Json (fasterxml) stackoverflow exception

Question

When trying to serialize a Category I get a stackoverflow.

Exception

Warning: StandardWrapperValve[dispatcher]: Servlet.service() for servlet dispatcher threw exception java.lang.StackOverflowError at java.lang.ClassLoader.defineClass1(Native Method) at java.lang.ClassLoader.defineClass(ClassLoader.java:760) at org.apache.felix.framework.BundleWiringImpl$BundleClassLoader.findClass(BundleWiringImpl.java:2279) at org.apache.felix.framework.BundleWiringImpl.findClassOrResourceByDelegation(BundleWiringImpl.java:1501) at org.apache.felix.framework.BundleWiringImpl.access$400(BundleWiringImpl.java:75) at org.apache.felix.framework.BundleWiringImpl$BundleClassLoader.loadClass(BundleWiringImpl.java:1955) at java.lang.ClassLoader.loadClass(ClassLoader.java:357) at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:660) at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:152) at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:100) at com.fasterxml.jackson.databind.ser.impl.IndexedListSerializer.serializeContents(IndexedListSerializer.java:21) at com.fasterxml.jackson.databind.ser.std.AsArraySerializerBase.serialize(AsArraySerializerBase.java:183) at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:541) at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:644) at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:152)

Category.java

@Entity
public class Category implements DataObject, Serializable {

    @Id
    @GeneratedValue 
    private Long id;
    private String title;
    private String description;

    @ManyToOne @JsonIgnore 
    private Category parent;


@Override
public long getId() {
    return id;
}

@Override
public void setId(long id) {
    this.id = id;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}
public String getDescription() {
    return description;
}

public void setDescription(String description) {
    this.description = description;
}

public Category getParent() {
   return null;//return parent;
}

public void setParent(Category parent) {
   // this.parent = parent;
}

public boolean isMainCategory()
{
   return true;// return this.parent == null;
}

/**
 * Returns the chain of parent categories with the main category on index 0
 * @return Chain of categories 
 */
public List<Category> getParentChain()
{

   List<Category> cats = new ArrayList<>();
    Category current = this;
    while(!current.isMainCategory())
    {
        cats.add(current);
        current = current.getParent();
    }
    cats.add(current);
    Collections.reverse(cats);
    return cats;
}

@Override
public String toString()
{
    return this.title;
}

@Override
public boolean equals(Object o)
{
    if(!(o instanceof Category))return false;
    Category c = (Category)o;

    return c.title.equals(this.title);
}

@Override
public int hashCode()
{
    return super.hashCode();
} 
}

Rest Controller function

@RequestMapping(value="/{id}", method=RequestMethod.GET)
public ResponseEntity<Category> get(@PathVariable("id") long categoryId)
{
    Category c  =  service.getCategoryRepository().ReadValue(categoryId);
    if(c == null)
        return new ResponseEntity<>(HttpStatus.NOT_FOUND);
    return new ResponseEntity<>(c,HttpStatus.OK);
}

Note

Even when I replace return new ResponseEntity<>(c,HttpStatus.OK); with return new ResponseEntity<>(new Category(),HttpStatus.OK); I will get a stackoverflow whilist none of the fields contain a value.

It works fine with my other classes it's only this class that causes a stackoverflow.

Answer 1

Probably if you comment private Category parent; you will not have the StackOverflow. I've got the same problem in a project with circular dependencies.

The best way to solve this problem is to use the id of the parent instead of the Class like:

private Long parentId;

Edit:

The problem is with getParentChain() that is trying to be serialized. By adding @JsonIgnore before the method the problem was resolved.

Answer 2

Sure thing, @JsonIgnore does the job. But what if we need ignored field in our JSON output?

The solution is very simple.

We annotate our 'guilty' field by @JsonManagedReference annotation on the one side of our relation (which means our @ManyToMany annotation).

And @JsonBackReference on the other side of relation (where @OneToMany has been placed).

And that's it. No more recursive loops.

Answer 3

One annotation solves your problem.

Add following annotation on class.

@JsonIdentityInfo(
      generator = ObjectIdGenerators.PropertyGenerator.class, 
      property = "id")

Other way is to annotate on Collections @JsonManagedReference for forward direction and @JsonBackReference. for backward direction in mapping.

example:

public class User{
    @JsonManagedReference
    @OneToMany(mappedBy = "user")
    Set<Address> s = new Hashset<>();
}

public class Address{
    @JsonBackReference
    @ManyToOne
    @JoinColumn
    User user;
}