[Complete Guide to Java] How to Create Your Own ORM Library | Built with Annotations and Reflection!

"I'm using Hibernate and JPA, but I want to know how they work."
"I want to create a process that automatically generates SQL."
For those who feel that way, we recommendMy experience creating an ORM library in Javais.

In this article, we will use reflection, annotations, and dynamic generation of SQL strings to
How to create a simple ORM library that even beginners can buildof,
Easy to understand explanation with codeI will do it.

What is an ORM Library?

A mechanism for easily handling SQL operations in code

Conclusion: ORM is a mechanism that links databases and Java classes.

Traditionally, database operations required writing SQL statements.
But with an ORM, you can do things like this without having to write SQL statements:

• Retrieving data (SELECT)
• Data registration (INSERT)
• Update data (UPDATE)
• Delete data (DELETE)

This means:Maintainability, readability, and safetywill be greatly improved.

Understand the basic structure of your own ORM

Design a minimally functional configuration

Conclusion: A simple ORM can be built with just the following four files.

src/ ├─ Entity.java // Annotations for entities ├─ Column.java // Annotations for columns ├─ OrmManager.java // ORM body (registration, retrieval, etc.) └─ User.java // Actual entity (User table)

Annotation definition: Entity.java / Column.java

import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.TYPE) public @interface Entity { String table(); }

import java.lang.annotation.*; @Retention(RetentionPolicy.RUNTIME) @Target(ElementType.FIELD) public @interface Column { String name(); }

Example of an entity class: User.java

@Entity(table = "users") public class User { @Column(name = "id") public int id; @Column(name = "name") public String name; @Column(name = "email") public String email; }

ORM main processing: OrmManager.java

import java.lang.reflect.*; public class OrmManager { public static String generateInsertSQL(Object obj) throws Exception { Class cls = obj.getClass(); if (!cls.isAnnotationPresent(Entity.class)) { throw new Exception("Entity annotation not found"); } Entity entity = cls.getAnnotation(Entity.class); StringBuilder columns = new StringBuilder(); StringBuilder values = new StringBuilder(); for (Field field : cls.getDeclaredFields()) { if (field.isAnnotationPresent(Column.class)) { Column col = field.getAnnotation(Column.class); field.setAccessible(true); Object value = field.get(obj); columns.append(col.name()).append(", "); values.append("'").append(value).append("', "); } } String columnStr = columns.substring(0, columns.length() - 2); String valueStr = values.substring(0, values.length() - 2); return String.format("INSERT INTO %s (%s) VALUES (%s);", entity.table(), columnStr, valueStr); } }

Usage example: Main.java

public class Main { public static void main(String[] args) throws Exception { User user = new User(); user.id = 1; user.name = "Taro"; user.email = "taro@example.com"; String sql = OrmManager.generateInsertSQL(user); System.out.println(sql); } }

Output:

INSERT INTO users (id, name, email) VALUES ('1', 'Taro', 'taro@example.com');

Common errors and solutions

• Annotations are ignored
  → @Retention(RetentionPolicy.RUNTIME) Not set
• IllegalAccessException in Field.get()
  → field.setAccessible(true) Be sure to execute
• Nulls in SQL
  → null Check it "NULL" Added conversion processing to

Enhance practicality with application ideas

• Added SQL execution function (Connection.prepareStatement() etc.)
• Supports generation of update, delete and search SQL
• Automatic primary key detection and validation added
• @IdDefine the primary key using annotations

Summary of the completed configuration

• Entity/Column.java: Class and DB bridging annotation
• OrmManager.java: Logic for constructing SQL from annotations
• User.java: Entity instance
• Main.java: Code for checking operation

Summary: If you understand the mechanism, you can apply it freely

In this article,How to create your own ORM library in JavaWe explained the following.

Lessons learned:

• How to use reflection
• Defining and Using Annotations
• Automation logic for constructing SQL

Advanced frameworks such as Hibernate are also supported.
The fundamental mechanism is a stack of structures like those discussed in this article.is.

Once you try making one yourself, your understanding of how to use it will be much deeper.
Please try it yourself!ORM development based on the mechanismGive it a try!