Can standard Sun javac do incremental compiling?

Question

Recently I started to use Eclipse's java compiler, because it is significantly faster than standard javac. I was told that it's faster because it performs incremental compiling. But I'm still a bit unsure about this since I can't find any authoritative documentation about both - eclispse's and sun's - compilers "incremental feature". Is it true that Sun's compiler always compiles every source file and Eclipse's compiler compile only changed files and those that are affected by such a change?

Edit: I'm not using Eclipse autobuild feature but instead I'm setting

-Dbuild.compiler=org.eclipse.jdt.core.JDTCompilerAdapter

for my ant builds.

Answer 1

Is it true that Sun's compiler always compiles every source file and Eclipse's compiler compile only changed files and those that are affected by such a change?

I believe that you are correct on both counts.

You can of course force Eclipse to recompile everything.

But the other part of the equation is that Java build tools like Ant and Maven are capable of only compiling classes that have changed, and their tree of dependent classes.

EDIT

In Ant, incremental compilation can be done in two ways:

• By default the <javac> task compares the timestamps of .java and corresponding .class files, and only tells the Java compiler to recompile source (.java) files that are newer than their corresponding target (.class) files, or that don't have a target file at all.

• The <depend> task also takes into account dependencies between classes, which it determines by reading and analysing the dependency information embedded in the .class files. Having determined which .class files are out of date, the <depend> task deletes them so a following <javac> task will recompile them. However, this is not entirely fool-proof. For example, extensive changes to the source code can lead to the <depend> task may be analysing stale dependencies. Also certain kinds of dependency (e.g. on static constants) are not apparent in the .class file format.

  To understand why Ant <depend> is not fool-proof, read the "Limitations" section of the documentation.

Answer 2

Restating what I've heard here and phrasing it for lazy folks like me:

You can achieve incremental builds with the javac task in ant, but you should use the depend task to clear out .class files for your modified .java AND you must not leave the includes statement unspecified in the javac task. (Specifying just src path in the javac task and leaving includes unspecified causes javac recompile all sources it finds.)

Here are my depends and javac tasks. With the standard Oracle java compiler, only .java files I modify are compiled. Hope this helps!

<depend srcdir="JavaSource" destdir="${target.classes}" cache="${dependencies.dir}" closure="yes">
    <classpath refid="compiler.classpath" />
    <include name="**/*.java"/>
</depend>

<javac destdir="${target.classes}" debug="true" debuglevel="${debug.features}" optimize="${optimize.flag}" fork="yes" deprecation="no" source="1.6" target="1.6" encoding="UTF-8" includeantruntime="no">
    <classpath refid="compiler.classpath"/>
    <src path="JavaSource"/>
    <include name="**/*.java" />   <!-- This enables the incremental build -->
</javac>

Answer 3

Javac only compiles source files that are either named on the command line or are dependencies and are out of date. Eclipse may have a finer-grained way of deciding what that means.

Answer 4

Eclipse certainly does this. Also it does it at save time if you have that option turned on (and it is by default). It looks like sun also doesn't do this (it is very easy to test, just make a small project where A is the main class that uses class B, but B doesn't use class A. Then change A and compile the project again, see if the timestamp for b.class has changed.

This is the way many compilers work (also gcc for instance). You can use tools like ant and make to compile only the part the project that has changed. Also note that these tools aren't perfect, sometimes eclipse just loses track of the changes and you'll need to do a full rebuild.