StackOverflowError Springboot OneToMany BiDirectional Mapping

Why is this happening

This is caused by toString() method being present in both the classes and forming cyclic dependency. This leads to infinite recursion calls between Account and Member's toString() methods.

Consider these 2 simple classes. Lombok's @Data generates toString method for all fields by default.

class Member {
    String name;
    Account account;

    @Override
    public String toString() {
        return "Member{" +
                "name='" + name + '\'' +
                ", account=" + account.toString() + //Although string concatenation calls toString() on instance, I thought of adding it to call it out
                '}';
    }
}

class Account {
    String id;
    Member member;

    @Override
    public String toString() {
        return "Account{" +
                "id='" + id + '\'' +
                ", member=" + member.toString() + //Although string concatenation calls toString on instance, I thought of adding it to call it out
                '}';
    }
}

public static void main(String[] args) {
        Account account = new Account();
        Member member  = new Member();
        account.id="1";
        account.member=member;

        member.name ="XYZ";
        member.account=account;

        System.out.println(account);
    }

When you call toString method on one of the classes, say Account it will in turn call toString method of Member and the cycle repeats until you run out of stack memory. Here's the stacktracee:

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.StringBuilder.append(StringBuilder.java:136)
    at com.javainuse.main.Member.toString(AnotherMain.java:23)
    at java.lang.String.valueOf(String.java:2994)
    at java.lang.StringBuilder.append(StringBuilder.java:131)
    at com.javainuse.main.Account.toString(AnotherMain.java:10)
    at java.lang.String.valueOf(String.java:2994)

How can I fix it?

Exclude toString generation for one of the classes from your lombok and that should work.refer this I would recommend not having toString at all for entity classes.