Chapter 47. Passing Information into Resource Classes and Methods

One of the best practices for designing a RESTful Web service is that each resource should have a unique URI. A developer can use this principle to provide a good deal of information to the underlying resource implementation. When designing URI templates for a resource, a developer can build the templates to include parameter information that can be injected into the resource implementation. Developers can also leverage query and matrix parameters for feeding information into the resource implementations.

One of the more common mechanisms for getting information about a resource is through the variables used in creating the URI templates for a resource. This is accomplished using the javax.ws.rs.PathParam annotation. The @PathParam annotation has a single parameter that identifies the URI template variable from which the data will be injected.

In Example 47.1, “Injecting data from a URI template variable” the @PathParam annotation specifies that the value of the URI template variable color is injected into the itemColor field.

import javax.ws.rs.Path;
import javax.ws.rs.PathParam
...

@Path("/boxes/{shape}/{color}")
class Box
{
  ...

  @PathParam("color")
  String itemColor;

  ...
}

The data types supported by the @PathParam annotation are different from the ones described in the section called “Supported data types”. The entity into which the @PathParam annotation injects data must be of one of the following types:

A common way of passing information on the Web is to use query parameters in a URI. Query parameters appear at the end of the URI and are separated from the resource location portion of the URI by a question mark(?). They consist of one, or more, name value pairs where the name and value are separated by an equal sign(=). When more than one query parameter is specified, the pairs are separated from each other by either a semicolon(;) or an ampersand(&). Example 47.2, “URI with a query string” shows the syntax of a URI with query parameters.

http://fusesource.org?name=value;name2=value2;...

The javax.ws.rs.QueryParam annotation extracts the value of a query parameter and injects it into a JAX-RS resource. The annotation takes a single parameter that identifies the name of the query parameter from which the value is extracted and injected into the specified field, bean property, or parameter. The @QueryParam annotation supports the types described in the section called “Supported data types”.

Example 47.3, “Resource method using data from a query parameter” shows a resource method that injects the value of the query parameter id into the method’s id parameter.

import javax.ws.rs.QueryParam;
import javax.ws.rs.PathParam;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
...

@Path("/monstersforhire/")
public class MonsterService
{
  ...
  @POST
  @Path("/{type}")
  public void updateMonster(@PathParam("type") String type,
                            @QueryParam("id") String id)
  {
    ...
  }
  ...
}

To process an HTTP POST to /monstersforhire/daikaiju?id=jonas the updateMonster() method’s type is set to daikaiju and the id is set to jonas.

URI matrix parameters, like URI query parameters, are name/value pairs that can provide additional information selecting a resource. Unlike query parameters, matrix parameters can appear anywhere in a URI and they are separated from the hierarchical path segments of the URI using a semicolon(;). /mostersforhire/daikaiju;id=jonas has one matrix parameter called id and /monstersforhire/japan;type=daikaiju/flying;wingspan=40 has two matrix parameters called type and wingspan.

The value of a matrix parameter is injected into a field, parameter, or bean property using the javax.ws.rs.MatrixParam annotation. The annotation takes a single parameter that identifies the name of the matrix parameter from which the value is extracted and injected into the specified field, bean property, or parameter. The @MatrixParam annotation supports the types described in the section called “Supported data types”.

Example 47.4, “Resource method using data from matrix parameters” shows a resource method that injects the value of the matrix parameters type and id into the method’s parameters.

import javax.ws.rs.MatrixParam;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
...

@Path("/monstersforhire/")
public class MonsterService
{
  ...
  @POST
  public void updateMonster(@MatrixParam("type") String type,
                            @MatrixParam("id") String id)
  {
    ...
  }
  ...
}

To process an HTTP POST to /monstersforhire;type=daikaiju;id=whale the updateMonster() method’s type is set to daikaiju and the id is set to whale.

By default all request URIs are decoded. So the URI /monster/night%20stalker and the URI /monster/night stalker are equivalent. The automatic URI decoding makes it easy to send characters outside of the ASCII character set as parameters.

If you do not wish to have URI automatically decoded, you can use the javax.ws.rs.Encoded annotation to deactivate the URI decoding. The annotation can be used to deactivate URI decoding at the following levels:

Example 47.5, “Disabling URI decoding” shows a resource whose getMonster() method does not use URI decoding. The addMonster() method only disables URI decoding for the type parameter.

@Path("/monstersforhire/")
public class MonsterService
{
  ...

  @GET
  @Encoded
  @Path("/{type}")
  public Monster getMonster(@PathParam("type") String type,
                            @QueryParam("id") String id)
  {
    ...
  }

  @PUT
  @Path("/{id}")
  public void addMonster(@Encoded @PathParam("type") String type,
                         @QueryParam("id") String id)
  {
    ...
  }
  ...
}

If an error occurs when attempting to inject data using one of the URI injection annotations a WebApplicationException exception wrapping the original exception is generated. The WebApplicationException exception’s status is set to 404.

In normal usage the HTTP headers in a request message pass along generic information about the message, how it is to be handled in transit, and details about the expected response. While a few standard headers are commonly recognized and used, the HTTP specification allows for any name/value pair to be used as an HTTP header. The JAX-RS APIs provide an easy mechanism for injecting HTTP header information into a resource implementation.

One of the most commonly used HTTP headers is the cookie. Cookies allow HTTP clients and servers to share static information across multiple request/response sequences. The JAX-RS APIs provide an annotation inject data directly from a cookie into a resource implementation.

The javax.ws.rs.HeaderParam annotation is used to inject the data from an HTTP header field into a parameter, field, or bean property. It has a single parameter that specifies the name of the HTTP header field from which the value is extracted and injected into the resource implementation. The associated parameter, field, or bean property must conform to the data types described in the section called “Supported data types”.

Injecting the If-Modified-Since header shows code for injecting the value of the HTTP If-Modified-Since header into a class' oldestDate field.

import javax.ws.rs.HeaderParam;
...
class RecordKeeper
{
  ...
  @HeaderParam("If-Modified-Since")
  String oldestDate;
  ...
}

Cookies are a special type of HTTP header. They are made up of one or more name/value pairs that are passed to the resource implementation on the first request. After the first request, the cookie is passes back and forth between the provider and consumer with each message. Only the consumer, because they generate requests, can change the cookie. Cookies are commonly used to maintain session across multiple request/response sequences, storing user settings, and other data that can persist.

The javax.ws.rs.CookieParam annotation extracts the value from a cookie’s field and injects it into a resource implementation. It takes a single parameter that specifies the name of the cookie’s field from which the value is to be extracted. In addition to the data types listed in the section called “Supported data types”, entities decorated with the @CookieParam can also be a Cookie object.

Example 47.6, “Injecting a cookie” shows code for injecting the value of the handle cookie into a field in the CB class.

import javax.ws.rs.CookieParam;
...
class CB
{
  ...
  @CookieParam("handle")
  String handle;
  ...
}

If an error occurs when attempting to inject data using one of the HTTP message injection annotations a WebApplicationException exception wrapping the original exception is generated. The WebApplicationException exception’s status is set to 400.

HTML forms are an easy means of getting information from a user and they are also easy to create. Form data can be used for HTTP GET requests and HTTP POST requests:

The javax.ws.rs.FormParam annotation extracts field values from form data and injects the value into resource method parameters. The annotation takes a single parameter that specifies the key of the field from which it extracts the values. The associated parameter must conform to the data types described in the section called “Supported data types”.

Injecting form data into resource method parameters shows a resource method that injects form data into its parameters. The method assumes that the client’s form includes three fields—title, tags, and body—that contain string data.

import javax.ws.rs.FormParam;
import javax.ws.rs.POST;

...
@POST
public boolean updatePost(@FormParam("title") String title,
                          @FormParam("tags") String tags,
                          @FormParam("body") String post)
{
  ...
}

To provide for a more robust service implementation, you may want to ensure that any optional parameters can be set to a default value. This can be particularly useful for values that are taken from query parameters and matrix parameters since entering long URI strings is highly error prone. You may also want to set a default value for a parameter extracted from a cookie since it is possible for a requesting system not have the proper information to construct a cookie with all the values.

The javax.ws.rs.DefaultValue annotation can be used in conjunction with the following injection annotations:

The @DefaultValue annotation specifies a default value to be used when the data corresponding to the injection annotation is not present in the request.

Syntax for setting the default value of a parameter shows the syntax for using the @DefaultValue annotation.

import javax.ws.rs.DefaultValue;
  ...
  void resourceMethod(@MatrixParam("matrix")
                      @DefaultValue("value)
                      int someValue, ... )
  ...

The annotation must come before the parameter, bean, or field, it will effect. The position of the @DefaultValue annotation relative to the accompanying injection annotation does not matter.

The @DefaultValue annotation takes a single parameter. This parameter is the value that will be injected into the field if the proper data cannot be extracted based on the injection annotation. The value can be any String value. The value should be compatible with type of the associated field. For example, if the associated field is of type int, a default value of blue results in an exception.

If the type of the annotated parameter, bean or field is List, Set, or SortedSet then the resulting collection will have a single entry mapped from the supplied default value.

Setting default values shows two examples of using the @DefaultValue to specify a default value for a field whose value is injected.

import javax.ws.rs.DefaultValue;
import javax.ws.rs.PathParam;
import javax.ws.rs.QueryParam;
import javax.ws.rs.GET;
import javax.ws.rs.Path;

@Path("/monster")
public class MonsterService
{

  @Get
  public Monster getMonster(@QueryParam("id") @DefaultValue("42") int id,
                            @QueryParam("type") @DefaultValue("bogeyman") String type)
  {
    ...
  }

  ...
}

The getMonster() method in Setting default values is invoked when a GET request is sent to baseURI/monster. The method expects two query parameters, id and type, appended to the URI. So a GET request using the URI baseURI/monster?id=1&type=fomóiri would return the Fomóiri with the id of one.

Because the @DefaultValue annotation is placed on both parameters, the getMonster() method can function if the query parameters are omitted. A GET request sent to baseURI/monster is equivalent to a GET request using the URI baseURI/monster?id=42&type=bogeyman.

When posting HTML forms over REST, a common pattern on the server side is to create a Java bean to encapsulate all of the data received in the form (and possibly data from other parameters and HTML headers, as well). Normally, creating this Java bean would be a two step process: a resource method receives the form values by injection (for example, by adding @FormParam annotations to its method parameters), and the resource method then calls the bean’s constructor, passing in the form data.

Using the JAX-RS 2.0 @BeanParam annotation, it is possible to implement this pattern in a single step. The form data can be injected directly into the fields of the bean class and the bean itself is created automatically by the JAX-RS runtime. This is most easily explained by example.

The @BeanParam annotation can be attached to resource method parameters, resource fields, or bean properties. A parameter target is the only kind of target that can be used with all resource class lifecycles, however. The other kinds of target are restricted to the per-request lifecycle. This situation is summarized in Table 47.1, “@BeanParam Injection Targets”.

The following example shows how you might go about capturing form data in a Java bean using the conventional approach (without using @BeanParam):

// Java
import javax.ws.rs.POST;
import javax.ws.rs.FormParam;
import javax.ws.rs.core.Response;
...
@POST
public Response orderTable(@FormParam("orderId")  String orderId,
                           @FormParam("color")    String color,
                           @FormParam("quantity") String quantity,
                           @FormParam("price")    String price)
{
    ...
    TableOrder bean = new TableOrder(orderId, color, quantity, price);
    ...
    return Response.ok().build();
}

In this example, the orderTable method processes a form that is used to order a quantity of tables from a furniture Web site. When the order form is posted, the form values are injected into the parameters of the orderTable method, and the orderTable method explicitly creates an instance of the TableOrder class, using the injected form data.

The previous example can be refactored to take advantage of the @BeanParam annotation. When using the @BeanParam approach, the form parameters can be injected directly into the fields of the bean class, TableOrder. In fact, you can use any of the standard JAX-RS parameter annotations in the bean class: including @PathParam, @QueryParam, @FormParam, @MatrixParam, @CookieParam, and @HeaderParam. The code for processing the form can be refactored as follows:

// Java
import javax.ws.rs.POST;
import javax.ws.rs.FormParam;
import javax.ws.rs.core.Response;
...
public class TableOrder {
    @FormParam("orderId")
    private String orderId;

    @FormParam("color")
    private String color;

    @FormParam("quantity")
    private String quantity;

    @FormParam("price")
    private String price;

    // Define public getter/setter methods
    // (Not shown)
    ...
}
...
@POST
public Response orderTable(@BeanParam TableOrder orderBean)
{
    ...
    // Do whatever you like with the 'orderBean' bean
    ...
    return Response.ok().build();
}

Now that the form annotations have been added to the bean class, TableOrder, you can replace all of the @FormParam annotations in the signature of the resource method with just a single @BeanParam annotation, as shown. Now, when the form is posted to the orderTable resource method, the JAX-RS runtime automatically creates a TableOrder instance, orderBean, and injects all of the data specified by the parameter annotations on the bean class.