Prentice Hall and Sun Microsystems. Personal use only; do not redistribute.

7.3 Persistent Servlet State and Auto Reloading Pages

155

apply to servlets. Servlets can also store persistent data in the 

Servlet 

Context

 object that is available through the 

getServletContext

 method.

ServletContext

 has 

setAttribute

 and 

getAttribute

 methods that let

you store arbitrary data associated with specified keys. The difference

between storing data in instance variables and storing it in the 

Servlet 

Context

 is that the 

ServletContext

 is shared by all servlets in the servlet

engine (or in the Web application, if your server supports such a capability).

Listing 7.1 shows the main servlet class. First, it receives a request that

specifies two parameters: 

numPrimes

 and 

numDigits

. These values are nor 

mally collected from the user and sent to the servlet by means of a simple

HTML form. Listing 7.2 shows the source code and Figure 7 1 shows the

result. Next, these parameters are converted to integers by means of a simple

utility that uses 

Integer.parseInt

 (see Listing 7.5). These values are then

matched by the 

findPrimeList

  method  to  a 

Vector

 of recent or ongoing

calculations to see if there is a previous computation corresponding to the

same two values. If so, that previous value (of type 

PrimeList

) is used; oth 

erwise, a new 

PrimeList

 is created and stored in the ongoing calculations

Vector

, potentially displacing the oldest previous list. Next, that 

PrimeList

is checked to determine if it has finished finding all of its primes. If not, the

client is sent a 

Refresh

  header  to  tell  it  to  come  back  in  five  seconds  for

updated results. Either way, a bulleted list of the current values is returned to

the client. 

Listings 7.3 (

PrimeList.java

) and 7.4 (

Primes.java

) present auxiliary

code used by the servlet. 

PrimeList.java

 handles the background thread

for the creation of a list of primes for a specific set of values. 

Primes.java

contains the low level algorithms for choosing a random number of a speci 

fied length and then finding a prime at or above that value. It uses built in

methods in the 

BigInteger

 class; the algorithm for determining if the num 

ber is prime is a probabilistic one and thus has a chance of being mistaken.

However, the probability of being wrong can be specified, and I use an error

value of 100. Assuming that the algorithm used in most Java implementations

is the Miller Rabin test, the likelihood of falsely reporting a composite num 

ber as prime is provably less than 2

100

. This is almost certainly smaller than

the likelihood of a hardware error or random radiation causing an incorrect

response in a deterministic algorithm, and thus the algorithm can be consid 

ered deterministic. 

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

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