path: root/dbus-java.tex

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\author{Matthew Johnson\\matthew.johnson@cl.cam.ac.uk}
\title{D-Bus programming in Java 1.5}

\begin{document}

\maketitle

\tableofcontents
\listoffigures
\listoftables

\section{Introduction}

This document describes how to use the Java bindings for D-Bus. D-Bus
is an IPC mechanism which at a low level uses message passing over
Unix Sockets or IP. D-Bus models its messages as either function
calls on remote objects, or signals emitted from them.

Java is an object-oriented language and these bindings attempt to
match the D-Bus IPC model to the Java object model. The bindings also
make heavy use of threads and exceptions. The programmer should be
careful to take care of synchronisation issues in their code. All
method calls by remote programs on exported objects and all signal
handlers are run in new threads. Any calls on remote objects may
throw {\tt DBusExecutionException}, which is a runtime exception and
so the compiler will not remind you to handle it.

The Java bindings API is also documented in the
JavaDoc\footnote{http://javadoc.matthew.ath.cx/}, D-Bus is
described in the
specification\footnote{http://dbus.freedesktop.org/doc/dbus-specification.html}
and the API
documentation\footnote{http://dbus.freedesktop.org/doc/api/html/}.

\subsection{Dependencies}

These bindings require Java 1.5-compatible VM and compiler (either
Sun, or ecj+jamvm with classpath-generics newer than 0.19), and D-Bus
0.60 or newer.

\subsection{D-Bus Terminology}

D-Bus has several notions which are exposed to the users of the Java
bindings.

\subsubsection{Service Names}

Programs on the bus are issued a unique identifier by the bus. This
is guaranteed to be unique within one run of the bus, but is
assigned sequentially to each new connection. 

There are also so called well-known service names which a device can
request on the bus. These are of the form {\em ``org.freedesktop.DBus''},
and any program can request them if they are not already owned.

\subsubsection{Interfaces}

All method calls and signals are specified using an interface,
similar to those in Java. When executing a method or sending a
signal you specify the interface the method belongs to. These are of
the form {\em ``org.freedesktop.DBus''}.

\subsubsection{Object Paths}

A program may expose more than one object which implements an
interface. Object paths of the form {\em ``/org/freedesktop/DBus''}
are used to distinguish objects.

\subsubsection{Member Names}

Methods and Signals have names which identify them within an
interface. D-Bus does not support method overloading, only one
method or signal should exist with each name.

\subsubsection{Errors}

A reply to any message may be an error condition. In which case you reply with
an error message which has a name of the form {\em
   ``org.freedesktop.DBus.Error.ServiceUnknown''}. 

\section{DBusConnection}

The {\tt
DBusConnection\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/DBusConnection.html}}
class provides methods for connecting to the bus, exporting objects,
sending signals and getting references to remote objects.

{\tt DBusConnection} is a singleton class, multiple calls to {\tt
getConnection} will return the same bus connection. 

\begin{verbatim}
conn = DBusConnection.getConnection(DBusConnection.SESSION);
\end{verbatim}

This creates a connection to the session bus, or returns the
existing connection.

\begin{verbatim}
conn.addSigHandler(TestSignalInterface.TestSignal.class,
                   new SignalHandler());
\end{verbatim}

This sets up a signal handler for the given signal type.
SignalHandler.handle will be called in a new thread with an instance
of TestSignalInterface.TestSignal when that signal is recieved.

\begin{verbatim}
conn.sendSignal(new TestSignalInterface.TestSignal(
                     "/foo/bar/com/Wibble", 
                     "Bar", 
                     new UInt32(42)));
\end{verbatim}

This sends a signal of type {\tt TestSignalInterface.TestSignal},
from the object {\em ``/foo/bar/com/Wibble''} with the arguments {\em
``Bar''} and {\tt UInt32(42)}.

\begin{verbatim}
conn.exportObject("/Test", new testclass());
\end{verbatim}

This exports the {\tt testclass} object on the path {\em ``/Test''}

\begin{verbatim}
Introspectable intro = (Introspectable) conn.getRemoteObject(
                              "foo.bar.Test", "/Test",
                              Introspectable.class);
\end{verbatim}

This gets a reference to a an object on a remote program. The object
implements the {\tt Introspectable} interface, and calls may be made
to methods in that interface as if it was a local object.

\begin{verbatim}
String data = intro.Introspect();
\end{verbatim}

The Runtime Exception {\tt DBusExecutionException} may be thrown
by any remote method if any part of the execution fails.

\subsection{Asynchronous Calls}

Calling a method on a remote object is synchronous, that is the thread will
block until it has a reply. If you do not want to block you can use an
asynchronous call. These are called using the {\tt callMethodAsync} function on
the connection object. This returns a {\tt
DBusAsyncReply\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/DBusAsyncReply.html}}
object which can be used to check for a reply and get the return value. This is demonstrated in figure \ref{fig:async}.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
DBusAsyncReply<Boolean> stuffreply = 
   conn.callMethodAsync(remote, "stuff", arg1, arg2);
...
if (stuffreply.hasReply()) {
   Boolean b = stuffreply.getReply();
   ...
}
\end{verbatim}
\end{center}
\caption{Calling an asynchronous method}
\label{fig:async}
\end{figure}


\section{DBusInterface}

To call methods or expose methods on D-Bus you need to define them
with their exact signature in a Java interface. The full name of
this interface must be the same as the D-Bus interface they
represent.

For example, if I want to expose methods on the interface
{\em ``org.freedesktop.DBus''} I would define a Java interface in
the package {\tt org.freedesktop} called {\tt DBus}. This would be
in the file {\tt org/freedesktop/DBus.java} as normal. Any object
wanting to export these methods would implement {\tt
org.freedesktop.DBus}.

Any interfaces which can be exported over D-Bus must extend {\tt
DBusInterface\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/DBusInterface.html}}.
A class may implement more than one exportable interface, all public
methods declared in an interface which extend {\tt DBusInterface}
will be exported.

A sample interface definition is given in
figure~\ref{fig:interface}, and a class which implements it in
figure~\ref{fig:class}. More complicated definitions can be seen in
the test
classes\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/test/TestRemoteInterface2.java
http://javadoc.matthew.ath.cx/org/freedesktop/dbus/test/TestRemoteInterface.java}.

All method calls by other programs on objects you export over D-Bus
are executed in their own thread.

{\tt DBusInterface} itself specifies one method \verb&boolean isRemote()&. If this is executed on a remote object it will always
return true. Local objects implementing a remote interface should
implement this method to return false.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
package org.freedesktop;
import org.freedesktop.dbus.UInt32;
import org.freedesktop.dbus.DBusInterface;

public interface DBus extends DBusInterface
{
   public boolean NameHasOwner(String name);
   public UInt32 RequestName(String name, UInt32 flags);
}
\end{verbatim}
\end{center}
\caption{An interface which exposes two methods}
\label{fig:interface}
\end{figure}

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
package my.real.implementation;
import org.freedesktop.dbus.DBus;
import org.freedesktop.dbus.UInt32;

public class DBusImpl implements DBus
{
   Vector<String> names;
   public boolean NameHasOwner(String name)
   {
      if (names.contains(name)) return true;
      else return false;
   }
   public UInt32 RequestName(String name, UInt32 flags)
   {
      names.add(name);
      return new UInt32(0);
   }
}
\end{verbatim}
\end{center}
\caption{A class providing a real implementation which can be exported}
\label{fig:class}
\end{figure}

\section{DBusSignal}

Signals are also declared as part of an interface. The Java API
models these as inner classes within an interface. The containing
interface must extend {\tt DBusInterface}, and the inner classes
representing the signals must extend {\tt
DBusSignal\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/DBusSignal.html}}.
The Signal name is derived from the name of this inner class, and
the interface from its containing interface.

Signals can take parameters as methods can (although they cannot return
      anything). For the reflection to work, a Signal declare a single
constructor of the correct type. The constructor must  take the object path
they are being emitted from as their first (String) argument, followed by the
other parameters in order. They must also call the superclass constructor with
the same parameters.  A full definition of a signal can be seen in
figure~\ref{fig:signal}. Again, more complicated definitions are available in
the test
classes\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/test/TestSignalInterface.html}.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
package org.freedesktop;
import org.freedesktop.dbus.DBusInterface;
import org.freedesktop.dbus.DBusSignal;

public interface DBus extends DBusInterface
{
   public class NameAquired extends DBusSignal
   {
      public final String name;
      public NameAquired(String path, String name) 
                              throws DBusException
      {
         super(path, name);
         this.name = name;
      }
   }
}
\end{verbatim}
\end{center}
\caption{A Signal with one parameter}
\label{fig:signal}
\end{figure}

\section{DBusExecutionException}

If you wish to report an error condition in a method call you can throw an
instance of {\tt
DBusExecutionException\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/DBusExecutionException.html}}.
This will be sent back to the caller as an error message, and the error name is
taken from the class name of the exception. For example, if you wanted to
report an unknown method you would define an exception as in figure
\ref{fig:exceptiondef} and then throw it in your method as in figure
\ref{fig:exceptioncall}.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
package org.freedesktop.DBus.Error;
import org.freedesktop.dbus.DBusExecutionException;

public class UnknownMethod extends DBusExecutionException
{
   public UnknownMethod(String message)
   {
      super(message);
   }
}
\end{verbatim}
\end{center}
\caption{An Exception}
\label{fig:exceptiondef}
\end{figure}

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
...
public void throwme() throws org.freedesktop.DBus.Error.UnknownMethod
{
   throw new org.freedesktop.DBus.Error.UnknownMethod("hi");
}
...
\end{verbatim}
\end{center}
\caption{Throwing An Exception}
\label{fig:exceptioncall}
\end{figure}

If you are calling a remote method and you want to handle such an error you can
simply catch the exception as in figure \ref{fig:exceptioncatch}. 

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
...
try {
   remote.throwme();
} catch (org.freedesktop.DBus.Error.UnknownMethod UM) {
   ...
}
...
\end{verbatim}
\end{center}
\caption{Catching An Exception}
\label{fig:exceptioncatch}
\end{figure}

\section{DBusSigHandler}

To handle incoming signals from other programs on the Bus you must
register a signal handler. This must implement {\tt
DBusSigHandler\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/DBusSigHandler.html}}
and provide an implementation for the handle method. An example Signal Handler is in figure~\ref{fig:handler}. Signal handlers will be run in their own thread.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
import org.freedesktop.dbus.DBusSignal;
import org.freedesktop.dbus.DBusSigHandler;

public class Handler extends DBusSigHandler
{
   public void handle(DBusSignal sig)
   {
      if (sig instanceof DBus.NameAquired) {
         ...
      }
   }
}
\end{verbatim}
\end{center}
\caption{A Signal Handler}
\label{fig:handler}
\end{figure}


\section{D-Bus Types}

D-Bus supports a number of types in its messages, some which Java
supports natively, and some which it doesn't. These bindings provide
a way of modelling the extra D-Bus Types in Java. The full list of
types and what D-Bus type they map to is in table \ref{table:types}.

\subsection{Basic Types}

All of Java's basic types are supported as parameters and return types to methods, and as parameters to signals. These can be used in either their primitive or wrapper types.

\subsubsection{Unsigned Types}

D-Bus, like C and similar languages, has a notion of unsigned numeric
types. These bindings supply {\tt
UInt16\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/UInt16.html}}, {\tt
UInt32} and {\tt UInt64} classes to represent these new basic types.

\subsection{Strings}

D-Bus also supports sending Strings. When mentioned below, Strings
count as a basic type.

\subsection{Arrays}

You can send arrays of any valid D-Bus Type over D-Bus. These can
either be declared in Java as arrays (e.g. \verb&Integer[]& or
\verb&int[]&) or as Lists (e.g. \verb&List<String>&). All lists {\bf
must} be parameterised with their type in the source (reflection on
this is used by the bindings to determine their type).

\subsection{Maps}

D-Bus supports a dictionary type analogous to the Java Map type. This
has the additional restriction that only basic types can be used as
the key (including String). Any valid D-Bus type can be the value. As
with lists, maps must be fully parameterised. (e.g.
\verb&Map<Integer, String>&).

\subsection{Variants}

D-Bus has support for a Variant type. This is similar to declaring that a method takes a parameter of type {\tt Object}, in that a Variant may contain any other type. Variants can either be declared using the {\tt Variant\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/Variant.html}} class, or as a Type Variable. In the latter case the value is automatically unwrapped and passed to the function. Variants in compound types (Arrays, Maps, etc) must be declared using the Variant class and manually unwrapped.

Both these methods use variants:

\begin{verbatim}
public void display(Variant v);
public <T> int hash(T v);
\end{verbatim}

\subsection{Structs}

D-Bus has a struct type, which is a collection of other types. Java
does not have an analogue of this other than fields in classes, and
due to the limitation of Java reflection this is not sufficient. The
bindings declare a {\tt
Struct\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/Struct.html}} class
which should be subclassed as neccessary to create structs of the
appropriate sizes. The types are then added by the functions which
declare them. This is best demonstrated by an example.
Figure~\ref{fig:struct} shows a Struct definition, and
figure~\ref{fig:structmethod} shows this being used as a parameter
to a method.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
package org.freedesktop.dbus.test;

import org.freedesktop.dbus.DBusException;
import org.freedesktop.dbus.Struct;

public final class Struct3<A, B, C> extends Struct
{
   public final A a;
   public final B b;
   public final C c;
   public Struct3(A a, B b, C c) throws DBusException
   {
      super(a, b, c);
      this.a = a;
      this.b = b;
      this.c = c;
   }
}
\end{verbatim}
\end{center}
\caption{A Struct of size 3}
\label{fig:struct}
\end{figure}

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
public void do(Struct3<String, UInt32, String> data);
\end{verbatim}
\end{center}
\caption{A struct as a parameter to a method}
\label{fig:structmethod}
\end{figure}


Section~\ref{sec:create} describes how these can be automatically
generated from D-Bus introspection data.

\subsection{Objects}

You can pass references to exportable objects round using their object paths.
To do this in Java you declare a type of {\tt DBusInterface}. When the bindings
receive an object path they will automatically convert it into the object you
are exporting with that object path. You can pass remote objects back to their
services in a similar fashion.


\subsection{Multiple Return Values}

D-Bus also allows functions to return multiple values, a concept not supported
by Java. This has been solved in a fashion similar to the struct, using a {\tt
Tuple\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/Tuple.html}} class, and
defining tuples of the appropriate length. This can be seen in
figure~\ref{fig:tuple}. Again, these can be automatically generated
from introspection data.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
public ThreeTuple<String, Integer, Boolean> status(int item);
\end{verbatim}
\end{center}
\caption{A Tuple being returned from a method}
\label{fig:tuple}
\end{figure}

\subsection{Full list of types}

Table \ref{table:types} contains a full list of all the Java types and their corresponding D-Bus types.

\begin{table}
\begin{center}
\begin{tabular}{l|l}
\bf Java Type & \bf D-Bus Type \\
\hline
Byte	&	DBUS\_TYPE\_BYTE	\\
byte	&	DBUS\_TYPE\_BYTE	\\
Boolean	&	DBUS\_TYPE\_BOOLEAN	\\
boolean	&	DBUS\_TYPE\_BOOLEAN	\\
Short	&	DBUS\_TYPE\_INT16	\\
short	&	DBUS\_TYPE\_INT16	\\
UInt16	&	DBUS\_TYPE\_UINT16	\\
int	&	DBUS\_TYPE\_INT32	\\
Integer	&	DBUS\_TYPE\_INT32	\\
UInt32	&	DBUS\_TYPE\_UINT32	\\
long	&	DBUS\_TYPE\_INT64	\\
Long	&	DBUS\_TYPE\_INT64	\\
UInt64	&	DBUS\_TYPE\_UINT64	\\
double	&	DBUS\_TYPE\_DOUBLE	\\
Double	&	DBUS\_TYPE\_DOUBLE	\\
String	&	DBUS\_TYPE\_STRING	\\
$<$T$>$	&	DBUS\_TYPE\_VARIANT	\\
Variant	&	DBUS\_TYPE\_VARIANT	\\
? extends Struct	&	DBUS\_TYPE\_STRUCT	\\
?$[$~$]$	&	DBUS\_TYPE\_ARRAY	\\
? extends List	&	DBUS\_TYPE\_ARRAY	\\
? extends Map	&	DBUS\_TYPE\_DICT	\\
? extends DBusInterface	&	DBUS\_TYPE\_OBJECT\_PATH	\\
\end{tabular}
\end{center}
\caption{Mapping between Java types and D-Bus types}
\label{table:types}
\end{table}

\section{CreateInterface}
\label{sec:create}

D-Bus provides a method to get introspection data on a remote object,
which describes the interfaces, methods and signals it provides.
This introspection data is in XML
format\footnote{http://dbus.freedesktop.org/doc/dbus-specification.html\#introspection-format}.
The Java bindings mechanism automatically provides XML introspection data on all
objects which are exported by it.  Introspection data can be used to create
Java interface definitions automatically.

The {\tt
CreateInterface\footnote{http://javadoc.matthew.ath.cx/org/freedesktop/dbus/CreateInterface.html}}
class will automatically create Java source files from an XML file
containing the introspection data, or by querying the remote object
over D-Bus.  CreateInterface can be called from Java code, or can be run as a
stand alone program.  

The syntax for the CreateInterface program is

\begin{verbatim}
CreateInterface [--system] [--session] [--create-files] 
                  <service> <object>
CreateInterface [--create-files] <introspection-file.xml>
\end{verbatim}

The Java source code interfaces will be written to the standard ouput. If the
{\tt --create-files} option is specified the correct files in the
correct directory structure will be created.

\section{Annotations}

You can annotate your D-Bus methods as in figure \ref{fig:annotation} to provide hints to other users of your API. Common annotations are listed in table \ref{tab:annotations}.

\begin{figure}[htb]
\begin{center}
\begin{verbatim}
package org.freedesktop;
import org.freedesktop.dbus.UInt32;
import org.freedesktop.dbus.DBusInterface;

@org.freedesktop.DBus.Description("Some Methods");
public interface DBus extends DBusInterface
{
   @org.freedesktop.DBus.Description("Check if the name has an owner")
   public boolean NameHasOwner(String name);
   @org.freedesktop.DBus.Description("Request a name")
   @org.freedesktop.DBus.Deprecated()
   public UInt32 RequestName(String name, UInt32 flags);
}
\end{verbatim}
\end{center}
\caption{An annotated method}
\label{fig:annotation}
\end{figure}


\begin{table}[htb]
\begin{tabular}{l|l}
{\bf Name} & {\bf Meaning} \\
\hline
org.freedesktop.DBus.Description & Provide a short 1-line description \\
      & of the method or interface. \\
org.freedesktop.DBus.Deprecated & This method or interface is Deprecated. \\
org.freedesktop.DBus.Method.NoReply & This method may be called and returned \\
   & without waiting for a reply. \\
org.freedesktop.DBus.Method.Error & This method may throw the listed Exception\\
   & in addition to the standard ones. \\
\end{tabular}
\caption{Common Annotations}
\label{tab:annotations}
\end{table}

\section{Debugging}

Since version 1.4 of the bindings there has been a facility to print all the
messages the bindings are sending or receiving on the bus. To enable this, set
the {\tt DBUS\_JAVA\_DEBUG} environment variable before running your program.

\section{Examples}

As an example here are a complete set of interfaces for the
bluemon\footnote{http://www.matthew.ath.cx/projects/bluemon} daemon,
which communicates over D-Bus. These interfaces were all created by
querying introspection data over the bus.

\newpage

\begin{figure}[!h]
\begin{center}
\begin{verbatim}
package cx.ath.matthew.bluemon;
import org.freedesktop.dbus.DBusInterface;
import org.freedesktop.dbus.UInt32;
public interface Bluemon extends DBusInterface
{
  public Triplet<String, Boolean, UInt32> 
  Status(String address);
}
\end{verbatim}
\end{center}
\caption{cx/ath/matthew/bluemon/Bluemon.java}
\end{figure}

\newpage

\begin{figure}[!h]
\begin{center}
\begin{verbatim}
package cx.ath.matthew.bluemon;
import org.freedesktop.dbus.DBusException;
import org.freedesktop.dbus.DBusInterface;
import org.freedesktop.dbus.DBusSignal;
public interface ProximitySignal extends DBusInterface
{
   public static class Connect extends DBusSignal
   {
      public final String address;
      public Connect(String path, String address) 
                                 throws DBusException
      {
         super(path, address);
         this.address = address;
      }
   }
   public static class Disconnect extends DBusSignal
   {
      public final String address;
      public Disconnect(String path, String address)
                                 throws DBusException
      {
         super(path, address);
         this.address = address;
      }
   }
}
\end{verbatim}
\end{center}
\caption{cx/ath/matthew/bluemon/ProximitySignal.java}
\end{figure}

\newpage

\begin{figure}[!h]
\begin{center}
\begin{verbatim}
package cx.ath.matthew.bluemon;
import org.freedesktop.dbus.Tuple;
/** Just a typed container class */
public final class Triplet <A,B,C> extends Tuple
{
   public final A a;
   public final B b;
   public final C c;
   public Triplet(A a, B b, C c)
   {
      super(a, b, c);
      this.a = a;
      this.b = b;
      this.c = c;
   }
}
\end{verbatim}
\end{center}
\caption{cx/ath/matthew/bluemon/Triplet.java}
\end{figure}

\newpage

\begin{figure}[!h]
\begin{center}
\begin{verbatim}
package cx.ath.matthew.bluemon;
import org.freedesktop.dbus.DBusConnection;
import org.freedesktop.dbus.DBusException;
import org.freedesktop.dbus.DBusSigHandler;
import org.freedesktop.dbus.DBusSignal;
import org.freedesktop.dbus.UInt32;

public class Query {
   public static void main(String[] args) {
      String btid;
      Triplet<String, Boolean, UInt32> rv = null;
      
      if (0 == args.length) btid = "";
      else btid = args[0];
      
      DBusConnection conn = null;
      try {
         conn = DBusConnection.getConnection(DBusConnection.SYSTEM);
      } catch (DBusException De) {
         System.exit(1);
      }     
      Bluemon b = (Bluemon) conn.getRemoteObject(
            "cx.ath.matthew.bluemon.server", 
            "/cx/ath/matthew/bluemon/Bluemon", Bluemon.class);
      try {
         rv = b.Status(btid);
      } catch (RuntimeException Re) {
         System.exit(1);
      }
      String address = rv.a;
      boolean status = rv.b;
      int level = rv.c.intValue();

      if (status)
         System.out.println("Device "+address+
                            " connected with level "+level);
      else
         System.out.println("Device "+address+" not connected");
      conn.disconnect();
   }
}
\end{verbatim}
\end{center}
\caption{cx/ath/matthew/bluemon/Query.java}
\end{figure}

\newpage

\begin{figure}[!h]
\begin{center}
\begin{verbatim}
/* cx/ath/matthew/bluemon/Client.java */
package cx.ath.matthew.bluemon;

import org.freedesktop.dbus.DBusConnection;
import org.freedesktop.dbus.DBusException;
import org.freedesktop.dbus.DBusSigHandler;
import org.freedesktop.dbus.DBusSignal;

public class Client implements DBusSigHandler
{
   public void handle(DBusSignal s)
   {
      if (s instanceof ProximitySignal.Connect)
         System.out.println("Got a connect for "
               +((ProximitySignal.Connect) s).address);
      else if (s instanceof ProximitySignal.Disconnect)
         System.out.println("Got a disconnect for "
               +((ProximitySignal.Disconnect) s).address);
   }
   public static void main(String[] args) 
   {
      System.out.println("Creating Connection");
      DBusConnection conn = null;
      try {
         conn = DBusConnection
                  .getConnection(DBusConnection.SYSTEM);
      } catch (DBusException DBe) {
         System.out.println("Could not connect to bus");
         System.exit(1);
      }
      
      try {
         conn.addSigHandler(ProximitySignal.Connect.class, 
         new Client());
         conn.addSigHandler(ProximitySignal.Disconnect.class, 
         new Client());
      } catch (DBusException DBe) {
         conn.disconnect();
         System.exit(1);
      }
   }
}
\end{verbatim}
\end{center}
\caption{cx/ath/matthew/bluemon/Client.java}
\end{figure}

\newpage

\section{Credits}

This document and the Java API were written by and are copyright to
Matthew Johnson. Much help and advice was provided by the members of
the \#dbus irc channel. Comments, corrections and patches can be sent
to dbus-java@matthew.ath.cx.

\end{document}