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17.5 Sending Data by POST and Processing the Results Directly

451

results). These two options are discussed in Sections 17.1 and 17.3, respec 

tively. With 

POST

 data, however, only the second option is available since the

URL

 constructor has no method to let you associate 

POST

 data with it. Sending

POST

 data has some of the same advantages and disadvantages as when

applets send 

GET

 data. The two main disadvantages are that the server side

program must be on the host from which the applet was loaded, and that the

applet is required to display all the results itself: it cannot pass HTML to the

browser in a portable manner. On the plus side, the server side program can

be simpler (not needing to wrap the results in HTML) and the applet can

update its display without requiring the page to be reloaded. Furthermore,

applets that communicate using 

POST

 can use serialized data streams to send

data  to a servlet, in addition to reading serialized data from servlets. This is

quite an advantage, since serialized data simplifies communication and

HTTP tunneling lets you piggyback on existing connections through firewalls

even when direct socket connections are prohibited. Applets using 

GET

 can

read serialized data (see Section 17.4) but are unable to send it since it is not

legal to append arbitrary binary data to URLs.

Thirteen steps are required for the applet to send 

POST

 data to the server

and read the results, as shown below. Although there are many required

steps, each step is relatively simple. The code is slightly simplified by the

omission of 

try

/

catch

 blocks around the statements.

1. Create a 

URL

 object referring to the applet's home host. 

As before, since the URL must refer to the host the applet came 

from, it makes the most sense to specify a URL suffix and con 

struct the rest of the URL automatically.

URL currentPage = getCodeBase();

String protocol = currentPage.getProtocol();

String host = currentPage.getHost();

int port = currentPage.getPort();

String urlSuffix = "/servlet/SomeServlet";

URL dataURL = 

new URL(protocol, host, port, urlSuffix);

2. Create a 

URLConnection

 object. This object will be used to 

obtain input and output streams that connect to the server.

URLConnection connection = dataURL.openConnection();

3. Instruct the browser not to cache the results. 

connection.setUseCaches(false); 

4. Tell the system to permit you to send data, not just read it. 

connection.setDoOutput(true);

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