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Oracle® XML DB Developer's Guide
11g Release 2 (11.2)

E23094-04
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32 Writing Oracle XML DB Applications in Java

This chapter describes how to write Oracle XML DB applications in Java. It includes design guidelines for writing Java applications including servlets, and how to configure the Oracle XML DB servlets.

This chapter contains these topics:

Overview of Oracle XML DB Java Applications

You can use Java code in these ways:

  • In the database using the Java Virtual Machine (VM).

  • In a client or application server, using the OCI driver for JDBC.

Because Java runs in the database in the context of the database server process, the ways you can deploy and run Java code are restricted to the following:

  • You can run Java code as a stored procedure invoked from SQL or PL/SQL.

  • You can run a Java servlet.

Stored procedures are easier to integrate with SQL and PL/SQL code. They require Oracle Net Services as the protocol to access Oracle Database.

Servlets work better as the top-level entry point into Oracle Database, and require using HTTP(S) as the protocol to access Oracle Database.

Which Oracle XML DB APIs Are Available Inside and Outside the Database?

All Oracle XML DB application program interfaces (APIs) are available to applications running both in the server and outside the database, including:

  • JDBC support for XMLType

  • XMLType class

  • Java DOM implementation

Design Guidelines: Java Inside or Outside the Database?

When choosing an architecture for writing Java Oracle XML DB applications, consider the following guidelines:

HTTP(S): Accessing Java Servlets or Directly Accessing XMLType Resources

If the downstream client wants to deal with XML in its textual representation, then using HTTP(S) to either access the Java servlets or directly access XMLType resources performs the best, especially if the XML node tree is not being manipulated much by the Java program.

The Java implementation in the server can natively move data from the database to the network without converting character data through UCS-2 Unicode (which is required by Java strings). In many cases data is copied directly from the database buffer cache to the HTTP(S) connection. There is no need to convert data from the buffer cache into the SQL serialization format used by Oracle Net Services, then move it to the JDBC client, and then convert to XML. Loading on demand and the LRU cache for XMLType are most effective inside the database server.

Accessing Many XMLType Object Elements: Use JDBC XMLType Support

If the downstream client is an application that programmatically accesses many or most of the elements of an XMLType instance using Java, then use JDBC XMLType support for best performance. It is often easier to debug Java programs outside of the database server, as well.

Use the Servlets to Manipulate and Write Out Data Quickly as XML

Oracle XML DB servlets are intended for writing HTTP stored procedures in Java that can be accessed using HTTP(S). They are not intended as a platform for developing an entire Internet application. In that case, the application servlet should be deployed in Oracle Application Server application server and access data in the database either using JDBC, or by using the java.net.* or similar APIs to get XML data through HTTP(S).

They are best used for applications that want to get into the database, manipulate the data, and write it out quickly as XML, not to format HTML pages for end-users.

Writing Oracle XML DB HTTP Servlets in Java

Oracle XML DB provides a protocol server that supports FTP, HTTP 1.1, WebDAV, and Java Servlets. Oracle XML DB supports Java Servlet version 2.2, with the following exceptions:

  • The servlet WAR file (web.xml) is not supported in its entirety. Some web.xml configuration parameters must be handled manually. For example, creating roles must be done using the SQL CREATE ROLE command.

  • RequestDispatcher and associated methods are not supported.

  • Method HTTPServletRequest.getCookies() is not supported.

  • Only one ServletContext (and one web-app) is currently supported.

  • Stateful servlets (and thus the HttpSession class methods) are not supported. Servlets must maintain state in the database itself.

Configuring Oracle XML DB Servlets

Oracle XML DB servlets are configured using the /xdbconfig.xml file in Oracle XML DB Repository. Many of the XML elements in this file are the same as those defined by the Java Servlet 2.2 specification portion of Java 2 Enterprise Edition (J2EE), and have the same semantics. Table 32-1 lists the XML elements defined for the servlet deployment descriptor by the Java Servlet specification, along with extension elements supported by Oracle XML DB.

Table 32-1 XML Elements Defined for Servlet Deployment Descriptors

XML Element Name Defined By Supported? Description Comment

auth-method

Java

no

Specifies an HTTP authentication method required for access

--

charset

Oracle

yes

Specifies an IANA character set name

For example: ISO8859, UTF-8

charset-mapping

Oracle

yes

Specifies a mapping between a filename extension and a charset

--

context-param

Java

no

Specifies a parameter for a Web application

Not yet supported

description

Java

yes

A string for describing a servlet or Web application

Supported for servlets

display-name

Java

yes

A string to display with a servlet or Web application

Supported for servlets

distributable

Java

no

Indicates whether or not this servlet can function if all instances are not running in the same Java virtual machine

All servlets running in Oracle Database MUST be distributable.

errnum

Oracle

yes

Oracle error number

See Oracle Database Error Messages

error-code

Java

yes

HTTP(S) error code

Defined by RFC 2616

error-page

Java

yes

Defines a URL to redirect to if an error is encountered.

Can be specified through an HTTP(S) error, an uncaught Java exception, or through an uncaught Oracle error message

exception-type

Java

yes

Classname of a Java exception mapped to an error page

--

extension

Java

yes

A filename extension used to associate with MIME types, character sets, and so on.

--

facility

Oracle

yes

Oracle facility code for mapping error pages

For example: ORA, PLS, and so on.

form-error-page

Java

no

Error page for form login attempts

Not yet supported

form-login-config

Java

no

Config spec for form-based login

Not yet supported

form-login-page

Java

no

URL for the form-based login page

Not yet supported

icon

Java

Yes

URL of icon to associate with a servlet

Supported for servlets

init-param

Java

Yes

Initialization parameter for a servlet

--

jsp-file

Java

No

Java Server Page file to use for a servlet

Not supported

lang

Oracle

Yes

IANA language name

For example: en-US

lang-mapping

Oracle

Yes

Specifies a mapping between a filename extension and language content

--

large-icon

Java

Yes

Large sized icon for icon display

--

load-on-startup

Java

Yes

Specifies if a servlet is to be loaded on startup

--

location

Java

Yes

Specifies the URL for an error page

Can be a local path name or HTTP(S) URL

login-config

Java

No

Specifies a method for authentication

Not yet supported

mime-mapping

Java

Yes

Specifies a mapping between filename extension and the MIME type of the content

--

mime-type

Java

Yes

MIME type name for resource content

For example: text/xml or application/octet-stream

OracleError

Oracle

Yes

Specifies an Oracle error to associate with an error page

--

param-name

Java

Yes

Name of a parameter for a Servlet or ServletContext

Supported for servlets

param-value

Java

Yes

Value of a parameter

--

realm-name

Java

No

HTTP(S) realm used for authentication

Not yet supported

role-link

Java

Yes

Specifies a role a particular user must have for accessing a servlet

Refers to a database role name. Make sure to capitalize by default!

role-name

Java

Yes

A servlet name for a role

Just another name to call the database role. Used by the Servlet APIs

security-role

Java

No

Defines a role for a servlet to use

Not supported. You must manually create roles using the SQL CREATE ROLE

security-role-ref

Java

Yes

A reference between a servlet and a role

--

servlet

Java

Yes

Configuration information for a servlet

--

servlet-class

Java

Yes

Specifies the classname for the Java servlet

--

servlet-language

Oracle

Yes

Specifies the programming language in which the servlet is written.

Either Java, C, or PL/SQL. Currently, only Java is supported for customer-defined servlets.

servlet-mapping

Java

Yes

Specifies a filename pattern with which to associate the servlet

All of the mappings defined by Java are supported

servlet-name

Java

Yes

String name for a servlet

Used by servlet APIs

servlet-schema

Oracle

Yes

The Oracle Schema in which the Java class is loaded. If not specified, then the schema is searched using the default resolver specification.

If this is not specified, then the servlet must be loaded into the SYS schema to ensure that everyone can access it, or the default Java class resolver must be altered. Note that the servlet schema is capitalized unless the value is enclosed in double-quotes.

session-config

Java

No

Configuration information for an HTTPSession

HTTPSession is not supported

session-timeout

Java

No

Timeout for an HTTP(S) session

HTTPSession is not supported

small-icon

Java

Yes

Small icon to associate with a servlet

--

taglib

Java

No

JSP tag library

JSPs currently not supported

taglib-uri

Java

No

URI for JSP tag library description file relative to file web.xml

JSPs currently not supported

taglib-location

Java

No

Path name relative to the root of the Web application where the tag library is stored

JSPs currently not supported

url-pattern

Java

Yes

URL pattern to associate with a servlet

See Section 10 of Java Servlet 2.2 spec

web-app

Java

No

Configuration for a Web application

Only one Web application is currently supported

welcome-file

Java

Yes

Specifies a welcome-file name

--

welcome-file-list

Java

Yes

Defines a list of files to display when a folder is referenced through an HTTP GET request

Example: index.html


Note:

  • The following parameters defined for the web.xml file by Java are usable only by J2EE-compliant Enterprise Java Bean containers, and are not required for Java Servlet containers that do not support a full J2EE environment: env-entry, env-entry-name, env-entry-value, env-entry-type, ejb-ref, ejb-ref-type, home, remote, ejb-link, resource-ref, res-ref-name, res-type, res-auth.

  • The following elements are used to define access control for resources: security-constraint, web-resource-collection, web-resource-name, http-method, user-data-constraint, transport-guarantee, auth-constrain. Oracle XML DB provides this functionality through access control lists (ACLs). An ACL is a list of access control entries (ACEs) that determines which principals have access to a given resource or resources. A future release will support using a web.xml file to generate ACLs.

See Also:

Chapter 34, "Administering Oracle XML DB" for more information about configuring the /xdbconfig.xml file

HTTP Request Processing for Oracle XML DB Servlets

Oracle XML DB handles an HTTP request using the following steps:

  1. If a connection has not yet been established, then Oracle Listener hands the connection to a shared server dispatcher.

  2. When a new HTTP request arrives, the dispatcher wakes up a shared server.

  3. The HTTP headers are parsed into appropriate structures.

  4. The shared server attempts to allocate a database session from the Oracle XML DB session pool, if available, but otherwise creates a new session.

  5. A new database call and a new database transaction are started.

  6. If HTTP(S) has included authentication headers, then the session is authenticated as that database user (just as if the user logged into SQL*Plus). If no authentication information is included, and the request is GET or HEAD, then Oracle XML DB attempts to authenticate the session as the ANONYMOUS user. If that database user account is locked, then no unauthenticated access is allowed.

  7. The URL in the HTTP request is matched against the servlets in the xdbconfig.xml file, as specified by the Java Servlet 2.2 specification.

  8. The Oracle XML DB Servlet container is invoked in the Java VM inside Oracle. If the specified servlet has not been initialized yet, then the servlet is initialized.

  9. The Servlet reads input from the ServletInputStream, and writes output to the ServletOutputStream, and returns from method service().

  10. If no uncaught Oracle error occurred, then the session is put back into the session pool.

Session Pool and Oracle XML DB Servlets

Oracle Database uses one Java VM for each database session. A session that is reused from the session pool retains any state that is left over in the Java VM (Java static variables) from the last time that session was used.

This can be useful in caching Java state that is not user-specific, such as metadata, but Do not store secure user data in Java static memory. This could turn into a security hole inadvertently introduced by your application if you are not careful.

Native XML Stream Support

Node class DOM has an Oracle-specific method called write(), which takes the following arguments, returning void:

  • java.io.OutputStream stream: A Java stream for writing the XML text.

  • String charEncoding: The character encoding for writing the XML text. If NULL, then the database character set is used.

  • Short indent The number of characters to indent nested XML elements.

Java method write() has a shortcut implementation if the stream provided is the ServletOutputStream provided inside the database. The contents of the Node are written in XML in native code directly to the output socket. This bypasses any conversions into and out of Java objects or Unicode (required for Java strings) and provides very high performance.

Oracle XML DB Servlet APIs

The APIs supported by Oracle XML DB servlets are defined by the Java Servlet 2.2 specification, the Javadoc for which is available, as of the time of writing this, online at: http://java.sun.com/products/servlet/2.2/javadoc/index.html

Table 32-2 lists Java Servlet 2.2 methods that are not implemented. They result in run-time exceptions.

Table 32-2 Java Servlet 2.2 Methods that Are Not Implemented

Interface Methods Not Implemented
HttpServletRequest

getSession(), isRequestedSessionIdValid()

HttpSession

all

HttpSessionBindingListener

all


Oracle XML DB Servlet Example

Example 32-1 shows a simple servlet that prints the content of file resource /public/test/foo1.text.

Example 32-1 An Oracle XML DB Servlet

import javax.servlet.http.*;
import javax.servlet.*;
import java.util.*;
import java.io.*;
import java.util.*;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.Reader;
import java.io.Writer;
import java.sql.DriverManager;
import java.sql.SQLException;
import oracle.jdbc.OracleConnection;
import oracle.jdbc.OracleDriver;
import oracle.jdbc.OraclePreparedStatement;
import oracle.jdbc.OracleResultSet;
import oracle.sql.CLOB;
import oracle.xdb.XMLType;
import oracle.xdb.spi.XDBResource;
 
public class test extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException,
           IOException {
      try {
        try {
          // Get the database connection for the current HTTP session
          DriverManager.registerDriver(new oracle.jdbc.OracleDriver());
          OracleDriver ora = new OracleDriver();
          OracleConnection databaseConnection =
            (OracleConnection) ora.defaultConnection();
          String statementText =
            "SELECT XDBURIType('/public/test/foo1.txt').getClob() FROM DUAL";
          OraclePreparedStatement statement =
            (OraclePreparedStatement)
            databaseConnection.prepareStatement(statementText);
          OracleResultSet resultSet = null;
          CLOB content = null;
          // Execute the statement
          resultSet = (OracleResultSet) statement.executeQuery();
          while (resultSet.next())
            {// The statement returns a CLOB.
             // Copy content of CLOB to server's output stream.
             content = resultSet.getCLOB(1);
             Reader reader = content.getCharacterStream();
             Writer writer =
               new OutputStreamWriter(response.getOutputStream());
             int bytesSent = 0;
             int n;
             char[] buffer = new char[CLOB.MAX_CHUNK_SIZE];
             while (-1 != (n = reader.read(buffer)))
               { bytesSent = bytesSent + n;
                 writer.write(buffer, 0, n); }
             writer.flush();
             if (content.isOpen()) { content.close(); }}
          resultSet.close();
          statement.close();
          databaseConnection.close();
          response.getOutputStream().write('\n'); }
        catch (SQLException sql)
          { throw new ServletException(sql); }}
      catch (ServletException se )
        { se.printStackTrace(); }
      finally
        { System.out.flush(); }}}

To install the servlet, you compile it, then load it into Oracle Database:

% loadjava –grant public –u quine/curry –r test.class

Finally, register and map the servlet, associating it with a URL, as shown in Example 32-2.

Example 32-2 Registering and Mapping an Oracle XML DB Servlet

EXEC DBMS_XDB.addServlet('TestServletFoo', 'Java', 'TestServletFoo',
                         NULL, NULL, 'test', NULL, NULL, 'XDB');

EXEC DBMS_XDB.addServletMapping('/public/test/foo1.txt', 'TestServletFoo');

COMMIT;