# Appendix 1: Code Overview: `AudioSystem.java`

For the benefit of service providers and API implementers, a brief overview of the Java Sound source code is provided.

`javax.sound.sampled.AudioSystem` is the entry point to JavaSound for obtaining resources; i.e., mixers, lines, etc. And each method of `AudioSystem` involves getting the providers of some service—`MixerProvider[]`, `FormatConversionProvider[]`, `AudioFileReader[]`, or `AudioFileWriter[]`. Then the method goes on to obtain some specific information or perform some task. There is much similarity in how these methods work. Let us take a look at how `getMixerInfo()` works.

The following is a sequence diagram of `getMixerInfo()` in `AudioSystem.java`:

`getMixerInfo()` in `AudioSystem` first calls `getMixerProviders()`, a `private static` method of `AudioSystem`. The first thing that `getMixerProviders()` does is attempt to load `sun.misc.Service`. If an exception is thrown, it means that a pre-1.3 JRE is in use and there is no service provider lookup mechanism present. If an exception is thrown, then `sun.media.sound.DefaultServices` is used to obtain service providers (not depicted in above diagram). If the 1.3 mechanims is present, then `getJDK13Serives()` is called (shown above) with `"javax.sound.sampled.spi.MixerProvider"` as the argument.

`getJDK13Services()` is another `private static` method of `AudioSystem`. It attempts to the load the class for `"com.sun.media.sound.JDK13Services"`, and if it succeeds it set `Class jdk13Services` equal to it. A bit of trickery is then performed by using `Class.getMethod()` to get the `getProviders()` method of `jdk13Services`, which is returned as object `Method m`. The method is then invoked, which has the effect of invoking `getProviders("javax.sound.sampled.spi.MixerProvider")` on `JDK13Services`. This in turn uses the `providers()` method of `sun.misc.Service` to return a vector of mixer providers, `MixerProvider[]`. The `getMixerInfo()` method of `MixerProvider` is then called on each element of the vector to return info (`Mixer.info`) for all mixers of all mixer providers.

Other services are handled in a similar way. For instance, `getTargetEncodings()` calls `getFormatConversionProviders()`, `getAudioFileFormat()` calls `getAudioFileReaders()`, etc., which are similarly structured to `getMixerProviders()`.