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# Spring-Hibernate1

## 6. Fetch Types: Eager vs Lazy Loading

* Eager will retrieve everything
* Lazy will retrieve on request

```java
@OneToMany(fetch=FetchType.LAZY, mappedBy="instructor")
private List<Course> courses;
```

| Mapping     | Default Fetch Type |
| ----------- | ------------------ |
| @OneToOne   | FetchType.EAGER    |
| @OneToMany  | FetchType.LAZY     |
| @ManyToOne  | FetchType.EAGER    |
| @ManyToMany | FetchType.LAZY     |

Note:

If the Hibernate session is closed, and you attempt to retrieve lazy data Hibernate will throw an exception.

### 6.1 Retrieve lazy data using

* option 1: session.get and call appropriate getter method(s)
* option 2: Hibernate query with HQL

these two are most common.

## 7 @OneToMany

## 8. Specialized Annotation for DAOs

#### @Component, @Repository, @Service and @Controller annotations

1\) The`@Component`annotation marks a java class as a bean so the component-scanning mechanism of spring can pick it up and pull it into the application context. To use this annotation, apply it over class as below:

| `@ComponentpublicclassEmployeeDAOImplimplementsEmployeeDAO {...}` |
| ----------------------------------------------------------------- |

2\) Although above use of`@Component`is good enough but you can use more suitable annotation that provides additional benefits specifically for DAOs i.e.`@Repository`annotation. The`@Repository`annotation is a specialization of the`@Component`annotation with similar use and functionality. In addition to importing the DAOs into the DI container,**it also makes the unchecked exceptions (thrown from DAO methods) eligible for translation**into Spring`DataAccessException`.

3\) The`@Service`annotation is also a specialization of the component annotation. It doesn’t currently provide any additional behavior over the`@Component`annotation, but it’s a good idea to use`@Service`over`@Component`in service-layer classes because**it specifies intent better**. Additionally, tool support and additional behavior might rely on it in the future.

4\)`@Controller`annotation marks a class as a Spring Web MVC controller. It too is a`@Component`specialization, so beans marked with it are automatically imported into the DI container. When you add the`@Controller`annotation to a class, you can use another annotation i.e.`@RequestMapping`; to map URLs to instance methods of a class.

### 8.1 Spring @Transactional

* Spring provides an @Transactional annotation
* Automagically begin and end a transaction for your Hibernate code
* * No need for you to explicitly do this in your code(no need for **session.beginTransaction(), session.getTransaction().commit()**).
* This Spring magic happens behind the scenes to handles transaction management.

### 8.2 @Repository

use for DAO Implementations.