Use a custom classloader at compile time

Question

Is it possible to specify a custom classloader for javac (or some alternative java compiler)?

I'd love such a feat because it would allow me to compile classes that use classes that are only found by my special classloader.

For the curious ones: I'd write a classloder that connects to a database and creates classes based on the tables it finds.

score 6 · Accepted Answer · answered Jun 03 '09 at 19:43

6

When you run javac you can specify the classloader like so:

javac -J-Djava.system.class.loader=org.awesome.classloader sourcefile.java

answered Jun 03 '09 at 19:43

Hardwareguy

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I keep getting a ClassNotFOundException I don't know what I'm doing wrong. – Edoardo Vacchi Sep 04 '12 at 13:53
set the classpath in a similar way: `javac -J-Djava.class.path=./my-jar.jar -J-Djava.system.class.loader=xxxx file.java` – David Nichols Dec 24 '20 at 09:39

score 4 · Answer 2 · answered Jun 03 '09 at 17:31

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It may be possible to initialize a custom classloader and then use it while calling the new Java 6 Compiler API in javax.tools.

answered Jun 03 '09 at 17:31

Michael Borgwardt

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score 1 · Answer 3 · answered Jun 03 '09 at 17:43

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The only two ways I know of plugging directly into javac (as run on the command line) are via the annotation processor or via a compiler-specific hack.

answered Jun 03 '09 at 17:43

McDowell

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Get a 404 for the compiler-specific hack link, here's the web.archive.org link: http://web.archive.org/web/20140819224858/http://www.iam.unibe.ch/~akuhn/blog/2008/roman-numerals-in-your-java/ – Grant Peters Jan 25 '16 at 00:58

score 0 · Answer 4 · answered Jun 03 '09 at 17:36

Just to expand on Michael's answer, if you cannot use Java6, look at the sun. packages--they have always been available to java apps and have always had a compiler in there, it's just not standard Java so you don't hear about it much and the API is subject to change (like moving it to the javax.tools package!)

score 0 · Answer 5 · answered Jun 03 '09 at 19:02

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Take a look at ClassLoader.defineClass. I used it myself for loading plugins into a program I created, in which I loaded a file's bytes into a new class.

answered Jun 03 '09 at 19:02

Patrick

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score 0 · Answer 6 · answered Jun 03 '09 at 21:45

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If the classes all conform to the same Interface you could just provide that at compile time..

If not then I don't see what you are gaining by not outputing .java files based on the DB and compiling that.

answered Jun 03 '09 at 21:45

Chris Nava

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The 'problem' with generation java files and compiling them is it is a separate step. So if someone changes the database the classes don't fit and one gets cryptic sql exceptions. – Jens Schauder Jun 05 '09 at 10:55

Use a custom classloader at compile time

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