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Deserialization part 2

Last Updated: June 21, 2024

This is part 6 of the Bleeding edge Java series. Start at the introduction if you haven’t already.

In the first part of the deserialization article we defined a framework for deserializing simple Java types. Now we add support for complex types.

First we update the implementation of deserializerFor to include the complex types we handle:

TypedDeserializer deserializerFor(TypedDeserializer parentTypedDeserializer, Type type)
{
    return switch (type) {
        case Class<?> clazz when clazz.equals(byte.class) || clazz.equals(Byte.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, numberToken -> numberToken.value().byteValue());
        case Class<?> clazz when clazz.equals(short.class) || clazz.equals(Short.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, numberToken -> numberToken.value().shortValue());
        case Class<?> clazz when clazz.equals(int.class) || clazz.equals(Integer.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, numberToken -> numberToken.value().intValue());
        case Class<?> clazz when clazz.equals(long.class) || clazz.equals(Long.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, numberToken -> numberToken.value().longValue());
        case Class<?> clazz when clazz.equals(float.class) || clazz.equals(Float.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, numberToken -> numberToken.value().floatValue());
        case Class<?> clazz when clazz.equals(double.class) || clazz.equals(Double.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, numberToken -> numberToken.value().doubleValue());
        case Class<?> clazz when clazz.equals(boolean.class) || clazz.equals(Boolean.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, BooleanToken.class, BooleanToken::value);
        case Class<?> clazz when Number.class.isAssignableFrom(clazz) -> 
                simpleTypedDeserializer(parentTypedDeserializer, NumberToken.class, NumberToken::value);
        case Class<?> clazz when clazz.equals(String.class) -> 
                simpleTypedDeserializer(parentTypedDeserializer, StringToken.class, StringToken::value);
        case ParameterizedType parameterizedType when (parameterizedType.getRawType() instanceof Class<?> clazz) && Collection.class.isAssignableFrom(clazz) -> 
                collectionTypedDeserializer(parentTypedDeserializer, clazz, parameterizedType.getActualTypeArguments()[0]);
        case ParameterizedType parameterizedType when (parameterizedType.getRawType() instanceof Class<?> clazz) && Optional.class.isAssignableFrom(clazz) -> 
                optionalTypedDeserializer(parentTypedDeserializer, parameterizedType.getActualTypeArguments()[0]);
        case Class<?> clazz when clazz.isRecord() -> 
                recordTypedDeserializer(parentTypedDeserializer, clazz);
        case Class<?> clazz when clazz.isEnum() -> 
                enumTypedDeserializer(parentTypedDeserializer, clazz);
        default -> throw new RuntimeException();
    };
}

Enumerations

Use simpleTypedDeserializer to accept a string and use the string to build the enum.

TypedDeserializer enumTypedDeserializer(TypedDeserializer parentTypedDeserializer, Class<?> enumClass)
{
    return simpleTypedDeserializer(parentTypedDeserializer, StringToken.class, stringToken ->
            Stream.of(enumClass.getEnumConstants()).filter(c -> c.toString().equals(stringToken.value())).findFirst().orElseThrow(RuntimeException::new));
}

Optional

The pattern matching in the deserializerFor() switch will have extracted the Optional’s component type. We get the deserializer for that component type, get its value and then pass that value through Optional.ofNullable().

TypedDeserializer optionalTypedDeserializer(TypedDeserializer parentTypedDeserializer, Type componentType)
{
    return new TypedDeserializer()
    {
        private final TypedDeserializer valueDeserializer = deserializerFor(parentTypedDeserializer, componentType);

        @Override
        public TypedDeserializer accept(JsonToken jsonToken)
        {
            return valueDeserializer.accept(jsonToken);
        }

        @Override
        public Object value()
        {
            return Optional.ofNullable(valueDeserializer.value());
        }
    };
}

Collections

For collections we keep a list of a deserializers for each value in the collection. When the parent deserializer asks for the collection value, we can then process each deserializer to get its final value. We must perform this delayed value resolution because all the tokens that apply to the collection must be processed before the value can be determined.

The deserializer does the following:

  • Wait for the BeginArrayToken to mark that the collection has started
  • Set a flag to indicate that a value is expected once the collection has started
  • Check the next token to see if it’s an EndArrayToken when the value-expected flag is set
    • If so, the collection is complete
    • If not, get a new deserializer for the list’s component type and add it to values and set it as the next deserializer
  • If a ValueSeparatorToken is accepted, reset for a new value
  • Otherwise wait for EndArrayToken
TypedDeserializer collectionTypedDeserializer(TypedDeserializer parentTypedDeserializer, Class<?> collectionClass, Type componentType)
{
    return new TypedDeserializer()
    {
        private final List<TypedDeserializer> values = new ArrayList<>();
        private boolean started;
        private boolean isDone;
        private boolean expectingValue;

        @Override
        public TypedDeserializer accept(JsonToken jsonToken)
        {
            TypedDeserializer nextTypedDeserializer = this;
            if (expectingValue) {
                expectingValue = false;
                switch (jsonToken) {
                    case EndArrayToken __ -> {
                        nextTypedDeserializer = accept(jsonToken);    // it's an empty array
                    }
                    default -> {
                        TypedDeserializer valueTypedDeserializer = deserializerFor(this, componentType);
                        values.add(valueTypedDeserializer);
                        nextTypedDeserializer = valueTypedDeserializer.accept(jsonToken);
                    }
                }
            }
            else {
                switch (jsonToken) {
                    case BeginArrayToken __ -> {
                        if (started) {
                            throw new RuntimeException();
                        }
                        started = true;
                        expectingValue = true;
                    }
                    case EndArrayToken __ -> {
                        if (!started || isDone) {
                            throw new RuntimeException();
                        }
                        nextTypedDeserializer = parentTypedDeserializer;
                        isDone = true;
                    }
                    case ValueSeparatorToken __ -> {
                        if (!started) {
                            throw new RuntimeException();
                        }
                        expectingValue = true;
                    }
                    default -> throw new RuntimeException();
                }
            }
            return nextTypedDeserializer;
        }

        @Override
        public Object value()
        {
            if (!isDone) {
                throw new RuntimeException();
            }
            // get the final value from each of the stored deserializers
            Stream<Object> valueStream = values.stream().map(TypedDeserializer::value);
            // create either a set or a list depending on the collection class
            return Set.class.isAssignableFrom(collectionClass) ? valueStream.collect(Collectors.toSet()) : valueStream.toList();
        }
    };
}

Records

For records we use the record’s array of RecordComponents to know the names and types of the fields to expect in the object. Similar to the collection deserializer we keep a map whose key is the name of the field and the value is the deserializer for the component’s type. Just like for collections, when the parent deserializer asks for the record value, we can then process each deserializer to get its final value. We must perform this delayed value resolution because all the tokens that apply to the record must be processed before the value can be determined.

The deserializer does the following:

  • Wait for BeginObjectToken to mark that the object has started
  • Expect that ObjectNameToken is the next token. Once the ObjectNameToken is accepted:
    • Get a new deserializer for the record component’s type and add it to values map
    • Set it as the next deserializer
  • If a ValueSeparatorToken is accepted, reset for another BeginObjectToken, otherwise, wait for EndObjectToken
  • Build the final value by using the RecordComponents to find the canonical Constructor, create an array of values and create the record via reflection
TypedDeserializer recordTypedDeserializer(TypedDeserializer parentTypedDeserializer, Class<?> recordClass)
{
    RecordComponent[] recordComponents = recordClass.getRecordComponents();
    Map<String, RecordComponent> recordComponentMap = Stream.of(recordComponents).collect(Collectors.toMap(RecordComponent::getName, Function.identity()));

    return new TypedDeserializer() {
        private final Map<String, TypedDeserializer> valuesMap = new HashMap<>();
        private String currentName;
        private boolean started;
        private boolean isDone;

        @Override
        public TypedDeserializer accept(JsonToken jsonToken)
        {
            TypedDeserializer nextTypedDeserializer = this;
            switch (jsonToken) {
                case BeginObjectToken __ -> {
                    if (started) {
                        throw new RuntimeException();
                    }
                    else {
                        started = true;
                    }
                }
                case ObjectNameToken(var name) -> {
                    if (!started || (currentName != null)) {
                        throw new RuntimeException();
                    }
                    currentName = name;
                    RecordComponent recordComponent = recordComponentMap.get(currentName);
                    if (recordComponent == null) {
                        throw new RuntimeException();
                    }
                    TypedDeserializer typedDeserializer = deserializerFor(this, recordComponent.getGenericType());
                    valuesMap.put(currentName, typedDeserializer);
                    nextTypedDeserializer = typedDeserializer;
                }
                case EndObjectToken __ -> {
                    if (!started || isDone) {
                        throw new RuntimeException();
                    }
                    nextTypedDeserializer = parentTypedDeserializer;
                    isDone = true;
                }
                case ValueSeparatorToken __ -> {
                    if (!started || (currentName == null)) {
                        throw new RuntimeException();
                    }
                    currentName = null;
                }
                default -> throw new RuntimeException();
            }
            return nextTypedDeserializer;
        }

        @Override
        public Object value()
        {
            if (!isDone) {
                throw new RuntimeException();
            }
            Class<?>[] argumentTypes = new Class[recordComponents.length];
            Object[] arguments = new Object[recordComponents.length];
            for (int i = 0; i < recordComponents.length; ++i) {
                RecordComponent recordComponent = recordComponents[i];
                argumentTypes[i] = recordComponent.getType();
                TypedDeserializer valueTypedDeserializer = valuesMap.get(recordComponent.getName());
                if (valueTypedDeserializer == null) {
                    if (Optional.class.isAssignableFrom(recordComponent.getType())) {
                        arguments[i] = Optional.empty();
                    }
                    // otherwise leave it null
                }
                else {
                    arguments[i] = valueTypedDeserializer.value();
                }
            }
            try {
                return recordClass.getConstructor(argumentTypes).newInstance(arguments);
            }
            catch (InstantiationException | IllegalAccessException | InvocationTargetException | NoSuchMethodException | IllegalArgumentException e) {
                throw new RuntimeException(e);
            }
        }
    };
}

Generics

Because of erasure, there is no way in Java to specify many object types that we might want to deserialize such as generic lists (e.g. List<String>). However generic type information is not completely lost at compile-time in Java. The Java runtime retains a surprising amount of generic type information. In fact, we can create a real generic type for any generic definition we need by using a very simple utility called a “type token”. Sadly, the JDK does not include a utility to create these type tokens. Fortunately, the code to create them is just a few Java lines. The included TypeToken.java utility provides this. To create a type token for a list of strings write:

// create a type token that represents a list of string
TypeToken<List<String>> typeToken = new TypeToken<>(){};

We can use this type token to deserialize a list of records, strings, optionals, and others:

var parser = JsonParser.instance();
var deserializer = JsonDeserializer.instance();

String jsonText = "[1, 2, 3]";
TypeToken<List<Integer>> typeToken = new TypeToken<>() {};
List<Integer> ints = parser.parse(jsonText.chars()).collect(JsonDeserializerCollector.deserializing(deserializer, typeToken));

Test it out for yourself!

In the previous articles we developed a serializer, a printer and a parser. Now we can do a complete round trip from a Java object, to JSON tokens, to JSON text, back to JSON tokens and back to the original Java object.

Let’s put this together in jshell. The example uses these files:

From a terminal with Java 19 installed, run the following (note you’ll need the wget utility):

wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/TypeToken.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/JsonToken.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/JsonSerializer.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/StringUtils.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/JsonPrinter.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/JsonParser.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/JsonDeserializer.java
wget -nc https://raw.githubusercontent.com/starburstdata/developer-blog-assets/main/bleeding-edge-java/code/JsonDeserializerCollector.java
jshell --enable-preview TypeToken.java JsonToken.java JsonSerializer.java StringUtils.java JsonPrinter.java JsonParser.java JsonDeserializer.java JsonDeserializerCollector.java

Inside jshell let’s serialize a Java record into JSON tokens, print those tokens into JSON text, parse that text back into a stream of JSON tokens and finally deserialize those tokens back into a Java record. We can compare the two records to ensure it worked properly.

var serializer = JsonSerializer.instance();
var printer = JsonPrinter.instance();
var parser = JsonParser.instance();
var deserializer = JsonDeserializer.instance();

record Person(String name, int age) {}
var person = new Person("someone", 28);

var deserializedPerson = serializer.serialize(person)        // serialize to stream of JsonToken
    .map(printer::print)            // map each JsonToken to a String (as a CharSequence)
    .map(CharSequence::chars)       // map each CharSequence to an IntStream
    .flatMap(parser::parse)         // pass each IntStream to the parser and flatten the resulting stream of tokens
    .collect(JsonDeserializerCollector.deserializing(deserializer, Person.class));  // use the collector to reduce the tokens into a Person via the deserializer

if (person.equals(deserializedPerson)) {
    System.out.println("It worked!");
}
System.out.println("Done.");

Summary

We rounded out deserialization nicely now and have a full library at our hands. Lets move on to our final thoughts in the series conclusion.

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