Table of Contents

• What You Will Build
• Starting with Spring Initializr
• Create a Domain Object
• Create a Person Repository
• Running the Application
• Test the Application
• Summary
• See Also

This guide walks you through the process of creating an application that accesses relational JPA data through a hypermedia-based RESTful front end.

What You Will Build

You will build a Spring application that lets you create and retrieve Person objects stored in a database by using Spring Data REST. Spring Data REST takes the features of Spring HATEOAS and Spring Data JPA and automatically combines them together.

Note	Spring Data REST also supports Spring Data Neo4j, Spring Data Gemfire, and Spring Data MongoDB as backend data stores, but those are not part of this guide.

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Starting with Spring Initializr

You can use this pre-initialized project and click Generate to download a ZIP file. This project is configured to fit the examples in this tutorial.

To manually initialize the project:

1. Navigate to https://start.spring.io. This service pulls in all the dependencies you need for an application and does most of the setup for you.

2. Choose either Gradle or Maven and the language you want to use. This guide assumes that you chose Java.

3. Click Dependencies and select Rest Repositories, Spring Data JPA, and H2 Database.

4. Click Generate.

5. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.

Note	If your IDE has the Spring Initializr integration, you can complete this process from your IDE.

Note	You can also fork the project from Github and open it in your IDE or other editor.

Create a Domain Object

Create a new domain object to present a person, as the following listing (in src/main/java/com/example/accessingdatarest/Person.java) shows:

link:complete/src/main/java/com/example/accessingdatarest/Person.java[role=include]

The Person object has a first name and a last name. (There is also an ID object that is configured to be automatically generated, so you need not deal with that.)

Create a Person Repository

Next, you need to create a simple repository, as the following listing (in src/main/java/com/example/accessingdatarest/PersonRepository.java) shows:

link:complete/src/main/java/com/example/accessingdatarest/PersonRepository.java[role=include]

This repository is an interface that lets you perform various operations involving Person objects. It gets these operations by extending the PagingAndSortingRepository interface that is defined in Spring Data Commons.

At runtime, Spring Data REST automatically creates an implementation of this interface. Then it uses the @RepositoryRestResource annotation to direct Spring MVC to create RESTful endpoints at /people.

Note	@RepositoryRestResource is not required for a repository to be exported. It is used only to change the export details, such as using /people instead of the default value of /persons.

Here you have also defined a custom query to retrieve a list of Person objects based on the lastName. You can see how to invoke it later in this guide.

Spring Boot automatically spins up Spring Data JPA to create a concrete implementation of the PersonRepository and configure it to talk to a back end in-memory database by using JPA.

Spring Data REST builds on top of Spring MVC. It creates a collection of Spring MVC controllers, JSON converters, and other beans to provide a RESTful front end. These components link up to the Spring Data JPA backend. When you use Spring Boot, this is all autoconfigured. If you want to investigate how that works, by looking at the RepositoryRestMvcConfiguration in Spring Data REST.

Running the Application

You can now run the application by executing the main method in AccessingDataRestApplication. You can run the program from your IDE, or by executing the following Gradle command in the project root directory:

./gradlew bootRun

Alternatively, you could use Maven to run the application using the command:

./mvnw spring-boot:run

Test the Application

Now that the application is running, you can test it. You can use any REST client you wish. The following examples use the *nix tool, curl.

First you want to see the top level service. The following example shows how to do so:

$ curl http://localhost:8080
{
  "_links" : {
    "people" : {
      "href" : "http://localhost:8080/people{?page,size,sort}",
      "templated" : true
    }
  }
}

The preceding example provides a first glimpse of what this server has to offer. There is a people link located at http://localhost:8080/people. It has some options, such as ?page, ?size, and ?sort.

Note	Spring Data REST uses the HAL format for JSON output. It is flexible and offers a convenient way to supply links adjacent to the data that is served.

The following example shows how to see the people records (none at present):

$ curl http://localhost:8080/people
{
  "_embedded" : {
    "people" : []
  },
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/people{?page,size,sort}",
      "templated" : true
    },
    "search" : {
      "href" : "http://localhost:8080/people/search"
    }
  },
  "page" : {
    "size" : 20,
    "totalElements" : 0,
    "totalPages" : 0,
    "number" : 0
  }
}

There are currently no elements and, hence, no pages. Time to create a new Person! The following listing shows how to do so:

$ curl -i -H "Content-Type:application/json" -d '{"firstName": "Frodo", "lastName": "Baggins"}' http://localhost:8080/people
HTTP/1.1 201 Created
Server: Apache-Coyote/1.1
Location: http://localhost:8080/people/1
Content-Length: 0
Date: Wed, 26 Feb 2014 20:26:55 GMT

• -i: Ensures you can see the response message including the headers. The URI of the newly created Person is shown.

• -H "Content-Type:application/json": Sets the content type so the application knows the payload contains a JSON object.

• -d '{"firstName": "Frodo", "lastName": "Baggins"}': Is the data being sent.

• If you are on Windows, the command above will work on WSL. If you can’t install WSL, you might need to replace the single quotes with double quotes and escape the existing double quotes, i.e. -d "{\"firstName\": \"Frodo\", \"lastName\": \"Baggins\"}".

Note

Notice how the response to the POST operation includes a Location header. This contains the URI of the newly created resource. Spring Data REST also has two methods (RepositoryRestConfiguration.setReturnBodyOnCreate(…) and setReturnBodyOnUpdate(…)) that you can use to configure the framework to immediately return the representation of the resource just created. RepositoryRestConfiguration.setReturnBodyForPutAndPost(…) is a shortcut method to enable representation responses for create and update operations.

You can query for all people, as the following example shows:

$ curl http://localhost:8080/people
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/people{?page,size,sort}",
      "templated" : true
    },
    "search" : {
      "href" : "http://localhost:8080/people/search"
    }
  },
  "_embedded" : {
    "people" : [ {
      "firstName" : "Frodo",
      "lastName" : "Baggins",
      "_links" : {
        "self" : {
          "href" : "http://localhost:8080/people/1"
        }
      }
    } ]
  },
  "page" : {
    "size" : 20,
    "totalElements" : 1,
    "totalPages" : 1,
    "number" : 0
  }
}

The people object contains a list that includes Frodo. Notice how it includes a self link. Spring Data REST also uses Evo Inflector to pluralize the name of the entity for groupings.

You can query directly for the individual record, as follows:

$ curl http://localhost:8080/people/1
{
  "firstName" : "Frodo",
  "lastName" : "Baggins",
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/people/1"
    }
  }
}

Note	This might appear to be purely web-based. However, behind the scenes, there is an H2 relational database. In production, you would probably use a real one, such as PostgreSQL.

Tip	In this guide, there is only one domain object. With a more complex system, where domain objects are related to each other, Spring Data REST renders additional links to help navigate to connected records.

You can find all the custom queries, as shown in the following example:

$ curl http://localhost:8080/people/search
{
  "_links" : {
    "findByLastName" : {
      "href" : "http://localhost:8080/people/search/findByLastName{?name}",
      "templated" : true
    }
  }
}

You can see the URL for the query, including the HTTP query parameter, name. Note that this matches the @Param("name") annotation embedded in the interface.

The following example shows how to use the findByLastName query:

$ curl http://localhost:8080/people/search/findByLastName?name=Baggins
{
  "_embedded" : {
    "persons" : [ {
      "firstName" : "Frodo",
      "lastName" : "Baggins",
      "_links" : {
        "self" : {
          "href" : "http://localhost:8080/people/1"
        }
      }
    } ]
  }
}

Because you defined it to return List<Person> in the code, it returns all of the results. If you had defined it to return only Person, it picks one of the Person objects to return. Since this can be unpredictable, you probably do not want to do that for queries that can return multiple entries.

You can also issue PUT, PATCH, and DELETE REST calls to replace, update, or delete existing records (respectively). The following example uses a PUT call:

$ curl -X PUT -H "Content-Type:application/json" -d '{"firstName": "Bilbo", "lastName": "Baggins"}' http://localhost:8080/people/1
$ curl http://localhost:8080/people/1
{
  "firstName" : "Bilbo",
  "lastName" : "Baggins",
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/people/1"
    }
  }
}

The following example uses a PATCH call:

$ curl -X PATCH -H "Content-Type:application/json" -d '{"firstName": "Bilbo Jr."}' http://localhost:8080/people/1
$ curl http://localhost:8080/people/1
{
  "firstName" : "Bilbo Jr.",
  "lastName" : "Baggins",
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/people/1"
    }
  }
}

Note	PUT replaces an entire record. Fields not supplied are replaced with null. You can use PATCH to update a subset of items.

You can also delete records, as the following example shows:

$ curl -X DELETE http://localhost:8080/people/1
$ curl http://localhost:8080/people
{
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/people{?page,size,sort}",
      "templated" : true
    },
    "search" : {
      "href" : "http://localhost:8080/people/search"
    }
  },
  "page" : {
    "size" : 20,
    "totalElements" : 0,
    "totalPages" : 0,
    "number" : 0
  }
}

A convenient aspect of this hypermedia-driven interface is that you can discover all the RESTful endpoints by using curl (or whatever REST client you like). You need not exchange a formal contract or interface document with your customers.

Summary

Congratulations! You have developed an application with a hypermedia-based RESTful front end and a JPA-based back end.

See Also

The following guides may also be helpful:

• Building a Hypermedia-Driven RESTful Web Service

• Accessing GemFire Data with REST

• Accessing MongoDB Data with REST

• Accessing data with MySQL

• Accessing Neo4j Data with REST

• Consuming a RESTful Web Service

• Securing a Web Application

• Building REST services with Spring

• Building an Application with Spring Boot

• Creating API Documentation with Restdocs

• Enabling Cross Origin Requests for a RESTful Web Service

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