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joplin notion extension/Web Development/Java/Spring/Extras Telusko.md

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+Extras telusko
+
+- for configuring h2 database(in memory database)
+  - make sure package is there in pom.xml
+  - add following in `application.properties`
+    ```yml
+	spring.h2.console.enabled=true
+	spring.datasource.platform=h2
+	spring.datasource.url=jdbc:h2:mem:telusko
+	```
+	here telusko is name of database
+  - now start your application and go to `localhost:8080/h2~console` and u will get a login page. now click on test connection button
+- for configuring JPA
+  - make sure respective dependency is there and at `application.properties` along with spring.datasource properties one more is mentioned for say mysql 
+`spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQLDialect`
+  - in POJO add annotation `Entity` above class defintion and in instance say `aid` add `@Id` annotation
+  - now a table with fields of those POJO instance name will be created
+  - u can add `data.sql` in `src/main/resources` to add initial data
+  - in order to do crud operations use CrudRepository which is a part of spring data commons which is part of JPA
+  - Create this interface
+   ```java
+	package com.example.demo;
+
+	import org.springframework.data.repository.CrudRepository;
+
+	public interface AlienRepo extends CrudRepository<Other,Integer> {
+
+
+	} 
+
+	```
+	and then in your main file/controller to save data use
+	```java
+	public class HomeController(){
+		@Autowire
+		Alienrepo repo;
+
+		 @RequestMapping("/addAlien")
+		public String addAlien(Alien alien){ 
+			repo.save(alien);
+			return "home";
+		}
+		
+		@RequestMapping("/getAlien")
+		public ModelAndView getAlien(@RequestParam int aid){ 
+			ModelAndView mv= new ModelAndView("showAlien");
+			Alien alien=repo.findById(aid).orElse(new Alien());
+			mv.addObject(alien);
+			return mv;
+		}
+	}
+	```
+- More query methods for interface Repo
+  - In the following code, these other query methods are defined by just declaring then by following certain conventions like `findBy<PropertyName><Operation>` here property like Tech,Aid(first letter must be capital and other as small)
+  - everything is not configured with conventions so  for complex queries `Query` annotation have to be used. `?1` is used instead of `?`  bcz there can be other placeholders too and its to index them.
+  ```java
+	 package com.example.demo;
+
+	 import org.springframework.data.repository.CrudRepository;
+
+	 public interface AlienRepo extends CrudRepository<Other,Integer> {
+
+		List<Alien> findByTech(String tech);
+		 
+		 List<Alien> findByAidGreaterThan(int aid);
+		 
+		 // jpql instead of sql
+		 @Query("from Alien where tech=?1 order by aname")
+		 List<Alien> findByTechSorted(String tech);
+	 } 
+	```