# javac - Java programming language compiler

- Synopsis
- Description
- Searching for Types
- File List
- Options
- Examples
- See Also

## SYNOPSIS

>     javac [ options ] [ sourcefiles ] [ @files ]
>     oldjavac [ options ] [ sourcefiles ] [ @files ]

> Arguments may be in any order.

> > `options`
> >   Command-line options.
> > `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:

> - For a small number of source files, simply list the file names on the command line.

> - 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 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 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:

> - 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. (The -Xdepend option specifies a slower but more reliable procedure.)

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

## FILE LIST

> To shorten or simplify the `javac` command, you may specify one or more files that themselves contain one filename per line. On the command line, use the '`@`' character with the filename to specify it as a file list. When javac encounters an argument beginning with the character ``@`', it operates on the filenames in that file as if they had been on the command line. This enables you to overcome the command-line length limitation of Windows.

> For example, you can list all of the source file names in a file named `sourcefiles`. This file might look like:

>          MyClass1.java
>          MyClass2.java
>          MyClass3.java

> You could then run the compiler with:

>          C:> javac @sourcefiles

## 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 and compiler implementations and are subject to change in the future. Non-standard options begin with -X.

## Standard Options

> -classpath classpath
>   Set the user class path, overriding the user class path in the CLASSPATH environment variable. If neither CLASSPATH 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
>   Set the destination directory for class files. 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 c:\myclasses and the class is called `com.mypackage.MyClass`, then the class file is called `c:\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
>   Show 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
>   Set the source file encoding name, such as `EUCJIS/SJIS`. If -encoding is not specified, the platform default converter is used.

> -g
>   Generate all debugging information, including local variables. By default, only line number and source file information is generated.

> -g:none
>   Do not generate any debugging information.

> -g:{keyword list}
>   Generate 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

> -nowarn
>   Disable warning messages.

> -O
>   Note: the -O option does nothing in the current implementation of javac and oldjavac.
>   Optimize code for execution time. Using the -O option may slow down compilation, produce larger class files, and make the program difficult to debug.

>   Prior to the Java 2 SDK, the -g and -O options of javac could not be used togther. As of the Java 2 SDK, v1.2, you can combine -g and -O, but you may get suprising results, such as missing variables or relocated or missing code. -O no longer automatically turns on -depend or turns off -g.

> -sourcepath sourcepath
>   Specify the source code path to search for class or interface definitions. As with the user class path, source path entries are separated by semicolons (;) 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.

> -target version
>   Generate class files that will work on VMs with the specified version. The default is to generate class files to be compatible with both 1.1 VMs and VMs of the Java 2 SDK. The versions supported by javac in the Java 2 SDK are:

>   1.1
>     Ensure that generated class files will be compatible with 1.1 and VMs in the Java 2 SDK. This is the default.
>   1.2
>     Generate class files that will run on VMs in the Java 2 SDK, v 1.2 and later, but will not run on 1.1 VMs.
>   1.3
>     Generate class files that will run on VMs in the Java 2 SDK, v 1.3 and later, but will not run on 1.1 or 1.2 VMs.

> -bootclasspath bootclasspath
>   Cross-compile against the specified set of boot classes. As with the user class path, boot class path entries are separated by semicolons (;) and can be directories, JAR archives, or ZIP archives.

> -extdirs directories
>   Cross-compile against the specified extension directories. Directories is a semicolon-separated list of directories. Each JAR archive in the specified directories is searched for class files.

### Non-Standard Options

> These options are supported only by oldjavac. They are not supported by the new javac compiler.

> -X
>   Display information about non-standard options and exit.

> -Xdepend
>   Recursively search all reachable classes for more recent source files to recompile. This option will more reliably discover classes that need to be recompiled, but can slow down the compilation process drastically.

> -Xstdout
>   Send compiler messages to `System.out`. By default, compiler messages go to `System.err`.

> -Xverbosepath
>   Describe how paths and standard extensions were searched to find source and class files.

### The -J option

> Both javac and oldjavac support the -J option.
> -Joption
>   Pass 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.

## 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.

> >     C:> dir
> >     greetings/
> >     C:> dir greetings
> >     Hello.java
> >     C:> 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]);
> >             }
> >         }
> >     }
> >     C:> javac greetings\Hello.java
> >     C:> dir greetings
> >     Hello.class   Hello.java
> >     C:> 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`.

> >     C:> dir
> >     greetings\
> >     C:> dir greetings
> >     Aloha.java         GutenTag.java      Hello.java         Hi.java
> >     C:> javac greetings\*.java
> >     C:> dir 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:

> >     C:> cd
> >     \examples
> >     C:> 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.

> >     C:>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.

> >     C:>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.

> >     C:>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.

> >     C:> dir
> >     classes\  lib\      src\
> >     C:> dir src
> >     farewells\
> >     C:> dir src\farewells
> >     Base.java      GoodBye.java
> >     C:> dir lib
> >     Banners.jar
> >     C:> dir classes
> >     C:> javac -sourcepath src -classpath classes;lib\Banners.jar \
> >            src\farewells\GoodBye.java -d classes
> >     C:> dir classes
> >     farewells\
> >     C:> dir 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 Java 2 SDK's javac to compile code that will run on a 1.1 VM.

> >     C:> 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. The Java 2 SDK's 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 Java 2 SDK's javac would also by default compile against its own 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 Java 2 Platform API that would not be present on a 1.1 VM and fail at runtime.

## SEE ALSO

> - java - the Java Application Launcher
> - jdb - Java Application Debugger
> - javah - C Header and Stub File Generator
> - javap - Class File Disassembler
> - javadoc - API Documentation Generator
> - jar - JAR Archive Tool
> - The Java Extensions Framework