Embedded Java with GCJ

After building the cross compiler and root filesystem, building your GCJ application will be a bit anticlimactic. We'll start with the traditional hello world:

Class hello {
 Static public void main(String argc[]) {
   System.out.println("hello from GCJ");
 }
}

Following Java convention, this class resides in the hello.class file. To compile the file, enter:

powerpc-750-linux-gnu-gcj hello.class --main=hello -o hello-java

What's going on with --main=hello? Any class could define a method with a suitable entry point. The --main=hello option tells the linker to use the main method in the hello class when linking. Leaving off this option results in a link error, “undefined reference to main”, which, to the uninitiated, is confusing, because your class contains a main.

Download this file to the target and run it from the chrooted shell. You'll see:

# ./java-test
Hello from GCJ

At this point, development carries on much like any other Java project, with the exception of invoking the GCJ cross compiler instead of the native javac compiler.

In this example, the root filesystem weighs in at more than 20MB. Because many embedded systems use Flash memory, which is considerably more expensive on a per-megabyte basis than disk-based storage systems, a minimally sized root filesystem is frequently an important business requirement. One easy way to reduce the size of your root filesystem is to link your application statically. Although this may seem counterintuitive at first, as you'll have an extra copy of libc code in your application, recall that libgcj.so contains the entire Java standard library. Most applications use a fraction of the standard Java library, so using static linking is a great way to winnow out the unused code in the library. Just be sure to strip the resulting binary; otherwise, you'll be shocked at the size due to the amount of debugging information in libgcj.so.

From the article, you've seen that creating software for an embedded system using GCJ is something that can be reasonably accomplished using tools already present in the Open Source community. Although there are a few minor nits, configuring the root filesystem doesn't require a heroic effort; you just need to get a few different libraries from what you otherwise would need. For applications requiring a smaller root filesystem, we've seen how you can use static linking of your application to reduce the root filesystem greatly. In short, GCJ is a practical solution for using Java on a resource-constrained embedded system—worthy of consideration for your next project.