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XML documents typically refer to external entities, for example the public and/or system ID for the Document Type Definition. These external relationships are expressed using URIs, typically as URLs. However, if they are absolute URLs, they only work when your network can reach them. Relying on remote resources makes XML processing susceptible to both planned and unplanned network downtime. Conversely, if they are relative URLs, they're only useful in the context where they were initially created. For example, the URL "../../xml/dtd/docbookx.xml" will usually only be useful in very limited circumstances. One way to avoid these problems is to use an entity resolver (a standard part of SAX) or a URI Resolver (a standard part of JAXP). A resolver can examine the URIs of the resources being requested and determine how best to satisfy those requests. The XML catalog is a document describing a mapping between external entity references and locally-cached equivalents. ==Example Catalog.xml== The following simple catalog shows how one might provide locally-cached DTDs for an XHTML page validation tool, for example. PUBLIC "-//OASIS//DTD Entity Resolution XML Catalog V1.0//EN" "http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd"> This catalog makes it possible to resolve -//W3C//DTD XHTML 1.0 Strict//EN to the local URI dtd/xhtml1/xhtml1-strict.dtd. Similarly, it provides local URIs for two other public IDs. Note that the document above includes a DOCTYPE - this may cause the parser to attempt to access the system ID URL for the DOCTYPE (i.e. http://www.oasis-open.org/committees/entity/release/1.0/catalog.dtd) before the catalog resolver is fully functioning, which is probably undesirable. To prevent this, simply remove the DOCTYPE declaration. The following example shows this, and also shows the equivalent <system/> declarations as an alternative to <public/> declarations. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「XML Catalog」の詳細全文を読む スポンサード リンク
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