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<a name="Making-Java-Classes-Available-1"></a>
<h4 class="subsection">A.4.1 Making Java Classes Available</h4>

<a name="index-classpath_002c-setting"></a>
<a name="index-classpath_002c-difference-between-static-and-dynamic"></a>
<a name="index-static-classpath"></a>
<a name="index-dynamic-classpath"></a>
<a name="index-javaclasspath_002etxt"></a>
<a name="index-classpath_002etxt"></a>
<a name="index-classes_002c-making-available-to-Octave"></a>

<p>Java finds classes by searching a <var>classpath</var> which is a list of Java
archive files and/or directories containing class files.  In Octave the
<var>classpath</var> is composed of two parts:
</p>
<ul>
<li> the <var>static classpath</var> is initialized once at startup of the JVM, and

</li><li> the <var>dynamic classpath</var> which can be modified at runtime.
</li></ul>

<p>Octave searches the <var>static classpath</var> first, and then the
<var>dynamic classpath</var>.  Classes appearing in the <var>static classpath</var>, as
well as in the <var>dynamic classpath</var>, will therefore be found in the
<var>static classpath</var> and loaded from this location.  Classes which will be
used frequently, or must be available to all users, should be added to the
<var>static classpath</var>.  The <var>static classpath</var> is populated once from the
contents of a plain text file named <samp>javaclasspath.txt</samp> (or
<samp>classpath.txt</samp> historically) when the Java Virtual Machine starts.  This
file contains one line for each individual classpath to be added to the
<var>static classpath</var>.  These lines can identify directories containing class
files, or Java archives with complete class file hierarchies.  Comment lines
starting with a &lsquo;<samp>#</samp>&rsquo; or a &lsquo;<samp>%</samp>&rsquo; character are ignored.
</p>
<p>The search rules for the file <samp>javaclasspath.txt</samp> (or
<samp>classpath.txt</samp>) are:
</p>
<ul>
<li> First, Octave tries to locate it in the current directory (where Octave was
started from).  If such a file is found, it is read and defines the initial
<var>static classpath</var>.  Thus, it is possible to define a static classpath on a
&rsquo;per Octave invocation&rsquo; basis.

</li><li> Next, Octave searches in the user&rsquo;s home directory.  If a file
<samp>javaclasspath.txt</samp> exists here, its contents are appended to the static
classpath (if any).  Thus, it is possible to build an initial static classpath
on a &rsquo;per user&rsquo; basis.

</li><li> Finally, Octave looks for a <samp>javaclasspath.txt</samp> in the m-file directory
where Octave Java functions live.  This is where the function
<samp>javaclasspath.m</samp> resides, usually something like
<samp><code><span class="nolinebreak">OCTAVE_HOME</span></code><!-- /@w -->/share/octave/<code><span class="nolinebreak">OCTAVE_VERSION</span></code><!-- /@w -->/m/java/</samp>.
You can find this directory by executing the command

<div class="example">
<pre class="example">which javaclasspath
</pre></div>

<p>If this file exists here, its contents are also appended to the
<var>static classpath</var>.  Note that the archives and class directories defined
in this last step will affect all users.
</p></li></ul>

<p>Classes which are used only by a specific script should be placed in the
<var>dynamic classpath</var>.  This portion of the classpath can be modified at
runtime using the <code>javaaddpath</code> and <code>javarmpath</code> functions.
</p>
<p>Example:
</p>
<div class="example">
<pre class="example">octave&gt; base_path = &quot;C:/Octave/java_files&quot;;

octave&gt; # add two JAR archives to the dynamic classpath
octave&gt; javaaddpath ([base_path, &quot;/someclasses.jar&quot;]);
octave&gt; javaaddpath ([base_path, &quot;/moreclasses.jar&quot;]);

octave&gt; # check the dynamic classpath
octave&gt; p = javaclasspath;
octave&gt; disp (p{1});
C:/Octave/java_files/someclasses.jar
octave&gt; disp (p{2});
C:/Octave/java_files/moreclasses.jar

octave&gt; # remove the first element from the classpath
octave&gt; javarmpath ([base_path, &quot;/someclasses.jar&quot;]);
octave&gt; p = javaclasspath;
octave&gt; disp (p{1});
C:/Octave/java_files/moreclasses.jar

octave&gt; # provoke an error
octave&gt; disp (p{2});
error: A(I): Index exceeds matrix dimension.
</pre></div>

<p>Another way to add files to the <var>dynamic classpath</var> exclusively for your
user account is to use the file <samp>.octaverc</samp> which is stored in your home
directory.  All Octave commands in this file are executed each time you start a
new instance of Octave.  The following example adds the directory <samp>octave</samp>
to Octave&rsquo;s search path and the archive <samp>myclasses.jar</samp> in this directory
to the Java search path.
</p>
<div class="example">
<pre class="example"># contents of .octaverc:
addpath (&quot;~/octave&quot;);
javaaddpath (&quot;~/octave/myclasses.jar&quot;);
</pre></div>

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