This guide walks you through the process of creating an application that accesses relational JPA data through a hypermedia-based RESTful front end.
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.
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Note
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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. |
For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the set up for you. This example needs the Rest Repositories, Spring Data JPA, and H2 dependencies.
The following listing shows the pom.xml file created when you choose Maven:
link:complete/pom.xml[role=include]
The following listing shows the build.gradle file created when you choose Gradle:
link:complete/build.gradle[role=include]
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.)
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.
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Note
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@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.
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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.
Logging output is displayed. The service should be up and running within a few seconds.
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.
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Note
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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 createdPersonis 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\"}".
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Note
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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.
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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"
}
}
}
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Note
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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. |
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Tip
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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"
}
}
}
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Note
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PUT replaces an entire record. Fields not supplied are replaced with null. You
can use PATCH to update a subset of items.
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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.
Congratulations! You have developed an application with a hypermedia-based RESTful front end and a JPA-based back end.
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