How-To Guide for ALFA Java

We are keen to add more examples. Please contact or goto for any questions not covered below.

Q1: Setup ALFA compiler in a Maven project

Refer to the ALFA Maven Plugin which contains a complete working example of a pom.xml file showing how to generate Java for an ALFA model file.

Q2: Build and populate an ALFA Java object

ALFA generated Java code supports the builder pattern for constructing objects. Given an ALFA definitions like:

namespace demo

record Person {
   Name : string
   Age : int(16,120)
   Friends : list< string >

The following snippet of code shows ways of creating an object.

import demo.Person;
// Create builder, set values and build in one line
Person p1 = Person.newBuilder().setName("Paul").setAge(20).addFriends("John").build();

// Create builder and separately set values and build
Person.PersonBuilder b = Person.newBuilder();
Person p2 =;

When list< T >, set< T > or map< K, V> fields are used, methods to add individual items or an entire collection are generated.

The object returned from build() is immutable, and further modifications on the original builder instance has no impact on the object returned from the build() method.

It is possible to have a partially constructed builder instance and iteratively vary some values and call build() to get a series of immutable objects.

Q3: Convert an ALFA object to JSON and back

This achieved using methods in the Codec class.

import alfa.rt.Codec;
Person p1 = Person p1 = Person.newBuilder().setName("Paul").setAge(20).build();
String json = Codec.toJsonString( p1 );
Person decoded = Codec.fromJsonString( json );
System.out.println( json );

Output : {"$type":"demo.Person","Name":"Paul","Age":20,"Friends":["John"]} Other methods to work with input and output streams are available.

Q4: Set JSON encode/decode flags to control JSON processing

Using the JsonWriterCodecConfig class, behaviour of the JSON encoding can be changed. In the example below, the skipRootType is set, therefore the output does not contain the $type field.

Note: Setting skipRootType for small objects (with only scalars for example), can significant cut down the size of the JSON. For example {"$type":"Inventory.Items.Price","Ccy":"EUR","Price":1.08} will be {"Ccy":"EUR","Price":1.08}.

JsonWriterCodecConfig cfg = JsonWriterCodecConfig.newBuilder().setSkipRootTypeInfo(true).build();
String j2 = Codec.toJsonString(cfg,p1);

Output : {"Name":"Paul","Age":20,"Friends":["John"]}

Likewise when reading JSON, there are configuration settings that control the reader, for example being able to read a JSON without a $type field.

JsonReaderCodecConfig rcfg = JsonReaderCodecConfig.newBuilder().
        setSkipUnknownFields(true).                     // Ignore unknown fields in the JSON
        setAssignableToClass(Person.class).build();     // Read the JSON as the specified class

String jsn = "{\"Name\":\"Paul\",\"Age\":20,\"Rating\":3,\"Friends\":[\"John\"]}\n";

// The JSON above will be read as a Person class, and ignore the 'Rating' field, as per the reader configuration.
Person p3 = Codec.fromJsonString(rcfg, jsn);

Q5: Decode JSON to a smaller object - trait used by serialised object

Consider the definition below, and a usecase where given a large object containing a Person object with 100 Friends entries, and we only want the value of Name. Given the Person is a subclass of Named, ideally we want an object representing just the fields from the trait Named.

namespace demo

trait Named {
   Name : string

record Person includes Named {
   Age : int(16,120)
   Friends : list< string >

This is possible by implementing the following:

JsonReaderCodecConfig rcfg = JsonReaderCodecConfig.newBuilder().

String jsn = "{\"Name\":\"Paul\",\"Age\":20,\"Rating\":3,\"Friends\":[\"John\"]}\n";
Named p3 = Codec.fromJsonString(rcfg, jsn);

The decoded object only will contain the Name field and all other values in the payload will be ignored. This is possible as ALFA traits when generated contain a default implementation containing just the fields in the trait.

Q6: Handling data constraint errors

When constraints are set on datatypes, those are enforced when build() is called. Consider the following example.

Person p1 = Person.newBuilder().setName("Paul").setAge(15).addFriends("John").build();

Note Age is assigned to 15, when in the ALFA model (from Q2) its declared as range from 16 onwards. The following exception is thrown when the above code is executed. It clearly indicates the location of the error and cause.

alfa.rt.AlfaValidationException: Validation failed on {
}. Minimum size 16, result size 15

If constraints are used, the build() method can throw alfa.rt.AlfaValidationException, which extends java.lang.RuntimeException.

Note: The assertion of constraints can be disabled when calling the build method by passing in a BuilderConfig with a RuntimeContext where the shouldValidateOnBuild() method returns false.

Q7: Estimate ALFA Java object size

ALFA Java runtime has a facility to estimate space consumed by an object. This can be particularly useful when making generic data processing decisions. The estimation is performed by traversing the object using lambda expressions in the TypeDescriptors, also keeping track of unique strings.

The example below shows how this may be used. For sizing multiple objects, a String pool can be passed to get a more accurate figure. It should be emphasized that this is an estimate so will not be accurate to the byte level give JVM level decisions. However for purposes of getting a consistent size estimate and comparing to other ALFA objects, this method will be highly effective.

import alfa.rt.utils.AlfaUtils;

Person p1 = Person.newBuilder().setName("Paul").setAge(20).addFriends("John").build();
long size = AlfaUtils.estimateSize(p1);

Q8: Generating a random object

ALFA runtime is able to generate a random object given a type name. This is particular useful for mock testing using the model objects.

Given the Person model from Q2, running the code below generates an object instance. The randomiser respects datatype constraints with the exception of pattern types. In the example below, the Age value returned will always be within 16 and 120.

By default a collection will be returned with 5 entries, and strings will be randomized to 5 characters. Additional features will be added to the randomizer, therefore if there is interest in particular features, please reach out to or goto

See AlfaRandomiser
AlfaRandomizer r = new AlfaRandomizer();
AlfaObject obj = r.random("demo.Person");


    "Friends":["gmtvx", "fthnu", "btoru", "cincj", "tzwjf"]

The Java Generator has an example of a matrix data type and value generated from the ALFA Randomizer.

Q9: See immutability in practice with ALFA Java objects

The following code can be written for the model from Q2 above.

Person.PersonBuilder b = Person.newBuilder().setName("John").setAge(20);
List< String > friends = new ArrayList<>();
Person john = b.addAllFriends(friends).build();

System.out.println("John.friends:" + john.getFriends().size());

System.out.println("John.friends:" + john.getFriends().size());
System.out.println("Friends size:" + friends.size());


Friends size:4

Having called build(), the contents of the Friends field remain unchanged even if the original List is modified. Given ALFA performs a deep clone of collections, even when the collection had nested collections, those too will be immutable.

Note: The deep cloning of collections can be disabled when calling the build method by passing in a BuilderConfig with a RuntimeContext where the shouldCloneCollectionsOnBuild() method returns false.

Q10: See data constraints applied on JSON stream

ALFA uses stream-based JSON decoding for optimum performance. As well as decoding, it asserts some constraints such as sizes of collections. This is particular useful when the model and application expects a limit on data a dataset is received which is significantly larger that it potentially causes the application to exceed memory available.

Size constraint assertion can be demonstrated with the following example.

Consider the Person definition in Q2 is updated with Friends : list< string >(0,5) to set an upper limit on number of friends. With that in place the following code can be executed. Note the Friends array contain 6 strings.

String json = "{\"$type\":\"demo.Person\",\"Name\":\"Paul\",\"Age\":20,\"Friends\":[\"A\",\"B\",\"C\",\"D\",\"E\",\"F\"]}\n";
Person p = Codec.fromJsonString(json);

The above code throws an AlfaMaxSizeExceedException exception.

alfa.rt.AlfaMaxSizeExceedException: Cannot insert to list. Maximum limit 5 reached on field 'Friends'.

Natually this validation is applied on deeply nested models too, and not just the top level.

Q11: Build try< T > result or failure or either< L, R > value

The try and either ALFA types are extremely useful to model results or unexpected results. To make constructing these types easier, the AlfaUtils class contains number of utility methods.

Try< Integer> t1 = AlfaUtils.createTryObject( 10 );
Try< Double> t2 = AlfaUtils.createTryFailure( "Failed to calculate price" );

Either< String, Double> e1 = AlfaUtils.createEitherLeftObject( "John");
Either< String, Double> e2 = AlfaUtils.createEitherRightObject( 100000.00 );

Q12: Using compressed< T > values

Keeping occasionally used large objects in memory can waste resources. However completely eliminating them from the model also creates delay when they are needed. Some data types such as strings can consume lot of data when stored in large quantities containing descriptions etc. To handle such usescases compressed<T> can be used.

ALFA compressed< T > can be used for such cases, where T can be any ALFA type.

Consider the data type below.

record Events {
    Log : compressed< list< string > >

This type can be used in ALFA to create a compressed set of values.

List<String> l = new ArrayList<>();
l.add("2019.12.12 18:00 INFO Starting");
l.add("2019.12.12 18:10 INFO Running");
l.add("2019.12.12 18:12 INFO Processing 1");
l.add("2019.12.12 18:15 INFO Processing 2");
l.add("2019.12.12 18:17 INFO Processing ...");
l.add("2019.12.12 18:20 INFO Saving");
l.add("2019.12.12 18:30 INFO Completed");

// Create events object containg a list of event strings.
// As soon as setLog() is called the list of strings are compressed and stored as a byte[ ] internally.
Events e = Events.newBuilder().setLog(l).build();

String json = Codec.toJsonString(e);
Events decoded = Codec.fromJsonString(json);

Compressed<List<String>> c = decoded.getLog();
List<String> val = c.getValue( BuilderConfig.getInstance() );

System.out.println( val.get( val.size() - 1 ));

Q13: Using encrypted values

Using an ALFA encrypted<T> is converted into a Java alfa.rt.Encrypted<T> instance. The Encrypted type internally uses the RuntimeContext class to create a byte[ ] from encrypting a serialised object of the value.

An Encrypted<T> object does not hold a reference to the object of type T. The object is encrypted in the constructor of Encrypted and only stored as a byte[ ].

Users can extend and supply an alternative alfa.rt.RuntimeContext object. By default the RuntimeContext uses a private-public keypair to encrypt and decrypt data. This is to demonstrate the capability, however for a real application, the RuntimeContext will need to securely source the keypair.

As an example, consdier the following ALFA definition.

record CreditCard {
    Pin : encrypted< string(4,6) >

With that definition in place, it is possible to write code that securely hold a PIN where reading is only permitted when a correct public/private keypair is used.

// Create a Card and set a PIN. The reference stored is immediately encrypted by the implemenation of RuntimeContext.encrypt()
CreditCard.CreditCardBuilder builder = CreditCard.newBuilder().setPin("2341");
CreditCard cc =;

String json = Codec.toJsonString(cc);

// When we need to access the PIN, we get a handle to an Encrypted<> object.
Encrypted<String> encPin = cc.getPin();

// The value can only be deciphered by supplying a BuilderConfig, which will use the RuntimeContext.decrypt() method
String pin = encPin.getValue(BuilderConfig.getInstance());
System.out.println( pin );

As output, the following is printed.


Q14: View ALFA to Java type mappings

Please refer to section titled ‘Type Mappings’ in Java Generator.

Q15: Define and code a native typedef

Creating a native type class is straight-forward. The first step is to declare a native type as shown below, where storage is a new type, which is implemented by the native type acme.types.Storage.

typedefs {
    storage = native acme.types.Storage

The Storage ‘native’ ALFA type class needs to implement alfa.rt.NativeAlfaObject. In addition it needs a TypeDescriptor and Builder inner classes which support encoding and decoding the native type to a string and vice versa.

This class needs to be compiled and available in the classpath of the ALFA runtime and generated Java code. The generated Java code will reference this type when the type ( storage below) is declared a field type.

Given this is the Java ‘How-To’, the example native type below is Java. Other languages can have similar implementations.

package acme.types;

import alfa.rt.*;
import alfa.rt.model.UdtDataType;
import alfa.rt.model.UdtMetaType;
import alfa.rt.utils.DefaultTypeDescriptor;

 * Storage is an example ALFA 'native' typedef type.
public class Storage implements NativeAlfaObject {
    private final String _size;

     * Create a new immutable Storage instance
     * @param size Size of storage
    public Storage(String size) {
        this._size = size;

    public int hashCode() {
        return _size.hashCode();

    public boolean equals(Object obj) {
        if ( obj instanceof Storage )
            Storage rhs = (Storage) obj;
            return rhs._size.equals(_size);
        return false;

    public String toString() {
        return _size;

    public TypeDescriptor descriptor() {
        return StorageDescriptor.INSTANCE;

     * Convert object to a string representation, which also will be the string
     * that can be passed to the constructor to create a new instance.
     * @return
    public String encodeToString() {
        return _size;

     * This class is required and used by ALFA Codec classes.
     * A Storage builder is able to create a new immutable instance of Storage.
    public static class StorageBuilder implements Builder {
        private String value;

        public <T extends AlfaObject> T build() {
            if ( value == null )
                throw new NullPointerException("Value not assigned to storage builder");

            return (T) new Storage(value);

        public void modify(String fieldName, Object val) {
            value = ( String ) val;

     * This class is required and used by ALFA Codec classes.
     * Descriptor for the Storage class.
    public static class StorageDescriptor extends DefaultTypeDescriptor {
        public static TypeDescriptor INSTANCE = new StorageDescriptor();
        private static UdtDataType udtDataType = UdtDataType.newBuilder().setUdtType(UdtMetaType.nativeUdtType).setFullyQualifiedName(Storage.class.getName()).build();

        public UdtDataType getUdtDataType() {
            return udtDataType;

        public Builder newBuilder(BuilderConfig cc) {
            return new StorageBuilder();