Development Mode (aka devMode)

Struts 2 has a setting (which can be set to true or false in struts.properties) called devMode (= development mode). When this setting is enabled, Struts 2 acts much friendlier, which can significantly speed up development.

What does it do ?

• When enabled, Struts 2 will reload your resource bundles on every request (meaning you can change your .properties files, save them, and see the changes reflected on the next request).
  Note: this option can also be set standalone via struts.i18n.reload = true

• It will also reload your xml configuration files (struts.xml), your validation files, etc, on every request. This is useful for testing or finetuning your configuration, without having to redeploy your application every time.
  Note: this option can also be set standalone via struts.configuration.xml.reload = true

• And thirdly, perhaps the setting which is less widely known, and therefore a source of much confusion: it will raise the level of debug or normally ignorable problems to errors. For example: when you submit a field which cannot be set on an action 'someUnknownField', it will normally be ignored. However, when you're in development mode, an exception will be thrown, telling you an invalid field was submitted. This is very useful for debugging or testing large forms, but can also be confusing if you're relying on parameters in your request that are not set on the action, but which you are using directly in your view layer (warning: bad practice, you should always validate input from the web).

Don't forget ..

By default, the development mode is disabled, because it has a significant impact on performance, since the entire configuration will be reloaded on every request.