In a Nutshell
- In Application Express 4.2.2, if your image prefix application attribute equals the instance-wide image prefix attribute, it will be set to null in your application.
- At runtime, if the image prefix application attribute is null, the APEX engine will use the instance-wide image prefix attribute as your application's image prefix.
- This was done to facilitate ease of movement of applications across varying instances.
Every instance of Oracle Application Express has an instance-wide image prefix. This image prefix is used by Oracle Application Express to specify the directory location of the Application Express images directory. This image prefix ultimately correlates to a virtual directory or equivalent on the Web server, and this virtual directory directly maps to the 'images' directory shipped with an APEX distribution and staged on the Web server. This image prefix is specified at APEX installation time, but it can be adjusted later if necessary. This image prefix is usually always /i/.
For example, if you go to the Login page of apex.oracle.com and look at the source of that page, you'll see numerous references to images and files of the form /i/... The HTML for the logo in the upper left is:
<img alt="Application Express" height="20" src="/i/apex_ui/apexlogo.png" width="280" />
There is a virtual directory on the Web server of apex.oracle.com, which maps /i/ to a virtual directory. This virtual directory maps to a physical directory on the Web server, which contains the entire 'apex/images' directory from the APEX distribution.
The 'apex/images' directory changes from release to release. The 'apex/images' directory from an APEX 4.1 release is only usable with APEX 4.1. The 'apex/images' directory fron an APEX 4.2 release is only usable with APEX 4.2. You can try and use it with APEX 4.1, but you'll definitely see odd results, if anything at all.
The image prefix is also an attribute of the user interface of an application. This image prefix is used during the runtime of the APEX application, and is used to determine the image prefix for that application. 99% of the time, this image prefix application attribute equals the instance-wide image prefix (typically, /i/), but it could be different too. It could even be wrong! If you change your image prefix attribute to '/foobar/' and then run your application, you'll see that all the references to images and static files in the HTML source of your application all reference '/foobar/'. I suspect you'd get a page with broken images and references everywhere, and in the worst case, you may get what appears to be a blank page.
Using the Image Prefix
Now here's where it gets interesting. Even though the documentation recommends an image prefix of /i/, it isn't absolutely mandatory. So let's say that for my APEX 4.1 instance, I use an image prefix of /i_old/, and for my APEX 4.2.2 instance, I use an image prefix of /i_new/. And also let's assume that these instances use totally distinct Web servers. If I export an application from my APEX 4.1 instance and import it into my APEX 4.2.2 instance, I will need to update the image prefix application attribute before I can run it in the APEX 4.2.2 instance. Why is that? Because my application image prefix is /i_old/, and that isn't defined on the Web server for my APEX 4.2.2 instance.
For those who have participated in the Application Express Early Adopter program before, you'll remember that the image prefix was /i_ea/ or something like that. When you imported your existing APEX application into the Early Adopter instance, you had to adjust the image prefix of your application to be equal to the instance-wide image prefix, /i_ea/.
For enterprises which host multiple APEX instances via one Web server, they have an equally difficult problem. They may have image prefixes of /i40/, /i41/, /i42, and wish to freely move and test applications without having to adjust the image prefix attribute every time they import an application. It's cumbersome and unnecessary.
What has changed in APEX 4.2.2 and Later
Upon upgrade to Application Express 4.2.2 or application of the 4.2.2 patch set, the image prefix attribute of applications will be set to null if their value is equal to the instance-wide image prefix. In the simplest example, if the instance-wide image prefix is /i/ and your application's image prefix attribute is /i/, the application attribute will be set to null in 4.2.2.
If you try to edit the image prefix application attribute and set it to the instance-wide image prefix (e.g., /i/), it won't be saved and it will be null. Only if you change it to something other than the instance-wide image prefix will a non-null value be saved.
The utility script to adjust the image prefix instance-wide setting, apex/utilities/reset_image_prefix.sql, now only adjusts the instance setting and nothing more. No image prefix application attributes are touched. It's unnecessary, because a null image prefix application attribute means "use the setting of the APEX instance".
Now, if you use a non-standard image prefix for your APEX instance, you can import your application into any other instance without having to be aware of what the image prefix is on the target instance.
Now with that said.....I wish you a nice Memorial Day.