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javac - Java compiler
SYNOPSIS
javac [ -bootclasspath bootclasspath ]
[ -classpath classpath ] [ -d directory ]
[ -deprecation ] [ -encoding encoding ]
[ -extdirs directories ]
[ -g | -g:none | -g:keyword-list ] [ -Joption ]
[ -nowarn ] [ -O ] [ -sourcepath sourcepath ]
[ -target version ] [ -verbose ] [ -X ]
[ -Xstdout filename ] [ sourcefiles ] [ @files ]
PARAMETERS
Options may be in any order. For a discussion of parameters which
apply to a specific option, see below.
sourcefiles One or more source files to be compiled (such as
MyClass.java).
@files One or more files that list source files.
DESCRIPTION
The javac tool reads class and interface definitions, written in the
Java programming language, and compiles them into bytecode class files.
There are two ways to pass source code file names to javac:
o For a small number of source files, simply list the file names on the
command line.
o For a large number of source files, list the the file names in a
file, separated by blanks or line breaks. Then use the list file name
on the javac command line, preceded by an @ character.
Source code file names must have .java suffixes, class file names must
have .class suffixes, and both source and class files must have root
names that identify the class. For example, a class called MyClass
would be written in a source file called MyClass.java and compiled into
a bytecode class file called MyClass.class.
Inner class definitions produce additional class files. These class
files have names combining the inner and outer class names, such as
MyClass$MyInnerClass.class.
You should arrange source files in a directory tree that reflects their
package tree. For example, if you keep all your source files in
/workspace, the source code for com.mysoft.mypack.MyClass should be in
/workspace/com/mysoft/mypack/MyClass.java.
By default, the compiler puts each class file in the same directory as
its source file. You can specify a separate destination directory with
-d (see OPTIONS, below).
Searching for Types
When compiling a source file, the compiler often needs information
about a type it does not yet recognize. The compiler needs type infor-
mation 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 CLASSPATH 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:
o Search produces a class file but no source file: javac uses the class
file.
o Search produces a source file but no class file: javac compiles the
source file and uses the resulting class file.
o 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.
OPTIONS
The compiler has a set of standard options that are supported on the
current development environment and will be supported in future
releases. An additional set of non-standard options are specific to
the current virtual machine implementation and are subject to change in
the future. Non-standard options begin with -X.
Standard Options
-classpath classpath
Sets the user class path, overriding the user class path in the
CLASSPATH environment variable. If neither CLASSPATH or -class-
path 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 destina-
tion directory. If a class is part of a package, javac puts the
class file in a subdirectory reflecting the package name, creat-
ing 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/mypack-
age/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 mem-
ber 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 gen-
erated.
-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 asser-
tions. 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.
Cross-Compilation Options
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 compati-
ble with 1.2 VMs, with one exception. When the -source 1.4
option is used, the default target is 1.4. The versions sup-
ported 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.
Non-Standard Options
-Joption
Passes option to the java launcher called by javac. For exam-
ple, -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 warn-
ing 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.
COMMAND LINE ARGUMENT FILES
To shorten or simplify the javac command line, you can specify one or
more files that themselves contain arguments to the javac command. This
enables you to create javac commands of any length on any operating
system.
An argument file can include javac options and source filenames in any
combination. The arguments within a file can be space-separated or
newline-separated. Filenames within an argument file are relative to
the current directory, not the location of the argument file. Wild-
cards (*) are not allowed in these lists (such as for specifying
*.java). Use of the @ character to recursively interpret files is not
supported.
When executing javac, pass in the path and name of each argument file
with the @ leading character. When javac encounters an argument begin-
ning with the character @, it expands the contents of that file into
the argument list.
Example - Single Arg File
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.
Example - Two Arg Files
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
Example - Arg Files with Paths
The argument files can have paths, but any filenames inside the
files are relative to the current working directory (not path1
or path2):
C:> javac @path1\options @path2\classes
EXAMPLES
Compiling a Simple Program
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
Compiling Multiple Source Files
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
Specifying a User Class Path
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
Separating Source Files and Class Files
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.
Cross-Compilation Example
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.
RELATED
jar(1), java(1), javadoc(1), javah(1), javap(1), jdb(1)
See or search the Java web site for the following:
The Java Extensions Mechanism @
http://java.sun.com/j2se/1.4/docs/guide/extensions/index.html
CATEGORY