Laura Liparulo's blog -

Spring – search through entities with ExampleMatchers

May 27, 2023

In the *.data.domain package in Spring you can find useful classes to search through the database (if you are using the JPA repository interface) rather quickly.

The Example and ExampleMatcher class will do the trick.

For example, if you’re looking for albums in a Table:

  public List<Album> search(AlbumSearchDTO search) {
      Album probe = new Album();
        if (StringUtils.hasText(search.value())) {
          probe.setTitle(search.value());
          probe.setArtist(search.value());
          probe.setDescription(search.value());
        }
        Example<Album> example = Example.of(probe, //
          ExampleMatcher.matchingAny() //
            .withIgnoreCase() //
            .withStringMatcher(StringMatcher.CONTAINING));
        return repository.findAll(example);
      }

You can pass the Example collection to a findAll method directly, and it will return a subset of rows containing the string …anywhere on the columns.

Check it out!

Spring Database Repository interfaces…

May 26, 2023May 26, 2023

java , Programming , spring

Spring offers you more than a way to get your database access operations easily ready out of the box.

You can find interfaces to extends to create your repository in two packages:

org.springframework.data.repository
org.springframework.data.jpa.repository

In the latter package you can the JPA specific extension of the Repository interface defined in org.springframework.data.repository:

https://docs.spring.io/spring-data/data-jpa/docs/current/api/org/springframework/data/jpa/repository/JpaRepository.html

You can simply create a Repository object as an interface, like:

public interface UserAccountRepository extends Repository<UserAccount,Long>{}

Or you can use a more specific one like:

CrudRepository<T,ID>	Interface for generic CRUD operations on a repository for a specific type
ListCrudRepository<T,ID>	Interface for generic CRUD operations on a repository for a specific type.
ListPagingAndSortingRepository<T,ID>	Repository fragment to provide methods to retrieve entities using the pagination and sorting abstraction.
PagingAndSortingRepository<T,ID>	Repository fragment to provide methods to retrieve entities using the pagination and sorting abstraction.
JpaRepository<T,ID>	JPA specific extension of `Repository`.

You can extend one of them or more together according to your needs.

Tje JPARepository interface extends the CrudRepository and provides you with additional methods, like findAll, saveAndFlush, etc.

.

Setting up Node and npm packages with frontend-maven-plugin

May 25, 2023May 25, 2023

java , Programming

An extraordinary way to get your nodejs, npm and all packages installed and running without any worries is provided by a maven plugin called “frontend-maven-plugin”.

In your pom.xml section, you need to add the plugin and its goal in the build section:

<build>
		<plugins>

			<plugin>
				...
			</plugin>

			<plugin>
				<groupId>com.github.eirslett</groupId>
				<artifactId>frontend-maven-plugin</artifactId>
				<version>1.12.1</version>

				<executions>

					<execution>
						<goals>
							<goal>install-node-and-npm</goal>
						</goals>
						<phase>generate-resources</phase>
					</execution>

					<execution>
						<id>npm install</id>
						<goals>
							<goal>npm</goal>
						</goals>
						<phase>generate-resources</phase>
						<configuration>
							<arguments>install</arguments>
						</configuration>
					</execution>

         		  <execution>
                    <id>npm run-script build</id>
                    <goals>
                        <goal>npm</goal>
                    </goals>
                    <phase>generate-resources</phase>
                    <configuration>
                        <!-- arguments>run-script build</arguments-->
                    </configuration>
                </execution>

				</executions>

				<configuration>
					<nodeVersion>v16.14.2</nodeVersion>
				</configuration>

			</plugin>
		</plugins>
	</build>

Make sure your package.json file is in the root folder.

Then run:

mvn generate-resources

That’s it!

You can find a working example on my githup repo:

https://github.com/lauraliparulo/spring-rest-react-demo

Spring Data – Optimistic locking with @Version

May 23, 2023May 23, 2023

java , Programming , spring , SQL

Spring Data provides pessimistic locking as default locking mechanism.

It means that the database record gets locked exclusively until the operation completes. This approach might be prone to deadlocks and requires a direct connection to the database to be used indipendently.

Optimistic locking comes in play if you have a high-volume system and you don’t want to mantain a database connection for the whole session. This way the client doesn’t maintain a lock and you can use a different connection from a connection pool from time to time, each time you access the resource.

Optimistic locking can be achieved with timestamps or versioning

If you want to use it with Spring, you can use the @Version annotation and add an additional attribute in your entity or base entity class, like:

@MappedSuperclass 

public abstract class Person{

@Version

private Long version;

//other attribututes, getters and setters

   public Long getVersion() {

      return version;

   }

//add setter

}

In the database table you also need to specify the version column:

ALTER TABLE person ADD COLUMN version int NOT NULL DEFAULT 0;

If you execute PUT requests / update the table row, a consistency (and isolation) check will tell you if you are working on the newest version.

If your request has an older version, you will get an error (probably a 500 – internal server one)

Golang – commands with flag

May 17, 2023

go , Programming

Flag is a golang library to parse commands on the command line.

For example, let’s create an Hello World program that gets a name as parameter and returns the string:

package main

import (
	"flag"
	"fmt"
	"os"
)

func main() {

	name := flag.String("name", "", "The name to create the hello world message")
	flag.Parse()

	if *name == "" {
		fmt.Println("Enter your name:")
		flag.Usage()
		os.Exit(1)
	}

	fmt.Println("Hello,", *name)
}

Ton run the command and see the “Hello, Laura” message, assuming the file is called “flags.go”, run:

go run flags.go -name Laura

Golang – using Maps

May 16, 2023May 16, 2023

go , Programming

Maps can be defined in Golang with the keywork map.

The type is defined in the square brackets, followed by the value:

For example:

package main

import "fmt"

func main() {

	//variable lenght arrays
	empty_map := map[string]string{}

	fmt.Println(empty_map)

	location_map := map[string]string{
		"city":    "Frankfurt",
		"state":   "Hessen",
		"country": "Germany",
	}

	fmt.Println(location_map)

	coordinates_map := map[string]float32{
		"latitude":  56.7,
		"longitude": 45.6,
	}

	fmt.Println(coordinates_map)

	//print specific values
	fmt.Println(coordinates_map["latitude"])
}

Golang – Slices are Arrays!

May 16, 2023May 16, 2023

go , Programming

Arrays in Golang are called slices:

package main

import "fmt"

func main() {

	//variable lenght arrays
	slice := []int{1, 2, 3, 4}

	slice = append(slice, 4)

	fmt.Println(slice)

	//fixed lenght arrays
	fixed_slice := [4]int{1, 2, 3, 4}
	//cannot append another element - compiler error!
	//fixed_slice = append(slice, 4)

	fmt.Println(fixed_slice)

	//another way to define a slice
	another_slice := make([]int, 10)

	//initially filled with zeros
	fmt.Println(another_slice)

	//add a different number in the first position
	another_slice[0] = 45

	fmt.Println(another_slice)
}

Golang – using pointers

May 16, 2023May 16, 2023

go , Programming

Golang also uses pointers, but it doesn’ need any memory allocation code or anything.

It’s very straightforward.

The ampersand & creates a pointer, and the asteryx * allows you to get the value referenced by the pointer.

Simply:

package main

import (
	"fmt"
)

func main() {
	s := "this is a string"

	string_pointer := &s

	fmt.Println(s)

	//reference the pointer with asteryx
	fmt.Println("reference by pointer: ", *string_pointer)
	//reference the pointer with asteryx
	fmt.Println("address: ", string_pointer)
}

Error Handling with Golang

May 16, 2023May 16, 2023

go , Programming

In Golang you have to specify an error handling.

For example, you can use the panic keyword:

import (
	"fmt"
	"os"
	"strconv"
)

func main() {

	var sum int

	//exclude for argument from Args
	for _, a := range os.Args[1:] {

		i, err := strconv.Atoi(a)

		if err != nil {
			panic(fmt.Sprintf("Invalid value : %v", err))
		}

		sum += i

	}

	fmt.Printf("Sum = %v\n", sum)

}

As you can see, if you run the code with string parameters you get an error:

go run error_handling.go 1 a c

Then you get:

panic: Invalid value : strconv.Atoi: parsing "a": invalid syntax

goroutine 1 [running]:
main.main()
        C:/Users/lliparul/Desktop/go/3_ERROR_HANDLING/error_handling.go:19 +0x159
exit status 2

Inspired by the course “Getting started with Go Programming Language” by Matthew Spaulding (Packt Publishing)

Introduction to Golang

May 16, 2023May 16, 2023

go , Programming

Go was developed by some developers at Google and meant to be a better system programming language than C and C++.

It’s compiled and not interpreted.

It’s fast and suitable for concurrency.

Go uses vendoring for depedepency management. It means that the cofe of the dependencies is included in the project, once the dependency is downloaded.

Hello world program

A simple Hello world program (a file called “hello_world.go”) looks like this:

package main

import "fmt"

func main() {
	fmt.Println("Hello, World!")
}

As you can see the main function is the entry point to run golang applications.

Once golang is installed on your machine, you can simply run:

go run hello_world.go

The package fmt is a fundamental library to deal with strings, etc.

Assignments, data types, control structures

To assign values, columns and equal simbols are used. You can use data types like int, but also omit them.

a := 5  
var b int = 4

Strings always use double quotes.

fmt.Println("a is negative!")

Of course you have conditional clauses like, if and switch:

if a < 0 {
		fmt.Println("a is negative!")
	} else if a > b {
		fmt.Println("a is bigger than b")
	}

switch a {
	case 10:
		fmt.Println("It's 10")
	case 5:
		fmt.Print("It's five")
	}

And three types of for loops.

Simple loop:


	for i := 0; i < 4; i++ {
		for i := 0; i < 4; i++ {
		fmt.Println(i)
	}

Range loop:

for _, i := range []int{1, 2, 3, 4} {
		fmt.Println(i)
	}

Infinite loop:

	for {
		fmt.Println("this is infinite!")
	 }

Laura Liparulo's blog

Spring – search through entities with ExampleMatchers

Spring Database Repository interfaces…

Setting up Node and npm packages with frontend-maven-plugin

Spring Data – Optimistic locking with @Version

Golang – commands with flag

Golang – using Maps

Golang – Slices are Arrays!

Golang – using pointers

Error Handling with Golang

Introduction to Golang

Linux Day 2011 Call for Papers (IT)

Discovering Sqlite by Banshee media player

First steps in Sqlite and Perl

Sqlite database backup: the .dump command

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