What is ServiceLoader and how is it used?

Question

I came across the documentation of ServiceLoader and am unclear as to what use cases it suits.

When would one use ServiceLoader?

VGR · Answer 1 · 2022-07-27T12:55:52.720

You use ServiceLoader when you want your program to have a “plugin” functionality. When you want to allow people to customize your application by adding jar files to the classpath which contain implementations of a specific subset of functionality, you can use a ServiceLoader to look for those implementations in the classpath.

ServiceLoader is itself an implementation of the jar SPI specification, which has been around for a long time. (I believe it was introduced in Java 1.3.)

Java SE already uses it for exactly that purpose in a lot of places, including:

additional charsets via CharsetProvider
additional filesystems via FileSystemProvider (see FileSystems)
additional file content type analyzers via FileTypeDetector (see Files.probeContentType)
additional preferences implementations via PreferencesFactory (see Preferences)
custom ResourceBundle.Control via ResourceBundleControlProvider
All subclasses of LocaleServiceProvider:
- additional calendars via CalendarDataProvider and CalendarNameProvider
- additional currency via CurrencyNameProvider
- additional locales via LocaleNameProvider
- additional timezones via TimeZoneNameProvider
additional image formats via IIORegistry
additional printers via PrintServiceLookup
custom scripting engines via ScriptEngineFactory

@AndrewTobilko Yes, ServiceLoader was introduced in 1.6 as a convenient way to accomplish what the jar specification had already documented for many years prior. Before ServiceLoader, one would have to do it manually with something like `classLoader.getResources("/META-INF/services/" + spiClass.getName())`. — VGR, Sep 06 '18 at 21:48

Andrew Tobilko · Answer 2 · 2018-09-06T13:40:40.117

Are you familiar with the principle "Inversion of Control"?

Java implemented it by ServiceLoader. The class is designed to locate implementation classes of an interface on the classpath. You pass in a service interface, you get an implementation(s) of that service.

You might find a good practical example here.

P.S: Even though it's an out-of-the-box solution and a quite simple tool, I think it's outdated and not flexible enough compared to the Spring IoC Container and Google Guice.

score 1 · Answer 3 · answered Sep 06 '18 at 13:19

ServiceLoader is Java's light weight alternative to a full blown IoC container such as Spring, Guice, etc. It has far less bells and whistles than those frameworks, but works well for basic use cases when you just want to find what classes implement an interface.

Most application servers will have some usages of ServiceLoader you can see in practice:

https://github.com/apache/tomee/search?q=ServiceLoader&unscoped_q=ServiceLoader

https://github.com/apache/tomcat/search?q=ServiceLoader&unscoped_q=ServiceLoader

https://github.com/wildfly/wildfly/search?q=ServiceLoader&unscoped_q=ServiceLoader

Thanks for the links. That helps a lot. – armandino Nov 18 '22 at 05:27 — armandino, Nov 18 '22 at 05:27

Artem · Answer 4 · 2019-03-19T10:41:47.490

-2

When I hear something about ServiceLoader, the first thing, which comes to my mind, is JDBC. This technology provides loading JDBC driver classes from a classpath without using Class.forName(Class<?>clazz).

Also, I am sure there are many examples of using ServiceLoader, beside JDBC

edited Mar 19 '19 at 10:41

answered Sep 06 '18 at 13:06

Artem

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What is ServiceLoader and how is it used?

4 Answers4