16.3. Providers Utility Class
javax.ws.rs.ext.Providers
is a simple injectable interface that lets you locate MessageBodyReader
s, MessageBodyWriter
s, ContextResolver
s and ExceptionMapper
s. It also lets you implement multi-part providers (content types that embed other content types).
public interface Providers { /** * Get a message body reader that matches a set of criteria. The set of * readers is first filtered by comparing the supplied value of * {@code mediaType} with the value of each reader's * {@link javax.ws.rs.Consumes}, ensuring the supplied value of * {@code type} is assignable to the generic type of the reader, and * eliminating those that do not match. * The list of matching readers is then ordered with those with the best * matching values of {@link javax.ws.rs.Consumes} (x/y > x/* > */*) * sorted first. Finally, the * {@link MessageBodyReader#isReadable} * method is called on each reader in order using the supplied criteria and * the first reader that returns {@code true} is selected and returned. * * @param type the class of object that is to be written. * @param mediaType the media type of the data that will be read. * @param genericType the type of object to be produced. E.g. if the * message body is to be converted into a method parameter, this will be * the formal type of the method parameter as returned by * <code>Class.getGenericParameterTypes</code>. * @param annotations an array of the annotations on the declaration of the * artifact that will be initialized with the produced instance. E.g. if the * message body is to be converted into a method parameter, this will be * the annotations on that parameter returned by * <code>Class.getParameterAnnotations</code>. * @return a MessageBodyReader that matches the supplied criteria or null * if none is found. */ <T> MessageBodyReader<T> getMessageBodyReader(Class<T> type, Type genericType, Annotation annotations[], MediaType mediaType); /** * Get a message body writer that matches a set of criteria. The set of * writers is first filtered by comparing the supplied value of * {@code mediaType} with the value of each writer's * {@link javax.ws.rs.Produces}, ensuring the supplied value of * {@code type} is assignable to the generic type of the reader, and * eliminating those that do not match. * The list of matching writers is then ordered with those with the best * matching values of {@link javax.ws.rs.Produces} (x/y > x/* > */*) * sorted first. Finally, the * {@link MessageBodyWriter#isWriteable} * method is called on each writer in order using the supplied criteria and * the first writer that returns {@code true} is selected and returned. * * @param mediaType the media type of the data that will be written. * @param type the class of object that is to be written. * @param genericType the type of object to be written. E.g. if the * message body is to be produced from a field, this will be * the declared type of the field as returned by * <code>Field.getGenericType</code>. * @param annotations an array of the annotations on the declaration of the * artifact that will be written. E.g. if the * message body is to be produced from a field, this will be * the annotations on that field returned by * <code>Field.getDeclaredAnnotations</code>. * @return a MessageBodyReader that matches the supplied criteria or null * if none is found. */ <T> MessageBodyWriter<T> getMessageBodyWriter(Class<T> type, Type genericType, Annotation annotations[], MediaType mediaType); /** * Get an exception mapping provider for a particular class of exception. * Returns the provider whose generic type is the nearest superclass of * {@code type}. * * @param type the class of exception * @return an {@link ExceptionMapper} for the supplied type or null if none * is found. */ <T extends Throwable> ExceptionMapper<T> getExceptionMapper(Class<T> type); /** * Get a context resolver for a particular type of context and media type. * The set of resolvers is first filtered by comparing the supplied value of * {@code mediaType} with the value of each resolver's * {@link javax.ws.rs.Produces}, ensuring the generic type of the context * resolver is assignable to the supplied value of {@code contextType}, and * eliminating those that do not match. If only one resolver matches the * criteria then it is returned. If more than one resolver matches then the * list of matching resolvers is ordered with those with the best * matching values of {@link javax.ws.rs.Produces} (x/y > x/* > */*) * sorted first. A proxy is returned that delegates calls to * {@link ContextResolver#getContext(java.lang.Class)} to each matching context * resolver in order and returns the first non-null value it obtains or null * if all matching context resolvers return null. * * @param contextType the class of context desired * @param mediaType the media type of data for which a context is required. * @return a matching context resolver instance or null if no matching * context providers are found. */ <T> ContextResolver<T> getContextResolver(Class<T> contextType, MediaType mediaType); }
You can inject an instance of
Providers
into MessageBodyReader
or MessageBodyWriter
like so:
@Provider @Consumes("multipart/fixed") public class MultipartProvider implements MessageBodyReader { private @Context Providers providers; ... }