Fixing 401 Error with CORS Preflights (with Spring boot)

              

When the web application runs on a different domain and the server runs on another domain,  we have to deal with cross origin requests. But, if we don't know the mechanism of cross origin resource sharing (CORS), we will end up with errors like 401.

Error in brief

    My Angular web application runs on port 4200, and my spring boot application runs on server on port 8080. I could not enable CORS in my backend, so, I got an error when sending a request from my application like this.

 

  

Reason

       My pre-flight request is not responded with status OK(200).

Solution

        I enabled CORS in my backend. To do this, there are several methods. Here in my post, I have shown the easiest way to do that. But, if you are interest in knowing other ways, click me )

By using Web Security in Spring Boot,

      You have to create a web security configuration class.  I hope everybody creates that class for backend authentication purposes. 

If you have already created that class, inside configure(HttpSecurity httpSecurity) method ,add this line httpSecurity.cors();

It's that simple !!!

Issue related with Infinite Recursive fetching of data from relationships between Entity classes (Spring Boot JPA Hibernate)

This article is all about how to avoid recursive fetching of data from relationships designed in Database. I’ll explain everything with an example so that you will all understand clearly. Here I’m working with Intellij IDEA and MySQL databases.

1.1 Brief about the tables and the relationship used

Here in my example, I have created two Entity classes and they are connected with a OnetoMany relationship.

One student can have more than one email and this is the scenario taken. StudentId in ContactModel refers to the id field in StudentModel and This is a OnetoMany relationship. StudentModel is the “One” side and ContactModel is the “Many” side.

1.2 The classes before resolving the error

How the Entity classes are designed in Spring boot is given below in images.

StudentModel.java

ContactModel.java

1.3 Issue looks like this

Some data is included in the database and I have run some queries to fetch the data from the contact table.This is how it appears after fetching data,

This is an infinite recursion. This is the issue after designing the entity classes as above.

1.4 How to solve

There are many annotations such as @JsonManagedReference, @JsonBackReference, @JsonIgnore etc, to avoid this infinite recursion.But, when used them creates some errors in fetching and inserting data from and to the entities.

After searching a lot in internet and trying a lot, I came across the best way to solve this. It’s very simple.I used only to one annotation in the “One” side of the relationship.

@JsonIgnoreProperties

This annotation ignores the fields from Jsonification. You need to use this in class level as below,

Actually when fetching data from StudentModel, you don’t need to fetch data from the field “contact”. So, I have asked to ignore that field . This annotation take the ignoring fields in a String array.So you need to give the field-names in an String array as above.

Note: You need to ignore two more fields, “hibernateLazyInitializer”, “handler”, otherwise it gives an error message like this when fetching data

“No serializer found for class org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.ArrayList[0]->com.project.inventoryManagement.Models.ContactModel[\”student\”]->com.project.inventoryManagement.Models.StudentModel$HibernateProxy$bYhYXmwK[\”hibernateLazyInitializer\”])”

Happy Coding without bugs!

Thank you.