Java Reference
In-Depth Information
13.1.2
Creating a framework instance
It's great to have a standard interface for framework implementations, but you can't
instantiate an interface; you need a way to get a concrete implementation class. It isn't
possible for the
OSG
i specification to define a standard class name, so it adopts the
standard Java approach of specifying service-provider implementations in
JAR
files:
META-INF
/services.
In this case,
META-INF
/services refers to a directory entry in a
JAR
file. Just as a
JAR
's
META-INF
/
MANIFEST
.
MF
file contains metadata about the
JAR
file, so does its
META-INF
/services directory. More specifically, it contains metadata about the service
providers contained in a
JAR
file. Here the term
service
isn't referring to an
OSG
i ser-
vice, but to well-known interfaces and/or abstract classes in general. All in all, the con-
cept is similar to the
OSG
i service concept.
The
META-INF
/services directory in a
JAR
file contains service-provider configura-
tion files, which refer to a concrete implementation class for a given service. Concrete
service implementations are connected to their abstract service type via the name of
the file entry in the directory, which is named after the fully qualified service it imple-
ments. For example, a service implementation for the
java.text.spi.DateFormat-
Provider
service would be named
META-INF/services/java.text.spi.DateFormatProvider
The content of this file is the name of the concrete service-implementation class:
org.foo.CustomDateFormatProvider
Figure 13.1 depicts this hypothetical example. At execution time, when a service pro-
vider is required, the code needing it queries the service-provider configuration file
CustomerDateFormatProvider.java
package org.foo;
..
public class CustomDateFormatProvider
implements DateFormatProvider {
...
}
Compile
java.text.spi.DateFormatProvider
org.foo.CustomDateFormatProvider
.class
jar
jar
META-INF/
MANIFEST.MF
services/
java.text.spi.DateFormatProvider
org/
foo/
CustomDateFormatProvider.class
Figure 13.1
The Java META-INF/services approach dis-
covers service providers at execution time by performing
lookups of well-known named resource files to acquire
concrete service-implementation class names.
