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34

Chapter 2 First Servlets

Figure 2 4

Result of the HelloWWW3 servlet.

2.6 The Servlet Life Cycle

Earlier in this book, I vaguely referred to the fact that only a single instance

of a servlet gets created, with each user request resulting in a new thread that

is handed off to 

doGet

 or 

doPost

 as appropriate. I'll now be more specific

about how servlets are created and destroyed, and how and when the various

methods are invoked. I'll give a quick summary here, then elaborate in the

following subsections. 

When the servlet is first created, its 

init

 method is invoked, so that is

where you put one time setup code. After this, each user request results in a

thread that calls the 

service

 method of the previously created instance.

Multiple concurrent requests normally result in multiple threads calling 

ser 

vice

 simultaneously, although your servlet can implement a special interface

that stipulates that only a single thread is permitted to run at any one time.

The 

service

 method then calls 

doGet

, 

doPost

, or another 

doXxx

 method,

depending on the type of HTTP request it received. Finally, when the server

decides to unload a servlet, it first calls the servlet's 

destroy

 method.

The init Method

The 

init

 method is called when the servlet is first created and is not called

again for each user request. So, it is used for one time initializations, just as

with the 

init

 method of applets. The servlet can be created when a user first

invokes a URL corresponding to the servlet or when the server is first started,

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

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