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Your Upload Project's Main Components
Let's begin with the mechanism of persisting the uploaded file and get answers to the
three questions noted above.
- Where? You will store uploaded files in a database.
- Why? In many cases, it really is the proper solution. Using this article's solution, you won't need to bother with syncronizing uploaded files once you're doing partial backup of the application—you just need to backup the database. Additionally, you won't need to keep clumsy associations of a user with their file on a file system, etc.
- How? Using a BLOB (Binary Large OBject) field. Such fields are used to store data that comes in large and often raw or binary formats. Hibernate allows you to operate with these fields very easily.
The enterprise application (EAR) consists of two parts: the Web-tier (WAR) and the business-tier (EJB). The business-tier contains one stateless session bean which persists data—with the help of Hibernate. Figure 1 shows the EJB's remote interface.
Figure 1. The HelloSession EJB's interface.
From the Web-tier, this EJB accesses by a business delegate.
Notice that the code uses an object of the class User. What does User mean? It is a "Plain Old Java Object" (POJO) of a user entity that is kept in the database. You will actively use this User POJO. Setting it's attributes and asking the EJB to persist it, fetches a list of all User entities already in the database. Or, it fetches a specific User entity into such POJO and uses a getter's to access it's attributes (see Figure 2).
Figure 2. All the Web-tier's servlets actively use the User POJO.
As you can see, the Web-tier consists only of three servlets (Struts Action's). One for uploading files, one for downloading files, and one to list all the User entities with their files.
- DownloadFileAction: This servlet takes an argument id with the id of a user in database. It then loads this user's entity and passes the file from the BLOB field to ServletOutputStream.
- UploadFileAction: This servlet takes data from an HTML form and populates DynaActionForm with it. It takes only the user name and the uploaded file.
- ListAllFilesAction: This servlet takes no input arguments or data. It simply loads all the available User entities from the database.
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