org.apache.derby.iapi.types
Class SqlXmlUtil

java.lang.Object
  org.apache.derby.iapi.types.SqlXmlUtil

public class SqlXmlUtil
extends java.lang.Object

This class contains "utility" methods that work with XML-specific objects that are only available if JAXP and/or Xalan are in the classpath. NOTE: This class is only compiled with JDK 1.4 and higher since the XML-related classes that it uses (JAXP and Xalan) are not part of earlier JDKs. Having a separate class for this functionality is beneficial for two reasons: 1. Allows us to allocate XML objects and compile an XML query expression a single time per statement, instead of having to do it for every row against which the query is evaluated. An instance of this class is created at compile time and then passed to the appropriate operator implementation method in XML.java. 2. By keeping all XML-specific references in this one class, we have a single "point of entry" to the XML objects--namely, the constructor for this class. Thus, if we always make sure to check for the required XML classes _before_ calling this class's constructor, we can detect early on whether some classes (ex. Xalan) are missing, and can throw a friendly error up front, instead of a ClassNotFoundException somewhere deeper in the execution codepath. The initial check for the required XML classes can be found in XML.checkXMLRequirements(). Note that we don't want to put references to XML-specific objects directly into XML.java because that class (XML.java) is instantiated anytime a table with an XML column is referenced. That would mean that if a user tried to select a non-XML column (ex. integer) from a table that had at least one XML column in it, the user would have to have JAXP and Xalan classes in his/her classpath--which we don't want. Instead, by keeping all XML-specific objects in this one class, and then only instantiating this class when an XML operator is used (either implicitly or explicitly), we make it so that the user is only required to have XML-specific classes in his/her classpath _if_ s/he is trying to access or operate on XML values.

Nested Class Summary

private class

SqlXmlUtil.XMLErrorHandler

Field Summary

private javax.xml.parsers.DocumentBuilder	dBuilder
private java.lang.String	opName
private XPathExpression	query
private java.lang.String	queryExpr
private boolean	recompileQuery
private javax.xml.transform.Transformer	serializer
private static java.lang.reflect.Method	TO_PLAIN_STRING An object representing the BigDecimal.toPlainString() method if it's available on the platform.

Constructor Summary

SqlXmlUtil()
Constructor: Initializes objects required for parsing and serializing XML values.

Method Summary

void	compileXQExpr(java.lang.String queryExpr, java.lang.String opName) Take the received string, which is an XML query expression, compile it, and store the compiled query locally.
protected java.util.List	evalXQExpression(XMLDataValue xmlContext, boolean returnResults, int[] resultXType) Evaluate this object's compiled XML query expression against the received xmlContext.
private void	loadSerializer() Create an instance of Xalan serializer for the sake of serializing an XML value according the SQL/XML specification for serialization.
private static java.lang.String	numberToString(double d) Convert a number returned by an XPath query to a string, following the rules for the XPath string function.
protected java.lang.String	serializeToString(java.util.List items, XMLDataValue xmlVal) Take an array list (sequence) of XML nodes and/or string values and serialize that entire list according to SQL/XML serialization rules, which ultimately point to XML serialization rules as defined by w3c.
protected java.lang.String	serializeToString(java.lang.String xmlAsText) Take a string representing an XML value and serialize it according SQL/XML serialization rules.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

dBuilder

private javax.xml.parsers.DocumentBuilder dBuilder

serializer

private javax.xml.transform.Transformer serializer

query

private XPathExpression query

queryExpr

private java.lang.String queryExpr

opName

private java.lang.String opName

recompileQuery

private boolean recompileQuery

TO_PLAIN_STRING

private static final java.lang.reflect.Method TO_PLAIN_STRING

An object representing the BigDecimal.toPlainString() method if it's available on the platform. If it's not available, this field will be initialized to null, and in that case the BigDecimal.toString() method should be used instead without reflection.

The behaviour of the toString() method changed when toPlainString() was introduced in Java SE 5. On older platforms, it behaves just like toPlainString() does on newer platforms. So when toPlainString() is not available, it is safe to fall back to toString(). It behaves differently on newer platforms, so we need to use toPlainString() when it is available in order to get consistent behaviour across all platforms.

See Also:

numberToString(double)

Constructor Detail

SqlXmlUtil

public SqlXmlUtil()
           throws StandardException

Constructor: Initializes objects required for parsing and serializing XML values. Since most XML operations that require XML-specific classes perform both parsing and serialization at some point, we just initialize the objects up front.

Throws:

StandardException

Method Detail

compileXQExpr

public void compileXQExpr(java.lang.String queryExpr,
                          java.lang.String opName)
                   throws StandardException

Take the received string, which is an XML query expression, compile it, and store the compiled query locally. Note that for now, we only support XPath because that's what Xalan supports.

Parameters:

queryExpr - The XPath expression to compile

Throws:

StandardException

serializeToString

protected java.lang.String serializeToString(java.lang.String xmlAsText)
                                      throws java.lang.Exception

Take a string representing an XML value and serialize it according SQL/XML serialization rules. Right now, we perform this serialization by first parsing the string into a JAXP Document object, and then applying the serialization semantics to that Document. That seems a bit inefficient, but neither Xalan nor JAXP provides a more direct way to do this.

Parameters:

xmlAsText - String version of XML on which to perform serialization.

Returns:

A properly serialized version of xmlAsText.

Throws:

java.lang.Exception

serializeToString

protected java.lang.String serializeToString(java.util.List items,
                                             XMLDataValue xmlVal)
                                      throws javax.xml.transform.TransformerException

Take an array list (sequence) of XML nodes and/or string values and serialize that entire list according to SQL/XML serialization rules, which ultimately point to XML serialization rules as defined by w3c. As part of that serialization process we have to first "normalize" the sequence. We do that by iterating through the list and performing the steps for "sequence normalization" as defined here: http://www.w3.org/TR/xslt-xquery-serialization/#serdm This method primarily focuses on taking the steps for normalization; for the rest of the serialization work, we just make calls on the DOMSerializer class provided by Xalan.

Parameters:

items - List of items to serialize

xmlVal - XMLDataValue into which the serialized string returned by this method is ultimately going to be stored. This is used for keeping track of XML values that represent sequences having top-level (parentless) attribute nodes.

Returns:

Single string holding the serialized version of the normalized sequence created from the items in the received list.

Throws:

javax.xml.transform.TransformerException

evalXQExpression

protected java.util.List evalXQExpression(XMLDataValue xmlContext,
                                          boolean returnResults,
                                          int[] resultXType)
                                   throws java.lang.Exception

Evaluate this object's compiled XML query expression against the received xmlContext. Then if returnResults is false, return an empty sequence (ArrayList) if evaluation yields at least one item and return null if evaluation yields zero items (the caller can then just check for null to see if the query returned any items). If returnResults is true, then return return a sequence (ArrayList) containing all items returned from evaluation of the expression. This array list can contain any combination of atomic values and XML nodes; it may also be empty. Assumption here is that the query expression has already been compiled and is stored in this.query.

Parameters:

xmlContext - The XML value against which to evaluate the stored (compiled) query expression

returnResults - Whether or not to return the actual results of the query

resultXType - The qualified XML type of the result of evaluating the expression, if returnResults is true. If the result is a sequence of exactly one Document node then this will be XML(DOCUMENT(ANY)); else it will be XML(SEQUENCE). If returnResults is false, this value is ignored.

Returns:

If returnResults is false then return an empty ArrayList if evaluation returned at least one item and return null otherwise. If returnResults is true then return an array list containing all of the result items and return the qualified XML type via the resultXType parameter.

Throws:

java.lang.Exception - thrown on error (and turned into a StandardException by the caller).

loadSerializer

private void loadSerializer()
                     throws javax.xml.transform.TransformerConfigurationException

Create an instance of Xalan serializer for the sake of serializing an XML value according the SQL/XML specification for serialization.

Throws:

javax.xml.transform.TransformerConfigurationException

numberToString

private static java.lang.String numberToString(double d)
                                        throws java.lang.IllegalAccessException,
                                               java.lang.reflect.InvocationTargetException

Convert a number returned by an XPath query to a string, following the rules for the XPath string function.

Parameters:

d - double representation of the number

Returns:

String representation of the number

Throws:

java.lang.IllegalAccessException

java.lang.reflect.InvocationTargetException