提供采样音频服务

As you know, the Java Sound API includes two packages, javax.sound.sampled.spi and javax.sound.midi.spi, that define abstract classes to be used by developers of sound services. By implementing and installing a subclass of one of these abstract classes, a service provider registers the new service, extending the functionality of the runtime system. This page tells you how to go about using the javax.sound.sampled.spi package to provide new services for handling sampled audio.

There are four abstract classes in the javax.sound.sampled.spi package, representing four different types of services that you can provide for the sampled-audio system:

• AudioFileWriterprovides sound file-writing services. These services make it possible for an application program to write a stream of audio data to a file of a particular type.
• AudioFileReaderprovides file-reading services. These services enable an application program to ascertain a sound file's characteristics, and to obtain a stream from which the file's audio data can be read.
• FormatConversionProviderprovides services for converting audio data formats. These services allow an application program to translate audio streams from one data format to another.
• MixerProviderprovides management of a particular kind of mixer. This mechanism allows an application program to obtain information about, and access instances of, a given kind of mixer.

To recapitulate earlier discussions, service providers can extend the functionality of the runtime system. A typical SPI class has two types of methods: ones that respond to queries about the types of services available from a particular provider, and ones that either perform the new service directly, or return instances of objects that actually provide the service. The runtime environment's service-provider mechanism provides registration of installed services with the audio system, and management of the new service provider classes.

In essence there is a double isolation of the service instances from the application developer. An application program never directly creates instances of the service objects, such as mixers or format converters, that it needs for its audio processing tasks. Nor does the program even directly request these objects from the SPI classes that administer them. The application program makes requests to the AudioSystem object in the javax.sound.sampled package, and AudioSystem in turn uses the SPI objects to process these queries and service requests.

The existence of new audio services might be completely transparent to both the user and the application programmer. All application references are through standard objects of the javax.sound.sampled package, primarily AudioSystem, and the special handling that new services might be providing is often completely hidden.

In this discussion, we'll continue the previous convention of referring to new SPI subclasses by names like AcmeMixer and AcmeMixerProvider.

Providing Audio File-Writing Services

Let's start with AudioFileWriter, one of the simpler SPI classes.

A subclass that implements the methods of AudioFileWriter must provide implementations of a set of methods to handle queries about the file formats and file types supported by the class, as well as provide methods that actually write out a supplied audio data stream to a File or OutputStream.

AudioFileWriter includes two methods that have concrete implementations in the base class:

boolean isFileTypeSupported(AudioFileFormat.Type fileType) 
boolean isFileTypeSupported(AudioFileFormat.Type fileType, AudioInputStream stream) 

The first of these methods informs the caller whether this file writer can write sound files of the specified type. This method is a general inquiry, it will return true if the file writer can write that kind of file, assuming the file writer is handed appropriate audio data. However, the ability to write a file can depend on the format of the specific audio data that's handed to the file writer. A file writer might not support every audio data format, or the constraint might be imposed by the file format itself. (Not all kinds of audio data can be written to all kinds of sound files.) The second method is more specific, then, asking whether a particular AudioInputStream can be written to a particular type of file.

Generally, you won't need to override these two concrete methods. Each is simply a wrapper that invokes one of two other query methods and iterates over the results returned. These other two query methods are abstract and therefore need to be implemented in the subclass:

abstract AudioFileFormat.Type[] getAudioFileTypes() 
abstract AudioFileFormat.Type[] getAudioFileTypes(AudioInputStream stream) 

These methods correspond directly to the previous two. Each returns an array of all the supported file types-all that are supported in general, in the case of the first method, and all that are supported for a specific audio stream, in the case of the second method. A typical implementation of the first method might simply return an array that the file writer's constructor initializes. An implementation of the second method might test the stream's AudioFormat object to see whether it's a data format that the requested type of file supports.

The final two methods of AudioFileWriter do the actual file-writing work:

abstract int write(AudioInputStream stream, 
     AudioFileFormat.Type fileType, java.io.File out) 
abstract int write(AudioInputStream stream, 
     AudioFileFormat.Type fileType, java.io.OutputStream out) 

These methods write a stream of bytes representing the audio data to the stream or file specified by the third argument. The details of how this is done depend on the structure of the specified type of file. The write method must write the file's header and the audio data in the manner prescribed for sound files of this format (whether it's a standard type of sound file or a new, possibly proprietary one).

Providing Audio File-Reading Services

The AudioFileReader class consists of six abstract methods that your subclass needs to implement-actually, two different overloaded methods, each of which can take a File, URL, or InputStream argument. The first of these overloaded methods accepts queries about the file format of a specified file:

abstract AudioFileFormat getAudioFileFormat(java.io.File file) 
abstract AudioFileFormat getAudioFileFormat(java.io.InputStream stream) 
abstract AudioFileFormat getAudioFileFormat(java.net.URL url) 

A typical implementation of getAudioFileFormat method reads and parses the sound file's header to ascertain its file format. See the description of the AudioFileFormat class to see what fields need to be read from the header, and refer to the specification for the particular file type to figure out how to parse the header.

Because the caller providing a stream as an argument to this method expects the stream to be unaltered by the method, the file reader should generally start by marking the stream. After reading to the end of the header, it should reset the stream to its original position.

The other overloaded AudioFileReader method provides file-reading services, by returning an AudioInputStream from which the file's audio data can be read:

abstract AudioInputStream getAudioInputStream(java.io.File file) 
abstract AudioInputStream getAudioInputStream(java.io.InputStream stream) 
abstract AudioInputStream getAudioInputStream(java.net.URL url) 

Typically, an implementation of getAudioInputStream returns an AudioInputStream wound to the beginning of the file's data chunk (after the header), ready for reading. It would be conceivable, though, for a file reader to return an AudioInputStream whose audio format represents a stream of data that is in some way decoded from what is contained in the file. The important thing is that the method return a formatted stream from which the audio data contained in the file can be read. The AudioFormat encapsulated in the returned AudioInputStream object will inform the caller about the stream's data format, which is usually, but not necessarily, the same as the data format in the file itself.

Generally, the returned stream is an instance of AudioInputStream; it's unlikely you would ever need to subclass AudioInputStream.

Providing Format-Conversion Services

A FormatConversionProvider subclass transforms an AudioInputStream that has one audio data format into one that has another format. The former (input) stream is referred to as the source stream, and the latter (output) stream is referred to as the target stream. Recall that an AudioInputStream contains an AudioFormat, and the AudioFormat in turn contains a particular type of data encoding, represented by an AudioFormat.Encoding object. The format and encoding in the source stream are called the source format and source encoding, and those in the target stream are likewise called the target format and target encoding.

The work of conversion is performed in the overloaded abstract method of FormatConversionProvider called getAudioInputStream. The class also has abstract query methods for learning about all the supported target and source formats and encodings. There are concrete wrapper methods for querying about a specific conversion.

The two variants of getAudioInputStream are:

abstract AudioInputStream getAudioInputStream(AudioFormat.Encoding targetEncoding, 
     AudioInputStream sourceStream) 

and

abstract AudioInputStream getAudioInputStream(AudioFormat targetFormat, 
     AudioInputStream sourceStream) 

These differ in the first argument, according to whether the caller is specifying a complete target format or just the format's encoding.

A typical implementation of getAudioInputStream works by returning a new subclass of AudioInputStream that wraps around the original (source) AudioInputStream and applies a data format conversion to its data whenever a read method is invoked. For example, consider the case of a new FormatConversionProvider subclass called AcmeCodec, which works with a new AudioInputStream subclass called AcmeCodecStream.

The implementation of AcmeCodec's second getAudioInputStream method might be:

public AudioInputStream getAudioInputStream
      (AudioFormat outputFormat, AudioInputStream stream) {
        AudioInputStream cs = null;
        AudioFormat inputFormat = stream.getFormat();
        if (inputFormat.matches(outputFormat) ) {
            cs = stream;
        } else {
            cs = (AudioInputStream)
                (new AcmeCodecStream(stream, outputFormat));
            tempBuffer = new byte[tempBufferSize];
        }
        return cs;
    }

The actual format conversion takes place in new read methods of the returned AcmeCodecStream, a subclass of AudioInputStream. Again, application programs that access this returned AcmeCodecStream simply operate on it as an AudioInputStream, and don't need to know the details of its implementation.

The other methods of a FormatConversionProvider all permit queries about the input and output encodings and formats that the object supports. The following four methods, being abstract, need to be implemented:

abstract AudioFormat.Encoding[] getSourceEncodings() 
abstract AudioFormat.Encoding[] getTargetEncodings() 
abstract AudioFormat.Encoding[] getTargetEncodings(
    AudioFormat sourceFormat) 
abstract  AudioFormat[] getTargetFormats(
    AudioFormat.Encoding targetEncoding, 
    AudioFormat sourceFormat) 

As in the query methods of the AudioFileReader class discussed above, these queries are typically handled by checking private data of the object and, for the latter two methods, comparing them against the argument(s).

The remaining four FormatConversionProvider methods are concrete and generally don't need to be overridden:

boolean isConversionSupported(
    AudioFormat.Encoding targetEncoding,
    AudioFormat sourceFormat) 
boolean isConversionSupported(AudioFormat targetFormat, 
    AudioFormat sourceFormat) 
boolean isSourceEncodingSupported(
    AudioFormat.Encoding sourceEncoding) 
boolean isTargetEncodingSupported(
    AudioFormat.Encoding targetEncoding) 

As with AudioFileWriter.isFileTypeSupported(), the default implementation of each of these methods is essentially a wrapper that invokes one of the other query methods and iterates over the results returned.

Providing New Types of Mixers

As its name implies, a MixerProvider supplies instances of mixers. Each concrete MixerProvider subclass acts as a factory for the Mixer objects used by an application program. Of course, defining a new MixerProvider only makes sense if one or more new implementations of the Mixer interface are also defined. As in the FormatConversionProvider example above, where our getAudioInputStream method returned a subclass of AudioInputStream that performed the conversion, our new class AcmeMixerProvider has a method getMixer that returns an instance of another new class that implements the Mixer interface. We'll call the latter class AcmeMixer. Particularly if the mixer is implemented in hardware, the provider might support only one static instance of the requested device. If so, it should return this static instance in response to each invocation of getMixer.

Since AcmeMixer supports the Mixer interface, application programs don't require any additional information to access its basic functionality. However, if AcmeMixer supports functionality not defined in the Mixer interface, and the vendor wants to make this extended functionality accessible to application programs, the mixer should of course be defined as a public class with additional, well-documented public methods, so that a program that wishes to make use of this extended functionality can import AcmeMixer and cast the object returned by getMixer to this type.

The other two methods of MixerProvider are:

abstract Mixer.Info[] getMixerInfo() 

and

boolean isMixerSupported(Mixer.Info info) 

These methods allow the audio system to determine whether this particular provider class can produce a device that an application program needs. In other words, the AudioSystem object can iterate over all the installed MixerProviders to see which ones, if any, can supply the device that the application program has requested of the AudioSystem. The getMixerInfo method returns an array of objects containing information about the kinds of mixer available from this provider object. The system can pass these information objects, along with those from other providers, to an application program.

A single MixerProvider can provide more than one kind of mixer. When the system invokes the MixerProvider's getMixerInfo method, it gets a list of information objects identifying the different kinds of mixer that this provider supports. The system can then invoke MixerProvider.getMixer(Mixer.Info) to obtain each mixer of interest.

Your subclass needs to implement getMixerInfo, as it's abstract. The isMixerSupported method is concrete and doesn't generally need to be overridden. The default implementation simply compares the provided Mixer.Info to each one in the array returned by getMixerInfo.