Javalin: Get the request attribute or compute it based on the context if absent

Context#attributeOrCompute method get an attribute or compute it based on the context if absent.

Example

String name = (String) ctx.attributeOrCompute("name", context -> {
	return context.attribute("default_name");
});

Find the below working application.

 

Step 1: Create new maven project ‘javalin-compute-attribute-demo’.

 

Step 2: Update pom.xml with maven dependencies.

 

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.sample.app</groupId>
	<artifactId>javalin-compute-attribute-demo</artifactId>
	<version>1.0.0</version>

	<properties>
		<packaging>jar</packaging>
		<jdk.version>17</jdk.version>
		<release.version>17</release.version>
	</properties>

	<dependencies>
		<dependency>
			<groupId>io.javalin</groupId>
			<artifactId>javalin</artifactId>
			<version>5.3.2</version>
		</dependency>

		<dependency>
			<groupId>org.slf4j</groupId>
			<artifactId>slf4j-simple</artifactId>
			<version>2.0.3</version>
		</dependency>


		<dependency>
			<groupId>com.fasterxml.jackson.core</groupId>
			<artifactId>jackson-databind</artifactId>
			<version>2.14.0</version>
		</dependency>

		<dependency>
			<groupId>com.fasterxml.jackson.core</groupId>
			<artifactId>jackson-core</artifactId>
			<version>2.14.0</version>
		</dependency>


	</dependencies>

	<build>
		<plugins>

			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.10.1</version>
				<configuration>
					<source>17</source>
					<target>17</target>
				</configuration>
			</plugin>
			<plugin>
				<groupId>org.apache.maven.plugins</groupId>
				<artifactId>maven-assembly-plugin</artifactId>
				<version>2.6</version>
				<configuration>
					<descriptorRefs>
						<descriptorRef>jar-with-dependencies</descriptorRef>
					</descriptorRefs>
					<archive>
						<manifest>
							<addClasspath>true</addClasspath>
							<mainClass>com.sample.app.App</mainClass>
						</manifest>
					</archive>
				</configuration>
				<executions>
					<execution>
						<id>assemble-all</id>
						<phase>package</phase>
						<goals>
							<goal>single</goal>
						</goals>
					</execution>
				</executions>
			</plugin>
		</plugins>
	</build>
</project>

 

Step 3: Define JsonUtil class.

 

JsonUtil.java

 

package com.sample.app.util;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JsonUtil {

	public static String getJson(Object obj) throws JsonProcessingException {
		ObjectMapper mapper = new ObjectMapper();
		return mapper.writeValueAsString(obj);
	}

}

Step 3: Define main application class.

 

App.java

package com.sample.app;

import java.util.HashMap;
import java.util.Map;

import com.sample.app.util.JsonUtil;

import io.javalin.Javalin;
import io.javalin.http.HttpStatus;

public class App {

	public static void main(String[] args) {

		Javalin javalin = Javalin.create();

		javalin.get("/", (ctx) -> {
			ctx.result("Welcome to Javalin programming!!!!");
		});

		javalin.get("/attributes-demo", (ctx) -> {

			ctx.attribute("id", 123);
			ctx.attribute("default_name", "no_name");

			Integer id = ctx.attribute("id");
			String name = (String) ctx.attributeOrCompute("name", context -> {
				return context.attribute("default_name");
			});

			Map<String, Object> map = new HashMap<>();

			map.put("id", id);
			map.put("name", name);

			String json = JsonUtil.getJson(map);

			ctx.result(json).status(HttpStatus.OK);
		});

		javalin.start(8080);
	}

}

Total project structure looks like below.

Build the project

Navigate to the folder, where pom.xml is located and execute the command ‘mvn package’.

 

Upon successful command execution, you can see a ‘javalin-compute-attribute-demo-1.0.0-jar-with-dependencies.jar’ file.

$ls ./target/
archive-tmp
classes
generated-sources
generated-test-sources
javalin-compute-attribute-demo-1.0.0-jar-with-dependencies.jar
javalin-compute-attribute-demo-1.0.0.jar
maven-archiver
maven-status
test-classes

 

Run the application

Execute below command to run the application.

 

java -jar ./target/javalin-compute-attribute-demo-1.0.0-jar-with-dependencies.jar

$java -jar ./target/javalin-compute-attribute-demo-1.0.0-jar-with-dependencies.jar
[main] INFO io.javalin.Javalin - Starting Javalin ...
[main] INFO org.eclipse.jetty.server.Server - jetty-11.0.13; built: 2022-12-07T20:47:15.149Z; git: a04bd1ccf844cf9bebc12129335d7493111cbff6; jvm 17.0.5+9-LTS-191
[main] INFO org.eclipse.jetty.server.session.DefaultSessionIdManager - Session workerName=node0
[main] INFO org.eclipse.jetty.server.handler.ContextHandler - Started i.j.j.@30ee2816{/,null,AVAILABLE}
[main] INFO org.eclipse.jetty.server.AbstractConnector - Started ServerConnector@26a7b76d{HTTP/1.1, (http/1.1)}{0.0.0.0:8080}
[main] INFO org.eclipse.jetty.server.Server - Started Server@1fc2b765{STARTING}[11.0.13,sto=0] @404ms
[main] INFO io.javalin.Javalin - 
       __                  ___          ______
      / /___ __   ______ _/ (_)___     / ____/
 __  / / __ `/ | / / __ `/ / / __ \   /___ \
/ /_/ / /_/ /| |/ / /_/ / / / / / /  ____/ /
\____/\__,_/ |___/\__,_/_/_/_/ /_/  /_____/

       https://javalin.io/documentation

[main] INFO io.javalin.Javalin - Listening on http://localhost:8080/
[main] INFO io.javalin.Javalin - You are running Javalin 5.3.2 (released January 22, 2023).
[main] INFO io.javalin.Javalin - Javalin started in 277ms \o/

Test the api

Execute below command to test the api.

curl --location --request GET 'http://localhost:8080/attributes-demo'

$curl --location --request GET 'http://localhost:8080/attributes-demo'
{"name":"no_name","id":123}

You can download the application from this link.

  

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