Relational

SQL

SQL Cheat Sheet
SQL Triggers & Stored Procedures
SQL Views & Materialized views

PgSQL Functions

PgSQL Functions (Stored Procedure)

MySQL

MySQL Administration
MySQL User Privileges

No-relational

Mongo

MongoDB

ElasticSearch

ElasticSearch

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This are my Java-related notes. Here I have all the knowledge I refer to when I have doubts about how to use or how to implement a framework / feature I’ve already implemented once.

Version changes

Interesting changes, new functionality and APIs that come to Java with each new version. They don’t include the full changes but the ones I deemed most useful or most interesting.

From Java 8 to Java 11
Java12
Java13

Experience

Small, functional snipets on how to implement a determined feature.

Java experience sheet
How to create a database intermediate table
Java date time API
New script files in Java

Frameworks

How to use and implement determined frameworks in a Java project (using Maven).

Spring in Action (Book)
Spring Cache
Spring Beans
Thymeleaf
Spring Cors

Maven (builder)
Testing (JUnit, TestNG, Mockito)
Vert.x (microservices)
Lombok (builder)
MapStruct (mapper)
Liquibase (database version control)

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MapStruct is a Java Bean Mapper.

It contains functions that automatically maps between two Java functions. We only need to create the interface, and the library will automatically create a concrete implementation.

Implementation

config

pom.xml modifications for maven

<dependency>
	<groupId>org.mapstruct</groupId>
	<artifactId>mapstruct</artifactId>
	<version>1.3.1.Final</version>
</dependency>

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(This are my notes taken from the book Spring in Action 5th Edition)

Spring parts

• Spring core Provides the core container, dependency injection framework, Spring MVC (Rest APIs), Spring’s web framework and support for template-based JDBC and reactive programming with Spring WebFlux.
• Spring boot Starter dependencies and autoconfiguration.
• Spring data Provides the ability to define your application’s data repository as Java interfaces. Works with relational (JPA), document (Mongo) and graph (Neo4j) databases.
• Spring security Authentication, authorization and API security.
• Spring integration & Batch helps integrate with other applications
• Spring cloud helps with microservices

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The main improvements over previous classes are Thread safety and ease of use.

LocalDate

Represents a date in ISO format (yyyy-MM-dd) without time.

// Declaration
final LocalDate ld1 = LocalDate.now();
final LocalDate ld2 = LocalDate.of(2015, 02, 20);
final LocalDate ld3 = LocalDate.parse("2015-02-20");

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Both text blocks and switch expressions still are preview features!

Text Blocks

New way to use multiline strings. They start with 3 " and a newline.

String jsonBlock = """
{
  greeting: "Hello",
  audience: "World"
}
"""

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CORS (Cross-Origin Resource Sharing)

It’s a mechanism to let a web application running at one domain, protocol or port have permission to access resources from a server at a different one.

This is needed if you have, for example, a Frontend running on port :3000 (React) consuming a Backend API running on port :34831 (custom port for Spring). Unless CORS are set, FE will not be able to access BE resources.

In Spring

It’s possible to enable them for a single RestResource or globally for the whole application.

(This example has been done in Kotlin)

By RestResource

@RestController
@RequestMapping("courses")
@CrossOrigin("http://localhost:3000")
class CourseRestResource {
  // dependencies and methods
}

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Is possible to auto-generate builders for a Java class using @Builder lombok annotation. They’re really simple though and do not provide auto-filling. They just create an API to fill them with test data.

All we need is to put the annotation into a Java class

@Builder
@Getter
@Setter
@NoArgsConstructor
@AllArgsConstructor
public class PersonDto {

  private long id;

  private String name;
  private int age;

  @Singular
  private Set<String> hobbies;

}

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(This was done with MySQL, Hibernate and Lombok)

Setting: We have two Entities, Category and Code. Some categories must contain x codes, others cannot contain y codes and we want to leave open the possibility to “must contain z but not p” at the same time.

How: At database level we’ll have 4 tables with the following structure

client_category

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Thymeleaf is an HTML template engine, which provides full Spring support.

<h3>Designate your wrap:<h3>
  <div th:each="ingredient: ${wrap}">
    <input name="ingredients" type="checkbox"
            th:value="${ingredient.id}" />
    <span th:text="${ingredient.name}">ING</span>
    <br/>
  </div>

Operators

• @{} produces a context-relative path to the /static/ folder

Simple Tags

• th:src="@{images/taco.png}" retrieves an image with relative path from /static/
• th:href="@{/styles.css}" retrieve a css file

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Switch expression

It has been revamped to act as an expression. It removes the usage of break.

switch(day) {
  case SATURDAY, SUNDAY -> System.out.println(1);
  case TUESDAY, FRIDAY -> System.out.println(2);
  case THURSDAY, MONDAY -> System.out.println(3);
  case WEDNESDAY -> System.out.println(4);
}

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A spring bean is the basic building block of a Spring App. In its basis, it’s an Object which Spring Framework manages at runtime.

This management includes:

• Creating an Object
• Filling dependencies
• Intercepting method calls
• Destroying the Object

Define a Spring Bean

There’re 3 ways:

• declare it with @Component annotation
• @Bean annotation at a Bean Factory
• .xml config. file (old way)

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(This uses > java11)

To run a file as Java, we don’t need to do anything special to the .java file. Just write a class with a main() method and call it with java --source 11 file.java

Linux Shebang

To start it as a script in Linux we need to add java’s shebang #!/opt/jdk-11/bin/java --source 11 and do it executable chmod +x file.java. The shebang may need to be replaced if the java path is different.

Important

If we’re starting a java file as a script, the file’s name cannot end with .java or it won’t work.

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Read files > 1 GB lazily

This reads big files (>200 MBs) sequentially and without loading the whole File in memory. This way we’re able to read text files on the Gigabyte level. This example was done reading from a remote SFTP server.

final ChannelSftp sftpClient = this.connect();
final InputStream is = sftpClient.get(file);
final InputStreamReader isReader
      = new InputStreamReader(is);  

try (final BufferedReader bffReader  
      = new BufferedReader(isReader)) {
  bffReader.lines()
        .forEach(this::doAction);
} catch(final IOException ex) {
  log.error("bla", ex);
}

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Compile & execute:

mvn clean install  
java -jar target/[substitute_with_name]-fat.jar -cluster

Standard vs Worker Verticle

Concurrency is handled completely by Vert.x

When created, a standard verticle will have one Event-loop assigned to it (it’ll always use the same) and the start method it’s called within that Event-loop. If it calls other Handlers, it can guarantee that they’ll be executed in the same Event-Loop
Meanwhile, a worker verticle will have a different Thread assigned to it, everytime it wants to perform a Job.
If you’re able to use a standard verticle for non-blocking jobs, you’ll save resources every time you execute code with it.

A standard verticle runs in an Event-Loop thread (one of many). If they’re completely blocked, the whole Program will be blocked and it will just halt. On the other side, the worker verticles run on a different Thread than the main event-loop, so they’re perfect to execute blocking code (another option is an inline .executeBlocking() call). They will never be executed by more than one Thread simultaneously, but they may be executed each time by different Threads.

The downside of using always workers, is that the max. concurrency achievable is much lesser than using normal verticles + workers. With a lot of blocking tasks, you may create a processing queue.

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Codearte’s Catch exception

The only thing it does, it’s to do a bit easier to test and assert for exceptions in a Test Driven Development-like way. To use together with AssertJ. It only has two methods which are useful to me:

// When
BDDCatchException.when(this.instance)  
	.methodWhichThrowsException();

// Then
Assertions  
.assertThat(BDDCatchException.caughtException())  
.isNotNull()  
.isExactlyInstanceOf(IndexOutOfBoundsException.class);

Reference

https://github.com/Codearte/catch-exception

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This is a list of the changes at Java’s API I found interesting or that I may use frecuently. Not all the changes from Java9, 10 & 11 are listed here.

Java 9

Java REPL (JShell)

It stands for Java Shell. It’s used to easily execute and test any Java construction like a class, interface, enum, etc.

Module System

The way we deploy Java-Based applications using jars has a lot of limitations & drawbacks. Some of these are: The JRE & JDK are too big; JAR files are too big to use in small devices and applications; There’s no strong encapsulation, public is open to everyone.

The new Module System introduces new features to avoid all this. More information here.

Factory Methods for Inmutable Collections (List, Map, Set & Map.Entry)

(I’ll use Lists as an example in this file, but this is valid for Maps and Sets too)

Until Java8 we could use Collections.unmodifiableList() to achieve this, but this is really verbose. Now we can do the same with

List inmutableList = List.of("bla", "ble", "bli");

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Spring Cache

A cache itself may be imagined as a key-value map. For a basic Cache we need:

• @EnableCaching tag in @Configuration class
• Declare a CacheManager Bean
• Tag the method to cache w. @Cacheable
• Create a method with @CacheEvict

We may declare +1 cache(s) at the cache manager and select the one we want to use in the method we tag. As key for the cache we may use any conjunction of the parameters given to the method, or if this is a class, any of it’s accessible variables. The cache will only be triggered when the exact key is given again. Then the method won’t be executed and the value will be directly given from the cache. If the parameters don’t match any key, the method will be executed as normal and then the value will be saved into the cache to be returned the next time.

Caution with logs in big apps as they need to be written accordingly.
The hard part is not knowing when to cache something, but to know when to Evict the cache.

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