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Ok, so I'm trying to test some stuffs with jackson json converter. I'm trying to simulate a graph behaviour, so these are my POJO entities
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class ParentEntity implements java.io.Serializable
{
private String id;
private String description;
private ParentEntity parentEntity;
private List<ParentEntity> parentEntities = new ArrayList<ParentEntity>(0);
private List<ChildEntity> children = new ArrayList<ChildEntity>(0);
// ... getters and setters
}
@JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "id")
public class ChildEntity implements java.io.Serializable
{
private String id;
private String description;
private ParentEntity parent;
// ... getters and setters
}
The tags are required in order to avoid exception on serialization. When I try to serialize an object (both on a file or on a simple string) all works fine. However, when I try to deserialize the object, it throws an exception. This is the simple test method (try/catch omitted for simplicity)
{
// create some entities, assigning them some values
ParentEntity pe = new ParentEntity();
pe.setId("1");
pe.setDescription("first parent");
</span><span class="typ">ChildEntity</span><span class="pln"> ce1 </span><span class="pun">=</span><span class="pln"> </span><span class="kwd"><span class="hljs-keyword">new</span></span><span class="pln"> </span><span class="typ">ChildEntity</span><span class="pun">();</span><span class="pln">
ce1</span><span class="pun">.</span><span class="pln">setId</span><span class="pun">(</span><span class="str"><span class="hljs-string">"1"</span></span><span class="pun">);</span><span class="pln">
ce1</span><span class="pun">.</span><span class="pln">setDescription</span><span class="pun">(</span><span class="str"><span class="hljs-string">"first child"</span></span><span class="pun">);</span><span class="pln">
ce1</span><span class="pun">.</span><span class="pln">setParent</span><span class="pun">(</span><span class="pln">pe</span><span class="pun">);</span><span class="pln">
</span><span class="typ">ChildEntity</span><span class="pln"> ce2 </span><span class="pun">=</span><span class="pln"> </span><span class="kwd"><span class="hljs-keyword">new</span></span><span class="pln"> </span><span class="typ">ChildEntity</span><span class="pun">();</span><span class="pln">
ce2</span><span class="pun">.</span><span class="pln">setId</span><span class="pun">(</span><span class="str"><span class="hljs-string">"2"</span></span><span class="pun">);</span><span class="pln">
ce2</span><span class="pun">.</span><span class="pln">setDescription</span><span class="pun">(</span><span class="str"><span class="hljs-string">"second child"</span></span><span class="pun">);</span><span class="pln">
ce2</span><span class="pun">.</span><span class="pln">setParent</span><span class="pun">(</span><span class="pln">pe</span><span class="pun">);</span><span class="pln">
pe</span><span class="pun">.</span><span class="pln">getChildren</span><span class="pun">().</span><span class="pln">add</span><span class="pun">(</span><span class="pln">ce1</span><span class="pun">);</span><span class="pln">
pe</span><span class="pun">.</span><span class="pln">getChildren</span><span class="pun">().</span><span class="pln">add</span><span class="pun">(</span><span class="pln">ce2</span><span class="pun">);</span><span class="pln">
</span><span class="typ">ParentEntity</span><span class="pln"> pe2 </span><span class="pun">=</span><span class="pln"> </span><span class="kwd"><span class="hljs-keyword">new</span></span><span class="pln"> </span><span class="typ">ParentEntity</span><span class="pun">();</span><span class="pln">
pe2</span><span class="pun">.</span><span class="pln">setId</span><span class="pun">(</span><span class="str"><span class="hljs-string">"2"</span></span><span class="pun">);</span><span class="pln">
pe2</span><span class="pun">.</span><span class="pln">setDescription</span><span class="pun">(</span><span class="str"><span class="hljs-string">"second parent"</span></span><span class="pun">);</span><span class="pln">
pe2</span><span class="pun">.</span><span class="pln">setParentEntity</span><span class="pun">(</span><span class="pln">pe</span><span class="pun">);</span><span class="pln">
pe</span><span class="pun">.</span><span class="pln">getParentEntities</span><span class="pun">().</span><span class="pln">add</span><span class="pun">(</span><span class="pln">pe2</span><span class="pun">);</span><span class="pln">
</span><span class="com"><span class="hljs-comment">// serialization</span></span><span class="pln">
</span><span class="typ">ObjectMapper</span><span class="pln"> mapper </span><span class="pun">=</span><span class="pln"> </span><span class="kwd"><span class="hljs-keyword">new</span></span><span class="pln"> </span><span class="typ">ObjectMapper</span><span class="pun">();</span><span class="pln">
</span><span class="typ">File</span><span class="pln"> f </span><span class="pun">=</span><span class="pln"> </span><span class="kwd"><span class="hljs-keyword">new</span></span><span class="pln"> </span><span class="typ">File</span><span class="pun">(</span><span class="str"><span class="hljs-string">"parent_entity.json"</span></span><span class="pun">);</span><span class="pln">
</span><span class="com"><span class="hljs-comment">// write to file</span></span><span class="pln">
mapper</span><span class="pun">.</span><span class="pln">writeValue</span><span class="pun">(</span><span class="pln">f</span><span class="pun">,</span><span class="pln"> pe</span><span class="pun">);</span><span class="pln">
</span><span class="com"><span class="hljs-comment">// write to string</span></span><span class="pln">
</span><span class="typ">String</span><span class="pln"> s </span><span class="pun">=</span><span class="pln"> mapper</span><span class="pun">.</span><span class="pln">writeValueAsString</span><span class="pun">(</span><span class="pln">pe</span><span class="pun">);</span><span class="pln">
</span><span class="com"><span class="hljs-comment">// deserialization</span></span><span class="pln">
</span><span class="com"><span class="hljs-comment">// read from file</span></span><span class="pln">
</span><span class="typ">ParentEntity</span><span class="pln"> pe3 </span><span class="pun">=</span><span class="pln"> mapper</span><span class="pun">.</span><span class="pln">readValue</span><span class="pun">(</span><span class="pln">f</span><span class="pun">,</span><span class="typ">ParentEntity</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">);</span><span class="pln">
</span><span class="com"><span class="hljs-comment">// read from string</span></span><span class="pln">
</span><span class="typ">ParentEntity</span><span class="pln"> pe4 </span><span class="pun">=</span><span class="pln"> mapper</span><span class="pun">.</span><span class="pln">readValue</span><span class="pun">(</span><span class="pln">s</span><span class="pun">,</span><span class="pln"> </span><span class="typ">ParentEntity</span><span class="pun">.</span><span class="kwd">class</span><span class="pun">);</span><span class="pln">
}
and this is the exception thrown (of course, repeated twice)
com.fasterxml.jackson.databind.JsonMappingException: Already had POJO for id (java.lang.String) [com.fasterxml.jackson.annotation.ObjectIdGenerator$IdKey@3372bb3f] (through reference chain: ParentEntity["children"]->java.util.ArrayList[0]->ChildEntity["id"])
...stacktrace...
Caused by: java.lang.IllegalStateException: Already had POJO for id (java.lang.String) [com.fasterxml.jackson.annotation.ObjectIdGenerator$IdKey@3372bb3f]
...stacktrace...So, what is the cause of the problem? How can I fix it? Do I need some other annotation?
Answers
I have done this using org.codehaus.jackson.annotate.JsonManagedReference and org.codehaus.jackson.annotate.JsonBackReference in this way...
look at how i used @JsonManagedReference
@Id
@TableGenerator(name="CatId", table="catTable",pkColumnName="catKey",pkColumnValue="catValue", allocationSize=1)
@GeneratedValue(strategy=GenerationType.TABLE, generator="CatId")
@Column(name = "CategId", unique = true, nullable = false)
private long categoryId;
private String productCategory;
@JsonManagedReference("product-category")
@OneToMany(targetEntity=ProductDatabase.class,mappedBy="category", cascade=CascadeType.ALL, fetch=FetchType.EAGER)
private List<ProductDatabase> catProducts;and then at the other end i used @JsonBackReference as shown below.
@Id@GeneratedValue
private int productId;
private String description;
private int productPrice;
private String productName;
private String ProductImageURL;
@JsonBackReference("product-category")
@ManyToOne(fetch = FetchType.EAGER)
@JoinColumn(name = "CategId")
private Category category;just apply these annotations and check if it works for you.
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Actually, it seems that the problem was with the "id" property. Because the name of the property is equal for the two different entities, there were some problems during deserialization. Don't know if it makes sense at all, but I solved the problem changing the JsonIdentityInfo tag of ParentEntity to
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property = "id", scope = ParentEntity.class))Of course, I also changed the scope of ChildEntity with scope=ChildEntity.class
as suggested here
I'm open to new answer and suggestions by the way.