Chapter 4 XML Processing

191

face. Implementing this interface enables you to retrieve the resources referred

to in the style sheets by the 

xsl:import

 or 

xsl:include

 statements. For an ap 

plication using a large set of componentized style sheets, this may be used to

implement a cache in much the same way as the 

EntityResolver

. You can use

EntityResolver

 and 

URIResolver

 to implement: 

I

A caching mechanism in the application itself, or 

I

A custom URI lookup mechanism that may redirect system and public refer 

ences to a local copy of a public repository. 

You can use both caching approaches together to ensure even better perfor 

mance. Use a proxy cache for static entities whose lifetime is greater than the

application's lifetime. This particularly works with public schemas, which include

the version number in their public or system identifier, since they evolve through

successive versions. A custom entity resolver may first map public identifiers

(usually in the form of a URI) into system identifiers (usually in the form of an

URL). Afterwards, it applies the same techniques as a regular cache proxy when

dealing with system identifiers in the form of an URL, especially checking for

updates and avoiding caching dynamic content. Using these caching approaches

often results in a significant performance improvement, especially when external

entities are located on the network. Code Example 4.13 illustrates how to

implement a caching entity resolver using the SAX API.

import java.util.Map;

import java.util.WeakHashMap;

import java.lang.ref.SoftReference;

import org.xml.sax.*;

public class CachingEntityResolver implements EntityResolver {

public static final int MAX_CACHE_ENTRY_SIZE = ...;

private EntityResolver parentResolver;

private Map entities = new WeakHashMap();

static private class Entity {

String name;

byte[] content;

}

public CachingEntityResolver(EntityResolver parentResolver) {

this.parentResolver = parentResolver;