Section 3.4. The Java Compiler

3.4. The Java Compiler

In this section, we'll say a few words about javac, the Java compiler in the JDK. The javac compiler is written entirely in Java, so it's available for any platform that supports the Java runtime system. javac turns Java source code into a compiled class that contains Java bytecode. By convention, source files are named with a .java extension; the resulting class files have a .class extension. Each source code file is considered a single compilation unit. As you'll see in Chapter 6, classes in a given compilation unit share certain features, such as package and import statements.

javac allows 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. Avoid packing too many classes into a single source file. Packing classes together in a .java file only superficially associates them. In Chapter 6 we'll talk about inner classes, classes that contain other classes and interfaces.

As an example, place the following source code in file BigBird.java:

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

Next, compile it with:

     % javac BigBird.java

Unlike the Java interpreter, which takes just a class name as its argument, javac needs a filename (with the .java extension) to process. The previous command produces the class file BigBird.class in the same directory as the source file. While it's nice to see the class file in the same directory as the source for this example, for most real applications, you need to store the class file in an appropriate place in the classpath.

You can use the -d option with javac to specify an alternative directory for storing the class files javac 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 whether the class is contained in a package. (The compiler creates intermediate subdirectories automatically, if necessary.) For example, we can use the following command to create the BigBird.class file at /home/vicky/Java/classes/animals/birds/BigBird.class:

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

You can specify multiple .java files in a single javac command; the compiler creates a class file for each source file. But you don't need to list other classes your class references, as long as they are in the classpath in either source or compiled form. During compilation, Java resolves all other class references using the classpath.

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

By default, however, javac checks only source files that are referenced directly from other source files. This means that if you have an out-of-date class file that is referenced only by an up-to-date class file, it may not be noticed and recompiled. You can force javac to walk the entire tree of objects using the -depend option, but this can increase compilation time significantly. This technique still won't help if you want to keep class libraries or other collections of classes up to date even if they aren't being referenced at all. For that and many other reasons, most projects use a real build utility such as Apache's Ant to manage builds, packaging, and more. We discuss Ant in Chapter 15.

Finally, it's important to note that javac can compile an application even if only the compiled (binary) versions of some of the classes are available. You don't need source code for all your objects. Java class files contain all the data type and method signature information that source files contain, so compiling against binary class files is as typesafe (and exception safe) as compiling with Java source code.