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DynamicObject is a library that makes Clojure's powerful data modeling capabilities available to Java developers in an idiomatic way with minimal boilerplate. It reflects the belief that values should be immutable, cheap to specify, powerful to work with, and easy to convey to other processes. Browse the Javadoc online.

The Problem

The Java programming language is flawlessly incompetent at representing data. Classes are the main programming language construct in Java, and the entire point of classes is to combine data and code such that data is "encapsulated" and can only be accessed through an operational interface, if at all. Creating a class that does nothing but hold data requires the creation of pages of boilerplate--getters, setters, constructors, clone, equals, hashCode, toString, and so on. Not only is this labor-intensive to do by hand, it's surprisingly error prone: copy and paste can result in getters/setters touching the wrong fields, methods like equals can be overlooked when a new field is added, pervasive unmoderated mutability is the default, the equals/hashCode/compareTo contract is far more subtle than most realize, implementation inheritance is almost intractably complex, and concurrency and serialization have to be figured out from scratch.

There have been countless attempts to address various subsets (or supersets) of these problems: IDE code generation, compile-time code generation, object mapping, test case generators, annotation processors, public fields, and so on. These tools are usually helpful in the right context, but they're really only workarounds for a fundamentally broken approach to working with data. Instead of trying to fix a broken approach, why not switch to a better one altogether?

Anyone who has used Clojure's data structures knows that there is a night-and-day difference between the Java and Clojure approaches to modeling data, and that Clojure's approach yields dramatically more leverage. One reason for Clojure's flexibility and power in working with data is its philosophy that data belongs in maps. Immutable persistent maps can be safely shared, compose well, can be sent to other programs (serialized and deserialized), are expressive enough to model almost anything, and can be generically processed by a collections library, which makes for excellent code reuse. Unfortunately, Clojure is radically different from what most developers are familiar with, in both superficial and profound ways. As a consequence, Clojure adoption is usually an uphill battle at best, and outright impossible at worst. But is it possible to get the benefits of Clojure without writing a line of Clojure code?

Because Clojure targets the JVM, it is possible for Java shops to use Clojure data without actually writing any Clojure code. However, there is a significant impedance mismatch between the two languages, and trying to call Clojure directly from Java results in a number of ergonomic problems, such as loss of type information, pervasive downcasting, and a Java API that is largely internal and undocumented. The goal of DynamicObject is to bridge the gap between the two languages and retain the advantages of both.

A Simple Example

Consider a Clojure map that describes an album:

{:artist "Meshuggah", :album "Chaosphere", :tracks 8, :year 1998}

Getting access to this data from Java is kind of a mess if you go directly through the Clojure runtime. It might look something like this:

String edn = "{:artist \"Meshuggah\", :album \"Chaosphere\", :tracks 8, :year 1998}";
Map albumDescription = (Map) Clojure.var("clojure.edn/read-string").invoke(edn);
String album = (String) albumDescription.get(":album"));

This is where DynamicObject comes in. The first thing to do is to create an Album type:

public interface Album extends DynamicObject<Album> {
  @Key(":artist") String getArtist();
  @Key(":album")  String getAlbum();
  @Key(":tracks") int getTracks();
  @Key(":year")   int getYear();

  @Key(":artist") Album withArtist(String artist);
  @Key(":album")  Album withAlbum(String album);
  @Key(":tracks") Album withTracks(int tracks);
  @Key(":year")   Album withYear(int year);

The purpose of this type is not to create a class that laboriously defines operations on data, but rather to establish a schema that tells DynamicObject which types are associated with each key. With DynamicObject, the above deserialization example looks like this:

String edn = "{:artist \"Meshuggah\", :album \"Chaosphere\", :tracks 8, :year 1998}";
Album album = DynamicObject.deserialize(edn, Album.class);
String artist = album.getArtist();
assertEquals("Meshuggah", artist);

DynamicObjects can also be constructed in-memory through the use of builder methods (prefixed with with- in the above schema).

Album album = DynamicObject.newInstance(Album.class)
// {:year 1998, :tracks 8, :album "Chaosphere", :artist "Meshuggah"}


Some More Examples

Creating DynamicObject types

A DynamicObject type is a Java interface. The simplest possible DynamicObject type is this:

public interface Empty extends DynamicObject<Empty> {}

Although this type doesn't declare any particular fields, it is still usable as a map. For example:

Empty empty = DynamicObject.deserialize("{144 233}", Empty.class);

assertTrue(empty instanceof Map);
assertEquals(Long.valueOf(233), empty.get(144));
assertEquals(1, empty.size());

In addition to the interface itself, it is strongly recommended that reader tags be created for each DynamicObject type that will be serialized. Otherwise, serialized DynamicObjects look indistinguishable from regular maps, and the full type information cannot be recovered without some additional out-of-band information (such as the second argument to deserialize).

public interface T1 extends DynamicObject<T1> {}
public interface T2 extends DynamicObject<T2> {}

DynamicObject.registerTag(T1.class, "T1");
DynamicObject.registerTag(T2.class, "T2");

// Note the lack of type hints given to deserialize:
assertTrue(DynamicObject.deserialize("#T1{}", Object.class) instanceof T1);
assertTrue(DynamicObject.deserialize("#T2{}", Object.class) instanceof T2);

Extending Edn

In addition to Edn's built-in data types (sets, maps, vectors, #inst, #uuid, and so forth), DynamicObject offers full support for reader tags, Edn's extension mechanism. This makes it possible to include any Java value class in a DynamicObject without compromising serializability or requiring any modifications to the class. This is done through the EdnTranslator mechanism.

For instance, consider the following Edn data:

{:name "Mike Jones",
 :phone #phonenumber "(330) 281-8004"}

The :name field is an ordinary string, but the :phone field is tagged. When we deserialize this data, we don't want the phone number to be returned as a string, but rather as an instance of the following type:

public class PhoneNumber {
  public final String areaCode;
  public final String firstThree;
  public final String lastFour;

  // constructor, etc omitted

To do this, we'll define a class that translates this type to and from Edn:

public class PhoneNumberTranslator implements EdnTranslator<PhoneNumber> {
  // Translate the tagged Edn data (in this case, a String) to our actual type.
  public PhoneNumber read(Object obj) {
    String str = (String) obj;
    Pattern pattern = Pattern.compile("\\((\\d+)\\) (\\d+)-(\\d+)");
    Matcher matcher = pattern.matcher(str);
    if (!matcher.matches())
      throw new IllegalArgumentException("Malformed phone number: " + str);
    return new PhoneNumber(,,;

  // Return an Edn string representing the serialized phone number.
  public void write(PhoneNumber phoneNumber) {
    return String.format("\"(%s) %s-%s\"",
      phoneNumber.areaCode, phoneNumber.firstThree, phoneNumber.lastFour);

  // Return the reader tag associated with this type.
  public String getTag() {
    return "phonenumber";

Finally, we'll register this translator with DynamicObject:

DynamicObject.registerType(PhoneNumber.class, new PhoneNumberTranslator());

The PhoneNumber type is now fully interoperable with DynamicObject and Edn serialization. It can even be deserialized à la carte:

PhoneNumber actual = DynamicObject.deserialize("#phonenumber \"(330) 281-8004\"", PhoneNumber.class);
PhoneNumber expected = new PhoneNumber("330", "281", "8004");
assertEquals(expected, actual);

Fressian serialization

DynamicObject exposes Fressian's FressianReader and FressianWriter classes directly, in order to maximize flexibility. Instances of these classes are obtained by calling createFressianReader and createFressianWriter. These methods return readers and writers that understand:

Convenience methods are also offered: toFressianByteArray takes an Object and returns a byte array, and fromFressianByteArray does the reverse.

Conceptually, extending Fressian is no different from extending Edn. The PhoneNumber class from above can be translated to and from Fressian like so:

public class PhoneNumberEncoder implements ReadHandler, WriteHandler {
  public Object read(Reader r, Object tag, int componentCount) throws IOException {
    String areaCode = (String) r.readObject();
    String firstThree = (String) r.readObject();
    String lastFour = (String) r.readObject();
    return new PhoneNumber(areaCode, firstThree, lastFour);

  public void write(Writer w, Object instance) throws IOException {
    PhoneNumber phoneNumber = (PhoneNumber) instance;
    // In addition to the reader tag, we must specify how many fields we will write.
    w.writeTag("phonenumber", 3);

As with Edn, this type must be registered with DynamicObject:

PhoneNumberEncoder encoder = new PhoneNumberEncoder();
DynamicObject.registerType(PhoneNumber.class, "phonenumber", encoder, encoder);

PhoneNumber before = new PhoneNumber("123", "456", "7890");
byte[] byteArray = DynamicObject.toFressianByteArray(PhoneNumber);
PhoneNumber after = DynamicObject.fromFressianByteArray(byteArray);
assertEquals(before, after);

Schema Validation

Traditional object mappers pretend to transparently put static types on the wire. As a consequence, it is difficult to get them to accept any data that does not exactly match the object type they are expecting to see. For instance, if they see an unknown field, they will likely discard it, or they might just throw an exception, causing deserialization to fail altogether. The problem is usually not obvious right away; generally, it only becomes obvious once the software is running in production and needs to accommodate changes.

DynamicObject takes a completely different approach in which deserialization and validation are decoupled into two separate phases, each of which is independently available to the user. Any well-formed Edn data can be deserialized into any given DynamicObject type, and all of the data that was present on the wire will be preserved in its entirety in memory. Validation of the data can then proceed as a separate step. (Note that validation can also be performed on objects that were created using builders, rather than deserialized. This can be a way to ensure that none of the @Required fields were overlooked during construction.)

Validation checks that all @Required fields are present (they must not be null), and that all of the types are correct. Successful validation is a guarantee that any getter method can be invoked without resulting in a ClassCastException. For example, consider the following type:

interface Validated extends DynamicObject<Validated> {
  @Required int x();
  @Required int y();
  String str();

After deserializing instances of this type, we can use validation to ensure that they are correct. The validate() method will throw an exception if an instance doesn't validate. The exception message will give a detailed description of what went wrong:

DynamicObject.deserialize("{}", Validated.class).validate();
//  Exception in thread "main" java.lang.IllegalStateException: The following @Required fields were missing: x, y

DynamicObject.deserialize("{:x 1, :y 2, :str 3}", Validated.class).validate();
//  Exception in thread "main" java.lang.IllegalStateException: The following fields had the wrong type:
//    str (expected String, got Long)

DynamicObject.deserialize("{:x 1, :y 2, :str \"hello\"}", Validated.class).validate();
//  Success!

It is possible to add custom validation logic to a type by implementing validate() as a custom method. For example:

interface Custom extends DynamicObject<Custom> {
  @Required int oddsOnly();

  default Custom validate() {
    if (oddsOnly() % 2 == 0)
      throw new IllegalStateException("Odd number expected");
    return this;

This validation logic will be run in addition to the standard validation checks:

DynamicObject.deserialize("{:oddsOnly 4}", Custom.class).validate();
//  Exception in thread "main" java.lang.IllegalStateException: Odd number expected

DynamicObject.deserialize("{:oddsOnly nil}", Custom.class).validate();
//  Exception in thread "main" java.lang.IllegalStateException: The following @Required fields were missing: oddsOnly

DynamicObject.deserialize("{:oddsOnly 5}", Custom.class).validate();
//  Success!

Persistent Modification

DynamicObject makes it easy to leverage Clojure's immutable persistent data structures, which use structural sharing to enable cheap copying and "modification." A DynamicObject can declare builder methods, which are backed by assoc. These methods perform a functional update of the data structure, returning an updated instance and leaving the current instance unchanged. For example:

interface Buildable extends DynamicObject<Buildable> {
  @Key(":hello") String getHello();
  @Key(":hello") Buildable withHello(String hello);

public void invokeBuilderMethod() {
  Buildable hw = DynamicObject.newInstance(Buildable.class).withHello("world");
  assertEquals("{:hello \"world\"}", DynamicObject.serialize(hw)); // This is the original value

  Buildable hk = hw.withHello("kitty"); // Build a new value from the old one
  assertEquals("{:hello \"kitty\"}", DynamicObject.serialize(hk));
  assertEquals("{:hello \"world\"}", DynamicObject.serialize(hw)); // The original value is unchanged


DynamicObject allows direct access to Clojure's metadata facilities with the @Meta annotation. This allows information to be annotated in arbitrary ways without this information being part of the data itself. For example, if you're using DynamicObject to communicate across processes using a distributed queue like SQS, metadata is a great place to stash information about the messages themselves, such as the message receipt handle:

interface WorkerJob extends DynamicObject<WorkerJob> {
  UUID jobId();
  String inputLocation();
  // and so forth
  @Meta long messageAgeInSeconds(); // Use this for visibility purposes (e.g. are we falling behind?)
  @Meta String messageReceiptHandle(); // Use this later to delete the message once the job is done

  // The metadata fields can be set using builder methods:
  @Meta WorkerJob messageAgeInSeconds(long seconds);
  @Meta WorkerJob messageReceiptHandle(String handle);

Remember that metadata is never serialized, and is ignored for purposes of equality.

User-defined Methods

Thanks to Java 8's default methods, it is straightforward to declare custom methods on a DynamicObject, even though all DynamicObject types are interfaces. For example, we could extend the above Album example with an AlbumCollection type:

public interface AlbumCollection extends DynamicObject<AlbumCollection> {
  Set<Album> albums();

  default int totalTracksInCollection() {
    return albums().stream()

Custom Keys

DynamicObject does not have an elaborate system of conventions to map Java method names to Clojure map keys. By default, the name of the getter method is exactly the name of the keyword. str() maps to :str, and myString() maps to :myString, not :my-string. This default can be overridden with the @Key annotation. Suppose we want access to the following data:

{:camelCase 1,
 :kebab-case 2,
 "quoted string" 3}

The corresponding DynamicObject getters for each field look like this:

long camelCase(); // corresponds to the :camelCase field
@Key(":kebab-case") long kebabCase(); // corresponds to the :kebab-case field, as opposed to the default :kebabCase
@Key("quoted string") long quotedString(); // corresponds to the "quoted string" field

Note the convention: an initial colon yields a Clojure keyword, and anything else yields a string.

Custom keys have a second purpose: they decouple getters and setters. Without custom keys, the getter method for a given field must have the same name as the builder method for the same field. But if getters and builders both have @Key annotations, the methods can take on arbitrary names:

public interface MyType extends DynamicObject<MyType> {
  @Key(":flubber")    String getFlubber();     // Pretty standard for field access in Java...
  @Key(":is-flubbed") boolean isFlubbed();     // ...except for booleans, which generally use "is" and not "get"

  @Key(":flubber") MyType setFlubber(String flubber);   // You can prefix these with "set" if you want...
  @Key(":flubber") MyType withFlubber(String flubber);  // ...but "with" makes more sense for persistent updates

Clojure Interop

In addition to providing a sane alternative to POJOs, DynamicObject implements the map abstractions of both Java and Clojure. This allows the same values to be sent back and forth between Java and Clojure code without wrapping, unwrapping, conversion, or reflection. Java and Clojure code can both idiomatically inspect DynamicObject values and build new ones.

As an example, we'll use the RecursionTest's definition of a DynamicObject-based linked list:

public interface LinkedList extends DynamicObject<LinkedList> {
  long value();
  LinkedList next();

  LinkedList value(long value);
  LinkedList next(LinkedList linkedList);

The following REPL session shows the use of this type from Clojure (various imports omitted):

; We'll start by creating a three-node linked list. First, let's register a reader tag:
(DynamicObject/registerTag RecursionTest$LinkedList "LL")

; Now we'll deserialize the tail node from Edn:
(def tail (DynamicObject/deserialize "#LL{:value 3, :next nil}" RecursionTest$LinkedList))
=> #'user/tail

; We'll build the middle node by using standard Clojure functions:
(def middle (-> tail empty (assoc :value 2) (assoc :next tail)))
=> #'user/middle

; Finally, we'll construct the head node using DynamicObject's builder methods:
(def head (-> middle (.value 1) (.next middle)))
=> #'user/head

; Let's print the result (note the reader tags):
=> #LL{:next #LL{:next #LL{:value 3, :next nil}, :value 2}, :value 1}

; Let's navigate around a bit, first in a Clojure style:
(get-in head [:next :next :value])
=> 3

; Let's try the equivalent Java style as well:
(-> head .next .next .value)
=> 3

; We can use the DynamicObject instance as a function:
(tail :value)
=> 3

; Let's modify a deeply nested value:
(assoc-in head [:next :next :value] 19)
=> #LL{:next #LL{:next #LL{:value 19, :next nil}, :value 2}, :value 1}

; DynamicObject validation can be used from Clojure for runtime type checking:
(.validate (assoc head :next 4))
IllegalStateException The following fields had the wrong type:
  next (expected LinkedList, got Long)
  com.github.rschmitt.dynamicobject.internal.Validation.validateInstance (

; What about pretty printing? Let's make a value that wraps:
(def long-list (-> head (assoc :some-long-key :some-long-value :another-long-key :another-long-value)))
(pprint long-list)
#LL{:another-long-key :another-long-value,
    :some-long-key :some-long-value,
    :next #LL{:next #LL{:value 3, :next nil}, :value 2},
    :value 1}

; Just for grins, let's round-trip the output:
(DynamicObject/deserialize (with-out-str (pprint long-list)) RecursionTest$LinkedList)
=> #LL{:another-long-key :another-long-value, :some-long-key :some-long-value, :next #LL{:next #LL{:value 3, :next nil}, :value 2}, :value 1}
(= *1 long-list)
=> true

; Let's play with Fressian:
(def head-bytes (DynamicObject/toFressianByteArray head))
=> #'user/head-bytes
(vec head-bytes)
=> [-17 -36 76 76 1 -64 -24 -54 -9 -51 -34 110 101 120 116 -96 -64 -24 -54 -9 -128 -96 -64 -24 -54 -9 -51 -33 118 97 108 117 101 3 -54 -9 -128 -9 -54 -9 -127 2 -54 -9 -127 1]
(DynamicObject/fromFressianByteArray head-bytes)
=> #LL{:next #LL{:next #LL{:value 3, :next nil}, :value 2}, :value 1}


Constraints and Limitations


DynamicObject should work out-of-the-box with IntelliJ 14.1; just import it as a Maven project using the pom.xml file. The Community Edition is sufficient. You'll need JDK8 installed and configured as an SDK within IntelliJ.

You can also run the build from the command line using mvn package. To just run the unit tests, use mvn test.

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