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I am told that javac is smart enough that it does not recompile the .java, in condition that this .java has a timestamp before that of .class. I just find it does not work like that in my machine, am I certainly wrong somewhere?

my P.java is located under .../eg/access, with P.java declared as a package of access

eg 
 |- access 
      |- P.class
      |- P.java

Then when I run javac P.java, each time it is recompiled, is that normal or am I wrong?

Michael Berry
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zell
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    I wouldn't use `javac` directly. Instead I would use a build tool like maven, ant or an IDE which are a lot smarter all round. – Peter Lawrey Nov 25 '11 at 16:17
  • Just a note: If you have class `P` with `package access;` as the first declaration, you should compile it as `javac access/P.java` from the `eg` directory. – Paŭlo Ebermann Nov 26 '11 at 01:19

3 Answers3

7

Javac by default recompiles everything each time. The behaviour you observe is correct, and by design.

It looks like you can specify an option for this behaviour though:

-Xprefer:{newer,source}

Specify which file to read when both a source file and class file are found for a type. (See Searching For Types). If -Xprefer:newer is used, it reads the newer of the source or class file for a type (default). If the -Xprefer:source option is used, it reads source file. Use -Xprefer:source when you want to be sure that any annotation processors can access annotations declared with a retention policy of SOURCE.

Taken from: http://docs.oracle.com/javase/6/docs/technotes/tools/windows/javac.html

You may also be thinking of Eclipse's compiler, which I'm pretty sure does do incremental compilation - or the javac task in ant which is smart enough to figure out such cases.

Community
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Michael Berry
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    Actually, -Xprefer modifies the search preferences for classes *not specified on the command line*. So if I do javac Foo.java and it references a class Bar, and the java compiler finds both a Bar.java *and* a Bar.class, this option specifies which one it will use to compile Foo. It doesn't affect whether or not Foo.java is recompiled or not. See http://docs.oracle.com/javase/7/docs/technotes/tools/windows/javac.html#searching – Matthew Farwell Nov 25 '11 at 16:31
5

It's normal that it is recompiled each time. javac doesn't do incremental compilation.

Matthew Farwell
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  • But, "The javac compiler looks in the class path for both a source file and a class file, recompiling the source (and regenerating the class file) if it is newer. " from http://www.cis.upenn.edu/~bcpierce/courses/629/jdkdocs/tooldocs/win32/javac.html – zell Nov 25 '11 at 15:44
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    @zell not sure how old that document is, but it looks very old. It's probably been replaced with the command line option I've referred to in my answer, that clause isn't in the newer equivalent docs as far as I can see. – Michael Berry Nov 25 '11 at 15:59
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    @zell, Looking at the headers on the page: Wed, 10 Sep 1997 20:40:24 GMT was the last time that page was modified. Personally, I'd trust the Sun/Oracle documentation. – Matthew Farwell Nov 25 '11 at 16:27
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    @zell: You have to distinguish between *source files mentioned on the command line* (which will always be compiled), and *classes needed by these source files*, and found in both classpath and sourcepath (here this "newer" heuristic or the `-Xprefer` option mentioned by berry120 will be used). (Matthew: Feel free to add this to your answer, to make it more clear.) – Paŭlo Ebermann Nov 26 '11 at 01:16
2

I think you might be confusing javac as the command line executable with the javac task in Apache Ant. javac recompiles the source file always, no matter whether it is older or newer than the binary file. Ant is smart enough to figure out when the file has to be recompiled, but of course you have to use the Ant build system in order to make use of this feature.

Tamás
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