Spring Boot Basics.txt

Things to learn more about:
- beans
- component scanning
- class path



Problems w/ Traditional Spring
-----------------------------------------
- keeping up with dependecies, module availability, and version compatibablity is burdensome for us
- deployment has to be done to external web containers/servers


Advantages of SpringBoot
-----------------------------------

[ AutoConfiguration ]
- sets up/configures everything we need within our app
- don't have to deal with xml, java, or annotation based configuration anymore
- if we use the web or "mvc" module, SpringBoot automatically configures the dispatcher servlet and takes care of request mapping for us
- if we use the "orm" module, we no longer have to configure the DataSource or TransactionManager

[ SpringBoot Starters ]
- take care of module availability and version compatiability for us; 
- every project has a spring-boot-starter-parent that contains all of the version information for all of the libraries that will be contained in our project

[ Embedded Servlet Containers ]
- provides embedded web containers for us to deploy to; default is Tomcat but also supports Jetty and Undertow

[ SpringBoot Actuators ]
- provides utility routes/endpoints that we can consume in our app (autoconfig, mappings, info, health, metrics)
- can quickly intergrate 3rd party apps (prometheus) using these endpoints


Configuring JPA


Deploying/Running A SpringBoot Application
-----------------------------------------------------------

[ Inside SpringBoot - Embeded Tomcat Server ]


[ Outsude of SpringBoot - Standalone JAR ]
- in order to run the application on any system we want we must compile our application and get a "jar" artifact. This is the articfact that we will pass to java to run on any remote system that has java installed
 
 // Process
 Execute maven "install" goal
   this compiles out project and spits our a fully packaged jar file for us to execute
   
 Run java and reference the jar file
    java -jar <name_of_project>.jar
    
This will cause our app to run just like it did when we ran it inside of Spring Boot


[ Using JAR Layers - layers.idx - Docker ] 
- build an optimized container image from the contents (layers) of the jar file
- helps slim down the overall size of a Springboot based images; instead of copying "fat" jar

// JAR Layers
Dependencies - all dependencies whose version does NOT contain SNAPSHOT
Spring-Boot-Loader - springboot loader classes
Snapshot-Dependencies - all dependencies whose version DOES contain SNAPSHOT
Application - application classes, resources

// Process 
java -Djarmode=layertools jar <jar> list
java -Djarmode=layertools -jar <jar> extract -- destination layers // performs an extraction to directory called "layers"
    
    



Logging
----------

In Spring Boot all of the starter packages are included for us to log. The default implementation is sl4j.

Create an instance of a Logger in your class
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 private static Logger LOGGER = LoggerFactory.getLogger(ProductController.class);

Set the log level for the application in the application.properties file
  log.level=warn
  log.level.org.org.springframework=error // log for a particular framework in your app
  log.com.com.lowsandbox.restapi.controllers.ProductRestController=error // log a particular package
  
  
  
Health Checks + Metrics
--------------------------------

Spring Boot uses "actuators" to provide us with REST Endpoints that we can consume that provides us rich telemtry about 
our app

Add the actuator starter pack to your pom.xml
        <dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-actuator</artifactId>
		</dependency>
		
Check the endpoint that is exposed

  2021-05-24 08:15:46.038  INFO 39728 --- [           main] o.s.b.a.e.web.EndpointLinksResolver      : Exposing 1 endpoint(s) beneath base path '/actuator' // log line relevant to actuator running 
  
  http://localhost:8080/api/actuator
  
  Response:
    	
 {
  "_links": {
    "self": {
      "href": "http://localhost:8080/api/actuator",
      "templated": false
    },
    "health-path": {
      "href": "http://localhost:8080/api/actuator/health/{*path}",
      "templated": true
    },
    "health": {
      "href": "http://localhost:8080/api/actuator/health",
      "templated": false
    }
  }
}

Update application.properties to expose more health details
  management.endpoint.health.show-details=always
  re-run application and check the /health endpoint again
  
Add the following line to expose ALL of the actuator endpoints
  management.endpoints.web.exposure.include=*
  Visit http://localhost:8080/api/actuator/metrics endpoint
  http://localhost:8080/api/actuator/metrics/<metric_name> // for more details
  
  
 Enabling Security On Endpoints/Routes
 ----------------------------------------------------
 
 It is very simple to protect routes in Spring Boot
 
 Add security starter pack in pom.xml
   <dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-security</artifactId>
  </dependency>
  
Look for line in the console output
  Using generated security password: c8abbda3-9d80-4a11-9ee7-a96589abfb48
  

When you attempted to access any routes, you will be met with a "Login" page
  The username is "user"
  Password is the generated string you see in the output above
  
  
  
Thymeleaf Templates (Server-Side Rendering -- MVC)
-----------------------------------------------------------------------

Thymeleaf is the new standard template engine for rendering static and dynamic html pages. In the past, JSP (Java Server Pages) was the standard, but the language has since labeled this standard deprecated in favor of Thymeleaf

Tymeleaf is very similar to other language specific template engines such as Razor (C#), and Django (Python). These engines make implementing the MVC architectures in each of the respective languages easier


MVC Controller
--------------------

When creating a controller it is important to note the diffierence between the annotations @RestController and @Controller

@RestController - used specifically to declare that your controller will function as a RESTful service providing JSON or XML to it's clients
@Controller - used specifically to declare that your controller will function in a MVC fashion providing views to it's clients in the form of HTML

Create a controller class

@Controller
public class HelloController {

    @GetMapping("/hello")
    public String hello() {
        return "hello"; // name of the "template" you will return to the browser
    }
    
}

Create a hello.html template file

<html xmlns:th="http://www.thymeleaf.org">
    <title>Thymeleaf</title>
    <body>
        <b>Hello Thymeleaf!</b>
    </body>
</html>


Passing Data To A Template
----------------------------------------

Define a new function that will handle the passing data from the model to the view
  @GetMapping("sendData")
    public ModelAndView sendData() {
       ModelAndView mav = new ModelAndView("data"); // name of the template
       mav.addObject("message", "Getting started with Tymeleaf!"); // inject data into the view
       return mav; // return the "data.html" template to the client
    }
    
    
Disabling Caching In Thymeleaf
------------------------------------------

By default, thymeleaf caches responses on the client side so that sometime when changes are made to the template, the client will not pick up those new changes due to caching.

Disabe caching my adding this to application.properties
  spring.thymeleaf.cache=false
  
  
  
Messaging in Java
-------------------------

MOM - Message Oriented Middleware (ActiveMQ, RabbitMQ, AWS SQS)
Sender -----> MOM (Messaging Server) <------ Receiver
(App1)																  (App2)