Development experience based on J2EE

1. CMP's findByPrimaryKey sometimes returns NULL:
If a key value that does not exist in the database is sent to findByPrimaryKey, an exception is thrown.
If the parameter is a keyword in the database, the corresponding primarykey is returned.
2. For BMP, a create method must be called Remotely. Otherwise, JDBC may occur only once. If this create () statement is not used to return an object, you can write a value that returns the primayKey.
3. response. setContentType ("text/xml; charset = UTF-8 ");
PrintWriter out = response. getWriter ();
However, when encoding needs to be specified, the two statements must be in this order; otherwise, an error will occur.
Because the default encoding for response. getWriter () is ISO-5988-1, it is not transcoded into GB2312, UTF-8.
In addition, this encoding does not support Chinese characters.
4. For jsp pages, you can access them by link without configuring them in the web. xml file. Only for specific access needs
Configure it to the web. xml file to control the page direction.
5. For the error handling page, you only need to set the following in the web. xml file:
<Error-page>
<Error-code> 404 </error-code>
<Location>/error. jsp </location>
</Error-page>
That is, when a page access error occurs, the page is displayed. There is no way to handle errors in the request!
6. For jsp and servlet display and processing, garbled characters may often occur.
Usually
For jsp, <% @ page contentType = "text/html; charset = gb2312" %>
For servlets, Response. setContentType ("text/html; charset = gb2312 ");
Generally, it is an insurance method for displaying Chinese characters, and other codes may often contain garbled characters.
7. Sometimes the request content must be encoded. For example
Request. setCharacterEncoding ("GBK ");
8. Read files from the server database in the servlet (the fileupload component smartupload can be used for uploading). Example:
String SQL = "SELECT FileSize, FileStore FROM ResourseStore where MagazineNumber = '" + MIDValue + "'";
Stmt = con. prepareStatement (SQL );
ResultSet rs1_stmt.exe cuteQuery ();