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Class Loading
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Class Loading
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About:
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This section describes the order in which the class
loader searches for classes on the file system and how additional
classes are automatically loaded. This section is also
relevant to any command in the java package that makes a reference to
a java class (i.e. <b><code>java::new</code></b>).
References to unresolved classes will implicitly use the
TclClassLoader to find unresolved classes.
<P>
The process of resolving a class is done by searching for the class
in the following locations.
<P>
<ol>
<li>
Search the cache of previously loaded classes.
</li>
<br>
<li>
Search for the class using the context class loader.
Typically, this means searching on the <b><code>CLASSPATH</code></b>
via the system class loader.
</li>
<br>
<li>
Search the <I>pathList</I> list, looking only in directories or jar
files specified by <I>pathList</I>.
(Available only for the <B><code>java::load</code></B> command)
</li>
<br>
<li>
Search <I>pathList</I> again, inspecting all jar files found in
each directory. (Available only for the <B><code>java::load</code></B> command)
</li>
<br>
<li>
Search the <B><code>env(TCL_CLASSPATH)</code></B> list, looking only in directories or jar
files specified by <B><code>env(TCL_CLASSPATH)</code></B>.
</li>
<br>
<li>
Search <B><code>env(TCL_CLASSPATH)</code></B> again, inspecting all jar files found in
each directory.
</li>
</ol>
</P>
<DL><B>Note</B>
<DD>
<code>env(CLASSPATH)</code> is a <U>string</U> of directories or jar files, separated by ":" (unix) or ";" (windows).
<BR>
<code>env(TCL_CLASSPATH)</code> is a <U>list</U> of directories or jar files.
</DD>
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<P>
If the context class loader fails to load the class (step [2]),
this typically means that the class could not be located on
the <b><code>CLASSPATH</code></b>. The <b><code>CLASSPATH</code></b> is normally
set before the JVM is launched, and is not changed after that.
If the user needs to add a class search path at runtime, the
<b><code>env(TCL_CLASSPATH)</code></b> environment variable should be used.
A security check is done to assure that the class does not have access
to package-protected variables. If the extension name begins with
"java." or "tcl.", a Tcl error is generated.
<P>
If a path in <I>pathList</I> or <B><code>env(TCL_CLASSPATH)</code></B> is not an
absolute path, it is assumed to be a relative path from the
interpreter's current directory.
<P>
Steps [3] and [5] only use the paths in <I>pathList</I> or
<B><code>env(TCL_CLASSPATH)</code></B> to locate the class file. If the path is a
directory, the loader looks in that directory for the class file. If
the path is a jar file, the jar file is inspected to see if it
contains the class. Steps [4] and [6] search <I>pathList</I> and
<B><code>env(TCL_CLASSPATH)</code></B> again. If a path in the list is a directory,
the loader looks for jar files in that directory, inspecting any jar
files found to locate the class. The loader is capable of extracting
files from a compressed jar file.
<P>
Any class referenced by <I>extensionName</I> will also be loaded into
the VM. This includes classes used in new expressions as well as
those whose instances are reported (i.e. the use of static methods.)
Due to the recursive nature of class loading, any class referenced
must be located on the <b><code>CLASSPATH</code></b>, <B><code>env(TCL_CLASSPATH)</code></B> or
<I>pathList</I>. If a class cannot be located in one of the three
locations, a Tcl error is generated.
<P>
Currently it is not possible to unload or reload an extension.
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<PRE>
<A HREF="../license.html">Copyright</A> © 1997-1998 Sun Microsystems, Inc.
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