Spring MVC

October 02, 2010

This post is about Java Web application development using the Spring MVC framework. It distills much of what I have learned from developing enterprise applications with Spring MVC, guiding usage of the components of Spring MVC that I most frequently encounter in practice and on discussion forums. This post provides a brief overview of these components by taking you through the development of an example Spring MVC application from scratch.

My goals in writing this post are to guide developers who are unfamiliar with Spring MVC, and to supply a convenient reference to more seasoned developers.

Intended audience

This post is written for developers who are interested in working with Spring MVC, whether they are newcomers or have been working with it for years. It assumes basic familiarity with Spring and Java Web applications, though I have included links to reference information to help fill these gaps.

I generally don't build a Spring MVC application from scratch, opting instead to build upon a framework I have already prepared, or a sample I have already developed. Since understanding the fundamental concepts is in no way the same as generating boilerplate code from memory, I find it valuable to maintain a barebones framework from which to springboard new project development.

Topics covered

The following components of Spring MVC are covered:

This post does not cover advanced, uncommon, or deep-dive topics into Spring MVC, Spring Web Flow, Spring JavaScript, JavaScript frameworks, etc., although there is some basic usage of both Spring JavaScript and the Dojo Toolkit's UI library Dijit. Instead, this post focuses on the development of an example application which features the common Spring MVC components.

Example application

This post is based on a minimal example application which serves two goals: to provide a useful foundation in something not unlike a real-world application, and to avoid diving into a specific problem domain which eclipses the concepts as they are presented.

The example application is a limited employee directory. A user interacts with a Web front-end to view a list of employees, add new employees, and edit or delete existing employees.

Employees view
Employees view

This shows the main view of the Employee Directory. It lists all employees in the directory, provides links to view and edit the details of each, and includes a button for adding a new employee to the directory.

Employee view
Employee view

This shows a detailed view of a single employee. When the details of an employee are viewed, or when a new employee is to be added, this view is presented.

Application skeleton

The Employee Directory is structured as a Maven project. At the root of the project is the POM, where dependencies and other project configuration is maintained. Application source code is placed in src/main/java, with corresponding configuration in src/main/resources. Web application content and configuration is within src/main/webapp. This includes the web.xml deployment descriptor, the JSP view templates, and other Web content (stylesheets, images, etc.).

Source code

Source code included in this post is formatted as follows.

CodeConventions.java:

package com.earldouglas.barebones.springmvc;

public class CodeConventions {

  public static void main(String[] arguments) {
    System.out.println("Hello World!");
  }
}

Section references

Core application

Components

The core of the Employee Directory is the largest single segment of its construction. It consists of an in-memory repository of employees, with an HTML front-end for user interaction, using the following components:

Classes

Views

Configuration

Classes

At the architectural bottom of the code is the domain class Employee.

Employee class
Employee class

Employee.java:

public class Employee {

  private Long id;
  private String name;
  private String title;

  public Edmployee() {
  }

  public Employee(Long id, String name, String title) {
    this.id = id;
    this.name = name;
    this.title = title;
  }

  // Getters and setters omitted for brevity.
}

A service interface is defined by EmployeeService, which provides the standard CRUD behavior.

EmployeeService interface
EmployeeService interface

EmployeeService.java:

public interface EmployeeService {

  public Collection<Employee> get();

  public Employee get(Long id);

  public Employee save(Employee employee);

  public void delete(Long id);
}

A simple in-memory EmployeeService is implemented by InMemoryEmployeeService.

EmployeeService interface and InMemoryEmployeeService class
EmployeeService interface and InMemoryEmployeeService class

InMemoryEmployeeService.java:

@Repository
public class InMemoryEmployeeService implements EmployeeService {

  private long maxId = 3L;

  private final Map<Long, Employee> employees = new HashMap<Long, Employee>() {
    private static final long serialVersionUID = 1L;
    {
      put(1L, new Employee(1L, "Professor Jefe", "The Boss"));
      put(2L, new Employee(2L, "Number Two", "Number Two"));
      put(3L, new Employee(3L, "Johnny McDoe", "Work Man"));
    }
  };

  @Override
  public Employee get(Long id) {
    return employees.get(id);
  }

  @Override
  public Collection<Employee> get() {
    return employees.values();
  }

  private synchronized long nextId() {
    return ++maxId;
  }

  @Override
  public Employee save(Employee employee) {
    if (employee.getId() == null) {
      employee.setId(nextId());
    }
    employees.put(employee.getId(), employee);
    return employee;
  }

  @Override
  public void delete(Long employeeId) {
    employees.remove(employeeId);
  }
}

This service implementation will provide core services to the Web front-end, implemented as EmployeeController. The EmployeeController is responsible for handling HTTP requests, and translating between the UI model and the domain model classes.

BindableEmployee, EmployeeController, and Employee classes
BindableEmployee, EmployeeController, and Employee classes

EmployeeController.java:

@Controller
@RequestMapping("/employees")
public class EmployeeController {

  @Autowired
  private EmployeeService employeeService;

  @RequestMapping(method = RequestMethod.GET)
  public Collection<BindableEmployee> get() {
    return BindableEmployee.bindableEmployees(employeeService.get());
  }

  @RequestMapping(value = "/new", method = RequestMethod.GET)
  public String get(Model model) {
    return get(null, model);
  }

  @RequestMapping(value = "/{employeeId}", method = RequestMethod.GET)
  public String get(@PathVariable Long employeeId, Model model) {
    Employee employee = employeeService.get(employeeId);
    if (employee != null) {
      model.addAttribute(new BindableEmployee(employee));
    } else {
      model.addAttribute(new BindableEmployee());
    }
    return "employee";
  }

  @RequestMapping(value = "/{employeeId}/delete", method = RequestMethod.GET)
  public String deleteViaGet(@PathVariable Long employeeId) {
    return delete(employeeId);
  }

  @RequestMapping(value = "/{employeeId}", method = RequestMethod.DELETE)
  public String delete(@PathVariable Long employeeId) {
    employeeService.delete(employeeId);
    return "redirect:../../employees";
  }

  @RequestMapping(method = RequestMethod.POST)
  public String save(BindableEmployee bindableEmployee) {
    employeeService.save(bindableEmployee.asEmployee());
    return "redirect:employees";
  }
}

There are several things going on in EmployeeController. The class is annotated with @Controller to indicate to Spring its function as an MVC controller and its candidacy for component scanning by the Spring container. In addition, it is annotated with a class-level @RequestMapping to base all of its method-level @RequestMappings on a top-level URL pattern. Each method is also annotated with @RequestMapping to further constrain their specific associated request patterns.

The get(Long, Model), deleteViaGet(Long), and delete(Long) methods are each configured to map to RESTful URLs which contain the identifier of the Employee object on which to operate.

The save(BindableEmployee method contains no URL information in its @RequestMapping, so it will map simply to /employees, that of the class-level annotation. This is in contrast to the other methods, such as get(Model), which specifies /new. This combines with the class-level annotation to map to /employees/new.

All of the methods specify a HTTP request method in the RESTful style.

EmployeeController presents Employee-like data to the user both as textual data and as an HTML form. This is cause for a special class to be designed with the Web UI in mind, specifically to bind to the HTML form. This role is filled by BindableEmployee.

BindableEmployee.java:

public class BindableEmployee {

  private Long id;

  private String name;

  private String title;

  public BindableEmployee() {
  }

  public BindableEmployee(Employee employee) {
    this.id = employee.getId();
    this.name = employee.getName();
    this.title = employee.getTitle();
  }

  // Getters and setters omitted for brevity.

  public static Collection<BindableEmployee> bindableEmployees(
      Collection<Employee> employees) {
    Collection<BindableEmployee> bindableEmployees = new ArrayList<BindableEmployee>();
    for (Employee employee : employees) {
      bindableEmployees.add(new BindableEmployee(employee));
    }
    return bindableEmployees;
  }

  public Employee asEmployee() {
    return new Employee(id, name, title);
  }

}

BindableEmployee knows both how to convert itself into the domain class Employee via its asEmployee() method and how to convert a collection of instances of Employee into a collection of instances of BindableEmployee. This is a convenient location for this functionality, and an important one as well. Because this conversion is only concerned with connecting the domain to a thin Web layer, the appropriate location for related computation is in the Web layer and out of the domain.

That's all the Java code there is to write. Simple!

View templates

Next, the view templates are defined.

employee.jsp uses Spring's form tag library to build a form with text inputs for the name and title of an employee.

Employee view
Employee view

employee.jsp:

<c:url var="formUrl" value="/employees" />
<form:form action="${formUrl}" modelAttribute="bindableEmployee">
  <ul class="button">
    <c:if test="${not empty bindableEmployee.id}">
      <li><a href="<c:url value="/employees/${bindableEmployee.id}/delete" />">
        Delete
      </a></li>
    </c:if>
    <li><input id="submit" type="submit" value="Save" /></li>
  </ul>
  <form:hidden path="id" />
  <ul>
    <li>
      <label for="name">Name</label>
      <form:input path="name" />
    </li>
    <li>
      <label for="title">Title</label>
      <form:input path="title" />
    </li>
  </ul>
</form:form>

employees.jsp displays a list of the employees in the system, provides links to edit each, and includes a button to add a new employee to the system.

Employees view
Employees view

employees.jsp:

<ul class="button">
  <li><a href="<c:url value="/employees/new" />">Add Employee</a></li>
</ul>
<table cellpadding="0" cellspacing="0">
  <thead>
    <tr>
      <th>Name</th>
      <th>Title</th>
    </tr>
  </thead>
  <tbody>
    <c:forEach items="${bindableEmployeeList}" var="employee">
      <tr>
        <td>
          <a href="<c:url value="/employees/${employee.id}" />">
            <c:out value="${employee.name}" />
          </a>
        </td>
        <td><c:out value="${employee.title}" /></td>
      </tr>
    </c:forEach>
  </tbody>
</table>

Spring configuration

Next, the Spring configuration is defined.

spring-mvc-servlet.xml:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:mvc="http://www.springframework.org/schema/mvc"
  xmlns:context="http://www.springframework.org/schema/context"
  xsi:schemaLocation="
    http://www.springframework.org/schema/mvc
    http://www.springframework.org/schema/mvc/spring-mvc-3.0.xsd
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
    http://www.springframework.org/schema/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">

  <context:annotation-config />
  <mvc:annotation-driven />
  <mvc:default-servlet-handler />

  <bean class="com.earldouglas.barebones.springmvc.web.EmployeeController" />

</beans>

The <mvc:annotation-driven /> element tells Spring to create a DefaultAnnotationHandlerMapping bean to set up handling of the @RequestMapping annotations in EmployeeController, while the lone bean definition registers a view resolver which looks for JSPs by view name. The EmployeeController is picked up by the component-scan, instantiated, and mapped to its applicable requests by DefaultAnnotationHandlerMapping.

Next, we have our Web deployment descriptor.

web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
    http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd"
  version="2.5">

  <servlet>
    <servlet-name>spring-mvc</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>spring-mvc</servlet-name>
    <url-pattern>/</url-pattern>
  </servlet-mapping>

</web-app>

Note that since the DispatcherServlet is named spring-mvc, by convention the Spring configuration is retrieved from /WEB-INF/spring-mvc-servlet.xml.

Section references

Server-side validation

Form validation goes hand-in-hand with Web applications, and server-side form validation is an easy addition to the core application. JSR-303 Bean Validation specifies annotations for declarative validation rules, which can be standardized across the layers of an enterprise application from the database to the user interface.

Additions

The following additions are required:

Classes

There is only one controller method with input: save(BindableEmployee). The BindableEmployee parameter is annotated with @Valid, which will trigger Spring will use its configured JSR-303 Validator to validate the BindableEmployee.

Spring needs a place to put the result of the validation, so a BindingResult is added to the controller method immediately after the corresponding BindableEmployee parameter. This will make binding errors available to the view.

EmployeeController.java:

@Controller
@RequestMapping("/employees")
public class EmployeeController {

  // Some methods omitted for brevity.

  @RequestMapping(method = RequestMethod.POST)
  public String save(@Valid BindableEmployee bindableEmployee,
      BindingResult bindingResult) {

    if (bindingResult.hasErrors()) {
      return "employee";
    }

    employeeService.save(bindableEmployee.asEmployee());
    return "redirect:employees";
  }
}

JSR-303 annotations are added to BindableEmployee.java to limit the pattern of name to two words and the pattern of title to at least one word.

BindableEmployee.java:

public class BindableEmployee {

  private Long id;

  @Pattern(regexp = "\\w+ \\w+")
  private String name;

  @Pattern(regexp = "\\w+( \\w+)?")
  private String title;

  // Methods omitted for brevity.

}

The Spring MVC namespace will automatically configure a JSR-303-backed Validator as long as it is present on the classpath.

View templates

Next, the Spring <form:errors /> element is added to the view to show validation errors.

employee.jsp:

<ul>
  <li>
    <label for="name"><spring:message code="heading.name" /></label>
    <form:input path="name" />
    <form:errors path="name" />
  </li>
  <li>
    <label for="title"><spring:message code="heading.title" /></label>
    <form:input path="title" />
    <form:errors path="title" />
  </li>
</ul>

When the form is submitted, the inputs are automatically validated, and any validation errors are displayed next to each corresponding input field in the form.

Server-side validation error
Server-side validation error

Section references

Rich client-side validation

The counterpart to server-side validation is client-side validation, which is made easy by Spring JavaScript.

Additions

The following additions are required:

Web configuration

Spring JavaScript includes ResourceServlet, which provides various scripts and CSS layouts from both Dojo and Spring JavaScript. These add the functionality and look-and-feel needed for rich client-side validation. The ResourceServlet must be added to the Web deployment descriptor.

web.xml:

<servlet>
  <servlet-name>Resources Servlet</servlet-name>
  <servlet-class>org.springframework.js.resource.ResourceServlet</servlet-class>
</servlet>
<servlet-mapping>
  <servlet-name>Resources Servlet</servlet-name>
  <url-pattern>/resources/*</url-pattern>
</servlet-mapping>

View templates

The Dojo and Spring JavaScript scripts and layout must be added to each view which will provide rich client behavior.

employee.jsp:

<head>
  <title><spring:message code="application.title" /></title>
  <script type="text/javascript"
    src="<c:url value="/resources/dojo/dojo.js" />"></script>
  <script type="text/javascript"
    src="<c:url value="/resources/spring/Spring.js" />"></script>
  <script type="text/javascript"
    src="<c:url value="/resources/spring/Spring-Dojo.js" />"></script>
  <link type="text/css" rel="stylesheet"
    href="<c:url value="/resources/dijit/themes/tundra/tundra.css" />" />
  <link type="text/css" rel="stylesheet"
    href="<c:url value="/style.css" />" />
</head>

Spring JavaScript uses the decorator pattern to cleanly introduce rich client behavior into views. Script-free HTML is first built to create a fully functioning application, and Spring JavaScript decorators are added on top of the existing DOM to introduce rich behavior. This means that a view is fully functional on its own, which allows the application to run in an environment where JavaScript support might be limited or non-existent.

This practice, known as progressive enhancement, allows a Web application to remain functional across a wealth of browsers, which may vary in their level of support of JavaScript and CSS. The most important takeaway from this idea is that the onclick attribute is never directly used in HTML code. It is only accessed by a decorator, meaning its behavior is only used when the decorator script itself is supported.

The form in employee.jsp is updated to insert Spring JavaScript decorators.

employee.jsp:

  </form:form>
</div>
<script type="text/javascript">
  Spring.addDecoration(new Spring.ValidateAllDecoration( {
    elementId : "submit",
    event : "onclick"
  }));

  Spring.addDecoration(new Spring.ElementDecoration( {
    elementId : "name",
    widgetType : "dijit.form.ValidationTextBox",
    widgetAttrs : {
      regExp : "\\w+ \\w+",
      required : true
    }
  }));

  Spring.addDecoration(new Spring.ElementDecoration( {
    elementId : "title",
    widgetType : "dijit.form.ValidationTextBox",
    widgetAttrs : {
      regExp : "\\w+( \\w+)?",
      required : true
    }
  }));
</script>

Field decorators have been added to all of the form input fields, and a global validation director has been added to the form submission button. These are just a few examples of the vast set of features provided by Dojo.

The employee form will now validate on the client, displaying any validation errors dynamically.

Client-side validation error
Client-side validation error

Section references

Security

A Web application would seldom be complete without at least a minimal security layer to prohibit unauthenticated access to protected resources. In this section, basic security is introduced by adding HTML form-based authentication using Spring Security.

Additions

The following additions are required:

Web configuration

Spring Security's DelegatingFilterProxy is essentially a J2EE Filter which nominally handles all requests and determines how to allow or reject access.

web.xml:

<filter>
  <filter-name>springSecurityFilterChain</filter-name>
  <filter-class>
    org.springframework.web.filter.DelegatingFilterProxy
  </filter-class>
</filter>
<filter-mapping>
  <filter-name>springSecurityFilterChain</filter-name>
  <url-pattern>/*</url-pattern>
</filter-mapping>

<context-param>
  <param-name>contextConfigLocation</param-name>
  <param-value>
    /WEB-INF/spring-mvc-security.xml
  </param-value>
</context-param>

<listener>
  <listener-class>
    org.springframework.web.context.ContextLoaderListener
  </listener-class>
</listener>

Spring configuration

Spring's ContextLoaderListener is needed because there is now a parent Spring context which is extended by the spring-mvc context of before. The contextConfigLocation parameter specifies the location of the new parent configuration file.

spring-mvc-security.xml:

<!-- Enable Spring Security with HTTP basic authentication. -->
<http auto-config="true">
  <http-basic />
  <intercept-url pattern="/**" access="ROLE_ADMIN" />
  <form-login />
</http>

<!-- An AuthenticationProvider with sample users and roles. -->
<authentication-manager>
  <authentication-provider>
    <user-service>
      <user name="admin" password="password"
        authorities="ROLE_ADMIN" />
    </user-service>
  </authentication-provider>
</authentication-manager>

This nearly minimal configuration sets up an in-memory repository of roles, and enforces access to every resource against this repository. Here, a form-based login page is provided by Spring.

Spring Security form-based login page
Spring Security form-based login page

Section references

Database integration

A major component of enterprise applications is information storage and retrieval via a relational database. Most of this behavior is confined to a special data tier, with a thin API exposed to the application tier for interaction with the domain. In this section, a Hibernate-based data tier is introduced for storage and retrieval of data.

Additions

The following additions are required:

Classes

A persistable type is created to represent the domain model. In this simple example, this will closely resemble the UI model, but it is important to draw a distinction between the two, as they serve two very different purposes.

The purpose of a domain model is to represent the domain model. The purpose of a UI model is to represent the UI model. This is intentionally redundant, because it is easy to forget. The domain model can include potentially complex object hierarchies as well as database-specific metadata. The UI model will have forms and other data structures which will tend to be very flat, and contain very specific information meant to be rendered in a view.

Attempting to merge the two models can get painfully cumbersome, as the domain model tends not to map directly to the UI model. Furthermore, the resulting tight coupling will force any changes in one to necessitate changes in the other. It is much simpler to create simple conversion logic in the service tier to translate between the two models.

The Employee class is made persistable with JPA annotations.

Employee.java:

@Entity
public class Employee {

  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  @Column
  private Long id;

  @Column
  private String name;

  @Column
  private String title;

  public Employee() {

  }

  public Employee(Long id, String name, String title) {
    this.id = id;
    this.name = name;
    this.title = title;
  }

  // Getters and setters omitted for brevity.
}

A repository serves the domain as an opaque entry point into the database. It provides accessors and mutators for database tables represented only by domain objects.

HibernateEmployeeService.java:

@Transactional
public class HibernateEmployeeService implements EmployeeService {

  @Autowired
  private SessionFactory sessionFactory;

  private Session session() {
    return sessionFactory.getCurrentSession();
  }

  @Override
  public void delete(Long employeeId) {
    session().delete(get(employeeId));
  }

  @Override
  public Employee get(Long employeeId) {
    return (Employee) session().createCriteria(Employee.class).add(
        Restrictions.idEq(employeeId)).uniqueResult();
  }

  @SuppressWarnings("unchecked")
  @Override
  public Collection<Employee> get() {
    return session().createCriteria(Employee.class).list();
  }

  @Override
  public Employee save(Employee employee) {
    session().saveOrUpdate(employee);
    return employee;
  }
}

This repository is meant to work with Hibernate, and so uses a Hibernate SessionFactory.

In this example, the service tier is contained entirely within EmployeeController, which is modified to interact with the new repository.

BindableEmployee provides an Employee-based constructor plus two helper methods, asEmployee() and bindableEmployee(Collection<Employee>), which do the mapping between the UI model and the domain model.

BindableEmployee.java:

public class BindableEmployee {

  private Long id;
  private String name;
  private String title;

  public BindableEmployee() {
  }

  public BindableEmployee(Employee employee) {
    this.id = employee.getId();
    this.name = employee.getName();
    this.title = employee.getTitle();
  }

  // Getters and setters omitted for brevity.

  public static Collection<BindableEmployee> bindableEmployees(
      Collection<Employee> employees) {
    Collection<BindableEmployee> bindableEmployees = new ArrayList<BindableEmployee>();
    for (Employee employee : employees) {
      bindableEmployees.add(new BindableEmployee(employee));
    }
    return bindableEmployees;
  }

  public Employee asEmployee() {
    return new Employee(id, name, title);
  }

}

Spring configuration

Next, a new global Spring context is created to manage the database-related objects.

persistence-context.xml:

<tx:annotation-driven />

<context:annotation-config />

<bean id="transactionManager"
  class="org.springframework.orm.hibernate3.HibernateTransactionManager">
  <property name="sessionFactory" ref="sessionFactory" />
</bean>

<bean class="com.earldouglas.barebones.springmvc.service.HibernateEmployeeService" />

<bean id="sessionFactory"
  class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean"
  init-method="createDatabaseSchema">
  <property name="dataSource" ref="hsqlDataSource" />
  <property name="packagesToScan" value="com.earldouglas.barebones.springmvc" />
  <property name="hibernateProperties">
    <props>
      <prop key="hibernate.dialect">
        org.hibernate.dialect.HSQLDialect
      </prop>
      <prop key="hibernate.show_sql">false</prop>
    </props>
  </property>
</bean>

<bean id="hsqlDataSource" class="org.apache.commons.dbcp.BasicDataSource"
  destroy-method="close">
  <property name="driverClassName" value="org.hsqldb.jdbcDriver" />
  <property name="url" value="jdbc:hsqldb:mem:spring-mvc" />
  <property name="username" value="sa" />
  <property name="password" value="" />
</bean>

This context is made a parent context via ContextLoaderListener in the Web deployment descriptor.

web.xml:

<context-param>
  <param-name>contextConfigLocation</param-name>
  <param-value>
    /WEB-INF/spring-mvc-security.xml
    /WEB-INF/persistence-context.xml  Barebones Spring MVC: Database Integration
  </param-value>
</context-param>

<listener>
  <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>

A minor change is required in spring-mvc-servlet.xml to enable service tier transaction management.

spring-mvc-servlet.xml:

<!--
  <bean
  class="com.earldouglas.barebones.springmvc.service.InMemoryEmployeeService"
  />
-->

The DataSource can optionally be externalized from the Spring configuration via JNDI. Configuration specifics, such as database username and password, are then kept out of the Spring configuration and delegated to the application server. This adds security by allowing the application server protect these sensitive data. For Apache Tomcat, the DataSource is added to META-INF/context.xml.

context.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Context>
    <Resource name="jdbc/hsqlDataSource"
             auth="Container"
             type="javax.sql.DataSource"
             username="sa"
             password=""
             driverClassName="org.hsqldb.jdbcDriver"
             factory="org.apache.commons.dbcp.BasicDataSourceFactory"
             url="jdbc:hsqldb:mem:mydatabase"/>
</Context>

The DataSource bean is removed from the Spring configuration, and replaced by a JNDI-lookup:

persistence-context.xml:

<jee:jndi-lookup id="hsqlDataSource" jndi-name="java:comp/env/jdbc/hsqlDataSource" />

Section references

Restful web services

One of the awesome features of the Spring MVC is its ability to easily support multiple types of request/response content. In fact, the same Spring MVC beans can be used to serve conventional HTML, RESTful XML, JSON, Atom, etc. usually with only some minor configuration changes.

This section introduces a RESTful Web service which utilizes the existing Spring MVC beans and configuration.

Additions

The following additions are required:

Classes

JAXB annotations are similar in use to Hibernate annotations. In this example, BindableEmployee is simple and flat enough that it will marshal easily with a few JAXB annotations.

BindableEmployee.java:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType
@XmlRootElement
public class BindableEmployee {

  @XmlElement
  private Long id;

  @XmlElement
  @Pattern(regexp = "\\w+ \\w+")
  private String name;

  @XmlElement
  @Pattern(regexp = "\\w+( \\w+)?")
  private String title;

  // Methods omitted for brevity.
}

Spring MVC needs the ability to choose an appropriate view resolver depending on the specifics of the request. When a conventional text/html request is made from a Web browser, Spring MVC uses an InternalResourceViewResolver to delegate to a JSP view template as before. When an application/xml request is made by a Web service consumer, Spring MVC uses a MarshallingView with a JAXB marshaller to provide an XML representation of the BindableEmployee.

Spring configuration

spring-mvc-servlet.xml:

<bean
  class="org.springframework.web.servlet.view.ContentNegotiatingViewResolver">
  <property name="viewResolvers">
    <list>
      <bean
        class="org.springframework.web.servlet.view.InternalResourceViewResolver">
        <property name="prefix" value="/WEB-INF/views/" />
        <property name="suffix" value=".jsp" />
      </bean>
      <bean class="org.springframework.web.servlet.view.BeanNameViewResolver" />
    </list>
  </property>
</bean>

<oxm:jaxb2-marshaller id="marshaller">
  <oxm:class-to-be-bound
    name="com.earldouglas.barebones.springmvc.web.BindableEmployee" />
</oxm:jaxb2-marshaller>

<bean name="employee"
  class="org.springframework.web.servlet.view.xml.MarshallingView">
  <constructor-arg ref="marshaller" />
</bean>

Testing

The HTML/XML duality of this example can be tested with curl:

> curl -H 'Accept: application/xml' localhost:8080/spring-mvc/employee
> curl -H 'Accept: text/html' localhost:8080/spring-mvc/employee

Section references

Externalization and internationalization

Message externalization in the Web view layer digs the various text out of view templates and keeps it centralized and manageable. It also provides a convenient launchpad for site internationalization. Spring MVC provides for easy introduction of message externalization and internationalization into the view layer.

Additions

The following additions are required:

Spring configuration

Spring needs to know where it will find externalized messages. This is done with Java's resource bundle functionality, encapsulated in a Spring ResourceBundleMessageSource.

spring-mvc-servlet.xml:

<bean id="messageSource"
  class="org.springframework.context.support.ResourceBundleMessageSource">
  <property name="basename" value="messages" />
</bean>

There isn't much in the way of messages in this example, but the little that is there is moved into a properties file named in the above ResourceBundleMessageSource.

message.properties:

application.title=Employee Directory

button.add-employee=Add employee
button.delete=Delete
button.save=Save

heading.employees=Employees
heading.employee=Employee
heading.name=Name
heading.title=Title

english=English
spanish=Español

This is also done in Spanish. Translations were performed with the help of Google Translate, so as far as I know nothing below says "Your mother was a hamster."

messages_es.properties:

application.title=Directorio de Empleados

button.add-employee=Añadir empleado
button.delete=Eliminar
button.save=Guardar

heading.employees=Empleados
heading.employee=Empleado
heading.name=Nombre
heading.title=Título

Next, a couple of beans are added to the Spring context to allow detection of a user's desire to switch languages, and the ability to store that preference in the user's session.

spring-mvc-servlet.xml:

<mvc:interceptors>
  <bean class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor" />
</mvc:interceptors>
<bean id="localeResolver" class="org.springframework.web.servlet.i18n.SessionLocaleResolver" />

A user simply includes the HTTP request parameter lang=es to change the language to Spanish.

Employee view in Spanish
Employee view in Spanish

References