Java Meta-Inf Services

What is the pu开发者_开发问答rpose of Meta-INF services in Java ?

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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

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Take a look at the ServiceLoader docs.

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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.