Java Glossary : javac.exe

Java definitions: 0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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Sun's free Java compiler that comes bundled with the JDK. If you install Java 1.4.2 in the default directory you should find it in J:\j2sdk1.4.2_04\bin\javac.exe. You normally invoke it from the command line, but you can also invoke it from within a Java program. Use the sun.tools.javac.Main class located in %java_home%/lib/tools.jar to pass it an array of Strings equivalent to the command line parameters. You might do this to bulk-compile large numbers of files without reloading the javac compiler for each file.

JDK Tools Documentation Pack : available:

on the web at java.Sun.com
on J: in the JDK 1.4.2_04: J:\j2sdk1.4.2_04\docs\tooldocs\windows\javac.html
on J: in the JDK 1.5 beta: J:\Program Files\Java\j2sdk1.5.0\docs\tooldocs\windows\javac.html

Class and File Naming Rules

A class name must start with a capital letter. The source for that class must live in a plain text file with the same name (including case), with a *.java extension.

You cannot put more than one public class inside a *.java file.

Classes in the same package are accessible to each other, even if they are in different files. To access other classes, you must import them.

The source code must be stored in a directory with the same name as the package declared at the top of the file, including case, with the dots replaced by \ or / or whatever your platform uses for directory element separators.

Package names must be lower case, usually beginning com.yourwebsite to ensure global uniqueness.

It is a good idea to put every class in some package. Only experiments you plan to keep for under an hour should be without a package.

CLASSPATH And File Naming Recipes

Here are my simplified rules for using CLASSPATH and naming the files on the javac.exe command line:

Use JDK 1.1 or later, why not the latest?. Configure your SET CLASSPATH= to clear it out. Avoid JDK 1.0 if you can because its CLASSPATH is more complicated since it has to help javac find the standard classses.
Download SmartJ. It forgives most problems with classpath and file naming, even if you use JDK1.1+. Unfortunately, it has not been updated to work with JDK 1.2 or 1.3.
In all that follows, everything is strictly case sensitive.
To compile a HelloWorld.java app in the default package in C:\MyDir , use
```
CD \MyDir
javac.exe -classpath . HelloWorld.java
```
To compile a HelloWorld.java app in C:\com\mindprod\mypackage , in package com.mindprod.mypackage, use
```
CD \
javac.exe -classpath . com\mindprod\mypackage\HelloWorld.java
```

The Javac.exe Command Line

Unlike with java.exe, you must specify the extension (.java).
The switches are case sensitive.

Javac.exe Command Line Switches
Option	Effect
-g	generate all debugging information
-o	optimise
-debug	add information to class files to aid in debugging. It still won't get them all.
-Xdepend	Use a much slower more conservative approach to deciding which files need to be recompiled.
-nowarn	suppress warning messages.
-verbose	long version of error messages.
-classpath /mydir:/place/myCollection.jar	overriding CLASSPATH, colon separated. Infuriatingly, javac.exe won't let you use the -cp shortcut.
-nowrite	don't actually generate code, just check the syntax.
-deprecation	warn of any use of any deprecated methods.
-d targetDir	Place the output class files in this directory rather than the usual same directory as source.
-encoding UTF-8	what encoding was used to create the source files. Lets you embed fancy characters you would otherwise need to encode with \uxxxx.
-J runtimeflag
@listOfFiles.txt	To shorten or simplify the javac command, you may specify one or more files that themselves contain one filename per line. This enables you to overcome the command-line length limitation of Windows.
-target 1.1	Specifies which JRE will be used. You can also specify -target 1.2 and 1.3. It is not smart enough to warn you if you use classes not part of that class library.
-Xstdout	Send error messages to stdout instead of stderr. This makes them easier to redirect to a file in Windows. For some idiotic reason this is no longer supported under JDK 1.3, (a result of intercorporate sniping at Microsoft?) It is supported with oldjavac.exe .
-help	Get a list of options and what they do. Trust what it says over what I say here. Knowledge keeps no better that fish.

Note there is no -cp or -jar option! Instead you must use -classpath for both functions.

Programmatic Invocation

You can also invoke the javac compiler as an undocumented class, e.g.:

sun.tools.javac.Main comp = new sun.tools.javac.Main( System.out, null );
boolean success = comp.compile( new String [] { "HelloWorld.java"} );

You might do this for speed to avoid reloading javac.exe , or to have arbitrarily long command lines. You can also get long command lines with the @listOfFiles technique.

Capturing the error messages from javac.exe can be a hassle since they scroll off the screen faster than you can read them. Ordinary > redirection won't capture them to a file because they are going to STDERR, not STDOUT. I use a program called 4NT to solve this problem. It lets you independently redirect STDOUT and STDERR. You can also use the -Xstdout to bypass the problem.

What Gets Recompiled?

javac.exe will recompile all the *.java files you mention on the command line, whether they need it or not. Further, if your java source references *.java files not mentioned on the command line and they are out of date, it will recompile them too. However, it is still possible to miss recompiling something that needs it. That is why you periodically should recompile the universe especially before any release or global testing. Here are two case where javac.exe fails to recompile.

If you recompile A, and A depends on B (which has not changed) and B depends on C (which has changed), Javac will not recompile C, if you invoke it on A.
If you change the value of a final static value in a class and recompile it, javac won't necessarily recompile any other dependent classes which reference that field, and have the old value burned into the class file as a literal.

Using a make utility to figure out what needs recompiling usually won't do any better. It will take much longer since it will load javac.exe once per source file. You might as well recompile everything with javac.exe *.java. Invoking javac.exe once per source file or even once per directory will really slow you down. It is much better to invoke javac.exe once, and feed it everything either via the command line or the programmatic interface.

My rule of thumb is to erase all class files before a full application test or release. Also erase them if you suspect you are getting an old class file somewhere along the line. Most of the time, selective recompilation works fine. Usually you make changes to only one java file at a time before recompiling, and none of the other files need it. Any time you change public static final constants is a good time to delete all the class files.

Periodically delete all class files. This cleans up class files from renamed, and deleted source files. It gets rid of unused anonymous inner class files. Strange things can happen when you rename a module, and fail to fix all the references, and still leave the old class file lying about even if you delete the source.

4NT ¤ CLASSPATH ¤ environment ¤ flipping between different JVMs ¤ HelloWorld ¤ java.exe ¤ javaw.exe ¤ Jikes ¤ PATH ¤ Source code to Javac.exe ¤ Tips on using Javac's main class ¤ tools

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