Facilities for loading service providers. A service is
defined by a set of interfaces or abstract classes, and
a service provider gives a concrete implementation of this.
Service providers may be installed as part of the runtime
environment using JAR files in the extension directories,
or may be simply supplied on the classpath.
In terms of loading a service, the service is defined by
a single interface or abstract class which the provider
implements. This may not constitute the entire service,
but is simply a mechanism by which a provider of the
service can be loaded and its capabilities determined.
The variety of possible services means that no more
requirements are made of the service provider other than
that it must have an accessible zero argument constructor
in order to allow an instance to be created.
Service providers are listed in a file named after the
service type in the directory
META-INF/services.
The file contains a list of classes, and must be encoded
using UTF-8. Whitespace is ignored. Comments can be
included by using a
'#' prefix; anything occurring
on the same line after this symbol is ignored. Duplicate classes
are ignored.
The classes are loaded using the same classloader that was
queried in order to locate the configuration file. As a result,
the providers do not need to reside in the same JAR file as the
resource; they merely have to be accessible to this classloader,
which may differ from the one that loaded the file itself.
Providers are located and instantiated lazily, as calls to the
iterator() are made. Providers are cached, and those in
the cache are returned first. The cache may be cleared by calling
reload(). Service loaders always execute in the security
context of the caller, so ideally calls should be made from a trusted
source.
Note that this class is not thread-safe, and that strange errors may
occur as the result of the use of remote URLs occurring on the classpath,
which lead to erroneous web pages.
ServiceLoader.java -- Allows loading of plug-in services.
Copyright (C) 2006, 2007 Free Software Foundation
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