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

greenmarker
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dialloc
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3 Answers3

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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

samabcde
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Marco Sigismondi
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17

Take a look at the ServiceLoader docs.

kschneid
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    Whilst this may theoretically answer the question, [it would be preferable](//meta.stackexchange.com/q/8259) to include the essential parts of the answer here, and provide the link for reference. – dan1st Dec 23 '19 at 13:51
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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.

Yuresh Karunanayake
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  • I think this is wrong. META-INF does not contain modules? Can you point to an example where they do? – vab2048 May 02 '22 at 22:04