/*

   Derby - Class org.apache.derby.iapi.types.XML

   Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to you under the Apache License, Version 2.0
   (the "License"); you may not use this file except in compliance with
   the License.  You may obtain a copy of the License at

      http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.

 */

package org.apache.derby.iapi.types;

import org.apache.derby.iapi.error.StandardException;

import org.apache.derby.iapi.services.cache.ClassSize;
import org.apache.derby.iapi.services.io.ArrayInputStream;
import org.apache.derby.iapi.services.io.StoredFormatIds;
import org.apache.derby.iapi.services.io.StreamStorable;
import org.apache.derby.iapi.services.io.Storable;
import org.apache.derby.iapi.services.io.TypedFormat;
import org.apache.derby.shared.common.sanity.SanityManager;
import org.apache.derby.iapi.sql.conn.ConnectionUtil;

import org.apache.derby.iapi.reference.SQLState;

import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Types;
import java.text.RuleBasedCollator;

import java.io.InputStream;
import java.io.IOException;
import java.io.ObjectOutput;
import java.io.ObjectInput;

import java.util.List;

/**
 * This type implements the XMLDataValue interface and thus is
 * the type on which all XML related operations are executed.
 *
 * The first and simplest XML store implementation is a UTF-8
 * based one--all XML data is stored on disk as a UTF-8 string,
 * just like the other Derby string types.  In order to make
 * it possible for smarter XML implementations to exist in
 * the future, this class always writes an "XML implementation
 * id" to disk before writing the rest of its data.  When
 * reading the data, the impl id is read first and serves
 * as an indicator of how the rest of the data should be
 * read.
 *
 * So long as there's only one implementation (UTF-8)
 * the impl id can be ignored; but when smarter implementations
 * are written, the impl id will be the key to figuring out
 * how an XML value should be read, written, and processed.
 */
public class XML
    extends DataType implements XMLDataValue, StreamStorable
{
    // Id for this implementation.  Should be unique
    // across all XML type implementations.
    protected static final short UTF8_IMPL_ID = 0;

    // Guess at how much memory this type will take.
    private static final int BASE_MEMORY_USAGE =
        ClassSize.estimateBaseFromCatalog(XML.class);

    // Some syntax-related constants used to determine
    // operator behavior.
    public static final short XQ_PASS_BY_REF = 1;
    public static final short XQ_PASS_BY_VALUE = 2;
    public static final short XQ_RETURN_SEQUENCE = 3;
    public static final short XQ_RETURN_CONTENT = 4;
    public static final short XQ_EMPTY_ON_EMPTY = 5;
    public static final short XQ_NULL_ON_EMPTY = 6;

    /* Per SQL/XML[2006] 4.2.2, there are several different
     * XML "types" defined through use of primary and secondary
     * "type modifiers".  For Derby we only support two kinds:
     *
     * XML(DOCUMENT(ANY)) : A valid and well-formed XML
     *  document as defined by W3C, meaning that there is
     *  exactly one root element node.  This is the only
     *  type of XML that can be stored into a Derby XML
     *  column.  This is also the type returned by a call
     *  to XMLPARSE since we require the DOCUMENT keyword.
     *
     * XML(SEQUENCE): A sequence of items (could be nodes or
     *  atomic values).  This is the type returned from an
     *  XMLQUERY operation.  Any node that is XML(DOCUMENT(ANY))
     *  is also XML(SEQUENCE).  Note that an XML(SEQUENCE)
     *  value is *only* storable into a Derby XML column
     *  if it is also an XML(DOCUMENT(ANY)).  See the
     *  normalize method below for the code that enforces
     *  this.
     */
    public static final int XML_DOC_ANY = 0;
    public static final int XML_SEQUENCE = 1;

    // The fully-qualified type for this XML value.
    private int xType;

    // The actual XML data in this implementation is just a simple
    // string, so this class really just wraps a SQLChar and
    // defers most calls to the corresponding calls on that
    // SQLChar.  Note that, even though a SQLChar is the
    // underlying implementation, an XML value is nonetheless
    // NOT considered comparable nor compatible with any of
    // Derby string types.
    private SQLChar xmlStringValue;

    /*
     * Status variable used to verify that user's classpath contains
     * required classes for accessing/operating on XML data values.
     */
    private static String xmlReqCheck = null;

    /*
     * Whether or not this XML value corresponds to a sequence
     * that has one or more top-level ("parentless") attribute
     * nodes.  If so then we have to throw an error if the user
     * attempts to serialize this value, per XML serialization
     * rules.
     */
    private boolean containsTopLevelAttr;

    private SqlXmlUtil tmpUtil;

    /**
     * Default constructor.
     */
    public XML()
    {
        xmlStringValue = null;
        xType = -1;
        containsTopLevelAttr = false;
    }

    /**
     * Private constructor used for the {@code cloneValue} method.
     * Returns a new instance of XML whose fields are clones
     * of the values received.
     *
     * @param val A SQLChar instance to clone and use for
     *  this XML value.
     * @param xmlType Qualified XML type for "val"
     * @param seqWithAttr Whether or not "val" corresponds to
     *  sequence with one or more top-level attribute nodes.
     * @param materialize whether or not to force materialization of the
     *      underlying source data
     */
    private XML(SQLChar val, int xmlType, boolean seqWithAttr,
            boolean materialize) {
        xmlStringValue = (val == null ? null
                                      : (SQLChar)val.cloneValue(materialize));
        setXType(xmlType);
        if (seqWithAttr)
            markAsHavingTopLevelAttr();
    }

    /* ****
     * DataValueDescriptor interface.
     * */

    /**
     * @see DataValueDescriptor#cloneValue
     */
    public DataValueDescriptor cloneValue(boolean forceMaterialization) {
        return new XML(xmlStringValue, getXType(), hasTopLevelAttr(),
                forceMaterialization);
    }

    /**
     * @see DataValueDescriptor#getNewNull
     */
    public DataValueDescriptor getNewNull()
    {
        return new XML();
    }

    /**
     * @see DataValueDescriptor#getTypeName
     */
    public String getTypeName()
    {
        return TypeId.XML_NAME;
    }

    /**
     * @see DataValueDescriptor#typePrecedence
     */
    public int typePrecedence()
    {
        return TypeId.XML_PRECEDENCE;
    }

    /**
     * @see DataValueDescriptor#getString
     */
    public String getString() throws StandardException
    {
        return (xmlStringValue == null) ? null : xmlStringValue.getString();
    }

    /**
     * @see DataValueDescriptor#getLength
     */
    public int    getLength() throws StandardException
    {
        return ((xmlStringValue == null) ? 0 : xmlStringValue.getLength());
    }

    /** 
     * @see DataValueDescriptor#estimateMemoryUsage
     */
    public int estimateMemoryUsage()
    {
        int sz = BASE_MEMORY_USAGE;
        if (xmlStringValue != null)
            sz += xmlStringValue.estimateMemoryUsage();
        return sz;
    }

    /**
     * @see DataValueDescriptor#readExternalFromArray
     */
    public void readExternalFromArray(ArrayInputStream in)
        throws IOException
    {
        if (xmlStringValue == null)
            xmlStringValue = new SQLChar();

        // Read the XML implementation id.  Right now there's
        // only one implementation (UTF-8 based), so we don't
        // use this value.  But if better implementations come
        // up in the future, we'll have to use this impl id to
        // figure out how to read the data.
        in.readShort();

        // Now just read the XML data as UTF-8.
        xmlStringValue.readExternalFromArray(in);

        // If we read it from disk then it must have type
        // XML_DOC_ANY because that's all we allow to be
        // written into an XML column.
        setXType(XML_DOC_ANY);
    }

    /**
     * @see DataType#setFrom
     */
    protected void setFrom(DataValueDescriptor theValue)
        throws StandardException
    {
        String strVal = theValue.getString();
        if (strVal == null)
        {
            xmlStringValue = null;

            // Null is a valid value for DOCUMENT(ANY)
            setXType(XML_DOC_ANY);
            return;
        }

        // Here we just store the received value locally.
        if (xmlStringValue == null)
            xmlStringValue = new SQLChar();
        xmlStringValue.setValue(strVal);

        /*
         * Assumption is that if theValue is not an XML
         * value then the caller is aware of whether or
         * not theValue constitutes a valid XML(DOCUMENT(ANY))
         * and will behave accordingly (see in particular the
         * XMLQuery method of this class, which calls the
         * setValue() method of XMLDataValue which in turn
         * brings us to this method).
         */
        if (theValue instanceof XMLDataValue)
        {
            setXType(((XMLDataValue)theValue).getXType());
            if (((XMLDataValue)theValue).hasTopLevelAttr())
                markAsHavingTopLevelAttr();
        }
    }

    /** 
     * @see DataValueDescriptor#setValueFromResultSet 
     */
    public final void setValueFromResultSet(
        ResultSet resultSet, int colNumber, boolean isNullable)
        throws SQLException
    {
        if (xmlStringValue == null)
            xmlStringValue = new SQLChar();

        String valAsStr = resultSet.getString(colNumber);

        /* As there is no guarantee that the specified column within
         * resultSet is well-formed XML (is there??), we have to try
         * to parse it in order to set the "xType" field correctly.
         * This is required to ensure that we only store well-formed
         * XML on disk (see "normalize()" method of this class).  So
         * create an instance of SqlXmlUtil and use that to see if the
         * text satisifies the requirements of a well-formed DOCUMENT.
         *
         * RESOLVE: If there is anyway to guarantee that the column
         * is in fact well-formed XML then we can skip all of this
         * logic and simply set xType to XML_DOC_ANY.  But do we
         * have such a guarantee...?
         */
        if (tmpUtil == null)
        {
            try {

                tmpUtil = new SqlXmlUtil();

            } catch (StandardException se) {

                if (SanityManager.DEBUG)
                {
                    SanityManager.THROWASSERT(
                        "Failed to instantiate SqlXmlUtil for XML parsing.");
                }

                /* If we failed to get a SqlXmlUtil then we can't parse
                 * the string, which means we don't know if it constitutes
                 * a well-formed XML document or not.  In this case we
                 * set the value, but intentionally leave xType as -1
                 * so that the resultant value canNOT be stored on disk.
                 */
                xmlStringValue.setValue(valAsStr);
                setXType(-1);
                return;

            }
        }

        try {

            /* The following call parses the string into a DOM and
             * then serializes it, which is exactly what we do for
             * normal insertion of XML values.  If the parse finishes
             * with no error then we know the type is XML_DOC_ANY,
             * so set it.
             */
            valAsStr = tmpUtil.serializeToString(valAsStr);
            xmlStringValue.setValue(valAsStr);
            setXType(XML_DOC_ANY);

        } catch (Throwable t) {

            /* It's possible that the string value was either 1) an
             * XML SEQUENCE or 2) not XML at all.  We don't know
             * which one it was, so make xType invalid to ensure this
             * field doesn't end up on disk.
             */
            xmlStringValue.setValue(valAsStr);
            setXType(-1);

        }
    }

    /**
     * Compare two XML DataValueDescriptors.  NOTE: This method
     * should only be used by the database store for the purpose of
     * index positioning--comparisons of XML type are not allowed
     * from the language side of things.  That said, all store
     * wants to do is order the NULLs, so we don't actually
     * have to do a full comparison.  Just return an order
     * value based on whether or not this XML value and the
     * other XML value are null.  As mentioned in the "compare"
     * method of DataValueDescriptor, nulls are considered
     * equal to other nulls and less than all other values.
     *
     * An example of when this method might be used is if the
     * user executed a query like:
     *
     * select i from x_table where x_col is not null
     *
     * @see DataValueDescriptor#compare
     */
    public int compare(DataValueDescriptor other)
        throws StandardException
    {
        if (SanityManager.DEBUG) {
            SanityManager.ASSERT(other instanceof XMLDataValue,
                "Store should NOT have tried to compare an XML value " +
                "with a non-XML value.");
        }

        if (isNull()) {
            if (other.isNull())
            // both null, so call them 'equal'.
                return 0;
            // This XML is 'less than' the other.
            return -1;
        }

        if (other.isNull())
        // This XML is 'greater than' the other.
            return 1;

        // Two non-null values: we shouldn't ever get here,
        // since that would necessitate a comparsion of XML
        // values, which isn't allowed.
        if (SanityManager.DEBUG) {
            SanityManager.THROWASSERT(
                "Store tried to compare two non-null XML values, " +
                "which isn't allowed.");
        }
        return 0;
    }

    /**
     * Normalization method - this method will always be called when
     * storing an XML value into an XML column, for example, when
     * inserting/updating.  We always force normalization in this
     * case because we need to make sure the qualified type of the
     * value we're trying to store is XML_DOC_ANY--we don't allow
     * anything else.
     *
     * @param desiredType   The type to normalize the source column to
     * @param source        The value to normalize
     *
     * @exception StandardException Thrown if source is not
     *  XML_DOC_ANY.
     */
    public void normalize(
                DataTypeDescriptor desiredType,
                DataValueDescriptor source)
                    throws StandardException
    {
        if (SanityManager.DEBUG) {
            SanityManager.ASSERT(source instanceof XMLDataValue,
                "Tried to store non-XML value into XML column; " +
                "should have thrown error at compile time.");
        }

        if (((XMLDataValue)source).getXType() != XML_DOC_ANY) {
            throw StandardException.newException(
                SQLState.LANG_NOT_AN_XML_DOCUMENT);
        }

        ((DataValueDescriptor) this).setValue(source);
        return;

    }

    /* ****
     * Storable interface, implies Externalizable, TypedFormat
     */

    /**
     * @see TypedFormat#getTypeFormatId
     *
     * From the engine's perspective, all XML implementations share
     * the same format id.
     */
    public int getTypeFormatId() {
        return StoredFormatIds.XML_ID;
    }

    /**
     * @see Storable#isNull
     */
    public boolean isNull()
    {
        return ((xmlStringValue == null) || xmlStringValue.isNull());
    }

    /**
     * @see Storable#restoreToNull
     */
    public void restoreToNull()
    {
        if (xmlStringValue != null)
            xmlStringValue.restoreToNull();
    }

    /**
     * Read an XML value from an input stream.
     * @param in The stream from which we're reading.
     */
    public void readExternal(ObjectInput in) throws IOException
    {
        if (xmlStringValue == null)
            xmlStringValue = new SQLChar();

        // Read the XML implementation id.  Right now there's
        // only one implementation (UTF-8 based), so we don't
        // use this value.  But if better implementations come
        // up in the future, we'll have to use this impl id to
        // figure out how to read the data.
        in.readShort();

        // Now just read the XML data as UTF-8.
        xmlStringValue.readExternal(in);

        // If we read it from disk then it must have type
        // XML_DOC_ANY because that's all we allow to be
        // written into an XML column.
        setXType(XML_DOC_ANY);
    }

    /**
     * Write an XML value. 
     * @param out The stream to which we're writing.
     */
    public void writeExternal(ObjectOutput out) throws IOException
    {
        // never called when value is null
        if (SanityManager.DEBUG)
            SanityManager.ASSERT(!isNull());

        // Write out the XML store impl id.
        out.writeShort(UTF8_IMPL_ID);

        // Now write out the data.
        xmlStringValue.writeExternal(out);
    }

    /* ****
     * StreamStorable interface
     * */

    /**
     * @see StreamStorable#returnStream
     */
    public InputStream returnStream()
    {
        return
            (xmlStringValue == null) ? null : xmlStringValue.returnStream();
    }

    /**
     * @see StreamStorable#setStream
     */
    public void setStream(InputStream newStream)
    {
        if (xmlStringValue == null)
            xmlStringValue = new SQLChar();

        // The stream that we receive is for an XML data value,
        // which means it has an XML implementation id stored
        // at the front (we put it there when we wrote it out).
        // If we leave that there we'll get a failure when
        // our underlying SQLChar tries to read from the
        // stream, because the extra impl id will throw
        // off the UTF format.  So we need to read in (and
        // ignore) the impl id before using the stream.
        try {
            // 2 bytes equal a short, which is what an impl id is.
            newStream.read();
            newStream.read();
        } catch (Exception e) {
            if (SanityManager.DEBUG)
                SanityManager.THROWASSERT("Failed to read impl id" +
                    "bytes in setStream.");
        }

        // Now go ahead and use the stream.
        xmlStringValue.setStream(newStream);

        // If we read it from disk then it must have type
        // XML_DOC_ANY because that's all we allow to be
        // written into an XML column.
        setXType(XML_DOC_ANY);
    }

    /**
     * @see StreamStorable#loadStream
     */
    public void loadStream() throws StandardException
    {
        getString();
    }

    /* ****
     * XMLDataValue interface.
     * */

    /**
     * Method to parse an XML string and, if it's valid,
     * store the _serialized_ version locally and then return
     * this XMLDataValue.
     *
     * @param stringValue The string value to check.
     * @param preserveWS Whether or not to preserve
     *  ignorable whitespace.
     * @param sqlxUtil Contains SQL/XML objects and util
     *  methods that facilitate execution of XML-related
     *  operations
     * @return If 'text' constitutes a valid XML document,
     *  it has been stored in this XML value and this XML
     *  value is returned; otherwise, an exception is thrown. 
     * @exception StandardException Thrown on error.
     */
    public XMLDataValue XMLParse(
            StringDataValue stringValue,
            boolean preserveWS,
            SqlXmlUtil sqlxUtil)
        throws StandardException
    {
        if (stringValue.isNull()) {
            setToNull();
            return this;
        }

        String text = stringValue.getString();
        try {

            if (preserveWS) {
            // Currently the only way a user can view the contents of
            // an XML value is by explicitly calling XMLSERIALIZE.
            // So do a serialization now and just store the result,
            // so that we don't have to re-serialize every time a
            // call is made to XMLSERIALIZE.
                text = sqlxUtil.serializeToString(text);
            }
            else {
            // We don't support this yet, so we shouldn't
            // get here.
                if (SanityManager.DEBUG)
                    SanityManager.THROWASSERT("Tried to STRIP whitespace " +
                        "but we shouldn't have made it this far");
            }

        } catch (Throwable t) {
        /* Couldn't parse the XML document.  Throw a StandardException
         * with the parse exception (or other error) nested in it.
         * Note: we catch "Throwable" here to catch as many external
         * errors as possible in order to minimize the chance of an
         * uncaught JAXP/Xalan error (such as a NullPointerException)
         * causing Derby to fail in a more serious way.  In particular,
         * an uncaught Java exception like NPE can result in Derby
         * throwing "ERROR 40XT0: An internal error was identified by
         * RawStore module" for all statements on the connection after
         * the failure--which we clearly don't want.  If we catch the
         * error and wrap it, though, the statement will fail but Derby
         * will continue to run as normal.
         */ 
            throw StandardException.newException(
                SQLState.LANG_INVALID_XML_DOCUMENT, t, t.getMessage());

        }

        // If we get here, the text is valid XML so go ahead
        // and load/store it.
        setXType(XML_DOC_ANY);
        if (xmlStringValue == null)
            xmlStringValue = new SQLChar();
        xmlStringValue.setValue(text);
        return this;
    }

    /**
     * The SQL/XML XMLSerialize operator.
     * Serializes this XML value into a string with a user-specified
     * character type, and returns that string via the received
     * StringDataValue (if the received StringDataValue is non-null
     * and of the correct type; else, a new StringDataValue is
     * returned).
     *
     * @param result The result of a previous call to this method,
     *    null if not called yet.
     * @param targetType The string type to which we want to serialize.
     * @param targetWidth The width of the target type.
     * @return A serialized (to string) version of this XML object,
     *  in the form of a StringDataValue object.
     * @exception StandardException    Thrown on error
     */
    public StringDataValue XMLSerialize(StringDataValue result,
        int targetType, int targetWidth, int targetCollationType) 
    throws StandardException
    {
        if (result == null) {
            switch (targetType)
            {
                case Types.CHAR:        result = new SQLChar(); break;
                case Types.VARCHAR:     result = new SQLVarchar(); break;
                case Types.LONGVARCHAR: result = new SQLLongvarchar(); break;
                case Types.CLOB:        result = new SQLClob(); break;
                default:
                // Shouldn't ever get here, as this check was performed
                // at bind time.

                    if (SanityManager.DEBUG) {
                        SanityManager.THROWASSERT(
                            "Should NOT have made it to XMLSerialize " +
                            "with a non-string target type: " + targetType);
                    }
                    return null;
            }
            // If the collation type is territory based, then we should use
            // CollatorSQLxxx rather than SQLxxx types for StringDataValue. 
            // eg
            // CREATE TABLE T_MAIN1 (ID INT  GENERATED ALWAYS AS IDENTITY 
            //       PRIMARY KEY, V XML);
            // INSERT INTO T_MAIN1(V) VALUES NULL;
            // SELECT ID, XMLSERIALIZE(V AS CLOB), XMLSERIALIZE(V AS CLOB) 
            //       FROM T_MAIN1 ORDER BY 1;
            // Following code is for (V AS CLOB) inside XMLSERIALIZE. The
            // StringDataValue returned for (V AS CLOB) should consider the 
            // passed collation type in determining whether we should
            // generate SQLChar vs CollatorSQLChar for instance. Keep in mind
            // that collation applies only to character string types.
            try {
                RuleBasedCollator rbs = ConnectionUtil.getCurrentLCC().getDataValueFactory().
                getCharacterCollator(targetCollationType);
                result = ((StringDataValue)result).getValue(rbs);
            }
            catch( java.sql.SQLException sqle)
            {
                throw StandardException.plainWrapException( sqle);
            }
        }

        // Else we're reusing a StringDataValue.  We only reuse
        // the result if we're executing the _same_ XMLSERIALIZE
        // call on multiple rows.  That means that all rows
        // must have the same result type (targetType) and thus
        // we know that the StringDataValue already has the
        // correct type.  So we're set.

        if (this.isNull()) {
        // Attempts to serialize a null XML value lead to a null
        // result (SQL/XML[2003] section 10.13).
            result.setToNull();
            return result;
        }

        /* XML serialization rules say that sequence "normalization"
         * must occur before serialization, and normalization dictates
         * that a serialization error must be thrown if the XML value
         * is a sequence with a top-level attribute.  We normalized
         * (and serialized) this XML value when it was first created,
         * and at that time we took note of whether or not there is
         * a top-level attribute.  So throw the error here if needed.
         * See SqlXmlUtil.serializeToString() for more on sequence
         * normalization.
         */
        if (this.hasTopLevelAttr())
        {
            throw StandardException.newException(
                SQLState.LANG_XQUERY_SERIALIZATION_ERROR);
        }

        // Get the XML value as a string.  For this UTF-8 impl,
        // we already have it as a UTF-8 string, so just use
        // that.
        result.setValue(getString());

        // Seems wrong to trunc an XML document, as it then becomes non-
        // well-formed and thus useless.  So we throw an error (that's
        // what the "true" in the next line says).
        result.setWidth(targetWidth, 0, true);
        return result;
    }

    /**
     * The SQL/XML XMLExists operator.
     * Checks to see if evaluation of the query expression contained
     * within the received util object against this XML value returns
     * at least one item. NOTE: For now, the query expression must be
     * XPath only (XQuery not supported) because that's what Xalan
     * supports.
     *
     * @param sqlxUtil Contains SQL/XML objects and util
     *  methods that facilitate execution of XML-related
     *  operations
     * @return True if evaluation of the query expression stored
     *  in sqlxUtil returns at least one node for this XML value;
     *  unknown if the xml value is NULL; false otherwise.
     * @exception StandardException Thrown on error
     */
    public BooleanDataValue XMLExists(SqlXmlUtil sqlxUtil)
        throws StandardException
    {
        if (this.isNull()) {
        // if the user specified a context node and that context
        // is null, result of evaluating the query is null
        // (per SQL/XML 6.17:General Rules:1.a), which means that we
        // return "unknown" here (per SQL/XML 8.4:General Rules:2.a).
            return SQLBoolean.unknownTruthValue();
        }

        // Make sure we have a compiled query (and associated XML
        // objects) to evaluate.
        if (SanityManager.DEBUG) {
            SanityManager.ASSERT(
                sqlxUtil != null,
                "Tried to evaluate XML xquery, but no XML objects were loaded.");
        }

        try {

            return new SQLBoolean(null !=
                sqlxUtil.evalXQExpression(this, false, new int[1]));

        } catch (StandardException se) {

            // Just re-throw it.
            throw se;

        } catch (Throwable xe) {
        /* Failed somewhere during evaluation of the XML query expression;
         * turn error into a StandardException and throw it.  Note: we
         * catch "Throwable" here to catch as many Xalan-produced errors
         * as possible in order to minimize the chance of an uncaught Xalan
         * error (such as a NullPointerException) causing Derby to fail in
         * a more serious way.  In particular, an uncaught Java exception
         * like NPE can result in Derby throwing "ERROR 40XT0: An internal
         * error was identified by RawStore module" for all statements on
         * the connection after the failure--which we clearly don't want.  
         * If we catch the error and wrap it, though, the statement will
         * fail but Derby will continue to run as normal. 
         */
            throw StandardException.newException(
                SQLState.LANG_XML_QUERY_ERROR, xe,
                "XMLEXISTS", xe.getMessage());
        }
    }

    /**
     * Evaluate the XML query expression contained within the received
     * util object against this XML value and store the results into
     * the received XMLDataValue "result" param (assuming "result" is
     * non-null; else create a new XMLDataValue).
     *
     * @param sqlxUtil Contains SQL/XML objects and util methods that
     *  facilitate execution of XML-related operations
     * @param result The result of a previous call to this method; null
     *  if not called yet.
     * @return An XMLDataValue whose content corresponds to the serialized
     *  version of the results from evaluation of the query expression.
     *  Note: this XMLDataValue may not be storable into Derby XML
     *  columns.
     * @exception StandardException thrown on error
     */
    public XMLDataValue XMLQuery(SqlXmlUtil sqlxUtil, XMLDataValue result)
            throws StandardException
    {
        if (this.isNull()) {
        // if the context is null, we return null,
        // per SQL/XML[2006] 6.17:GR.1.a.ii.1.
            if (result == null)
                result = (XMLDataValue)getNewNull();
            else
                result.setToNull();
            return result;
        }

        try {
 
            // Return an XML data value whose contents are the
            // serialized version of the query results.
            int [] xType = new int[1];
            List itemRefs = sqlxUtil.evalXQExpression(
                this, true, xType);

            if (result == null)
                result = new XML();
            String strResult = sqlxUtil.serializeToString(itemRefs, result);
            result.setValue(new SQLChar(strResult));

            // Now that we've set the result value, make sure
            // to indicate what kind of XML value we have.
            result.setXType(xType[0]);

            // And finally we return the query result as an XML value.
            return result;

        } catch (StandardException se) {

            // Just re-throw it.
            throw se;

        } catch (Throwable xe) {
        /* Failed somewhere during evaluation of the XML query expression;
         * turn error into a StandardException and throw it.  Note: we
         * catch "Throwable" here to catch as many Xalan-produced errors
         * as possible in order to minimize the chance of an uncaught Xalan
         * error (such as a NullPointerException) causing Derby to fail in
         * a more serious way.  In particular, an uncaught Java exception
         * like NPE can result in Derby throwing "ERROR 40XT0: An internal
         * error was identified by RawStore module" for all statements on
         * the connection after the failure--which we clearly don't want.  
         * If we catch the error and wrap it, though, the statement will
         * fail but Derby will continue to run as normal. 
         */
            throw StandardException.newException(
                SQLState.LANG_XML_QUERY_ERROR, xe,
                "XMLQUERY", xe.getMessage());
        }
    }

    /* ****
     * Helper classes and methods.
     * */

    /**
     * Set this XML value's qualified type.
     */
    public void setXType(int xtype)
    {
        this.xType = xtype;

        /* If the target type is XML_DOC_ANY then this XML value
         * holds a single well-formed Document.  So we know that
         * we do NOT have any top-level attribute nodes.  Note: if
         * xtype is SEQUENCE we don't set "containsTopLevelAttr"
         * here; assumption is that the caller of this method will
         * then set the field as appropriate.  Ex. see "setFrom()"
         * in this class.
         */
        if (xtype == XML_DOC_ANY)
            containsTopLevelAttr = false;
    }

    /**
     * Retrieve this XML value's qualified type.
     */
    public int getXType()
    {
        return xType;
    }

    /**
     * Take note of the fact this XML value represents an XML
     * sequence that has one or more top-level attribute nodes.
     */
    public void markAsHavingTopLevelAttr()
    {
        this.containsTopLevelAttr = true;
    }

    /**
     * Return whether or not this XML value represents a sequence
     * that has one or more top-level attribute nodes.
     */
    public boolean hasTopLevelAttr()
    {
        return containsTopLevelAttr;
    }

    /**
     * See if the required JAXP and Xalan classes are in the
     * user's classpath.  Assumption is that we will always
     * call this method before instantiating an instance of
     * SqlXmlUtil, and thus we will never get a ClassNotFound
     * exception caused by missing JAXP/Xalan classes.  Instead,
     * if either is missing we should throw an informative
     * error indicating what the problem is.
     *
     * NOTE: This method only does the checks necessary to
     * allow successful instantiation of the SqlXmlUtil
     * class.  Further checks (esp. the presence of a JAXP
     * _implementation_ in addition to the JAXP _interfaces_)
     * are performed in the SqlXmlUtil constructor.
     *
     * @exception StandardException thrown if the required
     *  classes cannot be located in the classpath.
     */
    public static void checkXMLRequirements()
        throws StandardException
    {
        // Only check once; after that, just re-use the result.
        if (xmlReqCheck == null)
        {
            xmlReqCheck = "";

            /* If the DocumentBuilderFactory class exists, then we
             * assume a JAXP implementation is present in
             * the classpath.  If this assumption is incorrect
             * then we at least know that the JAXP *interface*
             * exists and thus we'll be able to instantiate
             * the SqlXmlUtil class.  We can then do a check
             * for an actual JAXP *implementation* from within
             * the SqlXmlUtil class (see the constructor of
             * that class).
             *
             * Note: The JAXP API and implementation are
             * provided as part the JVM if it is jdk 1.4 or
             * greater.
             */
            if (!checkJAXPRequirement()) {
                xmlReqCheck = "JAXP";
            }

            /* If the XPathFactory class exists, then we assume that
             * our XML query processor is present in the classpath.
             */
            else if (!checkXPathRequirement()) {
                xmlReqCheck = "XPath";
            }
        }

        if (xmlReqCheck.length() != 0)
        {
            throw StandardException.newException(
                SQLState.LANG_MISSING_XML_CLASSES, xmlReqCheck);
        }
    }

    /**
     * Check if we have a JAXP implementation installed.
     *
     * @return {@code true} if JAXP is installed, or {@code false} otherwise
     */
    private static boolean checkJAXPRequirement() {
        try {
            Class.forName("javax.xml.parsers.DocumentBuilderFactory");
            return true;
        } catch (Throwable t) {
            // Oops... Couldn't load the DocumentBuilderFactory class for
            // some reason. Assume we don't have JAXP.
            return false;
        }
    }

    /**
     * Check if XPath is supported on this platform.
     *
     * @return {@code true} if XPath is supported, or {@code false} otherwise
     */
    private static boolean checkXPathRequirement() {
        try {
            Class.forName("javax.xml.xpath.XPathFactory");
            return true;
        } catch (Throwable t) {
            // Oops... Something went wrong when checking for XPath
            // support. Assume we don't have it.
            return false;
        }
    }
}
