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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
<html>
<head>
<title>OpenJava : Tutorial</title>
<meta http-equiv="Keywords" content="java, openjava, reflection">
</head>
<body bgcolor="white"
text="#000000" link="#007fff" vlink="#006fdf" alink="#ff0000">
<h2><!---------------------------------------------------------------------->
<center>
<h1><font color="Blue">OpenJava Tutorial</font></h1>
</center>
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<hr width="100%">
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4. <font color="blue">Generating New Methods</font>
</h2>
<p>
A MOP version of "hello world" is verbose objects, which print a message
for every method call.
We choose them as our first example.
<p>
The MOP programming in OpenJava is done through three steps :
(1) decide what the base-level program should look like,
(2) figure out what it should be translated into and what runtime support
code is needed,
and (3) write a meta-level program to perform the translation and also
write the runtime support code.
We implement the verbose objects through these steps.
<p>
<h3>2.1. What the base-level program should look like</h3>
<p>
Most of example programs given in the text are ready to be executed by
the OpenJava system and are similar in form to:
<br><blockquote><pre>
public class <b>Hello</b> instantiates <b>VerboseClass</b> {
public static void main( String[] args ) {
hello();
}
static void hello() {
System.out.println( "<i>Hello, world.</i>" );
}
}
</pre></blockquote>
This is an ordinary source code except for the first line.
The annotation in the first line :
<br><blockquote><pre>
instantiates <b>VerboseClass</b>
</pre></blockquote>
is a special annotation for OpenJava, and means that the semantics of
the class <b>Hello</b>, called metaobject, is specified to be extended
by the class <b>VerboseClass</b>, called metaclass. In practice, the
source code of the class <b>Hello</b> is translated by the object of
the metaclass <b>VerboseClass</b>.
<p>
<h3>2.2. What the base-level program should be translated</h3>
<p>
In this example, consider the metaclass <b>VerboseClass</b> to extend
the metaobjects to show messages for every call for its methods. In
practice, the statement in order to put the message into system
standard output is to be inserted at the first line of each methods'
body in the class <b>Hello</b> via the metaclass <b>VerboseClass</b>. Then
the first source code of class <b>Hello</b> should be translated into:
<br><blockquote><pre>
public class Hello {
public static void main( String[] args ) {
System.out.println( "<i>main is called.</i>" );
hello();
}
static void hello() {
System.out.println( "<i>hello is called.</i>" );
System.out.println( "<i>Hello, world.</i>" );
}
}
</pre></blockquote>
<p>
<h3>2.3. Write a meta-level program</h3>
<p>
Now, we write a meta-level program. What we should do is to translate
only method member in the class <b>Hello</b> in the way shown above.
We can easily do that if we use the MOP.
<p>
In OpenJava, classes are objects as in Smalltalk. We call them class
metaobjects when we refer to their meta-level representation. A
unique feature of OpenJava is that a class metaobject translates the
source code defining the class at compile time. For example, the
class metaobject for <b>Hello</b> translates a method declaration
<b>hello()</b>.
<p>
By default, class metaobjects are identity functions; they do not change
the program. So, to implement our translation, we define a new metaclass
- a new class for class metaobjects - and use it to make the class
metaobject for <b>Hello</b>.
<p>
Such a metaclass <b>VerboseClass</b> has been compiled and is similar
to in form to:
<br><blockquote><pre><font color=darkblue>
import openjava.mop.*;
import openjava.ptree.*;
public class <font color=black>VerboseClass</font> instantiates <font
color=black>Metaclass</font> extends <font color=black>OJClass</font>
{
public <font color=black>void</font> translatioDefinition() throws <font color=black>MOPException</font> {
<font color=black>OJMethod</font>[] methods = getDeclaredMethods();
for (<font color=black>int</font> i = 0; i < methods.length; ++i) {
<font color=black>Statement</font> printer = makeStatement(
"<font color=gray><i>System.out.println( \"</i></font>" + methods[i].getName() +
"<font color=gray><i> is called.\" );</i></font>"
);
methods[i].getBody().insertElementAt( printer, 0 );
}
}
}
</font></pre></blockquote>
The metaclass <b>VerboseClass</b> is just a regular Java class. It
inherits from <b>openjava.mop.OJClass</b> and overrides one member
function. <b>translateMethodDeclaration</b> takes an object of the
class <b>MethodDeclaration</b> and returns the translated one. Both
the given expression and the translated one are represented in the
form of parse tree.
<p>
<h3>2.4. Compile, debug, and run</h3>
<p>
On a Sun workstation, the first class, stored in the file
<tt>Hello.oj</tt>, can be translated into Java source code file named
<tt>Hello.java</tt> and the generated source code can be compiled into
byte code file named <tt>Hello.class</tt> by giving the commands :
<br><blockquote><pre>
% ojc VerboseClass.oj
% ojc Hello.oj
</pre></blockquote>
<p>
Execution of its bytecode on JVM (Java Virtual Machine) produces the
output :
<br><blockquote><pre>
main is called.
hello is called.
Hello, world.
</pre></blockquote>
<p>
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<hr width="100%">
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<center>
Please send any message to :
<address>
mich@acm.org
</address><BR>
</center>
<font size=1>Copyright (C) 1999 by Michaki Tatsubori.</font><br>
<font size=1>Java(TM) is a trademark of Sun Microsystems, Inc.</font>
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</body>
</html>
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