ServiceLoader

Remarks#

ServiceLoader can be used to get instances of classes extending a given type(=service) that are specified in a file packed in a .jar file. The service that is extended/implemented is often a interface, but this is not required.

The extending/implementing classes need to provide a zero argument constructor for the ServiceLoader to instantiate them.

To be discovered by the ServiceLoader a text file with the name of the fully qualified type name of the implemented service needs to be stored inside the META-INF/services directory in the jar file. This file contains one fully qualified name of a class implementing the service per line.

Logger Service

The following example shows how to instantiate a class for logging via the ServiceLoader.

Service

package servicetest;

import java.io.IOException;

public interface Logger extends AutoCloseable {
    
    void log(String message) throws IOException;
}

Implementations of the service

The following implementation simply writes the message to System.err

package servicetest.logger;

import servicetest.Logger;

public class ConsoleLogger implements Logger {

    @Override
    public void log(String message) {
        System.err.println(message);
    }

    @Override
    public void close() {
    }

}

The following implementation writes the messages to a text file:

package servicetest.logger;

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import servicetest.Logger;

public class FileLogger implements Logger {

    private final BufferedWriter writer;

    public FileLogger() throws IOException {
        writer = new BufferedWriter(new FileWriter("log.txt"));
    }

    @Override
    public void log(String message) throws IOException {
        writer.append(message);
        writer.newLine();
    }

    @Override
    public void close() throws IOException {
        writer.close();
    }

}

META-INF/services/servicetest.Logger

The META-INF/services/servicetest.Logger file lists the names of the Logger implementations.

servicetest.logger.ConsoleLogger
servicetest.logger.FileLogger

Usage

The following main method writes a message to all available loggers. The loggers are instantiated using ServiceLoader.

public static void main(String[] args) throws Exception {
    final String message = "Hello World!";

    // get ServiceLoader for Logger
    ServiceLoader<Logger> loader = ServiceLoader.load(servicetest.Logger.class);

    // iterate through instances of available loggers, writing the message to each one
    Iterator<Logger> iterator = loader.iterator();
    while (iterator.hasNext()) {
        try (Logger logger = iterator.next()) {
            logger.log(message);
        }
    }
}