Dirty Compiling

[Nitin Dubey]

Hello Friends,

I believed java supports Dirty Compiling(Where the class will be re-compiled only when the source has changed). But to my surprise I found it is compiling the sources each and every time we ask it to compile irrespective of whether it has changed or not.

Any reasons why Java does not support dirty Compilation ?

Cheers,

Nitin

[Barry Gaunt]

Not SCJP. Moving to Java In General (Intermediate)

[Hemal Pandya]

Originally posted by Nitin Dubey:
Hello Friends,

I believed java supports Dirty Compiling(Where the class will be re-compiled only when the source has changed). But to my surprise I found it is compiling the sources each and every time we ask it to compile irrespective of whether it has changed or not.

Any reasons why Java does not support dirty Compilation ?

Cheers,

Nitin

javac would compile all .java files specified on the command line, whether they are newer then the .class files or not.

There is a good reason for this, sometimes javac cannot correctly determine if a file needs to be recompiled, e.g. when it is referencing a static final of primitive type from a different class.

[Ernest Friedman-Hill]

The term you're looking for is "Incremental Compilation."

"Dirty Compiling" sounds like a now-forgotten movie starring Patrick Swayze and Ben Stein.

[Nitin Dubey]

Hemal,

I didn't quite understand what you are trying to say. Can you give an example ?

Why do we think that javac should be concerned about the internal contents of the class. Can it not be done by comparing the last modified timestamp of .java and .class ? Obviously we cannot compare the actual timestamp of the class file but we can definately store the timestamp of the java file somewhere inside the file for further comparison.

Do you think it is possible ?

Cheers,

Nitin

[Hemal Pandya]

Originally posted by Nitin Dubey:
Hemal,

I didn't quite understand what you are trying to say. Can you give an example ?

Why do we think that javac should be concerned about the internal contents of the class. Can it not be done by comparing the last modified timestamp of .java and .class ? Obviously we cannot compare the actual timestamp of the class file but we can definately store the timestamp of the java file somewhere inside the file for further comparison.

Do you think it is possible ?

Cheers,

Nitin

Create the following three files. Now compile and run Main.java, as follows.
javac Main.java
java Main

Now modify Settings.java and make DEBUG false.
Compile and run again:
javac Main.java
java Main

Notice that Worker.foo is still using the old value (true) of Settings.DEBUG. This is because compile time constants get inlined in the client classes. Now, compile everything and run:
javac *.java
java Main

And note that the debug output no longer appears in the output. This is because *.java forced Worker.java to be recompiled. When we compiled Main.java, javac did not realize that Worker.java needs to be recompiled.

hth.

// File Settings.java
public class Settings {
  public static final boolean DEBUG = true;
}
 
// File Work.java
public class Work {
  public void foo() {
    if (Settings.DEBUG) {
      System.out.println("In Work.foo, Settlings.DEBUG = " + 
    }
}
 
// File Main.java
public class Main
{
    public static void main(String args[]) {
        Worker w = new Worker();
        w.foo();
    }
}
 

[Nitin Dubey]

Hemal,

The example shows us that if java compiler finds a class file then it will not re-compile the source. Nothing special about dirty compilation. However I still believe we can achieve Dirty Compilation in the following manner:

CASE #1
Somewhere in Java compiler. (Present Implementation)

if Main.class is not found
compile Main.java
end if

exec main() from Main.class

CASE #2
If header for class file is modified to keep the timestamp of respective Java file. The following condition
will be executed for all the class files referenced within Main.java.

if Main.class is not found
OR
(Main.class is found
AND TIMESTAMP in Main.class header != Last Modified Timestamp of Main.java)
compile Main.java
end if

exec main() from Main.class

Cheers,

Nitin

[Jeff Albertson]

Originally posted by Ernest Friedman-Hill:
The term you're looking for is "Incremental Compilation."

"Dirty Compiling" sounds like a now-forgotten movie starring Patrick Swayze and Ben Stein.

And I thought it was an AC/DC song!