Java META-INF/services

Question

What is the purpose of META-INF/services in Java ?

Provincial

Related question: https://mcmap.net/q/65359/-what-39-s-the-purpose-of-meta-inf/2987755 – Hite

Answer 1

It's intended to store service provider configuration files.

A Service provider is an implementation of a Service Provider Interface packaged as JAR.

A Service loader discover and load all implementations declared in the service provider configuration file.

A configuration file is a file named as the fully qualified name of the interface and its content is a list of fully qualified names of implementations.

Following is an example of provider configuration file for javax.servlet.ServletContainerInitializer that is used by Servlet 3.0 at webapp startup.

org.apache.jasper.servlet.JasperInitializer
org.springframework.web.SpringServletContainerInitializer

In this example

• Tomcat is the Service Loader;
• javax.servlet.ServletContainerInitializer is the Service Provider Interface
• file named javax.servlet.ServletContainerInitializer is the Service Provider configuration file;
• org.apache.jasper.servlet.JasperInitializer and org.springframework.web.SpringServletContainerInitializer are Service providers

When tomcat startup webapp call both

onStartup(java.util.Set<java.lang.Class<?>> types, ServletContext context)

methods on JasperInitializer and SpringServletContainerInitializer classes

Indeterminate

Answer 2

Take a look at the ServiceLoader docs.

Nave

Whilst this may theoretically answer the question, it would be preferable to include the essential parts of the answer here, and provide the link for reference. – Recessive

Answer 3

Before Java 9, ServiceLoader find the implementations for a Service from the file in META-INF/services which has fully qualified name same as the Service interface. It contain list of fully qualified names of the implementations.

From Java 9 It have modules and modules have module descriptors. Those 'module' can define the services and their implementation that a ServiceLoader could load.

Quadriplegia

I think this is wrong. META-INF does not contain modules? Can you point to an example where they do? – Neville