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Chapter 5. Handling Exceptions

Abstract

When possible, exceptions caught by a resource method should cause a useful error to be returned to the requesting consumer. JAX-RS resource methods can throw a WebApplicaitonException exception. You can also provide ExceptionMapper<E> implementations to map exceptions to appropriate responses.

5.1. Using WebApplicaitonException exceptions to report errors
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Overview
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The JAX-RS API introduced the WebApplicationException runtime exception to provide an easy way for resource methods to create exceptions that are appropriate for RESTful clients to consume. WebApplicationException exceptions can include a Response object that defines the entity body to return to the originator of the request. It also provides a mechanism for specifying the HTTP status code to be returned to the client if no entity body is provided.

Creating a simple exception
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The easiest means of creating a WebApplicaitonException exception is to use either the no argument constructor or the constructor that wraps the original exception in a WebApplicationException exception. Both constructors create a WebApplicaitonException with an empty response.
When an exception created by either of these constructors is thrown, the runtime returns a response with an empty entity body and a status code of 500 Server Error.

Setting the status code returned to the client
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When you want to return an error code other than 500, you can use one of the four WebApplicaitonException constructors that allow you to specify the status. Two of these constructors, shown in Example 5.1, “Creating a WebApplicationException with a status code”, take the return status as an integer.

Example 5.1. Creating a WebApplicationException with a status code

WebApplicationException(int status);
WebApplicationException(java.lang.Throwable cause,
                        int status);
The other two, shown in Example 5.2, “Creating a WebApplicationException with a status code” take the response status as an instance of Response.Status.

Example 5.2. Creating a WebApplicationException with a status code

WebApplicationException(javax.ws.rs.core.Response.Status status);
WebApplicationException(java.lang.Throwable cause,
                        javax.ws.rs.core.Response.Status status);
When an exception created by either of these constructors is thrown, the runtime returns a response with an empty entity body and the specified status code.

Providing an entity body
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If you want a message to be sent along with the exception, you can use one of the WebApplicationException constructors that takes a Response object. The runtime uses the Response object to create the response sent to the client. The entity stored in the response is mapped to the entity body of the message and the status field of the response is mapped to the HTTP status of the message.
Example 5.3, “Sending a message with an exception” shows code for returning a text message to a client containing the reason for the exception and sets the HTTP message status to 409 Conflict.

Example 5.3. Sending a message with an exception

import javax.ws.rs.core.Response;
import javax.ws.rs.WebApplicationException;
import org.apache.cxf.jaxrs.impl.ResponseBuilderImpl;

...
ResponseBuilderImpl builder = new ResponseBuilderImpl();
builder.status(Response.Status.CONFLICT);
builder.entity("The requested resource is conflicted.");
Response response = builder.build();
throw WebApplicationException(response);
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Extending the generic exception
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It is possible to extend the WebApplicationException exception. This would allow you to create custom exceptions and eliminate some boiler plate code.
Example 5.4, “Extending WebApplicationException” shows a new exception that creates a similar response to the code in Example 5.3, “Sending a message with an exception”.

Example 5.4. Extending WebApplicationException

public class ConflicteddException extends WebApplicationException
{
  public ConflictedException(String message)
  {
    ResponseBuilderImpl builder = new ResponseBuilderImpl();
    builder.status(Response.Status.CONFLICT);
    builder.entity(message);
    super(builder.build());
     }
}

...
throw ConflictedException("The requested resource is conflicted.");
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