When compiling a source file, the compiler often needs information about a type it does not yet recognize. The compiler needs type information for every class or interface used, extended, or implemented in the source file. This includes classes and interfaces not explicitly mentioned in the source file but which provide information through inheritance.

For example, when you subclass java.applet.Applet, you are also using Applet's ancestor classes: java.awt.Panel, java.awt.Container, java.awt.Component, and java.awt.Object.

When the compiler needs type information, it looks for a source file or class file which defines the type. The compiler searches first in the bootstrap and extension classes, then in the user class path. The user class path is defined by setting the environment variable or by using the -classpath command line option. (For details, see Setting the Class Path.) If you use the -sourcepath option, the compiler searches the indicated path for source files; otherwise the compiler searches the user class path both for class files and source files. You can specify different bootstrap or extension classes with the -bootclasspath and -extdirs options; see Cross-Compilation Options below.

A successful type search may produce a class file, a source file, or both. Here is how javac handles each situation:

•

Search produces a class file but no source file: javac uses the class file.

•

Search produces a source file but no class file: javac compiles the source file and uses the resulting class file.

•

Search produces both a source file and a class file: javac determines whether the class file is out of date. If the class file is out of date, javac recompiles the source file and uses the updated class file. Otherwise, javac just uses the class file.

By default, javac considers a class file out of date only if it is older than the source file.

Note that javac can silently compile source files not mentioned on the command line. Use the -verbose option to trace automatic compilation.

-classpath classpath

Sets the user class path, overriding the user class path in the environment variable. If neither or -classpath is specified, the user class path consists of the current directory. See Setting the Class Path for more details.

If the -sourcepath option is not specified, the user class path is searched for source files as well as class files.

-d directory

Sets the destination directory for class files. The destination directory must already exist; javac will not create the destination directory. If a class is part of a package, javac puts the class file in a subdirectory reflecting the package name, creating directories as needed. For example, if you specify -d /home/myclasses and the class is called com.mypackage.MyClass, then the class file is called /home/myclasses/com/mypackage/MyClass.class.

If -d is not specified, javac puts the class file in the same directory as the source file.

Note that the directory specified by -d is not automatically added to your user class path.

-deprecation

Shows a description of each use or override of a deprecated member or class. Without -deprecation, javac shows the names of source files that use or override deprecated members or classes.

-encoding encoding

Sets the source file encoding name, such as EUCJIS/SJIS. If -encoding is not specified, the platform default converter is used.

-g

Generates all debugging information, including local variables. By default, only line number and source file information is generated.

-g:none

Does not generate any debugging information.

-g:keyword-list

Generates only some kinds of debugging information, specified by a comma separated list of keywords. Valid keywords are:

source

Source file debugging information

lines

Line number debugging information

vars

Local variable debugging information

-help

Prints a synopsis of standard options.

-nowarn

Disables warning messages.

-source release

Enables support for compiling source code containing assertions.

When release is set to 1.4, the compiler accepts code containing assertions. Assertions were introduced in J2SE 1.4.

When release is set to 1.3, the compiler does not support assertions. The compiler defaults to the 1.3-behavior if the -source flag is not used.

-sourcepath sourcepath

Specifies the source code path to search for class or interface definitions. As with the user class path, source path entries are separated by colons (:) and can be directories, JAR archives, or ZIP archives. If packages are used, the local path name within the directory or archive must reflect the package name.

Note that classes found through the classpath are subject to automatic recompilation if their sources are found.

-verbose

Verbose output. This includes information about each class loaded and each source file compiled.

By default, classes are compiled against the bootstrap and extension classes of the JDK that javac shipped with. But javac also supports cross-compiling, where classes are compiled against a bootstrap and extension classes of a different Java platform implementation. It is important to use -bootclasspath and -extdirs when cross-compiling; see Cross-Compilation Example below.

-bootclasspath bootclasspath

Cross-compiles against the specified set of boot classes. As with the user class path, boot class path entries are separated by colons (:) and can be directories, JAR archives, or ZIP archives.

-extdirs directories

Cross-compiles against the specified extension directories. Directories are a colon-separated list of directories. Each JAR archive in the specified directories is searched for class files.

-target version

Generates class files that will work on VMs with the specified version. The default is to generate class files to be compatible with 1.2 VMs, with one exception. When the -source 1.4 option is used, the default target is 1.4. The versions supported are:

1.1

Ensures that generated class files will be compatible with 1.1 and 1.2 VMs.

1.2

Generates class files that will run on 1.2 VMs, but will not run on 1.1 VMs. This is the default.

1.3

Generates class files that run on VMs in the Java 2 SDK, v1.3 and later, but will not run on 1.1 or 1.2 VMs.

1.4

Generates class files that are compatible only with 1.4 VMs.

-Joption

Passes option to the java launcher called by javac. For example, -J-Xms48m sets the startup memory to 48 megabytes. Although it does not begin with -X, it is not a `standard option' of javac. It is a common convention for -J to pass options to the underlying VM executing applications written in Java.

Note that CLASSPATH, -classpath, -bootclasspath, and -extdirs do not specify the classes used to run javac. Fiddling with the implementation of the compiler in this way is usually pointless and always risky. If you do need to do this, use the -J option to pass through options to the underlying java launcher.

-X

Displays information about non-standard options and exit.

-Xstdout filename

Send compiler messages to the named file. By default, compiler messages go to System.err.

-Xswitchcheck

Checks switch blocks for fall-through cases and provides a warning message for any that are found. Fall-through cases are cases in a switch block, other than the last case in the block, whose code does not include a break statement, allowing code execution to "fall through" from that case to the next case. For example, the code following the case 1 label in this switch block does not contain a break statement: switch (x) { case 1: System.out.println("1"); // No break; statement here. case 2: System.out.println("2"); } If the -Xswtichcheck flag were used when compiling this code, the compiler would emit a warning about "possible fall-through into case," along with the line number of the case in question.

You could use a single argument file named argfile to hold all javac arguments:

  C:> javac @argfile

This argument file could contain the contents of both files shown in the next example.

You can create two argument files -- one for the javac options and the other for the source filenames: (Notice the following lists have no line-continuation characters.)

Create a file named options containing:

-d classes
-g
-sourcepath \java\pubs\ws\1.3\src\share\classes

Create a file named

classes containing:

MyClass1.java
MyClass2.java
MyClass3.java

You would then run javac with:

C:> javac @options @classes

One source file, Hello.java, defines a class called greetings.Hello. The greetings directory is the package directory both for the source file and the class file and is off the current directory. This allows us to use the default user class path. It also makes it unnecessary to specify a separate destination directory with -d.

   example% ls
      greetings/
   example% ls greetings
      Hello.java
   example% cat greetings/Hello.java
      package greetings;
   
      public class Hello {
           public static void main(String[] args) {
              for (int i=0; i < args.length; i++) {
                  System.out.println("Hello " + args[i]);
              }
           }
      }
   example% javac greetings/Hello.java
   example% ls greetings
      Hello.class   Hello.java
   example% java greetings.Hello World Universe Everyone
      Hello World
      Hello Universe
      Hello Everyone

This example compiles all the source files in the package greetings.

   example% ls
      greetings/
   example% ls greetings
      Aloha.java     GutenTag.java     Hello.java      Hi.java
   example% javac greetings/*.java
   example% ls greetings
      Aloha.class    GutenTag.class    Hello.class     Hi.class
      Aloha.java     GutenTag.java     Hello.java      Hi.java

Having changed one of the source files in the previous example, we recompile it:

   example% pwd
      /examples
   example% javac greetings/Hi.java

Since greetings.Hi refers to other classes in the greetings package, the compiler needs to find these other classes. The example above works, because our default user class path happens to be the directory containing the package directory. But suppose we want to recompile this file and not worry about which directory we're in? Then we need to add /examples to the user class path. We can do this by setting CLASSPATH, but here we'll use the -classpath option.

   example% javac -classpath \examples /examples/greetings/Hi.java

If we change greetings.Hi again, to use a banner utility, that utility also needs to be accessible through the user class path.

   example% javac -classpath /examples:/lib/Banners.jar \
                     /examples/greetings/Hi.java

To execute a class in greetings, we need access both to greetings and to the classes it uses.

   example% java -classpath /examples:/lib/Banners.jar greetings.Hi

It often makes sense to keep source files and class files in separate directories, especially on large projects. We use -d to indicate the separate class file destination. Since the source files are not in the user class path, we use -sourcepath to help the compiler find them.

   example% ls
      classes/  lib/      src/
   example% ls src
      farewells/
   example% ls src/farewells
      Base.java      GoodBye.java
   example% ls lib
      Banners.jar
   example% ls classes
   example% javac -sourcepath src -classpath classes:lib/Banners.jar \
      src/farewells/GoodBye.java -d classes
   example% ls classes
      farewells/
   example% ls classes/farewells
      Base.class      GoodBye.class

Note that the compiler compiled src/farewells/Base.java, even though we didn't specify it on the command line. To trace automatic compiles, use the -verbose option.

Here we use the JDK 1.2 javac to compile code that will run on a 1.1 VM.

   example% javac -target 1.1 -bootclasspath jdk1.1.7/lib/classes.zip \
        -extdirs "" OldCode.java

The -target 1.1 option ensures that the generated class files will be compatible with 1.1 VMs. In JDK1.2, javac compiles for 1.1 by default, so this option is not strictly required. However, it is good form because other compilers may have other defaults.

The JDK 1.2 javac would also by default compile against its own 1.2 bootstrap classes, so we need to tell javac to compile against JDK 1.1 bootstrap classes instead. We do this with -bootclasspath and -extdirs. Failing to do this might allow compilation against a 1.2 API that would not be present on a 1.1 VM and fail at runtime.