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Developing Persistence Architectures Using EclipseLink Database Web Services, Release 2.5
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Beta Draft: 2013-04-15

Creating a DBWS Service from SQL Statements

This section includes information on:

Creating from Results Sets from Custom SQL SELECT Statements

EclipseLink DBWS can create a Web service that exposes the results of executing custom SQL SELECT statements, without exposing the actual SQL. There is no metadata to determine the structure of the returned data -- the Simple XML Format schema is used.

The SQL SELECT statements targeted for this service are in the DBWSBuilder builder XML file, as shown here:

Example 2-8 Sample DBWSBuilder XML File

<?xml version="1.0" encoding="UTF-8"?>
<dbws-builder xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <properties>
    <property name="projectName">testSql</property>
    ... database properties
  </properties>
  <sql
    name="count"
    simpleXMLFormatTag="aggregate-info"
    xmlTag="count"
    >
    <text><![CDATA[select count(*) from EMP]]></text>
  </sql>
  <sql
    name="countAndMaxSalary"
    simpleXMLFormatTag="aggregate-info"
    xmlTag="count-and-max-salary"
    >
    <text><![CDATA[select count(*) as "COUNT", max(SAL) as "MAX-Salary" from EMP]]></text>
  </sql>
</dbws-builder>
 

Use this command to create the web service:

prompt > dbwsbuilder.cmd -builderFile dbws-builder.xml -stageDir output_directory -packageAs wls testSql.war
 

where

  • dbws-builder.xml is the DBWS builder XML configuration file, as shown previously

  • output_directory is the output directory for the generated files

  • -packageAs the platform on which the web service will be deployed

The generated eclipselink-dbws-schema.xsd file is the schema for the Simple XML format:

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  >
  <xsd:complexType name="simple-xml-format">
    <xsd:sequence>
      <xsd:any minOccurs="0"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:schema>
 

The element tags simple-xml-format and simple-xml are customized in the SQL operations. For example, <simple-xml-format> = <aggregate-info>, <simple-xml> = <count-and-max-salary>.

Generated EclipseLink DBWS Service Descriptor

The SQL operations are included in the DBWS service descriptor file (eclipselink-dbws.xml) created by EclipseLink, as well as the settings to alter the default Simple XML Format <element-tag> name.

Example 2-9 Sample XML File

<?xml version="1.0" encoding="UTF-8"?>
<dbws xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:ns1="urn:testSql" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <name>testSql</name>
   <sessions-file>eclipselink-dbws-sessions.xml</sessions-file>
   <query>
      <name>count</name>
      <result>
         <type>simple-xml-format</type>
         <simple-xml-format>
            <simple-xml-format-tag>aggregate-info</simple-xml-format-tag>
            <simple-xml-tag>count</simple-xml-tag>
         </simple-xml-format>
      </result>
      <sql>
         <![CDATA[select count(*) from EMP]]>
      </sql>
   </query>
   <query>
      <name>countAndMaxSalary</name>
      <result>
         <type>simple-xml-format</type>
         <simple-xml-format>
            <simple-xml-format-tag>aggregate-info</simple-xml-format-tag>
            <simple-xml-tag>count-and-max-salary</simple-xml-tag>
         </simple-xml-format>
      </result>
      <sql>
         <![CDATA[select count(*) as "COUNT", max(SAL) as "MAX-Salary" from EMP]]>
      </sql>
   </query>
</dbws>
 

SOAP Messaging

The following SOAP Message invokes the <count> operation for the testSql DBWS service:

<?xml version="1.0" encoding="UTF-8"?>
<env:Envelope xmlns:env="http://schemas.xmlsoap.org/soap/envelope/">
  <env:Body>
    <count xmlns="urn:testSqlService" xmlns:urn="urn:testSql"/>
  </env:Body>
</env:Envelope>
 

returning:

<?xml version="1.0" encoding="utf-16"?>
<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
  <SOAP-ENV:Header />
  <SOAP-ENV:Body>
    <srvc:countResponse xmlns:srvc="urn:testSqlService">
      <srvc:result>
        <aggregate-info xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="simple-xml-format">
          <count>
            <COUNT_x0028__x002A__x0029_>14</COUNT_x0028__x002A__x0029_>
          </count>
        </aggregate-info>
      </srvc:result>
    </srvc:countResponse>
  </SOAP-ENV:Body>
</SOAP-ENV:Envelope>
 

NoteNote:

You should be aware of the resultSet for select count(*); the characters '(', '*' and ')' are not valid for XML element tags and are replaced by the well-known transformation, which documented as part of the SQL/X specification (SQL/XML:2003).


Custom SQL as Sub-operation of Table-based Web Service

The "SOAP Messaging" operation returns unstructured data. However, it is possible to nest such operations within the context of a Table-based operation; then, the nested operations can be configured to re-use the schema element type of the parent table and return structured data:

<dbws-builder>
  <properties>
    <property name="projectName">empSql</property>
    ... database properties
  </properties>
  <table
    catalogPattern="%"
    tableNamePattern="EMP"
    >
    <sql
      name="findEmpByName"
      isCollection="true"
      returnType="empType"
      >
      <text><![CDATA[select * from EMP where ENAME like ?]]></text>
      <binding name="ENAME" type="xsd:string"/>
    </sql>
  </table>
 </dbws-builder>
 

The generated EclipseLink DBWS service descriptor eclipselink-dbws.xml file:

Example 2-10 Sample eclipselink-dbws.xml File

<dbws xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:ns1="urn:testSql" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <name>empSql</name>
   <sessions-file>eclipselink-dbws-sessions.xml</sessions-file>
   <update>
      <name>update_empType</name>
      <parameter>
         <name>theInstance</name>
         <type>ns1:empType</type>
      </parameter>
   </update>
   ...
   <query>
      <name>findEmpByName</name>
      <parameter>
         <name>ENAME</name>
         <type>xsd:string</type>
      </parameter>
      <result isCollection="true">
         <type>ns1:empType</type>
      </result>
      <sql>
         <![CDATA[select * from EMP where ENAME like #ENAME]]>
      </sql>
   </query>
</dbws>

Creating based on Schema-formatted Results from Custom SQL SELECT Statements

EclipseLink can also create a web service in which the "shape" of the returned result is determined at design-time, not runtime. Normally, the custom SQL SELECT statement returns java.sql.ResultSets and the java.sql.ResultSetMetaData APIs (getColumnCount, getColumnLabel, getColumnType, etc.) can be used to determine the name and datatype of the returned information.

EclipseLink DBWS uses the Simplified XML Format (SXF) to create an XML document to describe the ResultSet's information. However, because this document can change arbitrarily, the SXF schema is extremely "loose" – the use of xsd:any places virtually no restriction on the document.

Example 2-11 Sample Schema

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema
  xmlns:xsd="http://www.w3.org/2001/XMLSchema"
  >
  <xsd:complexType name="simple-xml-format">
    <xsd:sequence>
      <xsd:any minOccurs="0"/>
    </xsd:sequence>
  </xsd:complexType>
</xsd:schema>
 

Example 2-12 Instance document:

<source lang="xml">
<?xml version = '1.0' encoding = 'UTF-8'?>
<simple-xml-format>
  <simple-xml>
    <EMPNO>7788</EMPNO>
    <ENAME>SCOTT</ENAME>
    <JOB>ANALYST</JOB>
    <MGR>7566</MGR>
    <HIREDATE>1987-04-19</HIREDATE>
    <SAL>3000</SAL>
    <DEPTNO>20</DEPTNO>
  </simple-xml>
  <simple-xml>
    <EMPNO>7369</EMPNO>
    <ENAME>SMITH</ENAME>
    <JOB>CLERK</JOB>
    <MGR>7902</MGR>
    <HIREDATE>1980-12-17</HIREDATE>
    <SAL>800</SAL>
    <DEPTNO>20</DEPTNO>
  </simple-xml>
</simple-xml-format>
 

Additional information at Design Time

As indicated previously, the java.sql.ResultSetMetaData APIs provide enough information, if available at design-time, from which you could generate a schema, as shown here:

Example 2-13 Sample Schema

<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema" targetNamespace="urn:emp" xmlns="urn:emp" elementFormDefault="qualified">
   <xsd:complexType name="empType">
      <xsd:sequence>
         <xsd:element name="empno" type="xsd:decimal"/>
         <xsd:element name="ename" type="xsd:string" minOccurs="0" nillable="true"/>
         <xsd:element name="job" type="xsd:string" minOccurs="0" nillable="true"/>
         <xsd:element name="mgr" type="xsd:decimal" minOccurs="0" nillable="true"/>
         <xsd:element name="hiredate" type="xsd:date" minOccurs="0" nillable="true"/>
         <xsd:element name="sal" type="xsd:decimal" minOccurs="0" nillable="true"/>
         <xsd:element name="comm" type="xsd:decimal" minOccurs="0" nillable="true"/>
         <xsd:element name="deptno" type="xsd:decimal" minOccurs="0" nillable="true"/>
      </xsd:sequence>
   </xsd:complexType>
   <xsd:element name="empType" type="empType"/>
</xsd:schema>
 

The DBWS sql operation is enhanced with an additional SQL statement that is executed at design-time -- the statement will not return any rows (such as when the WHERE clause evaluates to false in Example 2-14):

Example 2-14 Executing Additional SQL Statements

<?xml version="1.0" encoding="UTF-8"?>
<dbws-builder xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    <properties>
        <property name="projectName">emp</property>
        ...
    </properties>
    <sql
        name="Semployees"
        isCollection="false"
        returnType="empType"
        >
        <statement><![CDATA[select * from EMP where ENAME like 'S%']]></statement>
        <build-statement><![CDATA[select * from EMP where 0=1]]></build-statement>
    </sql>
</dbws-builder>
 

Understanding Issues and Limitations

Be aware of the following limitation:

Repeated labels

Valid SQL allows multiple identical columns. For example, consider the following SQL:

SELECT ENAME, ENAME FROM EMP WHERE LIKE 'S%'
ENAME ENAME

SMITH

SMITH

...

...

SCOTT

SCOTT


In this example, a SELECT statement that uses UNION could return a set of column labels where a label is repeated.

DBWSBuilder maintains a list of "already processed columns" and will throw an exception when it detects a duplicate.

Compatible column label sets

The runtime and design-time SQL statements must return compatible column label sets. EclipseLink performs no pre-processing to ensure that the column sets are the same; the error will be detected at runtime when the service is invoked.

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