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## Chapter 28. String marshalling for String based @*Param

## 28.1. Simple conversion

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 `String`parameter. 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".

## 28.2. ParamConverter

What if you have a class where `valueOf()`or this string constructor don't exist or are inappropriate for an HTTP request? JAX-RS 2.0 has the `javax.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.

## 28.3. StringParameterUnmarshaller

In addition to the JAX-RS `javax.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 `javax.ws.rs.ext.ParamConverter` except that

- it converts only from `String`s;
- it is configured with the annotations on the injected parameter, which allows for fine-grained control over the injection; and
- it is bound to a given parameter by an annotation that is annotated with the meta-annotation `org.jboss.resteasy.annotations.StringParameterUnmarshallerBinder:`

    @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()`.

## 28.4. Collections

For parameters and properties annotated with `@CookieParam`, `@HeaderParam`, `@MatrixParam`, `@PathParam,` or `@QueryParam`, the JAX-RS 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:

1. Types for which a `ParamConverter` is available via a registered `ParamConverterProvider`. See Javadoc for these classes for more information.
2. Primitive types.
3. Types that have a constructor that accepts a single `String` argument.
4. Types that have a static method named `valueOf` or `fromString` with a single `String` argument that return an instance of the type. If both methods are present then `valueOf` MUST be used unless the type is an enum in which case `fromString` MUST be used.
5. List<T>, Set<T>, or SortedSet<T>, where T satisfies 3 or 4 above.

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.

### 28.4.1. @QueryParam

For example, a multivalued query parameter is conventionally expressed like this:

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

In this case, there is a query with name "q" and value {1, 2, 3}. This notation is further supported in JAX-RS by the method

    public MultivaluedMap<String, String> getQueryParameters();

in `javax.ws.rs.core.UriInfo`.

### 28.4.2. @MatrixParam

There is no specified syntax for collections derived from matrix parameters, but

1. matrix parameters in a URL segment are conventionally separated by ";", and
2. the method

       MultivaluedMap<String, String> getMatrixParameters();

   in `javax.ws.rs.core.PathSegment` supports extraction of collections from matrix parameters.

RESTEasy adopts the convention that multiple instances of a matrix parameter with the same name are treated as a collection. For example,

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

is interpreted as a matrix parameter on path segment "sippycup" with name "m" and value {1, 2, 3}.

### 28.4.3. @HeaderParam

The HTTP 1.1 specification doesn't exactly specify that multiple components of a header value should be separated by commas, but commas are used in those headers that naturally use lists, e.g. Accept and Allow. Also, note that the method

    public MultivaluedMap<String, String> getRequestHeaders();

in `javax.ws.rs.core.HttpHeaders` returns a `MultivaluedMap`. It is natural, then, for RESTEasy to treat

    x-header: a, b, c

as mapping name "x-header" to set {a, b, c}.

### 28.4.4. @CookieParam

The syntax for cookies is specified, but, unfortunately, it is specified in multiple competing specifications. Typically, multiple name=value cookie pairs are separated by ";". However, unlike the case with query and matrix parameters, there is no specified JAX-RS method that returns a collection of cookie values. Consequently, if two cookies with the same name are received on the server and directed to a collection typed parameter, RESTEasy will inject only the second one. Note, in fact, that the method

    public Map<String, Cookie> getCookies();

in `javax.ws.rs.core.HttpHeaders` returns a `Map` rather than a `MultivaluedMap`.

### 28.4.5. @PathParam

Deriving a collection from path segments is somewhat less natural than it is for other parameters, but JAX-RS supports the injection of multiple `javax.ws.rs.core.PathSegment`s. There are a couple of ways of obtaining multiple `PathSegment`s. One is through the use of multiple path variables with the same name. For example, the result of calling `testTwoSegmentsArray()` and `testTwoSegmentsList()` in

    @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();
       }

is

    array segments: 2
    list segments: 2

An alternative is to use a wildcard template parameter. For example, the output of calling `testWildcardArray()` and `testWildcardList()` in

    @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();
       }

is

    array segments: 3
    list segments: 3

## 28.5. Extension to `ParamConverter` semantics

In the JAX-RS 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();
          }
       }

## 28.6. Default multiple valued `ParamConverter`

RESTEasy includes two built-in `ParamConverter`s in the resteasy-jaxrs module, one for `Collection`s:

       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 `Collection`s 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

- "," for a `@HeaderParam`, `@MatrixParam`, `@PathParam`, or `@QueryParam`, and
- "-" for a `@CookieParam`.

The `MultiValued*ParamConverter`s 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 JAX-RS specification. On the client side, `MultiValued*ParamConverter` turns a Java object into a string as follows:

1. look for a `ParamConverter`;
2. if there is no suitable `ParamConverter` and the parameter is labeled `@HeaderParam`, look for a `HeaderDelegate`; or
3. call `toString()`.

These `ParamConverter`s are meant to be fairly general, but there are a number of restrictions:

1. They don't handle nested `Collections` or arrays. That is, `List<String>` and `String[]` are OK, but `List<List<String>>` and `String[][]` are not.
2. The regular expression used in `Separator` must match the regular expression

       "\\p{Punct}|\\[\\p{Punct}+\\]"

   That is, it must be either a single instance of a punctuation symbol, i.e., a symbol in the set

       !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~

   or a class of punctuation symbols like "[-,;]".
3. For either of these `ParamConverter`s to be available for use with a given parameter, that parameter must be annotated with `@Separator`.

There are also some logical restrictions:

1. Cookie syntax, as specified in https://tools.ietf.org/html/rfc6265#section-4.1.1, assigns a meaning to ";", so it cannot be used as a separator.
2. If a separator character appears in the content of an element, then there will be problems. For example, if "," is used as a separator, then, if a proxy sends the array `["a","b,c","d"]`, it will turn into the string "a,b,c,d" on the wire and be reconstituted on the server as four elements.

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

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