Add Persistance to Bulletin Board Posts

Add Post Persistence to the Bulletin Board Provider

The current version of Bulletin Board looses all posts when the server (or webapp) shuts down. In this section we'll add code to save the posts and restore them when the server restarts.

There are a number of ways we could do this.
The most foolproof method would probably be to use JDBC and keep posts in a database.
Less elaborate schemes would keep the posts in a disk file:
We could open a file, make the change and close the file each time a post was added or removed. That would be fairly reliable, but would involve a lot of overhead.
We're going to assume bulletin board posts aren't mission critical, and just save the posts when the server shuts down. Besides being simple, this approach allows us to demonstrate the SrvcProvFtry quiesce method.

Source Backup

The current (V0.2.0) version of BBoardProvFtry isn't very different from V0.1.0, so we won't make a permanent backup.

Add Global Fields

Define a default path for the saved posts file
Add a private constants section after the public constants:

• // Private constants
• public static final String DEF_POST_FILE_PATH = "BBPosts.txt";

Create a global field to hold a file object

• Add a private File postFile = null; line to the main class private variables section

Create methods to save and restore the bulletin board.

Create private methods in the BBoardProvFtry main class:

• private void savePosts() { ... }
• private void restorePosts() { ... }

The methods don't need to be synchronized because they're only called at startup and shutdown.

Update the init method to load saved posts

Add local variables

• Initialize postFilePath to the default value:
  String postFilePath = DEF_POST_FILE_PATH;
• Define an iterator to loop through params:
  Iterator pitr;
• Define a variable to hold a tag-value pair:
  String[] pair;

Get the path to the saved posts file

Check the params list for a file parameter by iterating through it looking for a file tag.

• if (pair[0].toLowerCase().startsWith("file")) postFilePath = pair[1];

(We usually make parameter tags case-insensitive.)

Initialize the postFile field. We use a global variable because we'll use it again at shutdown.

• postFile = new File(postFilePath);

Restore the bulletin board from the saved posts file

First see if the file exists.
If it does we call restorePosts to restore the bulletin board.
If it doesn't we log a warning and continue:

• log.println("BBoardProvFtry WARNING: " + postFilePath + " Does not exist");

Implement the savePosts method

We'll leverage the BBoardProvImp.getPostsMsg method to save and restore the bulletin board. The TvList class has readFrom and writeTo methods to read and write the list to disk files. For savePosts, we'll copy the bulk of the getPostsMsg method to the main class and create a method used by both getPostsMsg and savePosts.

Create a getPostList method in the main class

• copy getPostsMsg to the end of the main class
• Change the name to getPostList
• Change the scope to private
• Rename the postMsg variable to postList
• Remove the (TAG_BB_POSTS, "") parameter from the postList constructor:
• TvList postList = new TvList();
• Change the javadoc comments:
• * Get the bulletin board posts, Sorted last post first.
• * @return Bulletin board posts

Fill in the savePosts method

• Get a PrintWriter for the post file
• Call the getPostList method to get a TVList containing the posts
• Use the TvList writeTo method to save the posts to the post file.

Update the BBoardProvImp.getPostsMsg method

The new getPostsMsg method is short, so we just replace the old content.

• Call the getPostList method to get a TVList containing the posts
• Use the TvList addFirst method to add the message header to the list
• postMsg.addFirst(TAG_BB_POSTS, "");
• Return the list

Implement the restorePosts method

• Get a BufferedReader for the post file
• Use the TvList readFrom method to read the posts from the file:
• postList.readFrom(postSrc);
• Loop through the post list, adding each post to the bbPosts map

(Details omitted. It's just plain java.)

Update the quiesce method

Replace the No action needed comment with a call to savePosts:

• savePosts();

The Bulletin Board Client

We don't have to make any changes to the client.

Deployment

For the native server version of EventWeb, we don't have to do anything.
If the BBPosts.txt file doesn't exist when the server shuts down, BBoardProvFtry will create it and save the posts in it. If it does exist, the old version will be overwritten.

For the servlet version, we want to keep configuration files in the root folder of the webapp. To do that, we use the File parameter to set the path to BBPosts.txt

Add the File parameter to the BBoardProvFtry line in RqstRouterServlet.ini:

• SrvcProv = eventweb.prov.BBoardProvFtry -File webapps/EvwDemosEc/BBPosts.txt