Liferay comes with a built-in Validator class (com.liferay.portal.kernel.util.Validator) that you can access from your plugin portlets. It features some basic static validation methods useable in your code like checks for Null and email address which are just two among many others. Check it Out.
Never place tags like <html> and <body> in your view markup (JSP, PHP, etc). Its the job of the portlet container to aggregate the final html code for all portlets and to pass that to the portal for rendering. So, using those tags can result in Done, with Errors on Page messages by your browser. [...]
The extension environment can also be used to replace existing Liferay javascripts. Just recreate the directory structure where the original javascript resides under the extension folder. Liferay will substitute your scripts for its own when its needed during runtime.
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