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Specifying a default value to inject

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.

Example 3.9 shows the syntax for using the @DefaultValue annotation.

Example 3.9. Syntax for setting the default value of a parameter

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.

Example 3.10 shows two examples of using the @DefaultValue to specify a default value for a field whose value is injected.

Example 3.10. Setting default values

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 Example 3.10 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.

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