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As the microservices style of system architecture (see, for example, Microservices by Martin Fowler) gains increasing traction, new API standards are coming along to support it. One set of such standards comes from the Microprofile Project supported by the Eclipse Foundation, and among those is one, MicroProfile Rest Client, of particular interest to RESTEasy and JAX-RS. In fact, it is intended to be based on, and consistent with, JAX-RS, and it includes ideas already implemented in RESTEasy. For a more detailed description of MicroProfile Rest Client, see https://github.com/eclipse/microprofile-rest-client. In particular, the API code is in https://github.com/eclipse/microprofile-rest-client/tree/master/api. and the specification is in https://github.com/eclipse/microprofile-rest-client/tree/master/spec.
One of the central ideas in MicroProfile Rest Client is a version of distributed object communication, a concept implemented in, among other places, CORBA, Java RMI, the JBoss Remoting project, and RESTEasy. Consider the resource
@Path("resource")
public class TestResource {
@Path("test")
@GET
String test() {
return "test";
}
}
The JAX-RS native way of accessing TestResource looks like
Client client = ClientBuilder.newClient();
String response = client.target("http://localhost:8081/test").request().get(String.class);
The call to TestResource.test() is not particularly onerous, but calling
test() directly allows a more natural syntax. That is exactly what Microprofile
Rest Client supports:
@Path("resource")
public interface TestResourceIntf {
@Path("test")
@GET
public String test();
}
TestResourceIntf service = RestClientBuilder.newBuilder()
.baseUrl("http://localhost:8081/")
.build(TestResourceIntf.class);
String s = service.test();
The first four lines of executable code are spent creating a proxy, service, that implements
TestResourceIntf, but once that is done, calls on TestResource
can be made very naturally in terms of TestResourceIntf, as illustrated by the call
service.test().
Beyond the natural syntax, another advantage of proxies is the way the proxy construction process quietly
gathers useful information from the implemented interface and makes it available for remote invocations.
Consider a more elaborate version of TestResourceIntf:
@Path("resource")
public interface TestResourceIntf2 {
@Path("test/{path}")
@Consumes("text/plain")
@Produces("text/html")
@POST
public String test(@PathParam("path") String path, @QueryParam("query") String query, String entity);
}
Calling service.test("p", "q", "e") results in an HTTP message that looks like
POST /resource/test/p/?query=q HTTP/1.1 Accept: text/html Content-Type: text/plain Content-Length: 1 e
The HTTP verb is derived from the @POST annotation, the request URI is derived from the
two instances of the @Path annotation (one on the class, one on the method) plus the
first and second parameters of test(), the Accept header is derived from the
@Produces annotation, and the Content-Type header is derived from the
@Consumes annotation,
Using the JAX-RS API, service.test("p", "q", "e") would look like the more verbose
Client client = ClientBuilder.newClient();
String response = client.target("http://localhost:8081/resource/test/p")
.queryParam("query", "q")
.request()
.accept("text/html")
.post(Entity.entity("e", "text/plain"), String.class);
One other basic facility offered by MicroProfile Rest Client is the ability to configure the client environment by registering providers:
TestResourceIntf service = RestClientBuilder.newBuilder()
.baseUrl("http://localhost:8081/")
.register(MyClientResponseFilter.class)
.register(MyMessageBodyReader.class)
.build(TestResourceIntf.class);
Naturally, the registered providers should be relevant to the client environment, rather than, say, a
ContainerResponseFilter.
So far, the MicroProfile Rest Client should look familiar to anyone who has used the RESTEasy client proxy facility (Section ""RESTEasy Proxy Framework"). The construction in the previous listing would look like
ResteasyClient client = (ResteasyClient) ResteasyClientBuilder.newClient();
TestResourceIntf service = client.target("http://localhost:8081/")
.register(MyClientResponseFilter.class)
.register(MyMessageBodyReader.class)
.proxy(TestResourceIntf.class);
in RESTEasy.
Beyond the central concept of the client proxy, some basic concepts in MicroProfile Client originate
in JAX-RS. Some of these have already been introduced in the previous section, since the interface
implemented by a client proxy represents the facilities provided by a JAX-RS server. For example, the
HTTP verb annotations and the @Consumes and @Produces annotations originate on the
JAX-RS server side. Injectable parameters annotated with @PathParameter, @QueryParameter,
etc., also come from JAX-RS.
Nearly all of the provider concepts supported by MicroProfile Client also originate in JAX-RS. These are:
Like JAX-RS, MicroProfile Client also has the concept of mandated providers. These are
MessageBodyReader and MessageBodyWriter
must be provided.MessageBodyReader and MessageBodyWriter
must be provided if the implementation supports JSON-B.MessageBodyReaders and MessageBodyWriters
must be provided for the following types:
Some concepts in MicroProfile Rest Client do not appear in either JAX-RS or RESTEasy.
Whenever no media type is specified by, for example, @Consumes or @Produces annotations,
the media type of a request entity or response entity is "application/json". This is different than JAX-RS, where
the media type defaults to "application/octet-stream".
In addition to programmatic registration of providers as illustrated above, it is also possible to
register providers declaratively with annotations introduced in MicroProfile Rest Client. In particular,
providers can be registered by adding the org.eclipse.microprofile.rest.client.annotation.RegisterProvider
annotation to the target interface:
@Path("resource")
@RegisterProvider(MyClientResponseFilter.class)
@RegisterProvider(MyMessageBodyReader.class)
public interface TestResourceIntf2 {
@Path("test/{path}")
@Consumes("text/plain")
@Produces("text/html")
@POST
public String test(@PathParam("path") String path, @QueryParam("query") String query, String entity);
}
Declaring MyClientResponseFilter and MyMessageBodyReader with
annotations eliminates the need to call RestClientBuilder.register().
One more way to register providers is by implementing one or both of the listeners in package
org.eclipse.microprofile.rest.client.spi:
public interface RestClientBuilderListener {
void onNewBuilder(RestClientBuilder builder);
}
public interface RestClientListener {
void onNewClient(Class<?> serviceInterface, RestClientBuilder builder);
}
which can access a RestClientBuilder upon creation of a new RestClientBuilder or
upon the execution of RestClientBuilder.build(), respectively. Implementations must
be declared in
META-INF/services/org.eclipse.microprofile.rest.client.spi.RestClientBuilderListener
or
META-INF/services/org.eclipse.microprofile.rest.client.spi.RestClientListener
One way of declaring a header to be included in a request is by annotating one of the resource method
parameters with @HeaderValue:
@POST @Produces(MediaType.TEXT_PLAIN) @Consumes(MediaType.TEXT_PLAIN) String contentLang(@HeaderParam(HttpHeaders.CONTENT_LANGUAGE) String contentLanguage, String subject);
That option is available with RESTEasy client proxies as well,
but in case it is inconvenient or otherwise inappropriate to include the necessary parameter,
MicroProfile Client makes a declarative alternative available through the use of the
org.eclipse.microprofile.rest.client.annotation.ClientHeaderParam annotation:
@POST @Produces(MediaType.TEXT_PLAIN) @Consumes(MediaType.TEXT_PLAIN) @ClientHeaderParam(name=HttpHeaders.CONTENT_LANGUAGE, value="en") String contentLang(String subject);
In this example, the header value is hardcoded, but it is also possible to compute a value:
@POST
@Produces(MediaType.TEXT_PLAIN)
@Consumes(MediaType.TEXT_PLAIN)
@ClientHeaderParam(name=HttpHeaders.CONTENT_LANGUAGE, value="{getLanguage}")
String contentLang(String subject);
default String getLanguage() {
return ...;
}
An instance of org.eclipse.microprofile.rest.client.ext.ClientHeadersFactory,
public interface ClientHeadersFactory {
/**
* Updates the HTTP headers to send to the remote service. Note that providers
* on the outbound processing chain could further update the headers.
*
* @param incomingHeaders - the map of headers from the inbound JAX-RS request. This will
* be an empty map if the associated client interface is not part of a JAX-RS request.
* @param clientOutgoingHeaders - the read-only map of header parameters specified on the
* client interface.
* @return a map of HTTP headers to merge with the clientOutgoingHeaders to be sent to
* the remote service.
*/
MultivaluedMap<String, String> update(MultivaluedMap<String, String> incomingHeaders,
MultivaluedMap<String, String> clientOutgoingHeaders);
}
if activated, can do a bulk transfer of incoming headers to an outgoing request. The default instance
org.eclipse.microprofile.rest.client.ext.DefaultClientHeadersFactoryImpl
will return a map consisting of those incoming headers listed in the comma separated configuration property
org.eclipse.microprofile.rest.client.propagateHeaders
In order for an instance of ClientHeadersFactory to be activated,
the interface must be annotated with
org.eclipse.microprofile.rest.client.annotation.RegisterClientHeaders. Optionally,
the annotation may include a value field set to an implementation class; without an explicit value, the
default instance will be used.
The org.eclipse.microprofile.rest.client.ext.ResponseExceptionMapper is the
client side inverse of the javax.ws.rs.ext.ExceptionMapper defined in JAX-RS. That is,
where ExceptionMapper.toResponse() turns an Exception thrown
during server side processing into a Response,
ResponseExceptionMapper.toThrowable() turns a
Response received on the client side with an HTTP error status into
an Exception. ResponseExceptionMappers can be registered
in the same manner as other providers, that is, either programmatically or declaratively. In the absence
of a registered ResponseExceptionMapper, a default ResponseExceptionMapper
will map any response with status >= 400 to a WebApplicationException.
MicroProfile Rest Client mandates that implementations must support CDI injection of proxies. At first, the concept might seem odd in that CDI is more commonly available on the server side. However, the idea is very consistent with the microservices philosophy. If an application is composed of a number of small services, then it is to be expected that services will often act as clients to other services.
CDI (Contexts and Dependency Injection) is a fairly rich subject and beyond the scope of this Guide. For more information, see JSR 365: Contexts and Dependency Injection for JavaTM 2.0 (the specification), Java EE 8 Tutorial, or WELD - CDI Reference Implementation.
The fundamental thing to know about CDI injection is that annotating a variable with
javax.inject.Inject will lead the CDI runtime (if it is present and enabled) to
create an object of the appropriate type and assign it to the variable. For example, in
public interface Book {
public String getTitle();
public void setTitle(String title);
}
public class BookImpl implements Book {
private String title;
@Override
public String getTitle() {
return title;
}
@Override
public void setTitle(String title) {
this.title = title;
}
}
public class Author {
@Inject private Book book;
public Book getBook() {
return book;
}
}
The CDI runtime will create an instance of BookImpl and assign it to the private field
book when an instance of Author is created;
In this example, the injection is done because BookImpl is assignable to book, but
greater discrimination can be imposed by annotating the interface and the field with qualifier
annotations. For the injection to be legal, every qualifier on the field must be present on the injected interface.
For example:
@Qualifier
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Text {}
@Qualifier
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Graphic {}
@Text
public class TextBookImpl extends BookImpl { }
@Graphic
public class GraphicNovelImpl extends BookImpl { }
public class Genius {
@Inject @Graphic Book book;
}
Here, the class TextBookImpl is annotated with the @Text qualifier and
GraphicNovelImpl is annotated with @Graphic. It follows that an instance
of GraphicNovelImpl is eligible for assignment to the field book in the
Genius class, but an instance of TextBookImpl is not.
Now, in MicroProfile Rest Client, any interface that is to be managed as a CDI bean must be annotated with
@RegisterRestClient:
@Path("resource")
@RegisterProvider(MyClientResponseFilter.class)
public static class TestResourceImpl {
@Inject TestDataBase db;
@Path("test/{path}")
@Consumes("text/plain")
@Produces("text/html")
@POST
public String test(@PathParam("path") String path, @QueryParam("query") String query, String entity) {
return db.getByName(query);
}
}
@Path("database")
@RegisterRestClient
public interface TestDataBase {
@Path("")
@POST
public String getByName(String name);
}
Here, the MicroProfile Rest Client implementation creates a proxy for a TestDataBase service,
allowing easy access by TestResourceImpl. Notice, though, that there's no indication of
where the TestDataBase implementation lives. That information can be supplied externally with
the system variable
<fqn of TestDataBase>/mp-rest/url=<URL>
For example,
com.bluemonkeydiamond.TestDatabase/mp-rest/url=https://localhost:8080/webapp
indicates that an implementation of com.bluemonkeydiamond.TestDatabase can be
accessed at https://localhost:8080/webapp. A number of other properties will be examined in the course
of creating the proxy, including, for example
com.bluemonkeydiamond.TestDatabase/mp-rest/providers
a comma separated list of provider classes to be registered with the proxy. See the MicroProfile Client documentation for more such properties.
These properties can be simplified through the use of the configKey field in
@RegisterRestClient. For example, setting the configKey as in
@Path("database")
@RegisterRestClient(configKey="bmd")
public interface TestDataBase { ... }
allows the use of properties like
bmd/mp-rest/url=https://localhost:8080/webapp
Note that, since the configKey is not tied to a particular interface name, multiple proxies can be configured with the same properties.
Proxies should be closed so that any resources they hold can be released. Every proxy created by
RestClientBuilder implements the java.io.Closeable
interface, so it is always possible to cast a proxy to Closeable and call
close(). A nice trick to have the proxy interface explicitly extend
Closeable, which not only avoids the need for a cast but also makes the proxy
eligible to use in a try-with-resources block:
@Path("resource")
public interface TestResourceIntf extends Closeable {
@Path("test")
@GET
public String test();
}
TestResourceIntf service = RestClientBuilder.newBuilder()
.baseUrl("http://localhost:8081/")
.build(TestResourceIntf.class);
try (TestResourceIntf tr = service) {
String s = service.test();
}
An interface method can be designated as asynchronous by having it return a
java.util.concurrent.CompletionStage. For example, in
public interface TestResourceIntf extends Closeable {
@Path("test")
@GET
public String test();
@Path("testasync")
@GET
public CompletionStage<String> testAsync();
}
the test() method can be turned into the asynchronous method testAsync() by having it return
a CompletionStage<String> instead of a String.
Asynchronous methods are made to be asynchronous by scheduling their execution on a thread distinct from
the calling thread. The MicroProfile Client implementation will have a default means of doing that, but
RestClientBuilder.executorService(ExecutorService) provides a way of substituting
an application specific ExecutorService.
The classes AsyncInvocationInterceptorFactory and
AsyncInvocationInterceptor in package
org.eclipse.microprofile.rest.client.ext provides a means of communication
between the calling thread and the asynchronous thread:
public interface AsyncInvocationInterceptorFactory {
/**
* Implementations of this method should return an implementation of the
* AsyncInvocationInterceptor interface. The MP Rest Client
* implementation runtime will invoke this method, and then invoke the
* prepareContext and applyContext methods of the
* returned interceptor when performing an asynchronous method invocation.
* Null return values will be ignored.
*
* @return Non-null instance of AsyncInvocationInterceptor
*/
AsyncInvocationInterceptor newInterceptor();
}
public interface AsyncInvocationInterceptor {
/**
* This method will be invoked by the MP Rest Client runtime on the "main"
* thread (i.e. the thread calling the async Rest Client interface method)
* prior to returning control to the calling method.
*/
void prepareContext();
/**
* This method will be invoked by the MP Rest Client runtime on the "async"
* thread (i.e. the thread used to actually invoke the remote service and
* wait for the response) prior to sending the request.
*/
void applyContext();
/**
* This method will be invoked by the MP Rest Client runtime on the "async"
* thread (i.e. the thread used to actually invoke the remote service and
* wait for the response) after all providers on the inbound response flow
* have been invoked.
*
* @since 1.2
*/
void removeContext();
}
The following sequence of events occurs:
AsyncInvocationInterceptorFactory.newInterceptor() is called
on the calling thread to get an instance of the AsyncInvocationInterceptor.
AsyncInvocationInterceptor.prepareContext() is executed on the calling
thread to store information to be used by the request execution.
AsyncInvocationInterceptor.applyContext() is executed on the
asynchronous thread.
All relevant outbound providers such as interceptors and filters are executed on the asynchronous thread, followed by the request invocation.
All relevant inbound providers are executed on the asynchronous thread, followed by executing
AsyncInvocationInterceptor.removeContext()
The asynchronous thread returns.
An AsyncInvocationInterceptorFactory class is enabled by registering it on the
client interface with @RegisterProvider.
The MicroProfile Client RestClientBuilder interface includes a number
of methods that support the use of SSL:
RestClientBuilder hostnameVerifier(HostnameVerifier hostnameVerifier); RestClientBuilder keyStore(KeyStore keyStore, String keystorePassword); RestClientBuilder sslContext(SSLContext sslContext); RestClientBuilder trustStore(KeyStore trustStore);
For example:
KeyStore trustStore = ... ;
HostnameVerifier verifier ... ;
TestResourceIntf service = RestClientBuilder.newBuilder()
.baseUrl("http://localhost:8081/")
.trustStore(trustStore)
.hostnameVerifier(verifier)
.build(TestResourceIntf.class);
It is also possible to configure HostnameVerifiers,
KeyStores, and TrustStores using configuration
properties:
com.bluemonkeydiamond.TestResourceIntf/mp-rest/hostnameVerifier
com.bluemonkeydiamond.TestResourceIntf/mp-rest/keyStore
com.bluemonkeydiamond.TestResourceIntf/mp-rest/keyStorePassword
com.bluemonkeydiamond.TestResourceIntf/mp-rest/keyStoreType
com.bluemonkeydiamond.TestResourceIntf/mp-rest/trustStore
com.bluemonkeydiamond.TestResourceIntf/mp-rest/trustStorePassword
com.bluemonkeydiamond.TestResourceIntf/mp-rest/trustStoreType
The values of the ".../mp-rest/keyStore" and "../mp-rest/trustStore" parameters can be either classpath resources (e.g., "classpath:/client-keystore.jks") or files (e.g., "file:/home/user/client-keystore.jks").