3.3 The Java Compiler

In this section, I'm going to say a few words about javac, the Java compiler that is supplied as part of Sun's JDK. (If you are happily working in another development environment, you may want to skip ahead to the next section.) The javac compiler is written entirely in Java, so it's available for any platform that supports the Java run-time system. The ability to support its own development environments is an important stage in a language's development. Java makes this bootstrapping automatic by supplying a ready-to-run compiler at the same cost as porting the interpreter.

javac turns Java source code into a compiled class that contains Java virtual machine byte-code. By convention, source files are named with a .java extension; the resulting class files have a .class extension. javac considers a file to be a single compilation unit. As you'll see in Chapter 5, Objects in Java, classes in a given compilation unit share certain features like package and import statements.

javac allows you one public class per file and insists the file have the same name as the class. If the filename and class name don't match, javac issues a compilation error. A single file can contain multiple classes, as long as only one of the classes is public. You should avoid packing lots of classes into a single file. The compiler lets you include extra non-public classes in a .java file, so that you can implement a class that is tightly coupled to another class without a lot of hassle. But you should have more than one class in a file if the public class in the file is the only one that ever uses additional classes.

Now for an example. The source code for the following class should be placed in a file called BigBird.java:

package animals.birds 
 
public class BigBird extends Bird { 
    ... 
} 

We can then compile it with:

% javac BigBird.java 

Unlike the Java interpreter, which takes a class name as its argument, javac requires an actual filename to process. The above command produces the class file BigBird.class and stores it in the same directory as the source file. While it's useful to have the class file in the same directory as the source when you are working on a simple example, for most real applications you'll need to store the class file in an appropriate place in the class path.

You can use the -d option to javac to specify an alternate directory for storing the class files it generates. The specified directory is used as the root of the class hierarchy, so .class files are placed in this directory or in a subdirectory below it, depending on the package name of the class. For example, we can use the following command to put our BigBird.class file in an appropriate location:

% javac -d /home/vicky/Java/classes BigBird.java 

When you use the -d option, javac automatically creates any directories needed to store the class file in the appropriate place. In the above command, the BigBird.class file is stored in /home/vicky/Java/classes/animals/birds.

You can specify multiple .java files in a single javac command; the compiler simply creates a class file for each source file. But you don't need to list source files for other classes your class references, as long as the other classes have already been compiled. During compilation, Java resolves other class references using the class path. If our class references other classes in animals.birds or other packages, the appropriate paths should be listed in the class path at compile time, so that javac can find the appropriate class files. You either make sure that the CLASSPATH environment variable is set or use the -classpath option to javac.

The Java compiler is more intelligent than your average compiler and replaces some of the functionality of a make utility. For example, javac compares the modification times of the source and class files for all referenced classes and recompiles them as necessary. A compiled Java class remembers the source file from which it was compiled, so as long as the source file is in the same directory as the class file, javac can recompile the source if necessary. If, in the above example, class BigBird references another class, animals.furry.Grover, javac looks for the source Grover.java in an animals.furry package and recompiles it if necessary to bring the Grover.class class file up to date.

It's important to note that javac can do its job even if only the compiled versions of referenced classes are available. Java class files contain all the data type and method signature information source files do, so compiling against binary class files is as type safe (and exception safe) as compiling with Java source code.