Is it possible to compile a large Java module in parallel?

Question

I know that multiple modules can be compiled using multiple threads where each thread compiles a single module but what if I have a single large module? Does Javac or the Eclipse Java Compiler support compiling single modules in parallel (using many threads)? Or is there any other Java compiler which supports it?

Update: I created a Java source file with ~50k simple methods (just for the purpose of this test) such as:

    static int add1(int a, int b, int c) {
        return 2 * a + 55 * b - c;
    }

   static int add2(int a, int b, int c) {
        return 2 * a + 55 * b - c;
    }

   static int add3(int a, int b, int c) {
        return 2 * a + 55 * b - c;
    }

These methods do not depend on each other so compilation could be done in parallel (at least in theory). Compiling this file with Javac on my 12 core + HT machine lead to an average 20% CPU usage with a really short spike of up to 50%. This leads me to believe that although there is some parralelization done inside Javac, it is really minor.

The interesting thing is that if I create 2, 3 or 4 classes with the same number of methods and compile them at the same time with a single Javac process, I cannot get a higher CPU usage. The compilation takes exactly 2x, 3x, 4x longer which shows that Javac doesn't compile these totally unrelated classes in parallel. But if I start separate Javac processes to compile these files separately, the CPU jumps to almost 100% when 4 files(=Javac processes) are used and the compilation time is just 5-10% higher than compiling a single file (compared to this, a single Javac process compiling all these 4 files, the compilation takes 400% longer).

So my opinion is that Javac does compile files using multiple threads but it is kind of limited to ~4 threads, it cannot fully utilize a 12 cores machine. Also to me it seems that Javac compiles multiple files in serial, it only uses cores/threads to compile a single file in parallel(I believe that when a single file is compiled, some parts can be done in parallel and this is what Javac does, but what about compiling multiple files in parallel? If I have 100 files which are independent I should be able to see my CPU jump to 100% which is not the case.)

Answer 1

Yes it is possible to build Java code in parallel.

The Java compiler (javac) itself doesn't do this, but both Maven and Ant (and some versions of Make) can run multiple javac instances in parallel.

Furthermore, the Eclipse Java compiler is multi-threaded and you can tell Maven to use it instead of javac; see https://stackoverflow.com/a/3727542/139985

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I note that your example involves compiling a single class with a huge number of methods. Parallel compiler instances won't help with that. The Eclipse compiler might help depending on how it is implemented.

However, I put it to you that that is an unrealistic example. People don't write code like that in real life¹, and code generators can (and should) be written to not emit source code like that.

^{1 - Their co-workers will rebel ...}

Answer 2

javac runs always single-threaded. There is a case for improving javac performance JDK-4229449 : RFE: Please multithread javac for better performance however Oracle does not intend to change the compilation architecture.