Building a XML gateway architecture for JDBC using JAX-RPC web services

Transcription

1 Analele Ştiinţifice ale Universităţii "Alexandru Ioan Cuza" din Iaşi Informatică, Tomul XIV, 2004 Scientific Annals of the "Alexandru Ioan Cuza" University of Iaşi Computer Science Section, Tome XIV, 2004 Building a XML gateway architecture for JDBC using JAX-RPC web services Florin Stoica Abstract In a utopian view all Web services data resides in XML but the reality is that most of corporate data are stored in existing relational and legacy databases and will continue to reside there. XML will likely become the common integrating medium for Web services applications, providing a de facto standard to transmit corporate data. Ultimately, data will typically start and end in a relational or legacy database. In this paper we show how to construct a simple XML gateway architecture for JDBC that allows to extract data from a JDBC data source and send disconnected ResultSets across Internet, using web services, to client applications. 1 Introduction A significant shift is occurring in data connectivity as we move from traditional 3GL/4GL client/server and Web application environments to distributed applications and Web services. Traditional client/server and Web application-environments relied on connection-based synchronous communication with the back-end database. Although this means tight control over locking and transaction behavior, it is not always possible to connect to a database server over the Internet, since proxies and firewalls do not often allow it. Even if they did, opening up databases over the Internet would pose a security threat. As a result the Java community, with WebRowSets, are focusing on XML and improved scalability with XML rowset-type architecture that is fundamentally disconnected and asynchronous in nature. The Rowset library allows to serialize and deserialize ResultSets across multi-layer applications, in between different applications, as well as over the web. This paper focuses on the WebRowSetImpl class in particular, a WebRowSet interface implementation that can serialize the data, metadata, and properties of a JDBC ResultSet to XML. 1

2 A WebRowSet can be used to provide Java clients with access to tabular data in situations where it would be inappropriate to use a JDBC driver directly in the client due to resource limitations or security considerations. 2 WebRowSet Defined The javax.sql package includes the RowSet interface. As its name implies, a rowset is an object that encapsulates a set of rows that have been retrieved from some tabular data source. The javax.sql.rowset interface extends the java.sql.resultset interface, so in many ways a rowset behaves just like a result set. A RowSet is simply a ResultSet that can function as a JavaBeans component. The standard implementations of the RowSet interface consist of two parts, the interfaces and the reference implementations. The interfaces are in the javax.sql.rowset package; the implementations are in the com.sun.rowset package. The javax.sql.rowset package includes the following interfaces: 1. CachedRowSet - a disconnected rowset that caches its data in memory; not suitable for very large data sets, but an ideal way to provide thin Java clients, such as a Personal Digital Assistant (PDA) or Network Computer (NC), with tabular data; 2. A JDBCRowSet class - a connected rowset that serves mainly as a thin wrapper around a ResultSet object to make a JDBC driver look like a JavaBeans component; 3. A WebRowSet class a disconnected rowset used to make it possible for web clients to retrieve and possibly update a set of rows. java.sql.resultset RowSet CachedRowSet JDBCRowSet WebRowSet Fig. 1 Rowset interface standard implementation 2

3 The standard implementation of the CachedRowSet interface (CachedRowSetImpl in the package com.sun.rowset) provides a container for a set of rows that is being cached in memory outside of a data source. The CachedRowSetImpl class can create a disconnected RowSet (as opposed to a connected ResultSet object). It is disconnected, serializable, updatable, and scrollable. Because a CachedRowSet object caches its own data, it does not need to maintain an open connection with a data source and is disconnected from its data source except when it is reading or writing data. A CachedRowSet object can populate itself with data from a tabular data source, and because it is updatable, it can also modify its data. As with all RowSet objects, in addition to getting data in, it can get data out, propagating its modifications back to the underlying data source. The WebRowSet interface extends the CachedRowSet interface and therefore has all of the same capabilities. What it adds is the ability to read and write a rowset in XML format. A WebRowSet object uses a WebRowSetXmlReader object to read a rowset in XML format and a WebRowSetXmlWriter object to write a rowset in XML format. The XML version of a WebRowSet object contains its metadata, including its properties, in addition to its data. A WebRowSet object is designed to work well in a distributed client/server application. A WebRowSet object uses HTTP/XML (Hypertext Transfer Protocol/ extensible Markup Language) to communicate with the middle tier, so that, for example, Web clients can talk to Java servlets that provide data access. XML has become more and more important for Web services because of the portability of data it provides. The JDBC RowSet implementations specification includes a standard WebRowSet XML Schema, available at to which a standard WebRowSet object adheres. This means that if two parties have the XML schema for a WebRowSet object, they can use it to exchange rowsets in a common format even though they may store their data internally in entirely different formats. This makes a WebRowSet object a powerful tool for data exchange using the Java platform for Web services. The advantages associated with the use of the WebRowSet feature include the following: Active connections with the datasource need not be maintained to pass the tabular data between tiers and components. The rows (tabular data) referred to as rowsets can be read and written in XML format, thus enabling rowsets to be sent over the Internet using the HTTP/XML/SOAP protocol. 3

4 3 Building Web Services With JAX-RPC The power of Web services is its reliance on simplicity, a foundation in current Internet technologies, and a focus on standards, including XML for data, SOAP for transport, WSDL for location, and UDDI for directories. JAX-RPC is a big part of Sun's strategy for embracing Web services. It is a best-of-breed technology, combining the familiar RPC programming model with the newest protocols, to create technology that is both productive and cuttingedge. WSDL Document JAX-RPC JAX-RPC Client Service Endpoint Java WSDL WSDL Java Generated code Container Client-side JAX-RPC Runtime System Container Server-side JAX-RPC Runtime System SOAP HTTP Fig. 2 JAX-RPC Architecture JAX-RPC stands for Java API for XML-based RPC. It is an API for building Web services and clients that use remote procedure calls (RPC) and XML. In JAX-RPC, a remote procedure call is represented by an XML-based protocol such as SOAP. The SOAP specification defines the envelope structure, encoding rules, and convention for representing remote procedure calls and responses. These calls and responses are transmitted as SOAP messages (XML files) over HTTP. With JAX-RPC, the developer does not generate or parse SOAP messages. It is the JAX-RPC runtime system that converts the API calls and responses to and from SOAP messages. The service s WSDL document enables clients and services that use very different technologies to map and convert their respective data objects. For services and clients that are based on JAX-RPC, the JAX-RPC runtime handles this mapping transparently. Clients can easily use the JAX-RPC-generated stub classes to access a Web service. 4

5 A JAX-RPC service is defined as a Java interface type. The developer codes one or more classes that implement interface methods. A client creates a proxy, a local object representing the service, and then simply invokes methods on the proxy. Stub communication is easy and allows for fast programmatic Java access to a Web service. This is the recommended approach for accessing services and their WSDL files when they are unlikely to change over time. For the stub model, a JAX-RPC runtime tool generates during development static stub classes that enable the service and the client to communicate. The stub class, which sits between the client and the client representation of the service endpoint interface, is responsible for converting a request from a client to a SOAP message and sending it to the service interface. The stub class also converts responses from the service endpoint, which it receives as SOAP messages, to a format understandable by the client. In a sense, a stub is a local object that acts as a proxy for the service endpoint. Figure 3 shows how a client application might access a Web service using a stub generated by a tool prior to the client s deployment and compilation. The WSDL file for the service served as input to the tool. The client-side service endpoint interface, which implements the service endpoint interface on the client-side of the communication channel, provides the client view of the Web service. WSDL JAX-RPC Mappings Stubgenerating tool Client Stub/Proxy Service endpoint interface JAX-RPC runtime Web service Fig. 3 Accessing a web service in stub communication approach The stub class, which is generated by the tool, implements the client-side service endpoint interface. The stub class acts as a proxy to the Web service for the client; that is, the stub is the client s view of the Web service. The tool that generates this stub class also generates all necessary helper classes for accessing the service. These helper classes define the parameters for calls to the service and the service s return values, and they ensure that the parameters and return values are all of the proper types expected by the JAX-RPC runtime. 5

6 The JAX-RPC tool generates these stub and helper classes using a WSDL document as well as a JAX-RPC mapping file. The mapping file supplies information regarding the Java-to-XML bindings, such as the correct package name for generated classes. The stubs approach, especially in a Java environment, is often the easiest to use, because the developer can work with generated class files representing the service method call parameters and return values. 4 XML gateway architecture for JDBC Database Web services leverage corporate existing server-side infrastructure, allowing to use a database as data service provider, thus enabling to share data and metadata across Internet. One of the principal reasons for implementing Web services is to achieve interoperability. Clients can access Web services regardless of the platform or operating system upon which the service or the client is implemented. In addition to interoperability, Web service clients can use standardized approaches to access services through firewalls. Such access extends the capabilities of clients. The transport protocol used by Web services enable clients to operate with systems through firewalls. In figure 4 is showed the XML gateway architecture for JDBC that allows to extract data from a JDBC data source, serialize the JDBC result set to XML and sending this XML version of extracted result set across Internet, using SOAP messages, to client applications. A client application can deserialize the data, metadata, and properties of a JDBC result set from XML, using the obtained WebRowSet object as data source in a well-known manner (as ResultSet). Client Web RowSet XML serialized data SOAP / HTTP Web service Web RowSet DBMS Firewall Fig. 4 XML gateway architecture for JDBC Using this architecture we can build a distributed JDBC application that allows to interact with a JDBC data source over the Internet using HTTP. 6

7 5 Web service implementation This example shows how we can connect to a database in a vendor independent manner and cache the database rows in a WebRowSet object. Firewalls do not often allow to connect to a database server over the Internet. Opening up databases over the Internet would pose a security threat. To avoid this lapse in security, the web service is designed to act as a proxy for the database. The client application can invoke the web service to retrieve tabular data. The service endpoint interface declares the methods that a remote client may invoke on the service. In this example, the interface WebServiceIF declares a single method named getxmlrowset: package webservice; import java.rmi.remote; import java.rmi.remoteexception; import com.sun.rowset.*; import javax.sql.rowset.*; public interface WebServiceIF extends Remote { public String getxmlrowset(string dburl, String dbdriver, String dbuser, String dbpassword, String sqlquery) throws RemoteException; Once a JAX-RPC service is defined in the form of service definition interface, it can be implemented. The class that implements the interface is called WebServiceImpl: package webservice; import java.sql.*; import javax.sql.rowset.*; import com.sun.rowset.*; import java.io.*; public class WebServiceImpl implements WebServiceIF { public String getxmlrowset(string dburl, String dbdriver, String dbuser, String dbpassword, String sqlquery) { StringWriter writer = new StringWriter(); WebRowSetImpl rowset = null; ResultSet resultset = null; 7

8 Connection con = null; try { Class.forName(dbDriver); con = DriverManager.getConnection(dbUrl, dbuser, dbpassword); Statement stmt = con.createstatement( ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_READ_ONLY); //Execute the select statement resultset = stmt.executequery(sqlquery); //create the WebRowSet rowset = new WebRowSetImpl(); //populate the WebRowSet from the resultset data rowset.populate(resultset); //close the resultset resultset.close(); rowset.writexml(writer); catch (ClassNotFoundException e) { e.printstacktrace(); catch (SQLException e) { e.printstacktrace(); return writer.tostring(); The above code fragment creates the WebRowSet object named rowset, populates it with the data of the ResultSet object resultset, and then output rowset in XML format to a StringWriter object. The web service method getxmlrowset returns string conversion of the StringWriter object, containing XML representation of the WebRowSet object. 6 Client implementation Client is a stand-alone program that calls the getxmlrowset method of the WebService. It makes this call through a stub, a local object which acts as a proxy for the remote service. Because the stub is created before runtime (by wscompile), it is usually called a static stub. package client; import javax.xml.rpc.stub; import javax.sql.rowset.*; import javax.sql.*; 8

9 import java.io.*; import com.sun.rowset.*; public class Client { private static String dburl = "jdbc:odbc:admitere"; private static String dbdriver = "sun.jdbc.odbc.jdbcodbcdriver"; private static String dbuser = "Admin"; private static String dbpassword = ""; private static String sqlquery = "select nume_pren, " + "legitim, media_fin from candidati"; public static void main(string[] args) { try { // create a Stub object Stub stub = createproxy(); // set the endpoint address that the stub uses to // access the service stub._setproperty( javax.xml.rpc.stub.endpoint_address_property, " // cast stub to the service endpoint interface WebServiceIF webdb = (WebServiceIF) stub; String xmlrowset = webdb.getxmlrowset(dburl, dbdriver, dbuser, dbpassword, sqlquery); StringReader reader = new StringReader(xmlRowSet); WebRowSetImpl wrs = new WebRowSetImpl(); wrs.readxml(reader); while (wrs.next()) { System.out.println("Nume:" + wrs.getstring(1)); catch (Exception ex) { ex.printstacktrace(); private static Stub createproxy() { return (Stub) (new WebService_Impl().getWebServiceIFPort()); Invoking the web service method getxmlrowset, the client receives a XML string containing XML representation of the remote WebRowSet object. Then, the client create a StringReader object using the passed XML string. The readxml() API is capable of creating the WebRowSet object (named wrs) using the StringReader object without connecting to the database. 9

10 Because WebRowSet interface extends the java.sql.resultset interface, the WebRowSet object can be used as any ResultSet object. In fact, most client applications that make use of a WebRowSet component will likely treat it as a ResultSet object. In the following application, we will use a nice feature of Crystal Reports, which can use a JavaBean class as data source for a report. 7 A data consumer application This section provides an example of how to use a WebRowSet received by a client of the database web service to report remote data. Reporting is the process of accessing data, formatting it, and delivering it as information inside and outside the organization. It serves as the foundation of broader business intelligence strategy by providing the most-requested pieces of information reliably and securely. The advent of the Web has provided yet another medium to distribute information - and with it developers are faced with a variety of challenges in getting actionable information into the hands of end users in a consumable format, as quickly as possible. Crystal Reports, a world standard for high-performance report design, is an intuitive reporting toolkit for creating flexible, feature-rich reports and tightly integrate them into web (and windows) applications. Adding the following method to the Client class described above, we can provide the returned WebRowSet as data source for a client report with a professional appearance. public WebRowSetImpl getwebrowset() { WebRowSetImpl wrs = null; try { Stub stub = createproxy(); stub._setproperty( javax.xml.rpc.stub.endpoint_address_property, " WebServiceIF webdb = (WebServiceIF) stub; String xmlrowset = webdb.getxmlrowset(dburl, dbdriver, dbuser, dbpassword, sqlquery); StringReader reader = new StringReader(xmlRowSet); wrs = new WebRowSetImpl(); wrs.readxml(reader); catch (Exception ex) { ex.printstacktrace(); return wrs; 10

11 Crystal Reports 9 is the first version that has the ability to use Java Beans as data sources. Any methods that return java.sql.resultset objects will be added as available tables under the Java Beans Connectivity folder, in the Database Expert dialog box. Figure 5 presents a report document generated with Crystal Reports Conclusions Rowsets make it easy to send tabular data over a network. Because XML and Web services are all about standardization and interoperability, it is important to look to standards-based techniques for data connectivity. Standardized data connectivity has been a key part of the Java platform from the early development of Java, through the Java Database Connectivity (JDBC) specification. JDBC 3.0 delivers developer productivity enhancements with fine-tuning capabilities, as well as new XML support. With this version, the Java Community is extending the platform with WebRowSets for XML. Fig. 5 A report document with retrieved WebRowSet as data source With WebRowSets, the Java strategy has been to focus on simplicity and portability of data with XML. For building XML Web services components and applications in Java, developers can use WebRowSets to share data in XML format under a disconnected model. 11

12 WebRowSet is a disconnected rowset in that it inherently provides disconnected data, and thereby does not require a continuous database connection to hold the data from the database. This feature enables disconnected applications such as Web Services clients to fetch a collection of rows from database tables (or other data sources) in XML format and to scroll through, update locally, and synchronize the rows back to the data source. This mechanism relies on HTTP and XML for exchanging commands and data between client and server-side implementations, thus eliminating the need for the client to connect to the database server over the Internet. References [1] Eckel, B.: Thinking in Java, 3rd Edition, Prentice-Hall, 2002 [2] Farley, J., Crawford, W., Flanagan, D.: Java Enterprise in a Nutshell, 2nd Edition, O Reilly, 2002 [3] Singh, I., Stearns, B., Johnson, M.: Designing Enterprise Applications with the J2EE Platform, Second Edition, Addison-Wesley, 2002 [4] Sheldon, B.: Crystal Enterprise - A report server solution, [5] Business Objects, Enterprise Reporting brochure, [6] Business Objects, Crystal Enterprise datasheet, [7] Skonnard, A.: The XML Files-Web Services and DataSets, MSDN Magazine, The Microsoft Journal for Developers, april 2003 [8] Sun Microsystems, Web Services Design, Early Access Draft, 2003 [9] Sun Microsystems, Designing Web Services with the J2EE 1.4 Platform, [10] McGovern, J., Tyagi S., Stevens, M., Mathew, S.: Java Web Services Architecture, [11] Fisher, M., Ellis, J., Bruce, J.: JDBC API Tutorial and Reference, Third Edition, Addison-Wesley,