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Auto-completion is available for virtually all tags and attributes, just give it a try:
Rename refactoring/Find Usages for all referencable elements
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Errors are highlighted immediately in the editor, additional validation is triggered for Make Project.
IDEA 7: see "Validation" tab in S2 facet for more options
IDEA 8+: see Settings->Compiler->Validation
Quickfixes
Various quickfixes to create missing elements etc.
struts.xml: restrict resolving to <result> value depending on result-type (e.g. Tiles, Velocity)
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(plugin "Velocity support" must be enabled)
IDEA 10.x+
Add all global macros (struts.vm in struts2-core.jar) automatically.
Autocomplete all global variables.
Blog post incl. screen-shots
Support struts2-jQuery plugin
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