Depth wise JSON Serializer
By: Abid Khan
In this blog I will show you, how you can serialize a entity depth wise.
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.
Labels: Java, JSON Serializer
posted by Talentica @ 7:00 AM
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