The java.util.ServiceLoader class in Java provides a simple mechanism for discovering and loading Implementations of a service interface from the classpath. It is part of the Java Standard Library and is often used in modular or plugin-based applications where different implementations of a service interface may be available.

Here's how ServiceLoader works:

1. Service Provider Interface

First, you define a service interface that represents a contract or API. This interface defines the methods or behaviour that implementations must adhere to.

To demonstrate the example, I defined a log-service project, and it has LogService interface.

LogService.java

package com.sample.app.log.service;

public interface LogService {
	void log(String msg);
}

2. Service Provider Implementations

Next, you create one or more implementations of the service interface. These implementations are typically packaged as JAR files and made available in the classpath.

To demonstrate the example, I defined two libraries sdk1, and sdk2 which implement LogService interface.

public class Sdk1Logger implements LogService {

	@Override
	public void log(String msg) {
		System.out.println("From Sdk1Logger " + new Date() + " : " + msg);
	}

}

3. Service Configuration File

Include a special file named META-INF/services/{fully-qualified-service-interface-name} in the JAR files containing the implementations. This file lists the fully qualified class names of the service provider implementations, one per line.

com.sample.app.log.service.LogService

com.sample.app.sdk1.log.service.Sdk1Logger

4. Service Loading

ServiceLoader logServicesLoader = ServiceLoader.load(LogService.class);

5. Iterating Over Implementations

Iterator logServiceIterator = logServicesLoader.iterator();

while (logServiceIterator.hasNext()) {
	LogService logService = logServiceIterator.next();
	logService.log("Hello World");
}

Find the below step-by-step procedure to build the working application.

log-service project setup

Create new maven project ‘log-service’ and update pom.xml like below.

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>4.0.0modelVersion>
	groupId>com.sample.appgroupId>
	artifactId>log-serviceartifactId>
	version>1.0.0version>

	build>
		plugins>


			plugin>
				groupId>org.apache.maven.pluginsgroupId>
				artifactId>maven-surefire-pluginartifactId>
				version>3.0.0-M6version>
			plugin>


			plugin>
				groupId>org.apache.maven.pluginsgroupId>
				artifactId>maven-release-pluginartifactId>
				version>2.5.3version>
				configuration>
					localCheckout>truelocalCheckout>
					autoVersionSubmodules>trueautoVersionSubmodules>
				configuration>
			plugin>

		plugins>
	build>
project>

Define LogService interface.

package com.sample.app.log.service;

public interface LogService {
	void log(String msg);
}

Setup sdk1

Create sdk1 maven project and update pom.xml like below.

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>4.0.0modelVersion>
	groupId>com.sample.appgroupId>
	artifactId>sdk1artifactId>
	version>1.0.0version>

	dependencies>

		dependency>
			groupId>com.sample.appgroupId>
			artifactId>log-serviceartifactId>
			version>1.0.0version>
		dependency>
	dependencies>

	build>
		plugins>


			plugin>
				groupId>org.apache.maven.pluginsgroupId>
				artifactId>maven-surefire-pluginartifactId>
				version>3.0.0-M6version>
			plugin>


			plugin>
				groupId>org.apache.maven.pluginsgroupId>
				artifactId>maven-release-pluginartifactId>
				version>2.5.3version>
				configuration>
					localCheckout>truelocalCheckout>
					autoVersionSubmodules>trueautoVersionSubmodules>
				configuration>
			plugin>

		plugins>
	build>
project>

Define Sdk1Logger class.

package com.sample.app.sdk1.log.service;

import java.util.Date;

import com.sample.app.log.service.LogService;

public class Sdk1Logger implements LogService {

	@Override
	public void log(String msg) {
		System.out.println("From Sdk1Logger " + new Date() + " : " + msg);
	}

}

Create a file 'com.sample.app.log.service.LogService' in src/main/resources/META-INF/services folder of sdk1 project.

com.sample.app.log.service.LogService

com.sample.app.sdk1.log.service.Sdk1Logger

sdk2 project setup

Create a new maven project sdk2 and 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>4.0.0modelVersion>
	groupId>com.sample.appgroupId>
	artifactId>sdk2artifactId>
	version>1.0.0version>

	dependencies>

		dependency>
			groupId>com.sample.appgroupId>
			artifactId>log-serviceartifactId>
			version>1.0.0version>
		dependency>
	dependencies>

	build>
		plugins>


			plugin>
				groupId>org.apache.maven.pluginsgroupId>
				artifactId>maven-surefire-pluginartifactId>
				version>3.0.0-M6version>
			plugin>


			plugin>
				groupId>org.apache.maven.pluginsgroupId>
				artifactId>maven-release-pluginartifactId>
				version>2.5.3version>
				configuration>
					localCheckout>truelocalCheckout>
					autoVersionSubmodules>trueautoVersionSubmodules>
				configuration>
			plugin>

		plugins>
	build>
project>

Define Sdk2Logger class.

Sdk2Logger.java

package com.sample.app.sdk2.log.service;

import com.sample.app.log.service.LogService;