Technical

Depth wise JSON Serializer

talentica

Administrator - 31 August 2014 - 12min

By: Abid Khan

In this blog I will show you, how you can serialize a entity depth wise.

Why we need it

Very often we land in situation with cyclic reference if target entity has self references. There are multiple solution available over internet, but none of these will solve your problem. The only option is left to write own depth wise serializer.

Solution

This example is based on spring-data-jpa. Hence forth I will use entity which is used in a spring-data-jpa project. Our target entity is User which extends AbstractAuditableEntity. Here we have two properties createdBy and lastModifiedBy referencing to User entity.

Entity

@SuppressWarnings("serial")
@Entity
@Table(name = "user")
public class User extends AbstractAuditableEntity {

    private String firstName;

    private String lastName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

}


@SuppressWarnings("serial")
@MappedSuperclass
@EntityListeners(AuditingEntityListener.class)
public class AbstractAuditableEntity extends AbstractPersistable<Long> implements Auditable<User, Long> {

    @Version
    protected Long version;

    @Type(type = "org.jadira.usertype.dateandtime.joda.PersistentDateTime")
    @DateTimeFormat(iso = ISO.DATE_TIME)
    protected DateTime createdDate;

    @Type(type = "org.jadira.usertype.dateandtime.joda.PersistentDateTime")
    @DateTimeFormat(iso = ISO.DATE_TIME)
    protected DateTime lastModifiedDate;

    protected User createdBy;

    protected User lastModifiedBy;

    @Enumerated(EnumType.STRING)
    protected StatusType status;

    public Long getVersion() {
        return version;
    }

    public void setVersion(Long version) {
        this.version = version;
    }

    public DateTime getCreatedDate() {
        return createdDate;
    }

    public void setCreatedDate(DateTime createdDate) {
        this.createdDate = createdDate;
    }

    public DateTime getLastModifiedDate() {
        return lastModifiedDate;
    }

    public void setLastModifiedDate(DateTime lastModifiedDate) {
        this.lastModifiedDate = lastModifiedDate;
    }

    public User getCreatedBy() {
        return createdBy;
    }

    public void setCreatedBy(User createdBy) {
        this.createdBy = createdBy;
    }

    public User getLastModifiedBy() {
        return lastModifiedBy;
    }

    public void setLastModifiedBy(User lastModifiedBy) {
        this.lastModifiedBy = lastModifiedBy;
    }

    public StatusType getStatus() {
        return status;
    }

    public void setStatus(StatusType status) {
        this.status = status;
    }

}

Test Case

Lets write a test case to serialize an instance of user. In this test case we have not used custom serializer.

@Test
public void serialize() throws JsonProcessingException {

    User user = new User();
    user.setFirstName("First Name");
    user.setLastName("Last Name");

    user.setCreatedBy(user);
    user.setLastModifiedBy(user);

    ObjectMapper objectMapper = new ObjectMapper();

    String userString = objectMapper.writeValueAsString(user);
    System.out.println("JSON String :: " + userString);
}

Once this tets case is executed, it will throw below exception.

com.fasterxml.jackson.databind.JsonMappingException: Direct self-reference leading to cycle (through reference chain: com.abid.learning.serialization.entity.User["createdBy"])
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter._handleSelfReference(BeanPropertyWriter.java:667)
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:540)

Serializer

We will write a custom serializer which will traverse depth wise to solve this issue.I will use thread local variable to determine depth of recursion with initial value 0.

private static ThreadLocal<Integer> depth = new ThreadLocal<Integer>() {

        @Override
        protected Integer initialValue() {
            return 0;
        }

    };

In this serializer if depth of recursion is more than 2, recursion will return null.

depth.set(depth.get() + 1);
if (depth.get() > 2) {
                jgen.writeNull();
} else {
  ...
}

And complete serializer…

public class UserJsonSerializer extends JsonSerializer<User> {

    private static ThreadLocal<Integer> depth = new ThreadLocal<Integer>() {

        @Override
        protected Integer initialValue() {
            return 0;
        }

    };

    @Override
    public void serialize(User value, JsonGenerator jgen,
            SerializerProvider provider) throws IOException,
            JsonProcessingException {

        depth.set(depth.get() + 1);
        try {
            if (depth.get() > 2) {
                jgen.writeNull();
            } else {

                jgen.writeStartObject();
                jgen.writeStringField("firstName", value.getFirstName());
                jgen.writeStringField("lastName", value.getLastName());

                User createdByUser = value.getCreatedBy();
                if (null != createdByUser) {
                    jgen.writeStringField(
                            "createdBy",
                            createdByUser.getFirstName() + " "
                                    + createdByUser.getLastName());
                }

                User lastModifiedByUser = value.getLastModifiedBy();
                if (null != lastModifiedByUser) {
                    jgen.writeStringField("lastModifiedBy",
                            lastModifiedByUser.getFirstName() + " "
                                    + lastModifiedByUser.getLastName());
                }

                jgen.writeEndObject();

                return;

            }
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }

}

Updated Test Case

We will rewrite the test case again. Will update object mapper with our custom serializer.

ObjectMapper objectMapper = new ObjectMapper();
SimpleModule userModule = new SimpleModule();
userModule.addSerializer(User.class, new UserJsonSerializer());
objectMapper.registerModule(userModule);

Complete test case…

public class UserTest {

    @Test
    public void serialize() throws JsonProcessingException {

        User user = new User();
        user.setFirstName("First Name");
        user.setLastName("Last Name");
        user.setCreatedBy(user);
        user.setLastModifiedBy(user);

        ObjectMapper objectMapper = new ObjectMapper();
        SimpleModule userModule = new SimpleModule();
        userModule.addSerializer(User.class, new UserJsonSerializer());
        objectMapper.registerModule(userModule);

        String userString = objectMapper.writeValueAsString(user);
        System.out.println("JSON String :: " + userString);
    }
}

Once we execute above test case, will get below output.

JSON String :: {"firstName":"First Name","lastName":"Last Name","createdBy":"First Name Last Name","lastModifiedBy":"First Name Last Name"}

Summary

This solution can be used for depth wise and normal custom json serializer. How deep object graph needs to serialize, fully depends on your requirement. Even you can customized the example as per your need.