Java Web services
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Web services allow software components to communicate over the web through standardized interfaces. There are two main types - RESTful web services which use HTTP methods to manipulate resources, and SOAP-based services which use XML messages over HTTP. A WSDL contract describes the operations, messages, and data types of a web service. JAX-WS and JAX-RS are Java APIs for creating web services that map Java methods to WSDL operations and SOAP/HTTP messages. RESTful services follow architectural constraints like using URIs to identify resources and HTTP methods to manipulate them.
Java Web services
- 1. Overview of Java Web Services Sujit Kumar Zenolocity LLC © 2013-2023
- 2. What are web services • Reusable web application components that provide software functions at a network address over the web with the service always on. • Interoperable between heterogeneous systems (language & platform independent) across the network. • Interface described by WSDL.
- 3. 2 Types • REST compliant web services. • SOAP based web services.
- 4. WSDL • Contract between caller and callee that describes a web service using XML format. • Set of operations available at a network address using a specific protocol and data format. • Each operation has an input request message and an output response message.
- 5. Analogies for a WSDL • WSDL – interface in java. • Operations – static methods in a java object. • Input Message of an operation – argument passed to a java method. • Output Message of an operation – argument returned from a java method. • Types – Classes in Java, without any methods.
- 6. WSDL Elements • Types – data type definitions • Messages – definition of data being exchanged. • Port Type – set of operations exposed at a network address. • Binding – protocol (SOAP over http) and data format specification.
- 7. SOAP • XML based communication protocol for accessing Web Services. • Language and Platform Independent. • SOAP provides a way for applications to communicate with each other over HTTP. The applications could be running on different operating systems with different technologies and programming languages.
- 8. SOAP XML Message Elements • Envelope – wraps everything • Header (optional) • Body – appears after Header. • Fault (optional) – part of the body
- 9. Example SOAP Message • <?xml version="1.0"?> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope" soap:encodingStyle="http://www.w3.org/2001/12/soap- encoding"> <soap:Body xmlns:m="http://www.example.org/stock"> <m:GetStockPrice> <m:StockName>IBM</m:StockName> </m:GetStockPrice> </soap:Body> </soap:Envelope>
- 10. JAXB • Java Architecture for XML Binding (JAXB) • Two main features: Marshall live java objects into XML data. Un-marshall XML data into live java objects. • Part of the Java SE platform. • One of the APIs in the Java EE platform. • Also, part of the Java Web Services Development Pack (JWSDP).
- 11. XML, XSD & Tools for JAXB • Analogy: XML is like an object, XSD is like a class or a type. XML should be well formed (syntactically correct) and valid (conform to an XSD). • xjc – convert xsd schemas to java classes. Address.xsd xjc Address.java • schemagen – convert annotated java classes to schema definitions. Address.java schemagen Address.xsd
- 12. JAX-WS • Java API for creating web services. • Part of Java EE platform. • Defines a standard for Java-to-WSDL mapping. • Determines how WSDL operations are bound to Java methods. • Determines how SOAP messages are mapped to java method arguments and return values. • Can be used in Java SE from version 6.0.
- 13. JAX-WS Tools • wsimport : top-down approach. Reads WSDL file & generates portable artifacts that includes SEI (service endpoint implementation), jaxb classes from schema types and exceptions from faults. • wsgen : bottom-up approach. Reads an SEI class and generates the portable artifacts for web service development and deployment. • Both are part of JDK 6 and above.
- 14. JAX-WS Implementations • Metro Project in Glassfish (open source) • Apache CXF • Apache Axis 2 • Oracle WebLogic • IBM WebSphere • JBossWS • Each of the above provides a wsdltojava utility and provide implementations for the interfaces in the JAX-WS api.
- 15. Converting WSDL to Java • Vendor provided wsldtojava utilitiy or wsimport utility in JDK 6. • Can be configured to run as an eclipse plugin, or invoked via command line utility or as an ANT/Maven task. • Output of Wsdl2Java is client stubs and skeletons. • Stubs are used to invoke the remote web services, actual business logic is written in the skeletons.
- 16. Comparison of JAX-WS Implementations • http://wiki.apache.org/ws/StackComparison • http://predic8.com/axis2-cxf-jax-ws- comparison.htm • http://www.ibm.com/developerworks/java/lib rary/j-jws19/index.html
- 17. Service Oriented Architecture (SOA) • Software architecture design pattern. • Based on discrete pieces of software providing application functionality as services to other applications. • Characteristics of a service: • Service Contract (defined via service description documents) • Loose Coupling (minimize dependencies) • Reusable • Abstraction (hide implementation logic) • Location transparency, discoverable, stateless & composable.
- 18. SOA and Web Services • Web Services are an implementation of SOA. • SOAP & REST are implementations of Web Services. • Web Service contract exposed via WSDL.
- 19. Web Services Implementation Mechanisms • Contract First or top-down : create the WSDL first and then generate the stubs and skeletons. WS Consumers use the stubs to invoke the service implemented by the WS Provider in the skeletons. • Contract Last or bottom-up. Implement the business logic first in the WS Provider. Extract WSDL after the implementation. • Contract first is usually preferred.
- 20. Benefits of Contract First • Looser Coupling between contract and implementation. Implementation logic can be changed without affecting clients. • Easier to keep contracts constant for longer periods of time. Contracts can be fragile and change constantly. In order for a contract to be useful, it must remain constant for as long as possible. • If a contract changes, you will have to contact all of the users of your service, and instruct them to get the new version of the contract. • Enables parallel development of clients and service.
- 21. RESTful Services • Architectural Style based on HTTP. • Client-Server : separation of responsibilities promotes portable clients & scalable servers. • Stateless – no client context saved on server. • Cacheable – clients can cache responses, improves performance & scalability. • Uniform Interface: identification of resources using URIs. The resources themselves are conceptually separate from the representations that are returned to the client. For example, the server does not send its database, but will send XML or JSON that represents some database records.
- 22. RESTful Services • Uniform Interface: Manipulation of resources through these representations held by the client. • Uniform Interface: Self-descriptive messages. Each message has enough information to describe how to process the message. • If a service violates any of the required constraints, it cannot be considered RESTful.
- 23. RESTful Services • Manipulate resources (CRUD) using HTTP methods. • GET – Retrieve • PUT – Create or Update • DELETE – Remove • POST - Create • GET, PUT and DELETE are idempotent, POST is not idempotent. • Idempotent implies: y = f(x), y = f(f(x)), y = f(f(f(x)))
- 24. Difference between PUT and POST • POST – create a child of the resource identified by the URI. • PUT – create or update the resource identified by the URI.
- 25. Considerations to use PUT vs POST • Do you name your URL objects you create explicitly, or let the server decide? If you name them then use PUT. If you let the server decide then use POST. • PUT is idempotent, so if you PUT an object twice, it has no effect, then use PUT when possible. • You can update or create a resource with PUT with the same object URI.
- 26. RESTful Methods applied to single resource or collection of resources STful API HTTP methods Resource GET PUT POST DELETE Collection URI tp://example.com /resources List the URIs and perhaps other details of the collection's members. Replace the entire collection with another collection. Create a new entry in the collection. The new entry's URI is assigned automatically and is usually returned by the operation. Delete the en collection. Element URI tp://example.com resources/item17 Retrieve the specific member of the collection. Replace the addressed member of the collection, or if it doesn't exist, create it. Not generally used. Treat the specific member as a collection & create a new entry in it. Delete the sp member of th collection.
- 27. Implementations of RESTful web services • Use JSR 311(JAX-RS) and its reference implementation Jersey. • Restlet framework • Spring MVC.
- 28. Tutorial Examples • Spring MVC based: IBM Developer Works Example Code Tutor Example • Jersey Implementation IBM Developer Works Example
- 30. Considerations for SOAP & RESTful web services • REST - How to model operations which are verbs like login, logout, transferFunds, etc? -- think of the resource on which you are applying these methods and use GET/PUT/POST operations on those resources. Need to map domain operations into CRUD. • SOAP doesn't require you to map domain operations onto CRUD. • SOAP is suited for complex request and responses where multiple objects may get updated within the same operation. • REST is particularly useful for mobiles and tablets where there may be a performance overhead of dealing with SOAP requests. • SOAP requires less plumbing code than REST services for things like transactions, security, coordination, addressing, trust, etc. • SOAP fault handling is richer, REST needs to use HTTP status codes.