The Data Axis and Filters in Java Produce pdf417 in Java The Data Axis and Filters

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2.7.4 The Data Axis and Filters use jdk pdf 417 encoding toaccess pdf417 with java Barcodes for Mobile Applications What we have descri Java pdf417 2d barcode bed so far is essentially similar to the architecture of every general-purpose webserver. There are, of course, differences in the details, but the request processing (metadata generator logger) phases are common. The major innovation in Apache 2, which transforms it from a mere webserver (like Apache 1.

3 and others) into a powerful applications platform, is the filter chain. The filter chain can be represented as a data axis, orthogonal to the requestprocessing axis (Figure 2-4). The request data may be processed by input filters before reaching the content generator, and the response may be processed by output filters before being sent to the client.

Filters enable a far cleaner and more efficient implementation of data processing than was possible in the past, as well as separating content generation from its transformation and aggregation. 2.7.

4.1 Handler or Filter Many applications can be implemented as either a handler or a filter. Sometimes it may be clear that one of these solutions is appropriate and the other would be nonsensical, but between these extremes lies a gray area.

How does one decide whether to write a handler or a filter When making this decision, there are several questions to consider: Feasibility: Can it be made to work in both cases If not, there s an instant decision. Utility: Is the functionality it provides more useful in one case than the other Filters are often far more useful than handlers, because they can be reused with different content generators and chained both with generators and other filters. But every request has to be processed by some handler, even if it does nothing!.

2.7 Request Processing in Apache Complexity: Is on PDF417 for Java e version substantially more complex than the other Will it take more time and effort to develop, and/or run more slowly Filter modules are usually more complex than the equivalent handler, because a handler is in full control of its data and can read or write at will, whereas a filter has to implement a callback that may be called several times with partial data, which it must treat as unstructured chunks. We will discuss this issue in detail in 8. For example, Apache 1.

3 users can do an XSLT transformation by building it into handlers, such as CGI or PHP. Alternatively, they can use an XSLT module, but this is very slow and cumbersome (this author tried an XSLT module for Apache 1.3, but found it many hundreds of times slower than running XSLT in a CGI script operating on temporary files).

Running XSLT in a handler works, but loses modularity and reusability. Any nontrivial application that needs it has to reinvent that. Data Axis Output Filters Accept Request Content Generator Logging Processing Axis Metadata Processing Input Filters FIGURE 2-4 Apache 2 introduces a new data axis enabling a new range of powerful applications 2 The Apache Platform and Architecture wheel, using whatev j2se pdf417 2d barcode er libraries are available for the programming or scripting language used and often resorting to ugly hacks such as temporary files. Apache 2, by contrast, allows us to run XSLT in a filter. Content handlers requiring XSLT can simply output the XML as is, and leave the transformation to Apache.

The first XSLT module for Apache 2, written by Phillip Dunkel and released while Apache 2.0 was still in beta testing, was initially incomplete, but already worked far better than XSLT in Apache 1.3.

It is now further improved, and is one of a choice of XSLT modules. This book s author developed another XSLT module. More generally, if a module has both data inputs and outputs, and if it may be used in more than one application, then it is a strong candidate for implementation as a filter.


2 Content Generator Examples The default handler sends a file from the local disk under the DocumentRoot. Although a filter could do that, there s nothing to be gained. CGI, the generic API for server-side programming, is a handler.

Because CGI scripts expect the central position in the webserver architecture, it has to be a handler. However, a somewhat similar framework for external filters is also provided by mod_ext_filter. The Apache proxy is a handler that fetches contents from a back-end server.

Any form-processing application will normally be implemented as a handler particularly those that accept POST data, or other operations that can alter the state of the server itself. Likewise, applications that generate a report from any back end are usually implemented as handlers. However, when the handler is based on HTML or XML pages with embedded programming elements, it can usefully be implemented as a filter.


3 Filter Examples mod_include implements server-side includes, a simple scripting language embedded in pages. It is implemented as a filter, so it can post-process content from any content generator, as discussed earlier with reference to XSLT..

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