Java, DynamoDB: Is it possible to custom-marsall a Set?

Question

I need to annotate a class that has a Set<UUID> property. Marshalling a scalar UUID is fine, and I can use the @DynamoDBMarshalling annotation to specify my converter, and everything works.

When trying to use the same converter on a Set<UUID>, I get "DynamoDBMappingException: Expected SS in value". I've tried creating a custom converter that expects Set<UUID>, but the same problem persists.

Is it possible to custom-marshall Sets?

@DynamoDBTable(tableName="djones-test")
public class UUIDRecommendation {
    private UUID id;
    private Set<UUID> recommendations;

    @DynamoDBHashKey
    @DynamoDBMarshalling(marshallerClass=UuidConverter.class)
    public UUID getId() {
        return id;
    }

    public void setId(UUID id) {
        this.id = id;
    }

    //Neither this nor UuidConverter.class work here    
    @DynamoDBMarshalling(marshallerClass=SetUuidConverter.class)
    public Set<UUID> getRecommendations() {
        return recommendations;
    }

    public void setRecommendations(Set<UUID> recommendations) {
        this.recommendations = recommendations;
    }
}

Here's the stack trace:

com.amazonaws.services.dynamodb.datamodeling.DynamoDBMappingException: Expected SS in value {SS: [1a841b97-ab9d-4425-a2c0-f9a81bebf0b4, 1a841b97-ab9d-4425-a2c0-f9a81bebf0b4, 1a841b97-ab9d-4425-a2c0-f9a81bebf0b4], } when invoking public void com.company.model.UUIDRecommendation.setRecommendations(java.util.Set)
    at com.amazonaws.services.dynamodb.datamodeling.SUnmarshaller.typeCheck(SUnmarshaller.java:26)
    at com.amazonaws.services.dynamodb.datamodeling.DynamoDBMapper.setValue(DynamoDBMapper.java:329)
    at com.amazonaws.services.dynamodb.datamodeling.DynamoDBMapper.marshallIntoObject(DynamoDBMapper.java:302)
    at com.amazonaws.services.dynamodb.datamodeling.DynamoDBMapper.load(DynamoDBMapper.java:253)
    at com.amazonaws.services.dynamodb.datamodeling.DynamoDBMapper.load(DynamoDBMapper.java:196)
    at com.mendeley.service.data.DynamoRecommendedItemsDataServiceTest.testObjectMapper(DynamoRecommendedItemsDataServiceTest.java:65)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
    at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:74)
    at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:83)
    at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:72)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

Answer 1

Try this. It handles the marshall/unmarshall inside the getter/setter. I use a similar technique to store Set<String> containing JSON, but access it as a Set<CustomObject> in my code. I leave the getter/setter alone and use convenience methods instead.

Here's the modified version:

@DynamoDBTable(tableName="djones-test")
public class UUIDRecommendation {
    private String id;
    // custom object representation in JVM
    private Set<UUID> recommendations;

    private static final JsonMarshaller<UUID> UUID_MARSHALLER = new JsonMarshaller<UUID>();

    @DynamoDBHashKey(attributeName="id")
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }

    // Set<String> holding JSON in DynamoDB
    @DynamoDBAttribute(attributeName="recommendations")
    public Set<String> getRecommendations() {
        if (recommendations != null) {
            Set<String> jsonSet = new HashSet<String>(recommendations.size());
            for (UUID recommendation : recommendations) {
                String json = UUID_MARSHALLER.marshall(recommendation);
                jsonSet.add(json);
            }
            return jsonSet;
        } else {
            return null;
        }
    }
    public void setRecommendations(Set<String> jsonSet) {
        if (jsonSet != null) {
            recommendations = new HashSet<UUID>(jsonSet.size());
            for (String json : jsonSet) {
                UUID recommendation = UUID_MARSHALLER.unmarshall(UUID.class, json);
                recommendations.add(recommendation);
            }
        }
    }

    // convenience methods
    @DynamoDBIgnore
    public UUID convenienceMethod(UUID recommendation) {
        return null;
    }

    // custom object
    public class UUID {

    }
}