fundas.txt

ORM Concepts:
- Used for saving data to database without any queries
- Java 		   ORM
  Class  -->  Table Schema
  Objects ->  Rows
- 3 steps to build hibernate application:
	1. Add the configuration(xml) file
	2. Define the entity class
	3. Define the main or application class to perform DB operations
	
Spring 5:
- Component/Bean based programming
- Dependecy Injection : Giving the control of object creation to Spring so that dynamicity is achieved. It is done 
						through auto wiring
- Autowiring : When an already dependency injected class/[NOT interface] has objects from another class then we
		don't instantiate it using new keyword rather we annotate the another class with @Component and it's object
		in the present class as @Autowired so that the object creation control is given to Spring.
	- autowiring setters and construtors are different cases. U have to autowire the setter and not the object
	  in order to reflect the new changes but incase of constructor u don't need to autowire the constructor 
	  rather u have to autowire object.
	  L-373 : Autowire Scenerios
	- @Qualifier("Component_name") -> u can specify which specific compnonent u want to autowire to in the
	"Compnent_name" to reduce ambiguity between several component. This is used for java files which are not the
	main or app.java. In main file we used context to distinguish between components.
- Bean: Java Object created by Spring
	- @Bean : Now u can remove the component annotation from different classes and use the @Bean annotaion for
		all of them in a single file i.e AppConfig.java
	- Constructor Injection : If two classes implementing the same interface are very similar, then u can combine 
	 them into one and use parameterized constructor to differentiate them 
- Server multiple context issue : https://stackoverflow.com/questions/7239613/multiple-contexts-with-the-same-path-error-running-web-service-in-eclipse-using
- Converting Dynamic Web Project to Maven : L384
- When creating maven project:
	1. group id -> package name
	2. artifact name -> project name

Spring MVC:
- In src/mainController.java
	@RequestMapping("displayname") //default method is get so we don't need to specify method and when there is 
	//only one parameter, it defaults to value
	//displayname is same as the name given in action param of form tag
- @RequestMapping() -> Responsible for handling URLs get and post request
- @RequestParam() -> Acts like bodyparser in node.js. Relieve us from using the legacy HttpServletRequest request.getParameter()
  					 method.
  	- U have to use model.addAttribute() now instead of request.setAtrribute()
  	- U also have to use ${} instead of <%= %> in ur view file now
- ModelAndView can be used instead of model and is the better approach although both do the same job.
- @GetMapping() -> Handles Get request Specifically
- used jstl core tag <c:> </c:> for implementing foreach loop in jsp view file -> L-387
		-u have to specify required taglib tag for using the core tag at top:
			<%@ taglib prefix = "c" uri = "http://java.sun.com/jsp/jstl/core" %>

***SPRING FORM ELEMENTS***:
*****************************
- spring-form JSP Tag Library : https://docs.spring.io/spring/docs/4.2.x/spring-framework-reference/html/spring-form-tld.html
- SPRING FORM TAGLIB: <%@ taglib prefix="form" uri="http://www.springframework.org/tags/form" %>
- ModelAndView : Can be use for both model and views.
				- U can add a class object here to pass it to your view path and later again pass it to different
					route view but now with some data in it.
				- i.e u can use ModelAndView to pass object or model to different views.
- How form works:
	-By path : u specify the class fields in these paths attr of forms so that they can be fields are given values
				when u submit the form. These are done automatically by spring. U have to specify getter and setter
				for this to work in ur model class
	- U can also use a Map<> to represent the classfields of model so u don't have to manually specify multiple
		classfields for multiple paths as in Project : Spring Form Radio Buttons
	- in case of Map, the value is displayed to the user while key is assigned to the classfield.
	
- Adding External Resource to Eclipse like Spring IDE:
	- now u can display without creating route for it and also access local files using the spring mvc plugin or
	 namespace:
		- Project : Spring Add External Resource
		- L-398
	- ${pageContext.request.contextPath} -> for getting the current context path for accessing local files
			- equivalent to __dirname.path in node

- Form Validation Using Hibernate:
	- Add hibernate validator to your maven
	- Add the condition to your model class
	- Add @valid and BindingResult parameter to your @PostMapping Function
	- Project : Spring Form Validation using Hibernate L-402

- How to let the user refill a form if the hibernate validatpr condition are not met:
	- Simply create a ModelAndView object and return if BindingResult has error
	- Project : Spring Form Repopulate; L-403

- Alternative to ModelAndView : 
	- U can also use ModelMap to pass values to views L-404

- Making the controller more simpler:
	- By shifting the map objects from the controller directly to the view files.
	- U should do this if u r dealing with STATIC values
	- STATIC CONTENT GOES TO VIEW FILE
	- DYNAMIC CONTENT WHICH REQUIRES LOGIC AND DATA FROM DATBASE SHOULD BE DECLARED IN CONTROLLER AND THEN 
		PASSED TO THE VIEW.
	- L-404
	
- How to add error message:
	1. Add @ModelAttribute("objectName") to ur ModelAndView object returning Post class
	2. Add <form:errors path="name" /> to ur form
	3. U can add a custom error message in the validator annotation parameter in ur model class

- Other validators : https://javaee.github.io/javaee-spec/javadocs/javax/validation/constraints/package-summary.html
	- most used - @Email for a correct email address; L-406
	
- Spring Database Concept:
	- Web Browser --> |Security Layer - Controller - Service Layer - DAO(Data Access Object)| --> Database
		


**RESTful Api(JAX-RS)**:
***************************
- REST -> Representational State Transfer
- Resource based URIs
- HTTP Status Codes:
	1xx -> Informational
	2xx -> Successful
	3xx -> Redirection
	4xx -> Client Error
	5xx -> Server Error
- HTTP		CRUD
  GET  -->	READ (Read)     Safe/Repeatable (Idempotent)
  DELETE -> DELETE (Write)	Safe/Repeatable (Idempotent)
  PUT  -->  UPDATE (Write)	Safe/Repeatable (Idempotent)
  POST -->  CREATE (Write)  NON REPEATABLE (Non-Idempotent)

- HATEOAS -> Hypermedia as the Engine of Application State 
	- Hypermedia(links to other media) derives the application.
- Richardson Maturity Model -> categorisation of restful APIs.
	- L-442
	- Level 0: Not a RESTful API
	- Level 1: Multiple URI and Single verb (POST) (Partially RESTful)
	- Level 2: Multiple URIs and Multiple Verbs (HTTP Methods) (Partially RESTful)
	- Level 3: URIs, HTTP and HATEOAS (Fully RESTful) 
- RESTful API are agreement between client and server

- PATH Params can be specified in a URI by 
	..something/{path_param}
	public String func(@PathParam(path_param) String x){
		return x; //x == path_param
	}
	
- A REST API
	1. Produces : return some response like GET
	2. Consumes : u must provide data in XML or Json format to perform operation in the server like POST and PUT

- For REST API to return XML response:
	1. Annotate the entity or the db schema class as @XmlRootElement
	2. In the controller or resource class, annotate every xml response returning methods as 
		@Produces(MediaType.APPLICATION_XML) or @Consumes(MediaType.APPLICATION_XML)

- For REST API to return JSON response:
	1. In pom.xml file uncomment:
		<dependency>
            <groupId>org.glassfish.jersey.media</groupId>
            <artifactId>jersey-media-json-binding</artifactId>
        </dependency>
	2. No need to Annotate the entity or the db schema class.
	3. In the controller or resource class, annotate every Json response returning or consuming methods as 
		@Produces(MediaType.APPLICATION_JSON) or @Consumes(MediaType.APPLICATION_JSON)
		
- For Rest API to handle filters ie /.../..?name=car
	1. Add @QueryParam() to ur resource method and change service and dao layer accordingly
	
- Reading header and cookie values using @HeaderParam & @CookieParam:
	L- 469

- Both header and cookie values as well MANY OTHER IMPORTANT VALUE like the uri path etc. can be retrieved
	using the @Context annotation : L-470  
	
- For setting the auto-increment Id of an entry properly annotate the id field in the entity class with:
 	@GeneratedValue(strategy=GenerationType.IDENTITY)

- How to add the location i.e the uri path to the header of an API response:
  L-472, Project: Rest Location Header
  
- To return multiple type of mediatype responses:
	@Produces(MediaType.TEXT_PLAIN, MediaType.APPLICATION_JSON)

- To map a Generic Exception Class method to any other class use the annotation :
	@Provider

- The generic exception mapper can be called by any exception. U can also build specific and custom 
	exception mapper but it can be called only by the specific Exception.

- How to explicitly tell hibernate that a class field is not a database column:
	@Transient

- To build a url for another class resource from a current class resource u can use .path() for class and method
	level paths:
		uri.getBaseUriBuilder()
				.path(ProductsResource.class) //to get the class level path
				.path(ProductsResource.class, "getProductsByBrand") //to get the method level path
				.resolveTemplate("brandId", brandId) //to resolve a dynamic or variable path value
				.toString();

- Jersey Moxy Maven : Helps to convert object to Json

- Alternate to web.xml configuration -> annotaion based java config file : L-484

- MessageBodyWriter is for making the server understand our code which may have different codebase.

- To make an object last longer in the memory i.e not creating new object everytime for get requests, use the
	annotation:
	@Singleton

- When u use @Singleton, then class level variables and annotations values should be initialised beforehand,
	but method level annotation values need not be initialised as they are just placeholder 
  SO if u want to use class level annotation variables in ur methods don't use @Singleton annotation.
  
- If u want to incorporate the value of query params into class object using its constructor then u have to
	use paramConverter and paramConverterprovider:
	L-490,491
	- U can also implement the logic in ParamConverter directly inside ParamConverterProvider so that u will
		have only one file for converters.

***Spring Boot***
====================
- How to generate web.xml files:
	1. Right click on ur project folder
	2. Click Java EE tools
	3. Click Generate Deployment Descriptor Sub
	
- SpringBoot app creates a embedded local tomcat server. So ur application will run on browser even if u stop or delete the server
- To incorporate jsp support to your springBoot app add the maven dependency:	
	tomcat-embed-jasper
		-THE VERSION MUST BE SAME AS YOUR LOCAL EMBEDDED TOMCAT. SO FIRST CHECK IT ON UR MAVEN DEPENDENCIES FOLDER
- If u want to move ur view files from web-inf to another subfolder of web-inf say views then u should specify the 
	prefix and suffix values inside ur application.properties which is in src/main/resources:
		spring.mvc.view.prefix=/WEB-INF/view/
		spring.mvc.view.suffix=.jsp
	-application.proerties is for overriding the springBoot defaults, see which properties u can override:
		https://docs.spring.io/spring-boot/docs/2.0.4.RELEASE/reference/html/common-application-properties.html
		
- HOW TO EXTRACT A WAR FILE FROM A MAVEN PROJECT:
	1. run as maven clean
	2. run as maven install
	3. Refresh ur project
	4. If no errors, .war file should be in target folder

- to tell springboot that a value is a variable in the url use the annotation:
	@PathVariable like @PathParam in jersey

- To make an object or variable singleton i.e only a single copy can persist, so that it can be retain by 
	the server temporarily declare it as STATIC
- spring doc for database connectivity : https://spring.io/guides/gs/accessing-data-mysql/

- if u don't have ssl functionality and want to avoid the warnings then in ur application.properties file:
	spring.datasource.url=jdbc:mysql://localhost:3306/blog?useSSL=false

- Easiest way to select port for a spring boot app:
	On ur application.propetries:
		server.port = 8081

- how to communicate between microservices:
	1. Create a rest template
	2. Store the result from another microcservice rest api in rest template
	3. add the result in a ModelAndView object
	4. Display the model and view object on a jsp file.

- How to redirect to a using ModelAndView:
	 return new ModelAndView("redirect: /error");
	 
- How to suppress spring boot default error:
   In ur application.properties:
   	server.error.whitelabel.enabled=false
   	
- To mark an application as an eureka server:
	@EnableEurekaServer
	
- To prevent eureka server from registering itself, on application.properties:
	eureka.client.register-with-eureka=false
	eureka.client.fetch-registry=false
	
- How to @LoadBalanced when u have 2 instances of the same application:
	1. Add the eureka client maven dependency in ur pom.xml
	2. Add the annotation to the rest template class because it is the consumer.
	
***Java***
===========

- Static variables, methods or classes can be accessed without object.
- Nested interface are by default static, so they can be accessed without creating an instance of outer
  class or outer interface.
- Similarly a nested class defined under an interface is static

- Factory method
  The method that returns the instance of a class is known as factory method.
- In java, garbage means unreferenced objects.
- There is only one instance of java.lang.Runtime class is available for one java application.
  The Runtime.getRuntime() method returns the singleton instance of Runtime class.
- Static Synchronization
  If you make any static method as synchronized, the lock will be on the class not on object.




***JUnit***
=============
- Junit4 --> Junit5
	- @Before() -> @BeforeEach
	- @RunWith(MockitoJUnitRunner.class) -> @ExtendWith(MockitoExtension.class)