APA style guidelines use the DOI (Digital Object Identifier) for locating journal articles.This alpha-numeric string usually appears on the article or in the database record. Its purpose is to identify content and provide a persistent link to the article's location on the Internet.

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• Include the digital object identifier (DOI) in the reference if one is assigned (see section 6.31 of the Publication Manual). 
• The DOI System provides a framework for persistent identification of an article.
• More and more publishers are assigning a DOI when the article is published and made available electronically.
• Because the article can be retrieved by using the DOI, no database name or date of retrieval is necessary. 

If no DOI has been assigned to the content or the database record, enter the citation information using the DOI Lookup. By entering information about the article in the lookup search, you will be able to determine whether a DOI has been assigned to the article.

These are areas to consider when no DOI has been assigned:

• Include the issue number if the journal is paginated by issue.
• If no DOI has been assigned to the content and you retrieved it online, provide the home page URL of the journal, book, or report publisher. If you are accessing the article from a private database, you may need to do a quick web search to locate this URL.
• In general it is not necessary to include database information. 
•  It is suggested that since DOI numbers are complex, you should copy and paste the DOI into your reference.

About retrieval dates: The date an electronic source was retrieved is important only if the content being cited is likely to be changed or updated.

About URLs: Do not insert a hyphen if you need to break a URL across lines; instead, break the URL before most punctuation (an exception would be http://). Do not add a period after the URL. 

APA Journal Article Citation with DOI

Author's Last Name(s), First Initial, Middle Initial. (year). Title of Article. Name of Journal, Volume, pages. doi:

Remember: When a DOI is used, no further retrieval information is needed to identify or locate the content.

Examples:

Grund, P., Kyoto, M, & Rissanen, M. (2004). Editing the Salem witchcraft records:

     An exploration of  linguistic treasury. American Speech, 79, 146-166. doi:

     10.1215/00031283-79-2-146

Jensen, S. K., Aars, J., Lydersen, C., Kovacs, K. M., & Asbakk, K. (2010). The prevalence

     of Toxoplasma gondii in polar bears and their marine mammal prey: Evidence for a

     marine transmission pathway? Polar Biology, 33, 599-606. doi:

     10.1007/s00300-009-0735-x   

Killacky, J., Thomas, C., & Accomando, A. (2002). Learning communities and community

     colleges: A case study.  Community College Journal of Research and Practice, 26,

     763-775.  doi: 10.1080/10668920290104859

Silverman, R., & Hines, S. (2009). The effects of multimedia-enhanced instruction on

     the vocabulary of English-language learners and non-English-language learners

     in pre-kindergarten through second grade. Journal of Educational Psychology,

     101, 305-314. doi: 10.1037/a0014217

APA Journal Article Citation without DOI

Author's Last Name(s), First Initial, Middle Initial. (year). Title of article. Name of Journal, Volume (issue), pages. Retrieved from http://xxxx

Section 6.32 of the Publication Manual states, "If you are accessing the article from a private database, you may need to do a quick web search to locate this [the publication's] URL." The url used in the citation should be for the publisher's website.

Examples:

Lenk, K. M., Toomey, T. L., Erickson, D. J., Kilian, G. R., Nelson, T. F., & Fabian, L. E.

     A. (2010). Alcohol control policies and practices at professional sports stadiums.

     Public Health Reports, 125, 665-673. Retrieved from

     http://www.publichealthreports.org/

Mantel, B. (2009, April 10). Business bankruptcy. CQ Researcher 19(14). Retrieved from 

      http://library.cqpress.com 

Warren, E. & Westbrook, J. L. (2005). Contracting out of bankruptcy: An empirical

     intervention. Harvard Law Review, 118(4), 1197-1254. Retrieved

     from http://www.harvardlawreview.org/index.php