Welkom bij de Web Services Interviewvragen met gedetailleerde antwoorden. Onlangs heb ik veel geschreven over webdiensten. Hoe we SOAP- en RESTful-webdiensten kunnen maken in Java.
Web Services Interviewvragen
Hier geef ik je een lijst met web services interviewvragen om je te helpen bij het solliciteren. Allereerst zijn er algemene vragen over het concept van webdiensten, omdat het niet specifiek is voor technologie of taal, en dan hebben we Java-webdiensten interviewvragen.
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Wat is het verschil tussen de Top Down en Bottom Up benadering bij SOAP Web Services?
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Welke verschillende HTTP-methoden worden ondersteund in Restful webdiensten?
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Wat is het gebruik van Accept en Content-Type Headers in een HTTP-verzoek?
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Noem enkele frameworks in Java om SOAP-webdiensten te implementeren?
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Noem belangrijke annotaties die worden gebruikt in de JAX-WS API?
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Wat is het verschil tussen RPC-stijl en Document-stijl SOAP-webdiensten?
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Noem belangrijke annotaties die worden gebruikt in de JAX-RS API?
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Hoe kan een verschillende statuscode worden ingesteld in het HTTP-antwoord?
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Wat is een webservice?
Webservices werken op het client-servermodel waarbij clienttoepassingen webservices kunnen benaderen via het netwerk. Webservices bieden eindpunt-URL’s en stellen methoden bloot die kunnen worden benaderd via het netwerk door clientprogramma’s geschreven in Java, shellscript of andere verschillende technologieën. Webservices zijn stateless en behouden geen gebruikerssessie zoals webtoepassingen.
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Wat zijn de voordelen van Web Services?
Sommige voordelen van web services zijn:
- Interoperabiliteit: Web services zijn toegankelijk via het netwerk en werken op het HTTP/SOAP-protocol en gebruiken XML/JSON om gegevens te transporteren, waardoor ze in elke programmeertaal kunnen worden ontwikkeld. Een web service kan bijvoorbeeld worden geschreven in Java en de client kan in PHP zijn, en vice versa.
- Herbruikbaarheid: Eén web service kan tegelijkertijd door veel clienttoepassingen worden gebruikt.
- Losse koppeling: De clientcode van web services is volledig onafhankelijk van de servercode, dus we hebben een losse koppeling in onze toepassing bereikt.
- Gemakkelijk te implementeren en integreren, net als webtoepassingen.
- Meerdere serviceversies kunnen tegelijkertijd worden uitgevoerd.
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Wat zijn de verschillende soorten webdiensten?
Er zijn twee soorten webdiensten:
- SOAP-webdiensten: Draaien op het SOAP-protocol en gebruiken XML-technologie voor het verzenden van gegevens.
- Restful webdiensten: Het is een architectuurstijl en draait bijna altijd op het HTTP/HTTPS-protocol. REST is een stateless client-server architectuur waar webdiensten bronnen zijn en kunnen worden geïdentificeerd aan de hand van hun URI’s. Clientapplicaties kunnen HTTP GET/POST-methoden gebruiken om Restful webdiensten aan te roepen.
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Wat is SOAP?
SOAP staat voor Simple Object Access Protocol. SOAP is een op XML gebaseerd industrienormprotocol voor het ontwerpen en ontwikkelen van webdiensten. Omdat het op XML gebaseerd is, is het platform- en taalonafhankelijk. Dus onze server kan gebaseerd zijn op JAVA en de client kan op .NET, PHP, enz. zijn, en vice versa.
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Wat zijn de voordelen van SOAP-webdiensten?
SOAP web services have all the advantages that web services has, some of the additional advantages are:
- WSDL document provides contract and technical details of the web services for client applications without exposing the underlying implementation technologies.
- SOAP uses XML data for payload as well as contract, so it can be easily read by any technology.
- SOAP protocol is universally accepted, so it's an industry standard approach with many easily available open source implementations.
Some of the disadvantages of SOAP protocol are:
- Only XML can be used, JSON and other lightweight formats are not supported.
- SOAP is based on the contract, so there is a tight coupling between client and server applications.
- SOAP is slow because payload is large for a simple string message, since it uses XML format.
- Anytime there is change in the server side contract, client stub classes need to be generated again.
- Can't be tested easily in browser
WSDL stands for Web Service Description Language. WSDL is an XML based document that provides technical details about the web service. Some of the useful information in WSDL document are: method name, port types, service end point, binding, method parameters etc.
Some of the different tags in WSDL xml are:
- xsd:import namespace and schemaLocation: provides WSDL URL and unique namespace for web service.
- message: for method arguments
- part: for method argument name and type
- portType: service name, there can be multiple services in a wsdl document.
- operation: contains method name
- soap:address for endpoint URL.
UDDI is acronym for Universal Description, Discovery and Integration. UDDI is a directory of web services where client applications can lookup for web services. Web Services can register to the UDDI server and make them available to client applications.
In Top Down approach first WSDL document is created to establish the contract between web service and client and then code is written, it's also termed as contract first approach. This is hard to implement because classes need to be written to confirm the contract established in WSDL. Benefit of this approach is that both client and server code can be written in parallel. In Bottom Up approach, first web service code is written and then WSDL is generated. It's also termed as contract last approach. This approach is easy to implement because WSDL is generated based on code. In this approach client code have to wait for WSDL from server side to start their work.
REST is the acronym for REpresentational State Transfer. REST is an architectural style for developing applications that can be accessed over the network. REST architectural style was brought in light by Roy Fielding in his doctoral thesis in 2000. REST is a stateless client-server architecture where web services are resources and can be identified by their URIs. Client applications can use HTTP GET/POST methods to invoke Restful web services. REST doesn’t specify any specific protocol to use, but in almost all cases it’s used over HTTP/HTTPS. When compared to SOAP web services, these are lightweight and doesn’t follow any standard. We can use XML, JSON, text or any other type of data for request and response.
Some of the advantages of REST web services are:
- Learning curve is easy since it works on HTTP protocol
- Supports multiple technologies for data transfer such as text, xml, json, image etc.
- No contract defined between server and client, so loosely coupled implementation.
- REST is a lightweight protocol
- REST methods can be tested easily over browser.
Some of the disadvantages of REST are:
- Since there is no contract defined between service and client, it has to be communicated through other means such as documentation or emails.
- Since it works on HTTP, there can't be asynchronous calls.
- Sessions can't be maintained.
Resource is the fundamental concept of Restful architecture. A resource is an object with a type, relationship with other resources and methods that operate on it. Resources are identified with their URI, HTTP methods they support and request/response data type and format of data.
Restful web services supported HTTP methods are - GET, POST, PUT, DELETE and HEAD.
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| SOAP | REST
|
| --- | --- |
| SOAP is a standard protocol for creating web services.
| REST is an architectural style to create web services.
|
| SOAP is acronym for Simple Object Access Protocol. | REST is acronym for REpresentational State Transfer. |
| SOAP uses WSDL to expose supported methods and technical details.
| REST exposes methods through URIs, there are no technical details.
|
| SOAP web services and client programs are bind with WSDL contract
| REST doesn't have any contract defined between server and client
|
| SOAP web services and client are tightly coupled with contract.
| REST web services are loosely coupled.
|
| SOAP learning curve is hard, requires us to learn about WSDL generation, client stubs creation etc.
| REST learning curve is simple, POJO classes can be generated easily and works on simple HTTP methods.
|
| SOAP supports XML data format only
| REST supports any data type such as XML, JSON, image etc.
|
| SOAP web services are hard to maintain, any change in WSDL contract requires us to create client stubs again and then make changes to client code.
| REST web services are easy to maintain when compared to SOAP, a new method can be added without any change at client side for existing resources.
|
| SOAP web services can be tested through programs or software such as Soap UI.
| REST can be easily tested through CURL command, Browsers and extensions such as Chrome Postman.
|
SOAP web services can be tested programmatically by generating client stubs from WSDL or through software such as Soap UI. REST web services can be tested easily with program, curl commands and through browser extensions. Resources supporting GET method can be tested with browser itself, without any program.
Web services are stateless so we can't maintain user sessions in web services.
Service Oriented Architecture (SOA) is an architectural pattern where applications are designed in terms of services that can be accessed through communication protocol over network. SOA is a design pattern and doesn't go into implementation. Web Services can be thought of as Services in SOAP architecture and providing means to implement SOA pattern.
These are important headers in Restful web services. Accept headers tells web service what kind of response client is accepting, so if a web service is capable of sending response in XML and JSON format and client sends Accept header as "application/xml" then XML response will be sent. For Accept header "application/json", server will send the JSON response. Content-Type header is used to tell server what is the format of data being sent in the request. If Content-Type header is "application/xml" then server will try to parse it as XML data. This header is useful in HTTP Post and Put requests.
Web Services work on client-server model and when it comes to choose between SOAP and REST, it all depends on project requirements. Let's look at some of the conditions affecting our choice:
- Do you know your web service clients beforehand? If Yes, then you can define a contract before implementation and SOAP seems better choice. But if you don't then REST seems better choice because you can provide sample request/response and test cases easily for client applications to use later on.
- How much time you have? For quick implementation REST is the best choice. You can create web service easily, test it through browser/curl and get ready for your clients.
- What kind of data format are supported? If only XML then you can go with SOAP but if you think about supporting JSON also in future then go with REST.
JAX-WS stands for Java API for XML Web Services. JAX-WS is XML based Java API to build web services server and client application. It’s part of standard Java API, so we don’t need to include anything else which working with it. Refer to [JAX-WS Tutorial](/community/tutorials/jax-ws-tutorial) for a complete example.
We can create SOAP web services using JAX-WS API, however some of the other frameworks that can be used are Apache Axis and Apache CXF. Note that they are not implementations of JAX-WS API, they are totally different framework that work on Servlet model to expose your business logic classes as SOAP web services. Read more at [Java SOAP Web Service Eclipse](/community/tutorials/soap-webservices-in-java-example-eclipse) example.
Some of the important annotations used in JAX-WS API are:
- @WebService
- @SOAPBinding
- @WebMethod
Endpoint class provides useful methods to create endpoint and publish existing implementation as web service. This comes handy in testing web services before making further changes to deploy it on actual server.
RPC style generate WSDL document based on the method name and it's parameters. No type definitions are present in WSDL document. Document style contains type and can be validated against predefined schema. Let's look at these with a simple program. Below is a simple test program where I am using Endpoint to publish my simple SOAP web service. `TestService.java`
```
package com.journaldev.jaxws.service;
import javax.jws.WebMethod;
import javax.jws.WebService;
import javax.jws.soap.SOAPBinding;
import javax.xml.ws.Endpoint;
@WebService
@SOAPBinding(style = SOAPBinding.Style.RPC)
public class TestService {
@WebMethod
public String sayHello(String msg){
return "Hello "+msg;
}
public static void main(String[] args){
Endpoint.publish("https://localhost:8888/testWS", new TestService());
}
}
```
When I run above program and then access the WSDL, it gives me below XML. `rpc.xml`
```
<?xml version='1.0' encoding='UTF-8'?>
<!-- Published by JAX-WS RI (https://jax-ws.java.net). RI's version is JAX-WS RI 2.2.10 svn-revision#919b322c92f13ad085a933e8dd6dd35d4947364b. --><!-- Generated by JAX-WS RI (https://jax-ws.java.net). RI's version is JAX-WS RI 2.2.10 svn-revision#919b322c92f13ad085a933e8dd6dd35d4947364b. -->
<definitions xmlns:wsu="https://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" xmlns:wsp="https://www.w3.org/ns/ws-policy" xmlns:wsp1_2="https://schemas.xmlsoap.org/ws/2004/09/policy" xmlns:wsam="https://www.w3.org/2007/05/addressing/metadata" xmlns:soap="https://schemas.xmlsoap.org/wsdl/soap/" xmlns:tns="https://service.jaxws.journaldev.com/" xmlns:xsd="https://www.w3.org/2001/XMLSchema" xmlns="https://schemas.xmlsoap.org/wsdl/" targetNamespace="https://service.jaxws.journaldev.com/" name="TestServiceService">
<types/>
<message name="sayHello">
<part name="arg0" type="xsd:string"/>
</message>
<message name="sayHelloResponse">
<part name="return" type="xsd:string"/>
</message>
<portType name="TestService">
<operation name="sayHello">
<input wsam:Action="https://service.jaxws.journaldev.com/TestService/sayHelloRequest" message="tns:sayHello"/>
<output wsam:Action="https://service.jaxws.journaldev.com/TestService/sayHelloResponse" message="tns:sayHelloResponse"/>
</operation>
</portType>
<binding name="TestServicePortBinding" type="tns:TestService">
<soap:binding transport="https://schemas.xmlsoap.org/soap/http" style="rpc"/>
<operation name="sayHello">
<soap:operation soapAction=""/>
<input>
<soap:body use="literal" namespace="https://service.jaxws.journaldev.com/"/>
</input>
<output>
<soap:body use="literal" namespace="https://service.jaxws.journaldev.com/"/>
</output>
</operation>
</binding>
<service name="TestServiceService">
<port name="TestServicePort" binding="tns:TestServicePortBinding">
<soap:address location="https://localhost:8888/testWS"/>
</port>
</service>
</definitions>
```
Notice that **types** element is empty and we can't validate it against any schema. Now just change the `SOAPBinding.Style.RPC` to `SOAPBinding.Style.DOCUMENT` and you will get below WSDL. `document.xml`
```
<?xml version='1.0' encoding='UTF-8'?>
<!-- Published by JAX-WS RI (https://jax-ws.java.net). RI's version is JAX-WS RI 2.2.10 svn-revision#919b322c92f13ad085a933e8dd6dd35d4947364b. --><!-- Generated by JAX-WS RI (https://jax-ws.java.net). RI's version is JAX-WS RI 2.2.10 svn-revision#919b322c92f13ad085a933e8dd6dd35d4947364b. -->
<definitions xmlns:wsu="https://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd" xmlns:wsp="https://www.w3.org/ns/ws-policy" xmlns:wsp1_2="https://schemas.xmlsoap.org/ws/2004/09/policy" xmlns:wsam="https://www.w3.org/2007/05/addressing/metadata" xmlns:soap="https://schemas.xmlsoap.org/wsdl/soap/" xmlns:tns="https://service.jaxws.journaldev.com/" xmlns:xsd="https://www.w3.org/2001/XMLSchema" xmlns="https://schemas.xmlsoap.org/wsdl/" targetNamespace="https://service.jaxws.journaldev.com/" name="TestServiceService">
<types>
<xsd:schema>
<xsd:import namespace="https://service.jaxws.journaldev.com/" schemaLocation="https://localhost:8888/testWS?xsd=1"/>
</xsd:schema>
</types>
<message name="sayHello">
<part name="parameters" element="tns:sayHello"/>
</message>
<message name="sayHelloResponse">
<part name="parameters" element="tns:sayHelloResponse"/>
</message>
<portType name="TestService">
<operation name="sayHello">
<input wsam:Action="https://service.jaxws.journaldev.com/TestService/sayHelloRequest" message="tns:sayHello"/>
<output wsam:Action="https://service.jaxws.journaldev.com/TestService/sayHelloResponse" message="tns:sayHelloResponse"/>
</operation>
</portType>
<binding name="TestServicePortBinding" type="tns:TestService">
<soap:binding transport="https://schemas.xmlsoap.org/soap/http" style="document"/>
<operation name="sayHello">
<soap:operation soapAction=""/>
<input>
<soap:body use="literal"/>
</input>
<output>
<soap:body use="literal"/>
</output>
</operation>
</binding>
<service name="TestServiceService">
<port name="TestServicePort" binding="tns:TestServicePortBinding">
<soap:address location="https://localhost:8888/testWS"/>
</port>
</service>
</definitions>
```
Open schemaLocation URL in browser and you will get below XML. `schemaLocation.xml`
```
<?xml version='1.0' encoding='UTF-8'?>
<!-- Published by JAX-WS RI (https://jax-ws.java.net). RI's version is JAX-WS RI 2.2.10 svn-revision#919b322c92f13ad085a933e8dd6dd35d4947364b. -->
<xs:schema xmlns:tns="https://service.jaxws.journaldev.com/" xmlns:xs="https://www.w3.org/2001/XMLSchema" version="1.0" targetNamespace="https://service.jaxws.journaldev.com/">
<xs:element name="sayHello" type="tns:sayHello"/>
<xs:element name="sayHelloResponse" type="tns:sayHelloResponse"/>
<xs:complexType name="sayHello">
<xs:sequence>
<xs:element name="arg0" type="xs:string" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="sayHelloResponse">
<xs:sequence>
<xs:element name="return" type="xs:string" minOccurs="0"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
```
So here WSDL document can be validated against the schema definintion.
WSDL document can be accessed by appending ?wsdl to the SOAP endoint URL. In above example, we can access it at `https://localhost:8888/testWS?wsdl` location.
This file is used to provide endpoints details when JAX-WS web services are deployed in servlet container such as Tomcat. This file is present in WEB-INF directory and contains endpoint name, implementation class and URL pattern. For example; `sun-jaxws.xml`
```
<?xml version="1.0" encoding="UTF-8"?>
<endpoints xmlns="https://java.sun.com/xml/ns/jax-ws/ri/runtime" version="2.0">
<endpoint
name="PersonServiceImpl"
implementation="com.journaldev.jaxws.service.PersonServiceImpl"
url-pattern="/personWS"/>
</endpoints>
```
Java API for RESTful Web Services (JAX-RS) is the Java API for creating REST web services. JAX-RS uses annotations to simplify the development and deployment of web services. JAX-RS is part of JDK, so you don’t need to include anything to use it’s annotations.
There are two major implementations of JAX-RS API.
1. Jersey: Jersey is the reference implementation provided by Sun. For using Jersey as our JAX-RS implementation, all we need to configure its servlet in web.xml and add required dependencies. Note that JAX-RS API is part of JDK not Jersey, so we have to add its dependency jars in our application.
2. RESTEasy: RESTEasy is the JBoss project that provides JAX-RS implementation.
We can use wsimport utility to generate the client stubs. This utility comes with standard installation of JDK. Below image shows an example execution of this utility for one of JAX-WS project. [](https://journaldev.nyc3.cdn.digitaloceanspaces.com/2015/10/wsimport-utility-parse-wsdl.png)
Some of the important JAX-RS annotations are:
- `@Path`: used to specify the relative path of class and methods. We can get the URI of a webservice by scanning the Path annotation value.
- `@GET`, `@PUT`, `@POST`, `@DELETE` and `@HEAD`: used to specify the HTTP request type for a method.
- `@Produces`, `@Consumes`: used to specify the request and response types.
- `@PathParam`: used to bind the method parameter to path value by parsing it.
-
Wat is het gebruik van de @XmlRootElement annotatie?
XmlRootElement annotation is used by JAXB to transform java object to XML and vice versa. So we have to annotate model classes with this annotation.
For setting HTTP status code other than 200, we have to use `javax.ws.rs.core.Response` class for response. Below are some of the sample return statements showing it's usage.
```
return Response.status(422).entity(exception).build();
return Response.ok(response).build(); //200
```
For a complete example, please read [RESTEasy Tutorial](/community/tutorials/resteasy-tutorial-eclipse-tomcat).
Dat is alles voor sollicitatievragen voor webdiensten. Laat me weten wat je ervan vindt via opmerkingen. Als je denkt dat ik enkele belangrijke vragen ben vergeten, laat het me dan weten via opmerkingen, en ik zal blij zijn om ze toe te voegen aan de lijst.
Source:
https://www.digitalocean.com/community/tutorials/web-services-interview-questions-soap-restful