Compile and create instance of a new java program from existing java program

I am writing a java program in eclipse to extract forms from lotusnotes db. Whenever I get a form I need to create a .java file containng a class with same name as of form. Then I have to compile this new .java class which gives me .class file & hence I can create an instance of that new class. I need to know whether this approach of creating,compiling & instantiating a new class is possible from a single existing java program.

My pseudocode goes below

import java.util.Iterator; import de.bea.domingo.*; import java.util.List; import java.io.*; public class LotusNotesDBExtraction { public static void main(String[] args) throws DNotesException,IOException { DNotesFactory factory = DNotesFactory.getInstance(); DSession session = factory.getSession(); DDatabase database = session.getDatabase("", "employee.nsf"); List formlist=database.getForms(); for(int i=0;i<formlist.size();i++) { DForm formn=(DForm)(formlist.get(i)); DForm formname= database.getForm(formn.getName()); [b]//Creating a class in a file [/b] FileWriter outputfile = new FileWriter("D:\\Lotusnotesformfiles\\"+formn.getName()+".java",true); BufferedWriter out = new BufferedWriter(outputfile); out.write("public class "+formn.getName()+" {"); out.newLine(); out.flush(); try { [b]//Compile the class here [/b] Process p1 = Runtime.getRuntime().exec("cmd javac \"D:\\Lotusnotesformfiles\\"+formn.getName()+".java\""); System.out.println("compiled"); } catch(IOException x) { x.printStackTrace(); } [b]//instantiate the new class here[/b] } }

But I am not able to get .class file for the new class. Can I instantiate the new class in the same program?? I was stuck at this point. Can any please help me

>

Check out URLClassLoader. You can use that to load the classes:
File dir = new File("D:\\Lotusnotesformfiles"); URLClassLoader ucl = new URLClassLoader(new URL[] {dir.toURI().toURL()}); Class<?> cls = ucl.loadClass(formn.getName()); // use cls to instantiate the class; newInstance() can be used if there is a constructor without arguments

You might want to look at CGLib to generate bytecode directly at runtime.

vikash talanki wrote:...I need to know whether this approach of creating,compiling & instantiating a new class is possible from a single existing java program.

The basic answer to your question is 'yes'. For the answer to your unasked (and possibly more important) question "is it a good idea?", we'd need to know a lot more about the requirements.

I guess my questions to you would be:

Why do you think you need to generate a program/class for each form?

What do these form classes do?

How do you plan on managing them?

Could your gazillion form classes be replaced by a single, or a few, slightly more complex classes, that use reflection?

Could you instead publish metadata that allows a single class to do the work of the reams you're banging out at the moment?

Without answers to those questions it's difficult to know what to advise, but your setup has a definite clunky feel to me.

I saw a few projects that went down this road back in the 80's and they were, almost uniformly, disasters. Program-writing programs (or Class-writing classes) have their uses, but I suspect they still belong mostly to the purview of research on Turing-completeness or other such nerdy pastimes. They are also, because of their explosive nature, a b*tch to manage, and probably even worse to test.

But I am not able to get .class file for the new class. Can I instantiate the new class in the same program?? I was stuck at this point. Can any please help me

Sounds like you need to look at ClassLoader and it's offspring, but I think Rob's already steered you that way.

Winston

I am running the similar program TestingRuntime from the command prompt and its creating both .java & .class files of RunTestTime.

class [b] TestingRuntime[/b] { public static void main(String[] args) throws IOException { FileWriter outputfile = new FileWriter("D:\\java learning\\[b]RunTimeTest[/b].java",true); BufferedWriter out = new BufferedWriter(outputfile); out.write("public class RunTimeTest {"); out.newLine(); out.write("int a=10;"); out.newLine(); out.write("void getInt(){ System.out.println(\"success\");}}"); out.flush();outputfile.close(); try { Process proc = Runtime.getRuntime().exec("javac \"D:\\java learning\\[b]RunTimeTest[/b].java\""); InputStream stderr = proc.getErrorStream(); InputStreamReader isr = new InputStreamReader(stderr); BufferedReader br = new BufferedReader(isr); String line = null; System.out.println("<ERROR>"); while ( (line = br.readLine()) != null) System.out.println(line); System.out.println("</ERROR>"); int exitVal = proc.waitFor(); System.out.println("Process exitValue: " + exitVal); System.out.println("compiled"); } catch(Throwable x) { x.printStackTrace(); } } }

The output I am getting is :
<ERROR> </ERROR> Process exitValue: 0 compiled


But When I am trying to run my original pgm(LotusNotesDBExtraction.java) & when I replace line 26 (I am running it from eclipse IDE)
Process p1 = Runtime.getRuntime().exec("cmd javac \"D:\\Lotusnotesformfiles\\"+formn.getName()+".java\"");

with
Process p1 = Runtime.getRuntime().exec("javac \"D:\\Lotusnotesformfiles\\"+formn.getName()+".java\"");

I am getting the error as
java.io.IOException: Cannot run program "javac": CreateProcess error=2, The system cannot find the file specified at java.lang.ProcessBuilder.start(Unknown Source) at java.lang.Runtime.exec(Unknown Source) at java.lang.Runtime.exec(Unknown Source) at java.lang.Runtime.exec(Unknown Source) at LotusNotesDBExtraction.main(LotusNotesDBExtraction.java:59)

Where is the actual problem? in Runtime.getRuntime().exec() or in eclipse IDE.

Please show me the way