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2.6 The Servlet Life Cycle

35

depending on how you have registered the servlet with the Web server. It will

be created for the first user request if it is not explicitly registered but is

instead just placed in one of the standard server directories. See the discus 

sion of Section 2.2 (A Simple Servlet Generating Plain Text) for details on

these directories.

There are two versions of 

init

: one that takes no arguments and one that

takes a 

ServletConfig

 object as an argument. The first version is used when

the servlet does not need to read any settings that vary from server to server.

The method definition looks like this:

public void init() throws ServletException {

// Initialization code...

}

For examples of this type of initialization, see Section 2.8 (An Example

Using Servlet Initialization and Page Modification Dates) later in this chap 

ter. Section 18.8 (Connection Pooling: A Case Study) in the chapter on JDBC

gives a more advanced application where 

init

 is used to preallocate multiple

database connections.

The second version of 

init

 is used when the servlet needs to read

server specific settings before it can complete the initialization. For example,

the servlet might need to know about database settings, password files,

server specific performance parameters, hit count files, or serialized cookie

data from previous requests. The second version of 

init

 looks like this:

public void init(ServletConfig config)

throws ServletException {

super.init(config);

// Initialization code...

}

Notice two things about this code. First, the 

init

 method takes a 

Servlet 

Config

 as an argument. 

ServletConfig

 has a 

getInitParameter

 method

with which you can look up initialization parameters associated with the servlet.

Just as with the 

getParameter

 method used in the 

init

 method of applets,

both the input (the parameter name) and the output (the parameter value) are

strings. For a simple example of the use of initialization parameters, see Section

2.7 (An Example Using Initialization Parameters); for a more complex exam 

ple, see Section 4.5 (Restricting Access to Web Pages) where the location of a

password file is given through the use of 

getInitParameter

. Note that

although you look up parameters in a portable manner, you set  them  in  a

server specific way. For example, with Tomcat, you embed servlet properties in

a file called 

web.xml

, with the JSWDK you use 

servlets.properties

, with

the WebLogic application server you use 

weblogic.properties

, and with

the Java Web Server you set the properties interactively via the administration

Second edition of this book: www.coreservlets.com; Sequel: www.moreservlets.com.

Servlet and JSP training courses by book's author: courses.coreservlets.com.