Google Git
Sign in
gdch-oss / third_party / eclipse-ee4j / jersey / bb5f0142f8a8aa92f294a50cd63d2842a4a1ea84 / . / docs / src / main / docbook / mbw.xml
blob: 09b997c9168d31e2f4a562deab24610729a4142d [file] [log] [blame]
<?xml version="1.0"?>
<!--
Copyright (c) 2012, 2021 Oracle and/or its affiliates. All rights reserved.
This program and the accompanying materials are made available under the
terms of the Eclipse Public License v. 2.0, which is available at
http://www.eclipse.org/legal/epl-2.0.
This Source Code may also be made available under the following Secondary
Licenses when the conditions for such availability set forth in the
Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
version 2 with the GNU Classpath Exception, which is available at
https://www.gnu.org/software/classpath/license.html.
SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
-->
<!DOCTYPE chapter [<!ENTITY % ents SYSTEM "jersey.ent" > %ents;]>
<chapter xmlns="http://docbook.org/ns/docbook"
version="5.0"
xml:lang="en"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:xi="http://www.w3.org/2001/XInclude"
xmlns:xlink="http://www.w3.org/1999/xlink"
xsi:schemaLocation="http://docbook.org/ns/docbook http://docbook.org/xml/5.0/xsd/docbook.xsd
http://www.w3.org/1999/xlink http://www.w3.org/1999/xlink.xsd"
xml:id="message-body-workers">
<title>JAX-RS Entity Providers</title>
<section>
<title>Introduction</title>
<para>
Entity payload, if present in an received HTTP message, is passed to Jersey from an I/O container as an input stream.
The stream may, for example, contain data represented as a plain text, XML or JSON document. However, in many JAX-RS
components that process these inbound data, such as resource methods or client responses, the JAX-RS API user can
access the inbound entity as an arbitrary Java object that is created from the content of the input stream based on
the representation type information. For example, an entity created from an input stream that contains data
represented as a XML document, can be converted to a custom JAXB bean.
Similar concept is supported for the outbound entities. An entity returned from the resource method in the form
of an arbitrary Java object can be serialized by Jersey into a container output stream as a specified representation.
Of course, while JAX-RS implementations do provide default support for most common combinations of Java type and it's
respective on-the-wire representation formats, JAX-RS implementations do not support the conversion described above
for any arbitrary Java type and any arbitrary representation format by default. Instead, a generic extension concept
is exposed in JAX-RS API to allow application-level customizations of this JAX-RS runtime to support for entity
conversions. The JAX-RS extension API components that provide the user-level extensibility are typically referred to
by several terms with the same meaning, such as <emphasis>entity providers</emphasis>,
<emphasis>message body providers</emphasis>, <emphasis>message body workers</emphasis> or
<emphasis>message body readers and writers</emphasis>. You may find all these terms used interchangeably throughout
the user guide and they all refer to the same concept.
</para>
<para>
In JAX-RS extension API (or SPI - service provider interface, if you like) the concept is captured in 2 interfaces.
One for handling inbound entity representation-to-Java de-serialization - &jaxrs.ext.MessageBodyReader; and the other
one for handling the outbound entity Java-to-representation serialization - &jaxrs.ext.MessageBodyWriter;.
A &lit.jaxrs.ext.MessageBodyReader;, as the name suggests, is an extension that supports reading the message body
representation from an input stream and converting the data into an instance of a specific Java type.
A &lit.jaxrs.ext.MessageBodyWriter; is then responsible for converting a message payload from an instance of a
specific Java type into a specific representation format that is sent over the wire to the other party as part of an
HTTP message exchange.
Both of these providers can be used to provide message payload serialization and de-serialization support on the
server as well as the client side. A message body reader or writer is always used whenever a HTTP request or
response contains an entity and the entity is either requested by the application code (e.g. injected as a parameter
of JAX-RS resource method or a response entity read on the client from a &jaxrs.core.Response;) or has to be
serialized and sent to the other party (e.g. an instance returned from a JAX-RS resource method or a request
entity sent by a JAX-RS client).
</para>
</section>
<section>
<title>How to Write Custom Entity Providers</title>
<para>
A best way how to learn about entity providers is to walk through an example of writing one. Therefore we will
describe here the process of implementing a custom &jaxrs.ext.MessageBodyWriter; and
&jaxrs.ext.MessageBodyReader; using a practical example. Let's first setup the stage by defining a JAX-RS
resource class for the server side story of our application.
<example>
<title>Example resource class</title>
<programlisting language="java" linenumbering="numbered">@Path("resource")
public class MyResource {
@GET
@Produces("application/xml")
public MyBean getMyBean() {
return new MyBean("Hello World!", 42);
}
@POST
@Consumes("application/xml")
public String postMyBean(MyBean myBean) {
return myBean.anyString;
}
}</programlisting>
</example>
The resource class defines &lit.http.GET; and &lit.http.POST; resource methods. Both methods work with an entity
that is an instance of <literal>MyBean</literal>.
</para>
<para>
The <literal>MyBean</literal> class is defined in the next example:
<example>
<title>MyBean entity class</title>
<programlisting language="java" linenumbering="numbered">@XmlRootElement
public class MyBean {
@XmlElement
public String anyString;
@XmlElement
public int anyNumber;
public MyBean(String anyString, int anyNumber) {
this.anyString = anyString;
this.anyNumber = anyNumber;
}
// empty constructor needed for deserialization by JAXB
public MyBean() {
}
@Override
public String toString() {
return "MyBean{" +
"anyString='" + anyString + '\'' +
", anyNumber=" + anyNumber +
'}';
}
}</programlisting>
</example>
</para>
<section>
<title>MessageBodyWriter</title>
<para>
The <literal>MyBean</literal> is a JAXB-annotated POJO. In &lit.http.GET; resource method we return
the instance of MyBean and we would like Jersey runtime to serialize it into XML and write
it as an entity body to the response output stream. We design a custom &lit.jaxrs.ext.MessageBodyWriter;
that can serialize this POJO into XML. See the following code sample:
<note>
<para>
Please note, that this is only a demonstration of how to write a custom entity provider.
Jersey already contains default support for entity providers that can serialize JAXB beans
into XML.
</para>
</note>
<example>
<title>MessageBodyWriter example</title>
<programlisting language="java" linenumbering="numbered"><![CDATA[@Produces("application/xml")
public class MyBeanMessageBodyWriter implements MessageBodyWriter<MyBean> {
@Override
public boolean isWriteable(Class<?> type, Type genericType,
Annotation[] annotations, MediaType mediaType) {
return type == MyBean.class;
}
@Override
public long getSize(MyBean myBean, Class<?> type, Type genericType,
Annotation[] annotations, MediaType mediaType) {
// deprecated by JAX-RS 2.0 and ignored by Jersey runtime
return -1;
}
@Override
public void writeTo(MyBean myBean,
Class<?> type,
Type genericType,
Annotation[] annotations,
MediaType mediaType,
MultivaluedMap<String, Object> httpHeaders,
OutputStream entityStream)
throws IOException, WebApplicationException {
try {
JAXBContext jaxbContext = JAXBContext.newInstance(MyBean.class);
// serialize the entity myBean to the entity output stream
jaxbContext.createMarshaller().marshal(myBean, entityStream);
} catch (JAXBException jaxbException) {
throw new ProcessingException(
"Error serializing a MyBean to the output stream", jaxbException);
}
}
}]]></programlisting>
</example>
</para>
<para>
The <literal>MyBeanMessageBodyWriter</literal> implements the &lit.jaxrs.ext.MessageBodyWriter;
interface that contains three methods. In the next sections we'll explore these methods more closely.
</para>
<section>
<title>
<literal>MessageBodyWriter.isWriteable</literal>
</title>
<para>
A method <literal>isWriteable</literal>
should return true if the &lit.jaxrs.ext.MessageBodyWriter; is able to write the given type. Method
does not decide only based on the Java type of the entity but also on annotations attached to the entity
and the requested representation media type.
</para>
<para>
Parameters <literal>type</literal> and <literal>genericType</literal> both define the entity,
where <literal>type</literal> is a raw Java type
(for example, a <literal>java.util.List</literal> class) and <literal>genericType</literal> is a
&jdk6.ParameterizedType; including generic information (for example <literal>List&lt;String&gt;</literal>).
</para>
<para>
Parameter <literal>annotations</literal> contains annotations that are either attached to the resource
method and/or annotations that are attached to the entity by building response like in the following piece
of code:
<example>
<title>Example of assignment of annotations to a response entity</title>
<programlisting language="java" linenumbering="numbered">@Path("resource")
public static class AnnotatedResource {
@GET
public Response get() {
Annotation annotation = AnnotatedResource.class
.getAnnotation(Path.class);
return Response.ok()
.entity("Entity", new Annotation[] {annotation}).build();
}
}</programlisting>
</example>
In the example above, the &lit.jaxrs.ext.MessageBodyWriter; would get
<literal>annotations</literal> parameter containing a JAX-RS <literal>@GET</literal> annotation
as it annotates the resource method and also a <literal>@Path</literal> annotation as it
is passed in the response (but not because it annotates the resource; only resource
method annotations are included). In the case of <literal>MyResource</literal>
and method <literal>getMyBean</literal> the annotations would contain the
&jaxrs.GET; and the &jaxrs.Produces; annotation.
</para>
<para>
The last parameter of the <literal>isWriteable</literal> method is the <literal>mediaType</literal>
which is the media type attached to the response entity by annotating the resource method with a
&lit.jaxrs.Produces; annotation or the request media type specified in the JAX-RS Client API.
In our example, the media type passed to providers for the resource <literal>MyResource</literal> and method
<literal>getMyBean</literal> would be <literal>"application/xml"</literal>.
</para>
<para>
In our implementation of the <literal>isWriteable</literal> method, we
just check that the type is <literal>MyBean</literal>. Please note, that
this method might be executed multiple times by Jersey runtime as Jersey needs to check
whether this provider can be used for a particular combination of entity Java type, media type, and attached
annotations, which may be potentially a performance hog. You can limit the number of execution by
properly defining the &lit.jaxrs.Produces; annotation on the &lit.jaxrs.ext.MessageBodyWriter;.
In our case thanks to &lit.jaxrs.Produces; annotation, the provider will be considered
as writeable (and the method <literal>isWriteable</literal> might be executed) only if the
media type of the outbound message is <literal>"application/xml"</literal>. Additionally, the provider
will only be considered as possible candidate and its <literal>isWriteable</literal> method will
be executed, if the generic type of the provider is either a sub class or super class of
<literal>type</literal> parameter.
</para>
</section>
<section>
<title>
<literal>MessageBodyWriter.writeTo</literal>
</title>
<para>
Once a message body writer is selected as the most appropriate (see the <xref linkend="providers-selection" />
for more details on entity provider selection), its <literal>writeTo</literal> method is invoked. This method
receives parameters with the same meaning as in <literal>isWriteable</literal> as well as a few additional ones.
</para>
<para>
In addition to the parameters already introduced, the <literal>writeTo</literal> method defies also
<literal>httpHeaders</literal> parameter, that contains HTTP headers associated with the
outbound message.
<note>
<para>
When a &lit.jaxrs.ext.MessageBodyWriter; is invoked, the headers still can be modified in this point
and any modification will be reflected in the outbound HTTP message being sent. The modification of
headers must however happen before a first byte is written to the supplied output stream.
</para>
</note>
</para>
<para>
Another new parameter, <literal>myBean</literal>, contains the entity instance to be serialized (the type of
entity corresponds to generic type of &lit.jaxrs.ext.MessageBodyWriter;). Related parameter
<literal>entityStream</literal> contains the entity output stream to which the method should serialize the entity.
In our case we use JAXB to marshall the entity into the <literal>entityStream</literal>. Note, that the
<literal>entityStream</literal> is not closed at the end of method; the stream will be closed by Jersey.
<important>
<para>
Do not close the entity output stream in the <literal>writeTo</literal> method of your
&lit.jaxrs.ext.MessageBodyWriter; implementation.
</para>
</important>
</para>
</section>
<section>
<title>
<literal>MessageBodyWriter.getSize</literal>
</title>
<para>
The method is deprecated since JAX-RS 2.0 and Jersey 2 ignores the return value. In JAX-RS 1.0 the
method could return the size of the entity that would be then used for "Content-Length" response
header. In Jersey 2.0 the "Content-Length" parameter is computed automatically using an internal
outbound entity buffering. For details about configuration options of outbound entity buffering see the javadoc
of &jersey.message.MessageProperties;, property <literal>OUTBOUND_CONTENT_LENGTH_BUFFER</literal>
which configures the size of the buffer.
<note>
<para>
You can disable the Jersey outbound entity buffering by setting the buffer size to 0.
</para>
</note>
</para>
</section>
<section>
<title>Testing a &lit.jaxrs.ext.MessageBodyWriter;</title>
<para>
Before testing the <literal>MyBeanMessageBodyWriter</literal>, the writer must
be registered as a custom JAX-RS extension provider. It should either be added to your application
&jersey.server.ResourceConfig;, or returned from your custom &jaxrs.core.Application; sub-class, or
annotated with &jaxrs.ext.Provider; annotation to leverage JAX-RS provider auto-discovery feature.
</para>
<para>
After registering the <literal>MyBeanMessageBodyWriter</literal> and <literal>MyResource</literal> class
in our application, the request can be initiated (in this example from Client API).
<example xml:id="client-get-call">
<title>Client code testing MyBeanMessageBodyWriter</title>
<programlisting language="java" linenumbering="numbered" >WebTarget webTarget = // initialize web target to the context root
// of example application
Response response = webTarget.path("resource")
.request(MediaType.APPLICATION_XML).get();
System.out.println(response.getStatus());
String myBeanXml = response.readEntity(String.class);
System.out.println(myBeanXml);</programlisting>
</example>
The client code initiates the &lit.http.GET; which will be matched to the resource method
<literal>MyResource.getMyBean()</literal>. The response entity is de-serialized as a <literal>String</literal>.
</para>
<para>
The result of console output is:
<example>
<title>Result of MyBeanMessageBodyWriter test</title>
<screen language="text" linenumbering="unnumbered"><![CDATA[200
<?xml version="1.0" encoding="UTF-8" standalone="yes"?><myBean>
<anyString>Hello World!</anyString><anyNumber>42</anyNumber></myBean>]]></screen>
</example>
The returned status is 200 and the entity is stored in the response in a <literal>XML</literal> format.
Next, we will look at how the Jersey de-serializes this XML document into a <literal>MyBean</literal> consumed by
our &lit.http.POST; resource method.
</para>
</section>
</section>
<section>
<title>MessageBodyReader</title>
<para>
In order to de-serialize the entity of <literal>MyBean</literal> on the server or the client, we need to implement
a custom &jaxrs.ext.MessageBodyReader;.
<note>
<para>
Please note, that this is only a demonstration of how to write a custom entity provider.
Jersey already contains default support for entity providers that can serialize JAXB beans
into XML.
</para>
</note>
</para>
<para>
Our &lit.jaxrs.ext.MessageBodyReader; implementation is listed in <xref linkend="mbw.ex.mbr" />.
<example xml:id="mbw.ex.mbr">
<title>MessageBodyReader example</title>
<programlisting language="java" linenumbering="numbered"><![CDATA[public static class MyBeanMessageBodyReader
implements MessageBodyReader<MyBean> {
@Override
public boolean isReadable(Class<?> type, Type genericType,
Annotation[] annotations, MediaType mediaType) {
return type == MyBean.class;
}
@Override
public MyBean readFrom(Class<MyBean> type,
Type genericType,
Annotation[] annotations, MediaType mediaType,
MultivaluedMap<String, String> httpHeaders,
InputStream entityStream)
throws IOException, WebApplicationException {
try {
JAXBContext jaxbContext = JAXBContext.newInstance(MyBean.class);
MyBean myBean = (MyBean) jaxbContext.createUnmarshaller()
.unmarshal(entityStream);
return myBean;
} catch (JAXBException jaxbException) {
throw new ProcessingException("Error deserializing a MyBean.",
jaxbException);
}
}
}]]></programlisting>
</example>
It is obvious that the &lit.jaxrs.ext.MessageBodyReader; interface is similar to &lit.jaxrs.ext.MessageBodyWriter;.
In the next couple of sections we will explore it's API methods.
</para>
<section>
<title><literal>MessageBodyReader.isReadable</literal></title>
<para>
It defines the method <literal>isReadable()</literal> which has a very similar meaning as method
<literal>isWriteable()</literal> in &lit.jaxrs.ext.MessageBodyWriter;. The method returns &lit.true;
if it is able to de-serialize the given type. The <literal>annotations</literal> parameter contains annotations
that are attached to the entity parameter in the resource method. In our &lit.http.POST; resource
method <literal>postMyBean</literal> the entity parameter <literal>myBean</literal> is not
annotated, therefore no annotation will be passed to the isReadable. The <literal>mediaType</literal>
parameter contains the entity media type. The media type, in our case, must be consumable by the &lit.http.POST;
resource method, which is specified by placing a JAX-RS &jaxrs.Consumes; annotation to the method.
The resource method <literal>postMyBean()</literal> is annotated with
<literal>@Consumes("application/xml")</literal>,
therefore for purpose of de-serialization of entity for the <literal>postMyBean()</literal> method,
only requests with entities represented as <literal>"application/xml"</literal>
media type will match the method. However, this method might be executed for entity types that are sub classes
or super classes of the declared generic type on the &lit.jaxrs.ext.MessageBodyReader; will be also considered.
It is a responsibility of the <literal>isReadable</literal> method to decide whether it is able
to de-serialize the entity and type comparison is one of the basic decision steps.
<tip>
<para>
In order to reduce number of <literal>isReadable</literal> executions, always define correctly the consumable
media type(s) with the &lit.jaxrs.Consumes; annotation on your custom &lit.jaxrs.ext.MessageBodyReader;.
</para>
</tip>
</para>
</section>
<section>
<title><literal>MessageBodyReader.readFrom</literal></title>
<para>
The <literal>readForm()</literal> method gets the parameters with the same meaning as in
<literal>isReadable()</literal>. The additional <literal>entityStream</literal> parameter provides a handle
to the entity input stream from which the entity bytes should be read and de-serialized into a Java entity which
is then returned from the method. Our <literal>MyBeanMessageBodyReader</literal> de-serializes the incoming
XML data into an instance of <literal>MyBean</literal> using JAXB.
<important>
<para>
Do not close the entity input stream in your &lit.jaxrs.ext.MessageBodyReader; implementation. The stream
will be automatically closed by Jersey runtime.
</para>
</important>
</para>
</section>
<section>
<title>Testing a &lit.jaxrs.ext.MessageBodyWriter;</title>
<para>
Now let's send a test request using the JAX-RS Client API.
<example>
<title>Testing MyBeanMessageBodyReader</title>
<programlisting language="java" linenumbering="numbered"><![CDATA[final MyBean myBean = new MyBean("posted MyBean", 11);
Response response = webTarget.path("resource").request("application/xml")
.post(Entity.entity(myBean, "application/xml"));
System.out.println(response.getStatus());
final String responseEntity = response.readEntity(String.class);
System.out.println(responseEntity);
]]></programlisting>
</example>
</para>
<para>
The console output is:
<example>
<title>Result of testing MyBeanMessageBodyReader</title>
<screen language="text" linenumbering="unnumbered"><![CDATA[200
posted MyBean]]></screen>
</example>
</para>
</section>
<section>
<title>Using Entity Providers with JAX-RS Client API</title>
<para>
Both, &lit.jaxrs.ext.MessageBodyReader; and &lit.jaxrs.ext.MessageBodyWriter; can be registered in a
configuration of JAX-RS Client API components typically without any need to change their code. The example
<xref linkend="mbw.ex.client.mbr.reg" /> is a variation on the <xref linkend="client-get-call" />
listed in one of the previous sections.
<example xml:id="mbw.ex.client.mbr.reg">
<title>MessageBodyReader registered on a JAX-RS client</title>
<programlisting language="java" linenumbering="numbered"><![CDATA[Client client = ClientBuilder.newBuilder()
.register(MyBeanMessageBodyReader.class).build();
Response response = client.target("http://example/comm/resource")
.request(MediaType.APPLICATION_XML).get();
System.out.println(response.getStatus());
MyBean myBean = response.readEntity(MyBean.class);
System.out.println(myBean);]]></programlisting>
</example>
</para>
<para>
The code above registers <literal>MyBeanMessageBodyReader</literal> to the &jaxrs.client.Client; configuration
using a &jaxrs.client.ClientBuilder; which means that the provider will be used for any &jaxrs.client.WebTarget;
produced by the <literal>client</literal> instance.
<note>
<para>
You could also register the JAX-RS entity (and any other) providers to individual
&lit.jaxrs.client.WebTarget; instances produced by the client.
</para>
</note>
Then, using the fluent chain of method invocations, a resource target pointing to our
<literal>MyResource</literal> is defined, a HTTP &lit.http.GET; request is invoked.
The response entity is then read as an instance of a <literal>MyBean</literal> type by invoking the
<literal>response.readEntity</literal> method, that internally locates the registered
<literal>MyBeanMessageBodyReader</literal> and uses it for entity de-serialization.
</para>
<para>
The console output for the example is:
<example>
<title>Result of client code execution</title>
<screen language="text" linenumbering="unnumbered"><![CDATA[200
MyBean{anyString='Hello World!', anyNumber=42}]]></screen>
</example>
</para>
</section>
</section>
</section>
<section xml:id="providers-selection">
<title>Entity Provider Selection</title>
<para>
Usually there are many entity providers registered on the server or client side (be default there must be
at least providers mandated by the JAX-RS specification, such as providers for primitive types, byte array,
JAXB beans, etc.).
JAX-RS defines an algorithm for selecting the most suitable provider for entity processing. This algorithm
works with information such as entity Java type and on-the-wire media type representation of entity, and searches
for the most suitable entity provider from the list of available providers based on the supported media type
declared on each provider (defined by &lit.jaxrs.Produces; or &lit.jaxrs.Consumes; on the provider class)
as well as based on the generic type declaration of the available providers. When a list of suitable candidate
entity providers is selected and sorted based on the rules defined in JAX-RS specification, a JAX-RS runtime
then it invokes <literal>isReadable</literal> or <literal>isWriteable</literal> method respectively on each
provider in the list until a first provider is found that returns &lit.true;. This provider is then used to
process the entity.
</para>
<para>
The following steps describe the algorithm for selecting a &lit.jaxrs.ext.MessageBodyWriter; (extracted
from JAX-RS with little modifications). The steps refer to the previously discussed example application.
The &lit.jaxrs.ext.MessageBodyWriter; is searched for purpose of deserialization of <literal>MyBean</literal>
entity returned from the method <literal>getMyBean</literal>. So, <emphasis>type is <literal>MyBean</literal>
and media type <literal>"application/xml"</literal></emphasis>. Let's assume the runtime contains also
registered providers, namely:
<simplelist>
<member>
<literal>A</literal>: <literal>@Produces("application/*")</literal> with generic type
<literal>&lt;Object&gt;</literal>
</member>
<member>
<literal>B</literal>: <literal>@Produces("*/*")</literal> with generic type <literal>&lt;MyBean&gt;</literal>
</member>
<member>
<literal>C</literal>: <literal>@Produces("text/plain")</literal> with generic type
<literal>&lt;MyBean&gt;</literal>
</member>
<member>
<literal>D</literal>: <literal>@Produces("application/xml")</literal> with generic type
<literal>&lt;Object&gt;</literal>
</member>
<member>
<literal>MyBeanMessageBodyWriter</literal>: <literal>@Produces("application/xml")</literal> with generic
type <literal>&lt;MyBean&gt;</literal>
</member>
</simplelist>
</para>
<para>
The algorithm executed by a JAX-RS runtime to select a proper &lit.jaxrs.ext.MessageBodyWriter; implementation
is illustrated in <xref linkend="mbw.writer.selection.algorithm" />.
</para>
<procedure xml:id="mbw.writer.selection.algorithm">
<title>&lit.jaxrs.ext.MessageBodyWriter; Selection Algorithm</title>
<step>
<para>
Obtain the object that will be mapped to the message entity body. For a return type of Response
or subclasses, the object is the value of the entity property, for other return types it is the returned
object.
</para>
<para>
So in our case, for the resource method <literal>getMyBean</literal> the type will
be <literal>MyBean</literal>.
</para>
</step>
<step>
<para>
Determine the media type of the response.
</para>
<para>
In our case, for resource method <literal>getMyBean</literal>
annotated with <literal>@Produces("application/xml")</literal>, the media type will be
<literal>"application/xml"</literal>.
</para>
</step>
<step>
<para>
Select the set of MessageBodyWriter providers that support the object and media
type of the message entity body.
</para>
<para>
In our case, for entity media type <literal>"application/xml"</literal>
and type <literal>MyBean</literal>, the appropriate &lit.jaxrs.ext.MessageBodyWriter; will
be the <literal>A</literal>, <literal>B</literal>, <literal>D</literal>
and <literal>MyBeanMessageBodyWriter</literal>. The provider <literal>C</literal> does
not define the appropriate
media type. <literal>A</literal> and <literal>B</literal> are fine as
their type is more generic and compatible with <literal>"application/xml"</literal>.
</para>
</step>
<step xml:id="mbw.writer.selection.algorithm.sortStep">
<para>
Sort the selected MessageBodyWriter providers with a primary key of generic type where providers
whose generic type is the nearest superclass of the object class are sorted first and a secondary key of
media type. Additionally, JAX-RS specification mandates that custom, user registered providers have to
be sorted ahead of default providers provided by JAX-RS implementation. This is used as a tertiary
comparison key. User providers are places prior to Jersey internal providers in to the final ordered list.
</para>
<para>
The sorted providers will be: <literal>MyBeanMessageBodyWriter</literal>,
<literal>B</literal>. <literal>D</literal>, <literal>A</literal>.
</para>
</step>
<step>
<para>
Iterate through the sorted &lit.jaxrs.ext.MessageBodyWriter; providers and, utilizing the
<literal>isWriteable</literal> method of each until you find a &lit.jaxrs.ext.MessageBodyWriter; that
returns &lit.true;.
</para>
<para>
The first provider in the list - our <literal>MyBeanMessageBodyWriter</literal> returns &lit.true; as
it compares types and the types matches. If it would return &lit.false;, the next provider
<literal>B</literal> would by check by invoking its <literal>isWriteable</literal> method.
</para>
</step>
<step>
<para>
If step 5 locates a suitable &lit.jaxrs.ext.MessageBodyWriter; then use its writeTo method to map the
object to the entity body.
</para>
<para>
<literal>MyBeanMessageBodyWriter.writeTo</literal> will be executed and it will serialize the
entity.
</para>
<stepalternatives>
<step>
<para>
Otherwise, the server runtime MUST generate an
<literal>InternalServerErrorException</literal>, a subclass of
<literal>WebApplicationException</literal> with its status set to 500, and no entity and the client
runtime MUST generate a <literal>ProcessingException</literal>.
</para>
<para>
We have successfully found a provider, thus no exception is generated.
</para>
</step>
</stepalternatives>
</step>
</procedure>
<note>
<para>
JAX-RS 3.x/2.x is incompatible with JAX-RS 1.x in one step of the entity provider selection algorithm.
JAX-RS 1.x defines sorting keys priorities in the <xref linkend="mbw.writer.selection.algorithm.sortStep" />
in exactly opposite order. So, in JAX-RS 1.x the keys are defined in the order: primary media type,
secondary type declaration distance where custom providers have always precedence to internal providers.
If you want to force Jersey to use the algorithm compatible with JAX-RS 1.x, setup the property
(to &jersey.server.ResourceConfig; or return from &jaxrs.core.Application; from its
<literal>getProperties</literal> method):
<programlisting language="java" linenumbering="unnumbered"><![CDATA[jersey.config.workers.legacyOrdering=true]]></programlisting>
Documentation of this property can be found in the javadoc of &jersey.message.MessageProperties;.
</para>
</note>
<para>
The algorithm for selection of &lit.jaxrs.ext.MessageBodyReader; is similar, including the incompatibility
between JAX-RS 3.x/2.x and JAX-RS 1.x and the property to workaround it. The algorithm is defined as follows:
</para>
<procedure xml:id="mbw.reader.selection.algorithm">
<title>&lit.jaxrs.ext.MessageBodyReader; Selection Algorithm</title>
<step>
<para>
Obtain the media type of the request. If the request does not contain a <literal>Content-Type</literal>
header then use <literal>application/octet-stream</literal> media type.
</para>
</step>
<step>
<para>
Identify the Java type of the parameter whose value will be mapped from the entity body. The
Java type on the server is the type of the entity parameter of the resource method. On the client
it is the <literal>Class</literal> passed to <literal>readFrom</literal> method.
</para>
</step>
<step>
<para>
Select the set of available &lit.jaxrs.ext.MessageBodyReader; providers that support the media type
of the request.
</para>
</step>
<step xml:id="mbw.reader.selection.algorithm.selectStep">
<para>
Iterate through the selected &lit.jaxrs.ext.MessageBodyReader; classes and, utilizing their
<literal>isReadable</literal> method, choose the first &lit.jaxrs.ext.MessageBodyReader; provider that
supports the desired combination of Java type/media type/annotations parameters.
</para>
</step>
<step>
<para>
If <xref linkend="mbw.reader.selection.algorithm.selectStep" /> locates a suitable
&lit.jaxrs.ext.MessageBodyReader;, then use its <literal>readFrom</literal> method to map the entity
body to the desired Java type.
</para>
<stepalternatives>
<step>
<para>
Otherwise, the server runtime MUST generate a <literal>NotSupportedException</literal>
(HTTP 415 status code) and no entity and the client runtime MUST generate an instance
of <literal>ProcessingException</literal>.
</para>
</step>
</stepalternatives>
</step>
</procedure>
</section>
<section>
<title>Jersey &lit.jersey.message.MessageBodyWorkers; API</title>
<para>
In case you need to directly work with JAX-RS entity providers, for example to serialize an entity in your resource
method, filter or in a composite entity provider, you would need to perform quite a lot of steps.
You would need to choose the appropriate &lit.jaxrs.ext.MessageBodyWriter; based on the type, media type and
other parameters. Then you would need to instantiate it, check it by <literal>isWriteable</literal> method and
basically perform all the steps that are normally performed by Jersey
(see <xref linkend="mbw.reader.selection.algorithm" />).
</para>
<para>
To remove this burden from developers, Jersey exposes a proprietary public API that simplifies the manipulation
of entity providers. The API is defined by &jersey.message.MessageBodyWorkers; interface and Jersey provides an
implementation that can be injected using the &jaxrs.core.Context; injection annotation. The interface declares
methods for selection of most appropriate &lit.jaxrs.ext.MessageBodyReader; and &lit.jaxrs.ext.MessageBodyWriter;
based on the rules defined in JAX-RS spec, methods for writing and reading entity that ensure proper and timely
invocation of interceptors and other useful methods.
</para>
<para>
See the following example of usage of &lit.jersey.message.MessageBodyWorkers;.
<example>
<title>Usage of MessageBodyWorkers interface</title>
<programlisting language="java" linenumbering="numbered"><![CDATA[@Path("workers")
public static class WorkersResource {
@Context
private MessageBodyWorkers workers;
@GET
@Produces("application/xml")
public String getMyBeanAsString() {
final MyBean myBean = new MyBean("Hello World!", 42);
// buffer into which myBean will be serialized
ByteArrayOutputStream baos = new ByteArrayOutputStream();
// get most appropriate MBW
final MessageBodyWriter<MyBean> messageBodyWriter =
workers.getMessageBodyWriter(MyBean.class, MyBean.class,
new Annotation[]{}, MediaType.APPLICATION_XML_TYPE);
try {
// use the MBW to serialize myBean into baos
messageBodyWriter.writeTo(myBean,
MyBean.class, MyBean.class, new Annotation[] {},
MediaType.APPLICATION_XML_TYPE, new MultivaluedHashMap<String, Object>(),
baos);
} catch (IOException e) {
throw new RuntimeException(
"Error while serializing MyBean.", e);
}
final String stringXmlOutput = baos.toString();
// stringXmlOutput now contains XML representation:
// "<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
// <myBean><anyString>Hello World!</anyString>
// <anyNumber>42</anyNumber></myBean>"
return stringXmlOutput;
}
}]]></programlisting>
</example>
</para>
<para>
In the example a resource injects &lit.jersey.message.MessageBodyWorkers; and uses it for selection
of the most appropriate &lit.jaxrs.ext.MessageBodyWriter;. Then the writer is utilized to serialize the entity
into the buffer as XML document. The <literal>String</literal> content of the buffer is then returned.
This will cause that Jersey will not use <literal>MyBeanMessageBodyWriter</literal>
to serialize the entity as it is already in the <literal>String</literal> type
(<literal>MyBeanMessageBodyWriter</literal> does not support <literal>String</literal>). Instead, a simple
<literal>String</literal>-based &lit.jaxrs.ext.MessageBodyWriter; will be chosen and it will only serialize the
<literal>String</literal> with XML to the output entity stream by writing out the bytes of the
<literal>String</literal>.
</para>
<para>
Of course, the code in the example does not bring any benefit as the entity could
have been serialized by <literal>MyBeanMessageBodyWriter</literal> by Jersey as in previous examples;
the purpose of the example was to show how to use &lit.jersey.message.MessageBodyWorkers; in a resource method.
</para>
</section>
<section>
<title>Default Jersey Entity Providers</title>
<para>
Jersey internally contains entity providers for these types with combination of media types (in brackets):
</para>
<simplelist>
<member>
<literal>byte[]</literal> (<literal>*/*</literal>)
</member>
<member>
&jdk6.String; (<literal>*/*</literal>)
</member>
<member>
&jdk6.InputStream; (<literal>*/*</literal>)
</member>
<member>
&jdk6.Reader; (<literal>*/*</literal>)
</member>
<member>
&jdk6.File; (<literal>*/*</literal>)
</member>
<member>
&jee9.DataSource; (<literal>*/*</literal>)
</member>
<member>
&jdk6.Source; (<literal>text/xml</literal>, <literal>application/xml</literal> and media types of the form
<literal>application/*+xml</literal>)
</member>
<member>
&jdk6.JAXBElement; (<literal>text/xml</literal>, <literal>application/xml</literal> and media types of the form
<literal>application/*+xml</literal>)
</member>
<member>
&jaxrs.core.MultivaluedMap; (<literal>application/x-www-form-urlencoded</literal>)
</member>
<member>
&jaxrs.core.Form; (<literal>application/x-www-form-urlencoded</literal>)
</member>
<member>
&jaxrs.core.StreamingOutput; ((<literal>*/*</literal>)) - this class can be used as an lightweight
&lit.jaxrs.ext.MessageBodyWriter; that can be returned from a resource method
</member>
<member>
&jdk6.Boolean;, &jdk6.Character; and &jdk6.Number; (<literal>text/plain</literal>) - corresponding
primitive types supported via boxing/unboxing conversion
</member>
</simplelist>
<para>
For other media type supported in jersey please see the <xref linkend="media" /> which describes
additional Jersey entity provider extensions for serialization to JSON, XML, serialization of collections,
&jersey.media.multipart; and others.
</para>
</section>
</chapter>