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6.3 A Front End to Various Search Engines

135

501 (Not Implemented)

The 501 (

SC_NOT_IMPLEMENTED

) status notifies the client that the 

server doesn't support the functionality to fulfill the request. It is used, 

for example, when the client issues a command like 

PUT

 that the server 

doesn't support. 

502 (Bad Gateway)

A value of 502 (

SC_BAD_GATEWAY

) is used by servers that act as proxies 

or gateways; it indicates that the initial server got a bad response from 

the remote server. 

503 (Service Unavailable)

A status code of 503 (

SC_SERVICE_UNAVAILABLE

) signifies that the 

server cannot respond because of maintenance or overloading. For 

example, a servlet might return this header if some thread or database 

connection pool is currently full. The server can supply a 

Retry After

header to tell the client when to try again. 

504 (Gateway Timeout)

A value of 504 (

SC_GATEWAY_TIMEOUT

) is used by servers that act as 

proxies or gateways; it indicates that the initial server didn't get a timely 

response from the remote server. This status code is new in HTTP 1.1. 

505 (HTTP Version Not Supported)

The 505 (

SC_HTTP_VERSION_NOT_SUPPORTED

) code means that the 

server doesn't support the version of HTTP named in the request line. 

This status code is new in HTTP 1.1.

6.3 A Front End to Various Search 

Engines

Listing 6.1 presents an example that makes use of the two most common sta 

tus codes other than 200 (OK): 302 (Found) and 404 (Not Found). The 302

code is set by the shorthand 

sendRedirect

 method of 

HttpServletRe 

sponse

, and 404 is specified by 

sendError

. 

In this application, an HTML form (see Figure 6 1 and the source code in

Listing 6.3) first displays a page that lets the user choose a search string, the

number of results to show per page, and the search engine to use. When the

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