Chapter 28. String marshalling for String based @*Param

Parameters and properties annotated with @CookieParam, @HeaderParam, @MatrixParam, @PathParam, or @QueryParam are represented as strings in a raw HTTP request. The specification says that any of these injected parameters can be converted to an object if the object's class has a valueOf(String) static method or a constructor that takes one Stringparameter. In the following, for example,

public static class Customer {
   private String name;

   public Customer(String name) {
      this.name = name;
   }
   
   public String getName() {
      return name;
   }
}

@Path("test")
public static class TestResource {

   @GET
   @Path("")
   public Response test(@QueryParam("cust") Customer cust) {
      return Response.ok(cust.getName()).build();
   }
}

@Test
public void testQuery() throws Exception {
   Invocation.Builder request = ClientBuilder.newClient().target("http://localhost:8081/test?cust=Bill").request();
   Response response = request.get();
   ...
}
   

the query "?cust=Bill" will be transformed automatically to an instance of Customer with name == "Bill".

What if you have a class where valueOf()or this string constructor don't exist or are inappropriate for an HTTP request? Jakarta RESTful Web Services has the jakarta.ws.rs.ext.ParamConverterProvider to help in this situation.

A ParamConverterProvider is a provider defined as follows:

public interface ParamConverterProvider {

   public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation annotations[]);
}
   

where a ParamConverter is defined:

public interface ParamConverter<T> {
   ...
   public T fromString(String value);
   public String toString(T value);
}
   

For example, consider DateParamConverterProvider and DateParamConverter:

@Provider
public class DateParamConverterProvider implements ParamConverterProvider {

   @SuppressWarnings("unchecked")
   @Override
   public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation[] annotations) {
      if (rawType.isAssignableFrom(Date.class)) {
         return (ParamConverter<T>) new DateParamConverter();
      }
      return null;
   }
}

public class DateParamConverter implements ParamConverter<Date> {

   public static final String DATE_PATTERN = "yyyyMMdd";

   @Override
   public Date fromString(String param) {
      try {
         return new SimpleDateFormat(DATE_PATTERN).parse(param.trim());
      } catch (ParseException e) {
         throw new BadRequestException(e);
      }
   }

   @Override
   public String toString(Date date) {
      return new SimpleDateFormat(DATE_PATTERN).format(date);
   }
}
   

Sending a Date in the form of a query, e.g., "?date=20161217" will cause the string "20161217" to be converted to a Date on the server.

In addition to the Jakarta RESTful Web Services jakarta.ws.rs.ext.ParamConverterProvider, RESTEasy also has its own org.jboss.resteasy.StringParameterUnmarshaller, defined

public interface StringParameterUnmarshaller<T>
{
   void setAnnotations(Annotation[] annotations);

   T fromString(String str);
}
   

It is similar to jakarta.ws.rs.ext.ParamConverter except that

@Target({ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
public @interface StringParameterUnmarshallerBinder
{
   Class<? extends StringParameterUnmarshaller> value();
}
   

For example,

   @Retention(RetentionPolicy.RUNTIME)
   @StringParameterUnmarshallerBinder(TestDateFormatter.class)
   public @interface TestDateFormat {
      String value();
   }

   public static class TestDateFormatter implements StringParameterUnmarshaller<Date> {
      private SimpleDateFormat formatter;

      public void setAnnotations(Annotation[] annotations) {
         TestDateFormat format = FindAnnotation.findAnnotation(annotations, TestDateFormat.class);
         formatter = new SimpleDateFormat(format.value());
      }

      public Date fromString(String str) {
         try {
            return formatter.parse(str);
         } catch (ParseException e) {
            throw new RuntimeException(e);
         }
      }
   }
   
   @Path("/")
   public static class TestResource {

      @GET
      @Produces("text/plain")
      @Path("/datetest/{date}")
      public String get(@PathParam("date") @TestDateFormat("MM-dd-yyyy") Date date) {
         Calendar c = Calendar.getInstance();
         c.setTime(date);
         return date.toString();
      }
   }
   

Note that the annotation @StringParameterUnmarshallerBinder on the annotation @TestDateFormat binds the formatter TestDateFormatter to a parameter annotated with @TestDateFormat. In this example, TestDateFormatter is used to format the Date parameter. Note also that the parameter "MM-dd-yyyy" to @TestDateFormat is accessible from TestDateFormatter.setAnnotations().

For parameters and properties annotated with @CookieParam, @HeaderParam, @MatrixParam, @PathParam, or @QueryParam, the Jakarta RESTful Web Services specification [https://jcp.org/aboutJava/communityprocess/final/jsr339/index.html] allows conversion as defined in the Javadoc of the corresponding annotation. In general, the following types are supported:

Items 1, 3, and 4 have been discussed above, and item 2 is obvious. Note that item 5 allows for collections of parameters. How these collections are expressed in HTTP messages depends, by default, on the particular kind of parameter. In most cases, the notation for collections is based on convention rather than a specification.

http://bluemonkeydiamond.com?q=1&q=2&q=3
   

public MultivaluedMap<String, String> getQueryParameters();
   

http://bluemonkeydiamond.com/sippycup;m=1;m=2;m=3
   

public MultivaluedMap<String, String> getRequestHeaders();
   

x-header: a, b, c
   

public Map<String, Cookie> getCookies();
   

@Path("")
public static class TestResource {

   @GET
   @Path("{segment}/{other}/{segment}/array")
   public Response getTwoSegmentsArray(@PathParam("segment") PathSegment[] segments) {
      System.out.println("array segments: " + segments.length);
      return Response.ok().build();
   }
   
   @GET
   @Path("{segment}/{other}/{segment}/list")
   public Response getTwoSegmentsList(@PathParam("segment") List<PathSegment> segments) {
      System.out.println("list segments: " + segments.size());
      return Response.ok().build();
   }
}

...

   @Test
   public void testTwoSegmentsArray() throws Exception {
      Invocation.Builder request = client.target("http://localhost:8081/a/b/c/array").request();
      Response response = request.get();
      Assert.assertEquals(200, response.getStatus());
      response.close();
   }
   
   @Test
   public void testTwoSegmentsList() throws Exception {
      Invocation.Builder request = client.target("http://localhost:8081/a/b/c/list").request();
      Response response = request.get();
      Assert.assertEquals(200, response.getStatus());
      response.close();
   }
   

array segments: 2
list segments: 2
   

@Path("")
public static class TestResource {

   @GET
   @Path("{segments:.*}/array")
   public Response getWildcardArray(@PathParam("segments") PathSegment[] segments) {
      System.out.println("array segments: " + segments.length);
      return Response.ok().build();
   }
   
   @GET
   @Path("{segments:.*}/list")
   public Response getWildcardList(@PathParam("segments") List<PathSegment> segments) {
      System.out.println("list segments: " + segments.size());
      return Response.ok().build();
   }
   
...

   @Test
   public void testWildcardArray() throws Exception {
      Invocation.Builder request = client.target("http://localhost:8081/a/b/c/array").request();
      Response response = request.get();
      response.close();
   }
   
   @Test
   public void testWildcardList() throws Exception {
      Invocation.Builder request = client.target("http://localhost:8081/a/b/c/list").request();
      Response response = request.get();
      response.close();
   }
   

array segments: 3
list segments: 3
   

In the Jakarta RESTful Web Services semantics, a ParamConverter is supposed to convert a single String that represents an individual object. RESTEasy extends the semantics to allow a ParamConverter to parse the String representation of multiple objects and generate a List<T>, Set<T>, SortedSet<T>, array, or, indeed, any multivalued data structure whatever. First, consider the resource

@Path("queryParam")
public static class TestResource {

   @GET
   @Path("")
   public Response conversion(@QueryParam("q") List<String> list) {
      return Response.ok(stringify(list)).build();
   }
}

private static <T> String stringify(List<T> list) {
   StringBuffer sb = new StringBuffer();
   for (T s : list) {
      sb.append(s).append(',');
   }
   return sb.toString();
}
   

Calling TestResource as follows, using the standard notation,

@Test
public void testQueryParamStandard() throws Exception {
   Client client = ClientBuilder.newClient();
   Invocation.Builder request = client.target("http://localhost:8081/queryParam?q=20161217&q=20161218&q=20161219").request();
   Response response = request.get();
   System.out.println("response: " + response.readEntity(String.class));
}
   

results in

response: 20161217,20161218,20161219,
   

Suppose, instead, that we want to use a comma separated notation. We can add

public static class MultiValuedParamConverterProvider implements ParamConverterProvider

   @SuppressWarnings("unchecked")
   @Override
   public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation[] annotations) {
      if (List.class.isAssignableFrom(rawType)) {
         return (ParamConverter<T>) new MultiValuedParamConverter();
      }
      return null;
   }   
}

public static class MultiValuedParamConverter implements ParamConverter<List<?>> {

   @Override
   public List<?> fromString(String param) {
      if (param == null || param.trim().isEmpty()) {
         return null;
      }
      return parse(param.split(","));
   }

   @Override
   public String toString(List<?> list) {
      if (list == null || list.isEmpty()) {
         return null;
      }
      return stringify(list);
   }
   
   private static List<String> parse(String[] params) {
      List<String> list = new ArrayList<String>();
      for (String param : params) {
         list.add(param);
      }
      return list;
   }
}
   

Now we can call

@Test
public void testQueryParamCustom() throws Exception {
   Client client = ClientBuilder.newClient();
   Invocation.Builder request = client.target("http://localhost:8081/queryParam?q=20161217,20161218,20161219").request();
   Response response = request.get();
   System.out.println("response: " + response.readEntity(String.class));
}
   

and get

response: 20161217,20161218,20161219,
   

Note that in this case, MultiValuedParamConverter.fromString() creates and returns an ArrayList, so TestResource.conversion() could be rewritten

@Path("queryParam")
public static class TestResource {

   @GET
   @Path("")
   public Response conversion(@QueryParam("q") ArrayList<String> list) {
      return Response.ok(stringify(list)).build();
   }
}
   

On the other hand, MultiValuedParamConverter could be rewritten to return a LinkList and the parameter list in TestResource.conversion() could be either a List or a LinkedList.

Finally, note that this extension works for arrays as well. For example,

  public static class Foo {
      private String foo;
      public Foo(String foo) {this.foo = foo;}
      public String getFoo() {return foo;}
   }
   
   public static class FooArrayParamConverter implements ParamConverter<Foo[]> {

      @Override
      public Foo[] fromString(String value)
      {
         String[] ss = value.split(",");
         Foo[] fs = new Foo[ss.length];
         int i = 0;
         for (String s : ss) {
            fs[i++] = new Foo(s);
         }
         return fs;
      }

      @Override
      public String toString(Foo[] values)
      {
         StringBuffer sb = new StringBuffer();
         for (int i = 0; i < values.length; i++) {
            sb.append(values[i].getFoo()).append(",");
         }
         if (sb.length() > 0) {
            sb.deleteCharAt(sb.length() - 1);
         }
         return sb.toString();
      }
   }
   
   @Provider
   public static class FooArrayParamConverterProvider implements ParamConverterProvider {

      @SuppressWarnings("unchecked")
      @Override
      public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType, Annotation[] annotations) {
         if (rawType.equals(Foo[].class));
         return (ParamConverter<T>) new FooArrayParamConverter();
      }
   }
   
   @Path("")
   public static class ParamConverterResource {

      @GET
      @Path("test")
      public Response test(@QueryParam("foos") Foo[] foos) {
         return Response.ok(new FooArrayParamConverter().toString(foos)).build();
      }
   }
   

RESTEasy includes two built-in ParamConverters in the resteasy-core module, one for Collections:

   org.jboss.resteasy.plugins.providers.MultiValuedCollectionParamConverter,

and one for arrays:

   org.jboss.resteasy.plugins.providers.MultiValuedArrayParamConverter,

which implement the concepts in the previous section.

In particular, MultiValued*ParamConverter.fromString() can transform a string representation coming over the network into a Collection or array, and MultiValued*ParamConverter.toString() can be used by a client side proxy to transform Collections or arrays into a string representation.

String representations are determined by org.jboss.resteasy.annotations.Separator, a parameter annotation in the resteasy-core module:

@Target({ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface Separator
{
   public String value() default "";
}

The value of Separator.value() is used to separate individual elements of a Collection or array. For example, a proxy implementing

@Path("path/separator/multi/{p}")
@GET
public String pathMultiSeparator(@PathParam("p") @Separator("-") List<String> ss); 

will turn

List<String> list = new ArrayList<String>();
list.add("abc");
list.add("xyz");
proxy.pathMultiSeparator(list);

and "path/separator/multi/{p}" into ".../path/separator/multi/abc-xyz". On the server side, the RESTEasy runtime will turn "abc-xyz" back into a list consisting of elements "abc" and "xyz" for

@Path("path/separator/multi/{p}")
@GET
public String pathMultiSeparator(@PathParam("p") @Separator("-") List<String> ss) {
   StringBuffer sb = new StringBuffer();
   for (String s : ss) {
      sb.append(s);
      sb.append("|");
   }
   return sb.toString();
}

which will return "abc|xyz|".

In fact, the value of the Separator annotations may be a more general regular expression, which is passed to String.split(). For example, "[-,;]" tells the server side to break up a string using either "-", ",", or ";". On the client side, a string will be created using the first element, "-" in this case.

If a parameter is annotated with @Separator with no value, then the default value is

The MultiValued*ParamConverters depend on existing facilities for handling the individual elements. On the server side, once it has parsed the incoming string into substrings, MultiValued*ParamConverter turns each substring into an Java object according to Section 3.2 "Fields and Bean Properties" of the Jakarta RESTful Web Services specification. On the client side, MultiValued*ParamConverter turns a Java object into a string as follows:

These ParamConverters are meant to be fairly general, but there are a number of restrictions:

There are also some logical restrictions:

These built-in ParamConverters have the lowest priority, so any user supplied ParamConverters will be tried first.