Flash is a concept which does not exist in standard Java API, but is very useful in web applications. Flash is a snippet of HTML whose life cycle is limited by the next HTTP request. In other words, a flash is created during a request, then it can be used in a subsequent request (of the same session), after which it dies.

Flash messages are useful in cases when a POST/Redirect to GET pattern is used.

Use case 1: pass message from controller

Flash messages are created in controllers (or filters) like so:

In a code of controller above, a new book is submitted in a POST request.

  • Line 5 is making attempt to save the book information to DB.

  • Lines 6, 7, 8 - in case of failure, three instances of flash are created: message, errors and params, where message is a generic message displayed at the top of page, params is a map with submitted values (it is used to re-populate input fields) and the error is a map with error messages

  • Line 11 - In case of success request is redirected to BooksController index method and the flash message is sent to view. The file /views/books/index.ftl would have a flash message displayed with flash tag:

For a complete example of using FlashTag, please refer to BooksController#create() and corresponding views. Also, please see ActiveJDBC Validation

The Flash tag automatically detects if a flash message exists and displays it. If a message is missing, nothing is rendered.

Since the user was redirected to the BooksController with an HTTP GET method, the page can simply be reloaded if a user presses Reload button on browser. However, if a user actually reloads the page, the flash message will disappear, because it cannot survive across more than one request.

In general, POST/Redirect to GET is a good programming pattern to use in case you need destructive operations. Leaving a user on a POSTed page is a bad idea, because the same request can be re-submitted if user presses Reload.

Use case 2: Named FlashTag with body

If you need to display a more complex HTML than a simple string, you can do so by placing a flash tag with body on the page:

and calling a single argument method inside the controller:

The body inside the flash tag lives by the same rules as any other in the template. You can use variables, normal syntax, lists or even partials:

It allows to organize code for error and warning messages into reusable componets.

Use case 3: Anonymous FlashTag with body

In case you only need one flash tag with body, no need to specify a name:

In controller:

Testing named flash with message

If you set this in the controller:

Then you can write a test like this:

Testing named flash with body

If you set this in the controller:

and your view looks like this:

Then you can write a test like this:

Rendering dynamic snippets of HTML (old method)

Older method is still operational, but not recommended:

The HTML code inside the IF condition has no restrictions to use any dynamic values from session of those passed into view by controller.

Internationalization of flash messages

The FlashTag is not internationalized. However, you can do this using one of two methods.

Method 1: Use class Messages in controller

Lets say your activeweb_messages_properties file contains this entry:

book_added=A book {0} was added to your library. Author: {1}

Please, refer to org.javalite.activejdbc.Messages

In controller write this:

In view:

This will print:

Method 2: Use MessageTag

Lets say your activeweb_messages_properties file contains this entry:

In controller:

In view:

This will print:

Method 1 should be used when you have parametrized messages. There is no way to pass a parameter to the MessageTag in method 2, because it is running in a different request (after redirect).

Choose a method that most appropriate for your app.

How to comment

The comment section below is to discuss documentation on this page.

If you have an issue, or discover bug, please follow instructions on the Support page