#54 - Added support for RFC5988 link headers.

Introduced new Links container element to group a list of links and allow parsing an RFC5988 compatible String representation of links into it. The parsing mechanism is currently regarding the rel attribute only.

Added valueOf(…) factory methods to allow Spring MVC to pick the types up out of the box.

src/main/java/org/springframework/hateoas/Link.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2012 the original author or authors.
+ * Copyright 2012-2013 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,11 +16,17 @@
 package org.springframework.hateoas;
 
 import java.io.Serializable;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
 
 import javax.xml.bind.annotation.XmlAttribute;
 import javax.xml.bind.annotation.XmlType;
 
 import org.springframework.util.Assert;
+import org.springframework.util.StringUtils;
 
 /**
  * Value object for links.
@@ -115,7 +121,8 @@ public class Link implements Serializable {
 		return this.href.equals(that.href) && this.rel.equals(that.rel);
 	}
 
-	/* (non-Javadoc)
+	/* 
+	 * (non-Javadoc)
 	 * @see java.lang.Object#hashCode()
 	 */
 	@Override
@@ -133,6 +140,62 @@ public class Link implements Serializable {
 	 */
 	@Override
 	public String toString() {
-		return String.format("{ rel : %s, href : %s }", rel, href);
+		return String.format("<%s>;rel=\"%s\"", href, rel);
+	}
+
+	/**
+	 * Factory method to easily create {@link Link} instances from RFC-5988 compatible {@link String} representations of a
+	 * link. Will return {@literal null} if an empty or {@literal null} {@link String} is given.
+	 * 
+	 * @param element an RFC-5899 compatible representation of a link.
+	 * @throws IllegalArgumentException if a non-empty {@link String} was given that does not adhere to RFC-5899.
+	 * @throws IllegalArgumentException if no {@code rel} attribute could be found.
+	 * @return
+	 */
+	public static Link valueOf(String element) {
+
+		if (!StringUtils.hasText(element)) {
+			return null;
+		}
+
+		Pattern uriAndAttributes = Pattern.compile("<(.*)>;(.*)");
+		Matcher matcher = uriAndAttributes.matcher(element);
+
+		if (matcher.find()) {
+
+			Map<String, String> attributes = getAttributeMap(matcher.group(2));
+
+			if (!attributes.containsKey("rel")) {
+				throw new IllegalArgumentException("Link does not provide a rel attribute!");
+			}
+
+			return new Link(matcher.group(1), attributes.get("rel"));
+
+		} else {
+			throw new IllegalArgumentException(String.format("Given link header %s is not RFC5988 compliant!", element));
+		}
+	}
+
+	/**
+	 * Parses the links attributes from the given source {@link String}.
+	 * 
+	 * @param source
+	 * @return
+	 */
+	private static Map<String, String> getAttributeMap(String source) {
+
+		if (!StringUtils.hasText(source)) {
+			return Collections.emptyMap();
+		}
+
+		Map<String, String> attributes = new HashMap<String, String>();
+		Pattern keyAndValue = Pattern.compile("(\\w+)=\\\"(\\p{Alnum}*)\"");
+		Matcher matcher = keyAndValue.matcher(source);
+
+		while (matcher.find()) {
+			attributes.put(matcher.group(1), matcher.group(2));
+		}
+
+		return attributes;
 	}
 }