In an earlier post, I blogged about the REST capabilities we added to Spring @MVC version 3.0. Later, Alef wrote about using the introduced functionality to add an Atom view to the Pet Clinic application. In this post, I would like to introduce the client-side capabilities we added in Milestone 2.

RestTemplate

The RestTemplate is the central Spring class for client-side HTTP access. Conceptually, it is very similar to the JdbcTemplate, JmsTemplate, and the various other templates found in the Spring Framework and other portfolio projects. This means, for instance, that the RestTemplate is thread-safe once constructed, and that you can use callbacks to customize its operations.

RestTemplate Methods

The main entry points of the template are named after the six main HTTP methods:

HTTP	RestTemplate
DELETE	delete(String, String...)
GET	getForObject(String, Class, String...)
HEAD	headForHeaders(String, String...)
OPTIONS	optionsForAllow(String, String...)
POST	postForLocation(String, Object, String...)
PUT	put(String, Object, String...)

The names of these methods clearly indicate which HTTP method they invoke, while the second part of the name indicates what is returned. For instance, getForObject() will perform a GET, convert the HTTP response into an object type of your choice, and returns that object. postForLocation will do a POST, converting the given object into a HTTP request, and returns the response HTTP Location header where the newly created object can be found. As you can see, these methods try to enforce REST best practices.

URI Templates

Each of these methods takes a URI as first argument. That URI can be a URI template, and variables can be used to expand the template to a normal URI. The template variables can be passed in two forms: as a String variable arguments array, or as a Map. The string varargs variant expands the given template variables in order, so that

String result = restTemplate.getForObject("http://example.com/hotels/{hotel}/bookings/{booking}", String.class, "42", "21");

will perform a GET on http://example.com/hotels/42/bookings/21. The map variant expands the template based on variable name, and is therefore more useful when using many variables, or when a single variable is used multiple times. For example:

Map<String, String> vars = new HashMap<String, String>();
vars.put("hotel", "42");
vars.put("booking", "21");
String result = restTemplate.getForObject("http://example.com/hotels/{hotel}/bookings/{booking}", String.class, vars);

will also perform a GET on http://example.com/hotels/42/rooms/42.

HttpMessageConverters

Objects passed to and returned from the methods getForObject(), postForLocation(), and put() and are converted to HTTP requests and from HTTP responses by HttpMessageConverters. Converters for the main mime types and Java types are registered by default, but you can also write your own converter and plug it in the RestTemplate. In the example below, I will show you how that's done.

Using the RestTemplate to retrieve photos from Flickr

Rather than going through the various methods of the RestTemplate, I will show you how to use it for retrieving pictures from Flickr, Yahoo!s online photo-sharing application. This sample application searches Flickr for photos that match a given search term. It then shows these pictures using a simple Swing UI. To run the application yourself, you will need to create a Flickr account and apply for an API key.

Searching for photos

Flickr exposes various APIs to manipulate its vast library of photos. The flickr.photos.search method allows you to search for photos, by issuing a GET request on http://www.flickr.com/services/rest?method=flickr.photos.search&api+key=xxx&tags=penguins, where you enter your API key and the thing to search for (penguins in this case). As a result, you get back a XML document, describing the photos that conform to your query. Something like:

<photos page="2" pages="89" perpage="10" total="881">
	<photo id="2636" owner="47058503995@N01"
		secret="a123456" server="2" title="test_04"
		ispublic="1" isfriend="0" isfamily="0" />
	<photo id="2635" owner="47058503995@N01"
		secret="b123456" server="2" title="test_03"
		ispublic="0" isfriend="1" isfamily="1" />
	<photo id="2633" owner="47058503995@N01"
		secret="c123456" server="2" title="test_01"
		ispublic="1" isfriend="0" isfamily="0" />
	<photo id="2610" owner="12037949754@N01"
		secret="d123456" server="2" title="00_tall"
		ispublic="1" isfriend="0" isfamily="0" />
</photos>

Using the RestTemplate, retrieving such a document is quite trivial:

final String photoSearchUrl =
   "http://www.flickr.com/services/rest?method=flickr.photos.search&api+key={api-key}&tags={tag}&per_page=10";
Source photos = restTemplate.getForObject(photoSearchUrl, Source.class, apiKey, searchTerm);

where apiKey and searchTerm are two Strings given on the command line. This method uses the SourceHttpMessageConverter to convert the HTTP XML response into a javax.xml.transform.Source (Note that the SourceHttpMessageConverter was introduced shortly after we released Spring 3.0 M2, so you will have to get a recent snapshot (or the upcoming M3) to use it. The sample project available below is set up to retrieve these via Maven).

Retrieving the photos

Next, we're going to use an XPath expression to retrieve all the photo elements of the document. For this, we are going to use the XPathTemplate from Spring Web Services. We are going to execute the //photo expressions, returning all photo elements occurring anywhere in the document. The NodeMapper is a callback interface, whose mapNode() method will be invoked for each photo element in the document. In this case, we are retrieving the server, id, and secret attributes of this element, and use those to fill up a Map. Finally, we use the RestTemplate again, to retrieve the photo as a java.awt.image.BufferedImage. Thus when the XPath evaluation is done, the resulting imageList will contain an image for each photo in the XML document.

List<BufferedImage> imageList = xpathTemplate.evaluate("//photo", photos, new NodeMapper() {
    public Object mapNode(Node node, int i) throws DOMException {
        Element photo = (Element) node;

        Map<String, String> variables = new HashMap<String, String>(3);
        variables.put("server", photo.getAttribute("server"));
        variables.put("id", photo.getAttribute("id"));
        variables.put("secret", photo.getAttribute("secret"));

        String photoUrl = "http://static.flickr.com/{server}/{id}_{secret}_m.jpg";
        return restTemplate.getForObject(photoUrl, BufferedImage.class, variables);
    }
});

For instance, given the XML document given above, the imageList will contain 4 images. The URL for the first image retrieved will be http://static.flickr.com/2/2636_ a123456_m.jpg, the second is http://static.flickr.com/2/2635_ b123456_m.jpg, etc.

Converting the images

There is one more thing that needs to be done in order for the code to work: we will need to write a HttpMessageConverter that is able to read from the HTTP response, and create a BufferedImagefrom that. Doing so with the Java Image I/O API is fairly simple, we just need to implement the read() method defined in the HttpMessageConverter interface. Overall, our simple converter looks like this:

public class BufferedImageHttpMessageConverter implements HttpMessageConverter<BufferedImage> {

    public List<MediaType> getSupportedMediaTypes() {
        return Collections.singletonList(new MediaType("image", "jpeg"));
    }

    public boolean supports(Class<? extends BufferedImage> clazz) {
        return BufferedImage.class.equals(clazz);
    }

    public BufferedImage read(Class<BufferedImage> clazz, HttpInputMessage inputMessage) throws IOException {
        return ImageIO.read(inputMessage.getBody());
    }

    public void write(BufferedImage image, HttpOutputMessage message) throws IOException {
        throw new UnsupportedOperationException("Not implemented");
    }

}

Note that we didn't implement write() because we are not uploading images, just downloading them. Now we just have to plug this converter into the RestTemplate. We do that in the Spring application context:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <bean id="flickrClient" class="com.springsource.samples.resttemplate.FlickrClient">
        <constructor-arg ref="restTemplate"/>
        <constructor-arg ref="xpathTemplate"/>
    </bean>

    <bean id="restTemplate" class="org.springframework.web.client.RestTemplate">
        <property name="messageConverters">
            <list>
                <bean class="org.springframework.http.converter.xml.SourceHttpMessageConverter"/>
                <bean class="com.springsource.samples.resttemplate.BufferedImageHttpMessageConverter"/>
            </list>
        </property>
    </bean>

    <bean id="xpathTemplate" class="org.springframework.xml.xpath.Jaxp13XPathTemplate"/>

</beans>

Showing the photos

The final stage is to show the photos in a simple GUI. For this, we use Swing:

JFrame frame = new JFrame(searchTerm + " photos");
frame.setLayout(new GridLayout(2, imageList.size() / 2));
for (BufferedImage image : imageList) {
    frame.add(new JLabel(new ImageIcon(image)));
}
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.pack();
frame.setVisible(true);

which gives us the following:

Overall, I hope this post showed you how simple it can be to use the RestTemplate to interact with HTTP servers. In just under 30 lines of Java code, we created a GUI that shows pictures of everybody's favorite bird: the penguin! Check out the RestTemplate and let us know what you think!

Downloads

A Maven project containing the code above can be downloaded here. Note that the project is based on a nightly snapshot build of Spring. The upcoming Milestone 3 of Spring will contain the necessary classes as well.

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