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435

answers:

8

Is it possible to make a Java program that prints its source code to a new file, and compiles it, and runs the compiled program?

+8  A: 

Yes, it is possible. A trivial implementation would be: have the source code contain itself in a string, save the string to a file and fill its own string with the same string (otherwise, the initial string would be of infinite size, due to the recursive manner of this implementation), compile the file, and run the compiled file (which will, in turn, do the very same).

Non-trivial implementations are significantly harder.

M.A. Hanin
"have the source code contain itself in a string" How? Since the string would be part of the source code, this would lead to an infinite string.
sepp2k
I edited my post to clarify this. The source code should contain itself in a string, including everythign except the content of this string, which is to be filled at runtime after the source-code was deployed to a text file (before it is being compiled).
M.A. Hanin
can you post an example?
Midhat
Have a look at my answer and follow the link to rosetta code. There's an example!
Andreas_D
+1  A: 

I don't know exactly what you want, but I think BeanShell is something you can use. BeanShell is an interpreter. You can run uncompiled Java-code (So you give it a String with code and he runs it).

Of course if you really want to do what you wrote, the machine where the program is running needs a JDK to compile your program.

Hope this helps

Martijn Courteaux
A: 

I dont think it will work in Java. Wouldn't that involve overwriting a running class file.

Suppose your program is in Quine.java compiled to Quine.class.

Now Quine.class will attempt to write its output to Quine.java (so far so good), and compile it to Quine.class. This is gonna be a problem as Quine.class is already running

Midhat
Erm ... he didn't require the new cloned program to be launched in the same VM (from the same classpath).
meriton
but it will be, as each copy of the program has to be identical, so you cannot change filenames every time
Midhat
Identical programs need not behave identically. They can use random number generators. They can interact with the host system, e.g. use the first unclaimed file name.
meriton
A: 

Yes - don't forget to use a JDK instead of a JRE:

  1. Bundle the app's source code files with the app. The app would copy the source files to a new set of source code files, compile the new source files, bundle the new source code with the new class files into a new app, and then spawn the new app.

    or

  2. Bundle a decompiler with the app. The app would run the decompiler on its own class files to generate new source code files, compile the new source files, bundle the decompiler with the new class files into a new app, and then spawn the new app.

Bert F
+3  A: 

Sure it works - Have a look at rosetta code and navigate to Quine, which is a self-referential program that can, without any external access, output its own source.

There's one example for a quine in Java.

Andreas_D
Direct link to the Java Quine program: http://rosettacode.org/wiki/Quine#Java
Ricket
I just wrote one up right here!
polygenelubricants
A: 

You could try The Java 6.0 Compiler API

01
+12  A: 

Update:

Okay, might as well make it autorun. Enjoy the madness. Run at your own risk.


Yes it's possible, because I actually wrote it up. It doesn't do the RUN part (that's just too crazy, because as others have mentioned, it will cause an infinite loop), but here it is: Quine.java

import java.io.*;
public class Quine {
   public static void main(String[] args) throws Exception {
      char q = 34;
      String out = "Quine$";
      String text = (
         "import java.io.*; " +
         "public class [OUT] { " +
           "public static void main(String[] args) throws Exception { " +
             "char q = 34; String out = `[OUT]$`; String text = `[TEXT]`; " +
             "PrintWriter pw = new PrintWriter(out + `.java`); " +
             "pw.format(text, 34, out, text); " +
             "pw.close(); Runtime runtime = Runtime.getRuntime(); " +
             "runtime.exec(`javac ` + out + `.java`).waitFor(); " +
             "runtime.exec(`java ` + out); " +
           "} " +
         "}"
      ).replace("`", "%1$c").replace("[OUT]", "%2$s").replace("[TEXT]", "%3$s");
      PrintWriter pw = new PrintWriter(out + ".java");
      pw.format(text, 34, out, text);
      pw.close();
      Runtime runtime = Runtime.getRuntime();
      runtime.exec("javac " + out + ".java").waitFor();
      runtime.exec("java " + out);
   }
}

So here's how to get the craziness to start:

  • javac Quine.java to compile
  • java Quine to run it
    • It will produce, compile and run Quine$
  • I've made sure Quine.java is as readable as possible, so the major difference from Quine$.java are formatting and the 3x replace. The minor difference is that Quine$.java has out set to Quine$$.
  • Quine$ will produce, compile and run Quine$$
  • Quine$$ will produce, compile and run Quine$$$
  • Quine$$$ will produce, compile and run Quine$$$$
  • ...

Do note that this doesn't do any reverse-engineering or cheat by reading the .java source code, etc. Quine is a quine-generator because it produces a different code differently formatted, but Quine$ is pretty much a true self-contained quine: it does reproduce itself, it just relabels it Quine$$ (which reproduces itself and relabels to Quine$$$ etc).

So technically there's no infinite loop: it will eventually come to a halt when the file system can't handle another $. I was able to manually stop the madness by forcefully deleting all Quine$* files, but run at your own risk!!!

polygenelubricants
+1  A: 

You could use the Java Compiler API (JSR-199) for this. Below, code from the JSR-199 that compiles code from a String (slightly modified to make it compile). The code actually compiles source code from the String into a byte array (i.e. it doesn't write to disk), loads it and then executes it via reflection:

That could be a starting point (credits to Peter Van der Ahé, the original author).

BTW, you need of course a JDK to use this API.

Pascal Thivent