A MOP version of "hello world" is verbose objects, which print a message for every method call. We choose them as our first example.
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.
Most of example programs given in the text are ready to be executed by
the OpenJava system and are similar in form to:
This is an ordinary source code except for the first line. The annotation in the first line :public class Hello instantiates VerboseClass { public static void main( String[] args ) { hello(); } static void hello() { System.out.println( "Hello, world." ); } }
is a special annotation for OpenJava, and means that the semantics of the class Hello, called metaobject, is specified to be extended by the class VerboseClass, called metaclass. In practice, the source code of the class Hello is translated by the object of the metaclass VerboseClass.instantiates VerboseClass
In this example, consider the metaclass VerboseClass 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 Hello via the metaclass VerboseClass. Then
the first source code of class Hello should be translated into:
public class Hello { public static void main( String[] args ) { System.out.println( "main is called." ); hello(); } static void hello() { System.out.println( "hello is called." ); System.out.println( "Hello, world." ); } }
Now, we write a meta-level program. What we should do is to translate only method member in the class Hello in the way shown above. We can easily do that if we use the MOP.
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 Hello translates a method declaration hello().
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 Hello.
Such a metaclass VerboseClass has been compiled and is similar
to in form to:
The metaclass VerboseClass is just a regular Java class. It inherits from openjava.mop.OJClass and overrides one member function. translateMethodDeclaration takes an object of the class MethodDeclaration and returns the translated one. Both the given expression and the translated one are represented in the form of parse tree.import openjava.mop.*; import openjava.ptree.*; public class VerboseClass instantiates Metaclass extends OJClass { public void translatioDefinition() throws MOPException { OJMethod[] methods = getDeclaredMethods(); for (int i = 0; i < methods.length; ++i) { Statement printer = makeStatement( "System.out.println( \"" + methods[i].getName() + " is called.\" );" ); methods[i].getBody().insertElementAt( printer, 0 ); } } }
On a Sun workstation, the first class, stored in the file
Hello.oj, can be translated into Java source code file named
Hello.java and the generated source code can be compiled into
byte code file named Hello.class by giving the commands :
% ojc VerboseClass.oj % ojc Hello.oj
Execution of its bytecode on JVM (Java Virtual Machine) produces the
output :
main is called. hello is called. Hello, world.