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38

Chapter 2 First Servlets

automatically uses the status line and header settings of 

doGet

 to answer

HEAD

 requests.

The SingleThreadModel Interface

Normally, the system makes a single instance of your servlet and then creates

a new thread for each user request, with multiple simultaneous threads run 

ning if a new request comes in while a previous request is still executing. This

means that your 

doGet

 and 

doPost

 methods must be careful to synchronize

access to fields and other shared data, since multiple threads may be trying to

access the data simultaneously. See Section 7.3 (Persistent Servlet State and

Auto Reloading Pages) for more discussion of this. If you want to prevent this

multithreaded access, you can have your servlet implement the 

SingleTh 

readModel

 interface, as below.

public class YourServlet extends HttpServlet

implements SingleThreadModel {

...

}

If you implement this interface, the system guarantees that there is never

more than one request thread accessing a single instance of your servlet. It

does so either by queuing up all the requests and passing them one at a time

to a single servlet instance, or by creating a pool of multiple instances, each of

which handles one request at a time. This means that you don't have to worry

about simultaneous access to regular fields (instance variables) of the servlet.

You  do, however, still have to synchronize access to class variables (

static

fields) or shared data stored outside the servlet. 

Synchronous access to your servlets can significantly hurt performance

(latency) if your servlet is accessed extremely frequently. So think twice

before using the 

SingleThreadModel

 approach.

The destroy Method

The server may decide to remove a previously loaded servlet instance, per 

haps because it is explicitly asked to do so by the server administrator, or per 

haps because the servlet is idle for a long time. Before it does, however, it

calls the servlet's 

destroy

 method. This method gives your servlet a chance

to close database connections, halt background threads, write cookie lists or

hit counts to disk, and perform other such cleanup activities. Be aware, how 

ever, that it is possible for the Web server to crash. After all, not all Web serv 

ers are written in reliable programming languages like Java; some are written

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

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