A registry for service provider instances. Service provider
classes may be detected at run time by means of meta-information in
the JAR files containing them. The intent is that it be relatively
inexpensive to load and inspect all available service provider
classes. These classes may them be used to locate and instantiate
more heavyweight classes that will perform actual work, in this
case instances of ImageReader,
ImageWriter, ImageTranscoder,
ImageInputStream, and ImageOutputStream.
Service providers found on the system classpath (e.g.,
the jre/lib/ext directory in Sun's implementation of
JDK) are automatically loaded as soon as this class is
instantiated.
When the registerApplicationClasspathSpis method
is called, service provider instances declared in the
meta-information section of JAR files on the application class path
are loaded. To declare a service provider, a services
subdirectory is placed within the META-INF directory
that is present in every JAR file. This directory contains a file
for each service provider interface that has one or more
implementation classes present in the JAR file. For example, if
the JAR file contained a class named
com.mycompany.imageio.MyFormatReaderSpi which
implements the ImageReaderSpi interface, the JAR file
would contain a file named:
The service provider classes are intended to be lightweight
and quick to load. Implementations of these interfaces
should avoid complex dependencies on other classes and on
native code.
It is also possible to manually add service providers not found
automatically, as well as to remove those that are using the
interfaces of the ServiceRegistry class. Thus
the application may customize the contents of the registry as it
sees fit.
For more details on declaring service providers, and the JAR
format in general, see the
JAR File Specification.
ImageReader,ImageWriter,ImageTranscoder,ImageInputStream, andImageOutputStream.Service providers found on the system classpath (e.g., the
jre/lib/extdirectory in Sun's implementation of JDK) are automatically loaded as soon as this class is instantiated.When the
containing the line:registerApplicationClasspathSpismethod is called, service provider instances declared in the meta-information section of JAR files on the application class path are loaded. To declare a service provider, aservicessubdirectory is placed within theMETA-INFdirectory that is present in every JAR file. This directory contains a file for each service provider interface that has one or more implementation classes present in the JAR file. For example, if the JAR file contained a class namedcom.mycompany.imageio.MyFormatReaderSpiwhich implements theImageReaderSpiinterface, the JAR file would contain a file named:The service provider classes are intended to be lightweight and quick to load. Implementations of these interfaces should avoid complex dependencies on other classes and on native code.
It is also possible to manually add service providers not found automatically, as well as to remove those that are using the interfaces of the
ServiceRegistryclass. Thus the application may customize the contents of the registry as it sees fit.For more details on declaring service providers, and the JAR format in general, see the JAR File Specification.