A DOI (digital object identifier) is a unique alphanumeric string assigned to a specific electronic resource which provides a persistent link to its location.
ex: doi: 10.1037/0278-6188.8.131.529
When available, include the DOI in your journal article references.
See pages 187-192 & 198-199 of the APA Publication Manual for more information
As indicated in the Publication Manual of the American Psychological Association (pg 198):
"If no DOI is assigned to the content and you retrieved it online, include the home page URL for the journal, newsletter, or magazine in the reference. Use this format: Retrieved from http://www.xxxxxxxx"