Chapter 13: Introduction to the Service Provider Interfaces

What Are Services?

Services are units of sound-handling functionality that are automatically available when an application program makes use of an implementation of the Java^TM Sound API. They consist of objects that do the work of reading, writing, mixing, processing, and converting audio and MIDI data. An implementation of the Java Sound API generally supplies a basic set of services, but mechanisms are also included in the API to support the development of new sound services by third-party developers (or by the vendor of the implementation itself). These new services can be "plugged into" an existing installed implementation to expand its functionality without requiring a new release. In the Java Sound API architecture, third-party services are integrated into the system in such a way that an application program's interface to them is the same as the interface to the "built-in" services. In some cases, application developers who use the javax.sound.sampled and javax.sound.midi packages might not even be aware that they are employing third-party services.

Examples of potential third-party, sampled-audio services include:

Sound file readers and writers
Converters that translate between different audio data formats
New audio mixers and input/output devices, whether implemented purely in software, or in hardware with a software interface

Third-party MIDI services might consist of:

MIDI file readers and writers
Readers for various types of soundbank files (which are often specific to particular synthesizers)
MIDI-controlled sound synthesizers, sequencers, and I/O ports, whether implemented purely in software, or in hardware with a software interface

How Services Work

The javax.sound.sampled and javax.sound.midi packages provide functionality to application developers who wish to include sound services in their application programs. These packages are for consumers of sound services, providing interfaces to get information about, control, and access audio and MIDI services. In addition, the Java Sound API also supplies two packages that define abstract classes to be used by providers of sound services: the javax.sound.sampled.spi and javax.sound.midi.spi packages.

Developers of new sound services implement concrete subclasses of the appropriate classes in the SPI packages. These subclasses, along with any additional classes required to support the new service, are placed in a Java^TM Archive (JAR) archive file with a description of the included service or services. When this JAR file is installed in the user's CLASSPATH, the runtime system automatically makes the new service available, extending the functionality of the Java^TM platform's runtime system.

Once the new service is installed, it can be accessed just like any previously installed service. Consumers of services can get information about the new service, or obtain instances of the new service class itself, by invoking methods of the AudioSystem and MidiSystem classes (in the javax.sound.sampled and javax.sound.midi packages, respectively) to return information about the new services, or to return instances of new or existing service classes themselves. Application programs need not—and should not—reference the classes in the SPI packages (and their subclasses) directly to make use of the installed services.

For example, suppose a hypothetical service provider called Acme Software, Inc. is interested in supplying a package that allows application programs to read a new format of sound file (but one whose audio data is in a standard data format). The SPI class AudioFileReader can be subclassed into a class called, say, AcmeAudioFileReader. In the new subclass, Acme would supply implementations of all the methods defined in AudioFileReader; in this case there are only two methods (with argument variants), getAudioFileFormat and getAudioInputStream. Then when an application program attempted to read a sound file that happened to be in Acme's file format, it would invoke methods of the AudioSystem class in javax.sound.sampled to access the file and information about it. The methods AudioSystem.getAudioInputStream and AudioSystem.getAudioFileFormat provide a standard API to read audio streams; with the AcmeAudioFileReader class installed, this interface is extended to support the new file type transparently. Application developers don't need direct access to the newly registered SPI classes: the AudioSystem object methods pass the query on to the installed AcmeAudioFileReader class.

What's the point of having these "factory" classes? Why not permit the application developer to get access directly to newly provided services? That is a possible approach, but having all management and instantiation of services pass through gatekeeper system objects shields the application developer from having to know anything about the identity of installed services. Application developers just use services of value to them, perhaps without even realizing it. At the same time this architecture permits service providers to effectively manage the available resources in their packages.

Often the use of new sound services is transparent to the application program. For example, imagine a situation where an application developer wants to read in a stream of audio from a file. Assuming that thePathName identifies an audio input file, the program does this:

    File theInFile = new File(thePathName);
    AudioInputStream theInStream = AudioSystem.getAudioInputStream(theInFile);

Behind the scenes, the AudioSystem determines what installed service can read the file and asks it to supply the audio data as an AudioInputStream object. The developer might not know or even care that the input audio file is in some new file format (such as Acme's), supported by installed third-party services. The program's first contact with the stream is through the AudioSystem object, and all its subsequent access to the stream and its properties are through the methods of AudioInputStream. Both of these are standard objects in the javax.sound.sampled API; the special handling that the new file format may require is completely hidden.

How Providers Prepare New Services

Service providers supply their new services in specially formatted JAR files, which are to be installed in a directory on the user's system where the Java runtime will find them. JAR files are archive files, each containing sets of files that might be organized in hierarchical directory structures within the archive. Details about the preparation of the class files that go into these archives are discussed in Chapters 14 and 15, which describe the specifics of the audio and MIDI SPI packages; here we'll just give an overview of the process of JAR file creation.

The JAR file for a new service or services should contain a class file for each service supported in the JAR file. Following the Java platform's convention, each class file has the name of the newly defined class, which is a concrete subclass of one of the abstract service provider classes. The JAR file also must include any supporting classes required by the new service implementation. So that the new service or services can be located by the runtime system's service provider mechanism, the JAR file must also contain special files (described below) that map the SPI class names to the new subclasses being defined.

To continue from our example above, say Acme Software, Inc. is distributing a package of new sampled-audio services. Let's suppose this package consists of two new services:

The AcmeAudioFileReader class, which was mentioned above, and which is a subclass of AudioFileReader
A subclass of AudioFileWriter called AcmeAudioFileWriter, which will write sound files in Acme's new format

Starting from an arbitrary directory—let's call it /devel—where we want to do the build, we create subdirectories and put the new class files in them, organized in such a manner as to give the desired pathname by which the new classes will be referenced:

    com/acme/AcmeAudioFileReader.class
    com/acme/AcmeAudioFileWriter.class

In addition, for each new SPI class being subclassed, we create a mapping file in a specially named directory META-INF/services. The name of the file is the name of the SPI class being subclassed, and the file contains the names of the new subclasses of that SPI abstract class.

We create the file

META-INF/services/javax.sound.sampled.spi.AudioFileReader, which consists of
    # Providers of sound file-reading services 
    # (a comment line begins with a pound sign)
    com.acme.AcmeAudioFileReader

and also the file

META-INF/services/javax.sound.sampled.spi.AudioFileWriter, which consists of
   # Providers of sound file-writing services 
    com.acme.AcmeAudioFileWriter

Now we run jar from any directory with the command line:

jar cvf acme.jar -C /devel .

The -C option causes jar to switch to the /devel directory, instead of using the directory in which the command is executed. The final period argument instructs jar to archive all the contents of that directory (namely, /devel), but not the directory itself.

This run will create the file acme.jar with the contents:

com/acme/AcmeAudioFileReader.class
com/acme/AcmeAudioFileWriter.class
META-INF/services/javax.sound.sampled.spi.AudioFileReader
META-INF/services/javax.sound.sampled.spi.AudioFileWriter
META-INF/Manifest.mf

The file Manifest.mf, which is generated by the jar utility itself, is a list of all the files contained in the archive.

How Users Install New Services

For end users (or system administrators) who wish to get access to a new service through their application programs, installation is simple. They place the provided JAR file in a directory in their CLASSPATH. Upon execution, the Java runtime will find the referenced classes when needed.

It's not an error to install more than one provider for the same service. For example, two different service providers might supply support for reading the same type of sound file. In such a case, the system arbitrarily chooses one of the providers. Users who care which provider is chosen should install only the desired one.