIELD_NAME in the action will be populated with the matched text, for example:

In this example, if the url /fiction/content/Frankenstein is requested, BookAction's field "type" will be set to "fiction", and the field "title" will be set to "Frankenstein".

The regular expression can also be in the form {FIELD_NAME:REGULAR_EXPRESSION}. The regular expression is a normal Java regular expression. For example:

In this example, if the url /philosophy/AynRand/list is requested, ListBooksAction's field "type" will be set to "philosophy" and "author" to "AynRand".

The matched groups can still be accessed using the {X} notation, like:

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3 Comments

  1. In the first code block, the curly braches are escaped in the <result> element. Is this necessary? If no, remove the backslashes, if yes, explain why it's needed.

    In the paragraph explaining \character, something is wrong at the end of the line, probably in the Wiki markup.

  2. The two paragraphs:

    "Also, the action mapping properties (set using the <set-property key="foo" value="bar"> syntax) will accept wildcard-matched strings in their value attribute.

    Like the action mapping, the action result properties (set using the <set-property key="foo" value="bar"> syntax) will accept wildcard-matched strings in their value attribute."

    should be changed to one:

    "Also, the action mapping and action result properties (set using the <set-property key="foo" value="bar"> syntax) will accept wildcard-matched strings in their value attribute."

  3. The line:

    "Mappings are matched against the request in the order they appear in the framework's configuration file. If more than one pattern matches the last one wins, so less specific patterns must appear before more specific ones."

    Needs to be changed to:

    "Mappings are matched against the request in the order they appear in the framework's configuration file. If more than one pattern matches the first match wins, so more specific patterns should be defined before less specific patterns."

    The pattern matching is carried out by the match method in com.opensymphony.xwork2.config.impl.ActionConfigMatcher class. The javadoc for this class mentions the above. Also, I have tested it out with the following configuration in struts.xml

    <action name="*/*" class="com.omkarpatil.{1}Action">
        <result>/Welcome.jsp</result>
    </action>
    <action name="*" class="com.omkarpatil.{1}Action">
        <result>/index.html</result>
    </action>

    When used this with the URL - "http://localhost:8090/firstapp/My.action", the page displayed is always the one corresponding to the action that is configured first.