32 Writing Oracle XML DB Applications in Java

Overview of Oracle XML DB Java Applications

Oracle XML DB provides two main architectures for the Java programmer:

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

• In a client or application server, using the OCI driver for JDBC. An application server is a server designed to host applications and their environments, permitting server applications to run. A typical example is Oracle Application Server, which is able to host Java, C, C++, and PL/SQL applications in cases where a remote client controls the interface. See also Oracle Application Server. The Oracle Application Server, that integrates all the core services and features required for building, deploying, and managing high-performance, n-tier, transaction-oriented Web applications within an open standards framework.

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 or

• You can run a Java servlet.

Stored procedures are easier to integrate with SQL and PL/SQL code, and require using 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.

Design Guidelines: Java Inside or Outside the Database?

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

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. The support for Java Servlets in this release is not complete, and provides a subset designed for easy migration to full compliance in a following release. Currently, 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.

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 is started, as well as a new database transaction.

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.

    See Also:

    Chapter 28, "Accessing the Repository using Protocols"

Session Pool and Oracle XML DB Servlets

Oracle Database keeps one Java VM for each database session. This means that a session reused from the session pool has any state left over in the Java VM (Java static variables) from the last time the 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.

HttpServletRequest

HttpSession

HttpSessionBindingListener

Oracle XML DB Servlet Example

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

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.

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

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

COMMIT;