You can use the javac
tool and its options to read Java class and interface definitions and compile them into bytecode and class files.
Synopsis
javac [ options ] [ sourcefiles ]
options
Command-line options. See Overview of javac Options.
sourcefiles
One or more source files to be compiled (such as MyClass.java
) or processed for annotations (such as MyPackage.MyClass
).
Description
The javac
command reads class and interface definitions, written in the Java programming language, and compiles them into bytecode class files. The javac
command can also process annotations in Java source files and classes.
In JDK 9, a new launcher environment variable, JDK_JAVAC_OPTIONS
, has been introduced that prepends its content to the command line to javac
. See Using JDK_JAVAC_OPTIONS Environment Variable.
There are two ways to pass source code file names to javac
.
For a small number of source files, you can list the file names on the command line.
For a large number of source files, you can use the @filename
option on the javac
command line to include a file that lists the source file names. See Standard Options for javac for a description of the option and javac Command-Line Argument Files for a description of javac
argument files.
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 that combine the inner and outer class names, such as MyClass$MyInnerClass.class
.
You should arrange the source files in a directory tree that reflects their package tree. For example:
Oracle Solaris, Linux, and OS X: If all of your source files are in /workspace
, then put the source code for com.mysoft.mypack.MyClass
in /workspace/com/mysoft/mypack/MyClass.java
.
Windows: If all of your source files are in \workspace
, then put the source code for com.mysoft.mypack.MyClass
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 the -d
option described in Standard Options for javac.
Programmatic Interface
The javac
command supports the new Java Compiler API defined by the classes and interfaces in the javax.tools
package.
Implicitly Loaded Source Files
To compile a set of source files, the compiler might need to implicitly load additional source files. See Searching for Types. Such files are currently not subject to annotation processing. By default, the compiler gives a warning when annotation processing occurs and any implicitly loaded source files are compiled. The -implicit
option provides a way to suppress the warning.
Using JDK_JAVAC_OPTIONS Environment Variable
The content of the JDK_JAVAC_OPTIONS
environment variable, separated by white-spaces ( ) or white-space characters (\n
, \t
, \r
, or \f
) is prepended to the command line arguments passed to javac
as a list of arguments.
The encoding requirement for the environment variable is the same as the javac
command line on the system. JDK_JAVAC_OPTIONS
environment variable content is treated in the same manner as that specified in the command line.
Single ('
) or double ("
) quotes can be used to enclose arguments that contain whitespace characters. All content between the open quote and the first matching close quote are preserved by simply removing the pair of quotes. In case a matching quote is not found, the launcher will abort with an error message. @files
are supported as they are specified in the command line. However, as in @files
, use of a wildcard is not supported.
Examples of quoting arguments containing white spaces:
export JDK_JAVAC_OPTIONS=‘@"C:\white spaces\argfile”'
export JDK_JAVAC_OPTIONS=‘"@C:\white spaces\argfile”'
export JDK_JAVAC_OPTIONS='@C:\"white spaces"\argfile'
Overview of javac Options
The compiler has sets of standard options, and cross-compilation options that are supported on the current development environment. The compiler also has a set of nonstandard options that are specific to the current virtual machine and compiler implementations but are subject to change in the future. The nonstandard options begin with -X
. The different sets of javac
options are described in the following sections:
Standard Options for javac
@filename
Reads options and file names from a file. To shorten or simplify the javac
command, you can specify one or more files that contain arguments to the javac
command (except -J
options). This let’s you to create javac
commands of any length on any operating system. See javac Command-Line Argument Files.
-Akey[=value]
Specifies options to pass to annotation processors. These options aren’t interpreted by javac
directly, but are made available for use by individual processors. The key
value should be one or more identifiers separated by a dot (.).
--add-modules module , module
Specifies root modules to resolve in addition to the initial modules, or all modules on the module path if module
is ALL-MODULE-PATH.
--boot-class-path path
or -bootclasspath path
Overrides the location of the bootstrap class files.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
--class-path path
, -classpath path
, or -cp path
Specifies where to find user class files and annotation processors. This class path overrides the user class path in the CLASSPATH
environment variable.
If --class-path
, -classpath
, or -cp
aren’t specified, then the user class path is the current directory.
If the -sourcepath
option isn’t specified, then the user class path is also searched for source files.
If the -processorpath
option isn’t specified, then the class path is also searched for annotation processors.
-d directory
Sets the destination directory for class files. If a class is part of a package, then javac
puts the class file in a subdirectory that reflects the package name and creates directories as needed. For example:
Oracle Solaris, Linux, and OS X: If you specify -d
/home/myclasses
and the class is called com.mypackage.MyClass
, then the class file is /home/myclasses/com/mypackage/MyClass.class
.
Windows: If you specify -d
C:\myclasses
and the class is called com.mypackage.MyClass
, then the class file is C:\myclasses\com\mypackage\MyClass.class
.
If the -d
option isn’t specified, then javac
puts each class file in the same directory as the source file from which it was generated.
Note:
The directory specified by the -d
option isn’t automatically added to your user class path.
-deprecation
Shows a description of each use or override of a deprecated member or class. Without the -deprecation
option, javac
shows a summary of the source files that use or override deprecated members or classes. The -deprecation
option is shorthand for -Xlint:deprecation
.
-encoding encoding
Specifies character encoding used by source files, such as EUC-JP and UTF-8. If the -encoding
option isn’t specified, then the platform default converter is used.
-endorseddirs directories
Overrides the location of the endorsed standards path.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
-extdirs directories
Overrides the location of the installed extensions. The directories
variable is a colon-separated list of directories. Each JAR file in the specified directories is searched for class files. All JAR files found become part of the class path.
If you are cross-compiling, then this option specifies the directories that contain the extension classes. See Cross-Compilation Options for javac.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
-g
Generates all debugging information, including local variables. By default, only line number and source file information is generated.
-g:[lines, vars, source],[lines, vars, source],[lines, vars, source]
Generates only the kinds of debugging information specified by the comma-separated list of keywords. Valid keywords are:
-g:none
Doesn’t generate any debugging information.
-h directory
Specifies where to place generated native header files.
When you specify this option, a native header file is generated for each class that contains native methods or that has one or more constants annotated with the java.lang.annotation.Native annotation. If the class is part of a package, then the compiler puts the native header file in a subdirectory that reflects the package name and creates directories as needed.
--help
or –help
Prints a synopsis of the standard options.
--help-extra
or -X
Prints the help for extra options.
-implicit:{[none, class],[none, class]}
Specifies whether or not to generate class files for implicitly referenced files:
-implicit:class
— Automatically generates class files.
-implicit:none
— Suppresses class file generation.
If this option isn’t specified, then the default automatically generates class files. In this case, the compiler issues a warning if any class files are generated when also doing annotation processing. The warning isn’t issued when the -implicit
option is explicitly set. See Searching for Types.
-Joption
Passes option
to the runtime system, where option
is one of the Java options described on javacommand. For example, -J-Xms48m
sets the startup memory to 48 MB.
Note:
The CLASSPATH
environment variable, -classpath
option, -bootclasspath
option, and -extdirs
option don’t specify the classes used to run javac
. Trying to customize the compiler implementation with these options and variables is risky and often doesn’t accomplish what you want. If you must customize the complier implementation, then use the -J
option to pass options through to the underlying Java launcher.
--limit-modules module , module
*Limits the universe of observable modules.
--module module-name
or -m module-name
Compiles only the specified module and checks time stamps.
--module-path path
or -p path
Specifies where to find application modules.
--module-source-path module-source-path
Specifies where to find input source files for multiple modules.
--module-version version
-nowarn
Disables warning messages. This option operates the same as the -Xlint:none
option.
-parameters
Generates metadata for reflection on method parameters. Stores formal parameter names of constructors and methods in the generated class file so that the method java.lang.reflect.Executable.getParameters
from the Reflection API can retrieve them.
-proc: [none, only] , [none, only]
Controls whether annotation processing and compilation are done. -proc:none
means that compilation takes place without annotation processing. -proc:only
means that only annotation processing is done, without any subsequent compilation.
-processor class1 [,class2,class3...]
Names of the annotation processors to run. This bypasses the default discovery process.
--processor-module—path path
or -p module-path
Specifies the module path used for finding annotation processors.
--processor—path path
or -processorpath path
Specifies where to find annotation processors. If this option isn’t used, then the class path is searched for processors.
-profile profile
Checks that the API used is available in the specified profile.
Note:
Not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
--release release
Compiles against the public, supported and documented API for a specific VM version. Supported release
targets are 6
, 7
, 8
, and 9
.
Note:
When using --release
for a version of the Java Platform that supports modules, you can’t use --add-modules
to access internal JDK modules, nor can you use --add-exports
to access internal JDK APIs in the modules.
-s directory
Specifies the directory used to place the generated source files. If a class is part of a package, then the compiler puts the source file in a subdirectory that reflects the package name and creates directories as needed. For example:
Oracle Solaris, Linux, and OS X:: If you specify -s /home/mysrc
and the class is called com.mypackage.MyClass
, then the source file is put in /home/mysrc/com/mypackage/MyClass.java
.
Windows: If you specify -s C:\mysrc
and the class is called com.mypackage.MyClass
, then the source file is put in C:\mysrc\com\mypackage\MyClass.java
.
-source release
Specifies the version of source code accepted. The following values for release
are allowed:
Note:
As of JDK 9, the javac
doesn’t support -source
release settings less than or equal to 5
. If settings less than or equal to 5
are used, then the javac
command behaves as if -source 6
were specified.
1.6
No language changes were introduced in Java SE 6. However, encoding errors in source files are now reported as errors instead of warnings as was done in earlier releases of Java Platform, Standard Edition.
6
Synonym for 1.6.
1.7
The compiler accepts code with features introduced in Java SE 7.
7
Synonym for 1.7.
1.8
The compiler accepts code with features introduced in Java SE 8.
8
Synonym for 1.8.
9
The default value. The compiler accepts code with features introduced in Java SE 9.
--source-path path
or -sourcepath path
Specifies where to find input source files. This is the source code path used to search for class or interface definitions. As with the user class path, source path entries are separated by colons (:) on Oracle Solaris and semicolons(;) on Windows. They can be directories, JAR archives, or ZIP archives. If packages are used, then the local path name within the directory or archive must reflect the package name.
Note:
Classes found through the class path might be recompiled when their source files are also found. See Searching for Types.
--system jdk | none
Overrides the location of system modules.
-target release
Generates class files for a specific VM version.
--upgrade-module—path path
Overrides the location of upgradeable modules.
-verbose
Outputs messages about what the compiler is doing. Messages include information about each class loaded and each source file compiled.
--version
or -version
Prints version information.
-Werror
Terminates compilation when warnings occur.
Cross-Compilation Options for javac
By default, for releases prior to JDK 9, classes were compiled against the bootstrap classes of the platform that shipped with thejavac
command. But javac
also supports cross-compiling, in which classes are compiled against bootstrap classes of a different Java platform implementation. It’s important to use the -bootclasspath
and -extdirs
options when cross-compiling.
Note:
Not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
Extra Options for javac
--add-exports module/package=other-module(,other-module)
*Specifies a package to be considered as exported from its defining module to additional modules or to all unnamed modules when the value of other-module
is ALL-UNNAMED
.
--add-reads module=other-module(,other-module)
*Specifies additional modules to be considered as required by a given module.
-Djava.endorsed.dirs=dirs
Overrides the location of the endorsed standards path.
Note:
Not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
-Djava.ext.dirs=dirs
Overrides the location of installed extensions.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
--doclint-format [html4|html5]
Specifies the format for documentation comments.
--patch-module module=file(:file)
*Overrides or augments a module with classes and resources in JAR files or directories.
-Xbootclasspath:path
Overrides the location of the bootstrap class files.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
-Xbootclasspath/a:path
Adds a suffix to the bootstrap class path.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
-Xbootclasspath/p:path
Adds a prefix to the bootstrap class path.
Note:
This option is not supported when using --release release
to compile for JDK 9. See the description of --release release
for details about compiling for versions other than JDK 9.
-Xdiags:[compact, verbose]
Selects a diagnostic mode.
-Xdoclint
Enables recommended checks for problems in javadoc
comments
-Xdoclint:(all|none|[-]group) [/access]
Enables or disables specific groups of checks, where group
is one of the following values:
accessibility
html
missing
reference
syntax
For more information about these groups of checks, see the -Xdoclint
option of the javadoc
command. The -Xdoclint
option is disabled by default in the javac
command.
The variable access
specifies the minimum visibility level of classes and members that the -Xdoclint
option checks. It can have one of the following values (in order of most to least visible:)
public
protected
package
private
The default access level is private
.
For example, the following option checks classes and members (with all groups of checks) that have the access level of protected and higher (which includes protected and public):
-Xdoclint:all/protected
The following option enables all groups of checks for all access levels, except it won’t check for HTML errors for classes and members that have the access level of package and higher (which includes package, protected and public :)
-Xdoclint:all,-html/package
Xdoclint/package:[-]packages(,[-]package)
*Enables or disables checks in specific packages. Each package
is either the qualified name of a package or a package name prefix followed by .*
, which expands to all sub-packages of the given package. Each package
can be prefixed with -
to disable checks for a specified package or packages.
-Xlint
Enables all recommended warnings. In this release, enabling all available warnings is recommended.
-Xlint:key(,key)
*Supplies warnings to enable or disable, separated by comma. Precede a key by a hypen (-) to disable the specified warning.
Supported values for key
are:
all
— Enables all warnings.
auxiliaryclass
— Warns about an auxiliary class that’s hidden in a source file, and is used from other files.
cast
— Warns about the use of unnecessary casts.
classfile
— Warns about the issues related to classfile contents.
deprecation
— Warns about the use of deprecated items.
dep-ann
— Warns about the items marked as deprecated in javadoc
but without the @Deprecated
annotation.
divzero
— Warns about the division by the constant integer 0.
empty
— Warns about an empty statement after if
.
exports
— Warns about the issues regarding module exports.
fallthrough
— Warns about the falling through from one case of a switch statement to the next.
finally
— Warns about finally
clauses that don’t terminate normally.
module
— Warns about the module system-related issues.
opens
— Warns about the issues related to module opens.
options
— Warns about the issues relating to use of command line options.
overloads
— Warns about the issues related to method overloads.
overrides
— Warns about the issues related to method overrides.
path
— Warns about the invalid path elements on the command l ine.
processing
— Warns about the issues related to annotation processing.
rawtypes
— Warns about the use of raw types.
removal
— Warns about the use of an API that has been marked for removal.
serial
— Warns about the serializable classes that don’t provide a serial version ID. Also warns about access to non-public members from a serializable element.
static
— Warns about the accessing a static member using an instance.
try
— Warns about the issues relating to the use of try blocks ( that is, try-with-resources).
unchecked
— Warns about the unchecked operations.
varargs
— Warns about the potentially unsafe vararg
methods.
none
— Disables all warnings.
-Xmaxerrs number
Sets the maximum number of errors to print.
-Xmaxwarns number
Sets the maximum number of warnings to print.
-Xpkginfo:[always, legacy, nonempty]
Specifies when and how the javac
command generates package-info.class
files from package-info.java
files using one of the following options:
always
Generates a package-info.class
file for every package-info.java
file. This option may be useful if you use a build system such as Ant, which checks that each .java
file has a corresponding .class
file.
legacy
Generates a package-info.class
file only if package-info.java
contains annotations. This option doesn't generate a package-info.class
file if package-info.java
contains only comments.
Note:
A package-info.class
file might be generated but be empty if all the annotations in the package-info.java
file have RetentionPolicy.SOURCE
.
nonempty
Generates a package-info.class
file only if package-info.java
contains annotations with RetentionPolicy.CLASS
or RetentionPolicy.RUNTIME
.
-Xplugin:name args
Specifies the name and optional arguments for a plug-in to be run.
-Xprefer:[source
or newer]
Specifies which file to read when both a source file and class file are found for an implicitly compiled class using one of the following options. See Searching for Types.
-Xprefer:newer
— Reads the newer of the source or class files for a type (default).
-Xprefer:source
— Reads the source file. Use -Xprefer:source
when you want to be sure that any annotation processors can access annotations declared with a retention policy of SOURCE
.
-Xprint
Prints a textual representation of specified types for debugging purposes. This doesn’t perform annotation processing or compilation. The format of the output could change.
-XprintProcessorInfo
Prints information about which annotations a processor is asked to process.
-XprintRounds
Prints information about initial and subsequent annotation processing rounds.
-Xstdout filename
Sends compiler messages to the named file. By default, compiler messages go to System.err
.
javac Command-Line Argument Files
An argument file can include javac
options and source file names in any combination. The arguments within a file can be separated by spaces or new line characters. If a file name contains embedded spaces, then put the whole file name in double quotation marks.
File names within an argument file are relative to the current directory, not to the location of the argument file. Wildcards (*) aren’t allowed in these lists (such as for specifying *.java
). Use of the at sign (@
) to recursively interpret files isn’t supported. The -J
options aren’t supported because they’re passed to the launcher, which doesn’t support argument files.
When executing the javac
command, pass in the path and name of each argument file with the at sign (@) leading character. When the javac
command encounters an argument beginning with the at sign (@
), it expands the contents of that file into the argument list.
Examples of Using javac @filename
You could use a single argument file named argfile
to hold all javac
arguments:
javac @argfile
This argument file could contain the contents of both files shown in Example 2.
You can create two argument files: one for the javac
options and the other for the source file names. Note that the following lists have no line-continuation characters.
Create a file named options that contains the following:
Oracle Solaris, Linux, and OS X::
-d classes -g -sourcepath /java/pubs/ws/1.3/src/share/classes
Windows:
-d classes -g -sourcepath C:\java\pubs\ws\1.3\src\share\classes
Create a file named classes that contains the following:
MyClass1.java MyClass2.java MyClass3.java
Then, run the javac
command as follows:
javac @options @classes
The argument files can have paths, but any file names inside the files are relative to the current working directory (not path1
or path2
):
javac @path1/options @path2/classes
Examples of Using -Xlint keys
cast
Warns about unnecessary and redundant casts, for example:
String s = (String) "Hello!"
classfile
Warns about issues related to class file contents.
deprecation
Warns about the use of deprecated items. For example:
java.util.Date myDate = new java.util.Date(); int currentDay = myDate.getDay();
The method java.util.Date.getDay
has been deprecated since JDK 1.1.
dep-ann
Warns about items that are documented with the @deprecated
Javadoc comment, but don’t have the @Deprecated
annotation, for example:
/** * @deprecated As of Java SE 7, replaced by {@link #newMethod()} */ public static void deprecatedMethod() { } public static void newMethod() { }
divzero
Warns about division by the constant integer 0, for example:
int divideByZero = 42 / 0;
empty
Warns about empty statements after if
statements, for example:
class E { void m() { if (true) ; } }
fallthrough
Checks the 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 doesn’t 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 doesn’t end with a break statement:
switch (x) { case 1: System.out.println("1"); // No break statement here. case 2: System.out.println("2"); }
If the -Xlint:fallthrough
option was used when compiling this code, then the compiler emits a warning about possible fall-through into case, with the line number of the case in question.
finally
Warns about finally
clauses that can’t be completed normally, for example:
public static int m() { try { throw new NullPointerException(); } catch (NullPointerException(); { System.err.println("Caught NullPointerException."); return 1; } finally { return 0; } }
The compiler generates a warning for the finally
block in this example. When the int
method is called, it returns a value of 0. A finally
block executes when the try
block exits. In this example, when control is transferred to the catch
block, the int
method exits. However, the finally
block must execute, so it’s executed, even though control was transferred outside the method.
options
Warns about issues that related to the use of command-line options. See Cross-Compilation Options for javac.
overrides
Warns about issues related to method overrides. For example, consider the following two classes:
public class ClassWithVarargsMethod { void varargsMethod(String... s) { } } public class ClassWithOverridingMethod extends ClassWithVarargsMethod { @Override void varargsMethod(String[] s) { } }
The compiler generates a warning similar to the following:.
warning: [override] varargsMethod(String[]) in ClassWithOverridingMethod overrides varargsMethod(String...) in ClassWithVarargsMethod; overriding method is missing '...'
When the compiler encounters a varargs
method, it translates the varargs
formal parameter into an array. In the method ClassWithVarargsMethod.varargsMethod
, the compiler translates the varargs
formal parameter String... s
to the formal parameter String[] s
, an array that matches the formal parameter of the method ClassWithOverridingMethod.varargsMethod
. Consequently, this example compiles.
path
Warns about invalid path elements and nonexistent path directories on the command line (with regard to the class path, the source path, and other paths). Such warnings can’t be suppressed with the @SuppressWarnings
annotation. For example:
Oracle Solaris, Linux, and OS X:: javac -Xlint:path -classpath /nonexistentpath Example.java
Windows: javac -Xlint:path -classpath C:\nonexistentpath Example.java
processing
Warns about issues related to annotation processing. The compiler generates this warning when you have a class that has an annotation, and you use an annotation processor that can’t handle that type of exception. For example, the following is a simple annotation processor:
Source file AnnocProc.java:
import java.util.*; import javax.annotation.processing.*; import javax.lang.model.*; import.javaz.lang.model.element.*; @SupportedAnnotationTypes("NotAnno") public class AnnoProc extends AbstractProcessor { public boolean process(Set<? extends TypeElement> elems, RoundEnvironment renv){ return true; } public SourceVersion getSupportedSourceVersion() { return SourceVersion.latest(); } }
Source file AnnosWithoutProcessors.java:
@interface Anno { } @Anno class AnnosWithoutProcessors { }
The following commands compile the annotation processor AnnoProc
, then run this annotation processor against the source file AnnosWithoutProcessors.java
:
javac AnnoProc.java javac -cp . -Xlint:processing -processor AnnoProc -proc:only AnnosWithoutProcessors.java
When the compiler runs the annotation processor against the source file AnnosWithoutProcessors.java
, it generates the following warning:
warning: [processing] No processor claimed any of these annotations: Anno
To resolve this issue, you can rename the annotation defined and used in the class AnnosWithoutProcessors
from Anno
to NotAnno
.
rawtypes
Warns about unchecked operations on raw types. The following statement generates a rawtypes
warning:
void countElements(List l) { ... }
The following example doesn’t generate a rawtypes
warning:
void countElements(List<?> l) { ... }
List
is a raw type. However, List<?>
is an unbounded wildcard parameterized type. Because List
is a parameterized interface, always specify its type argument. In this example, the List
formal argument is specified with an unbounded wildcard (?
) as its formal type parameter, which means that the countElements
method can accept any instantiation of the List
interface.
serial
Warns about missing serialVersionUID
definitions on serializable classes, for example:
public class PersistentTime implements Serializable { private Date time; public PersistentTime() { time = Calendar.getInstance().getTime(); } public Date getTime() { return time; } }
The compiler generates the following warning:
warning: [serial] serializable class PersistentTime has no definition of serialVersionUID
If a serializable class doesn’t explicitly declare a field named serialVersionUID
, then the serialization runtime environment calculates a default serialVersionUID
value for that class based on various aspects of the class, as described in the Java Object Serialization Specification. However, it’s strongly recommended that all serializable classes explicitly declare serialVersionUID
values because the default process of computing serialVersionUID
values is highly sensitive to class details that can vary depending on compiler implementations. As a result, might cause an unexpected InvalidClassExceptions
during deserialization. To guarantee a consistent serialVersionUID
value across different Java compiler implementations, a serializable class must declare an explicit serialVersionUID
value.
static
Warns about issues relating to the use of statics variables, for example:
class XLintStatic { static void m1() { } void m2() { this.m1(); } }
The compiler generates the following warning:
warning: [static] static method should be qualified by type name, XLintStatic, instead of by an expression
To resolve this issue, you can call the static
method m1
as follows:
XLintStatic.m1();
Alternately, you can remove the static
keyword from the declaration of the method m1
.
try
Warns about issues relating to the use of try
blocks, including try-with-resources statements. For example, a warning is generated for the following statement because the resource ac
declared in the try
block isn’t used:
try ( AutoCloseable ac = getResource() ) { // do nothing}
unchecked
Gives more detail for unchecked conversion warnings that are mandated by the Java Language Specification, for example:
List l = new ArrayList<Number>(); List<String> ls = l; // unchecked warning
During type erasure, the types ArrayList<Number>
and List<String>
become ArrayList
and List
, respectively.
The ls
command has the parameterized type List<String>
. When the List
referenced by l
is assigned to ls
, the compiler generates an unchecked warning. At compile time, the compiler and JVM can’t determine whether l
refers to a List<String>
type. In this case, l
doesn’t refer to a List<String>
type. As a result, heap pollution occurs.
A heap pollution situation occurs when the List
object l
, whose static type is List<Number>
, is assigned to another List
object, ls
, that has a different static type, List<String>
. However, the compiler still allows this assignment. It must allow this assignment to preserve backward compatibility with releases of Java SE that don’t support generics. Because of type erasure, List<Number>
and List<String>
both become List
. Consequently, the compiler allows the assignment of the object l
,
which has a raw type of List
, to the object ls
.
varargs
Warns about unsafe use of variable arguments (varargs
) methods, in particular, those that contain non-reifiable arguments, for example:
public class ArrayBuilder { public static <T> void addToList (List<T> listArg, T... elements) { for (T x : elements) { listArg.add(x); } } }
A non-reifiable type is a type whose type information isn’t fully available at runtime.
The compiler generates the following warning for the definition of the method ArrayBuilder.addToList
:
warning: [varargs] Possible heap pollution from parameterized vararg type T
When the compiler encounters a varargs method, it translates the varargs
formal parameter into an array. However, the Java programming language doesn’t permit the creation of arrays of parameterized types. In the method ArrayBuilder.addToList
, the compiler translates the varargs
formal parameter T...
elements to the formal parameter T[]
elements, an array. However, because of type erasure, the compiler converts the varargs
formal parameter to Object[]
elements. Consequently, there’s a possibility of heap pollution.
Example of Compiling by Providing Command-Line Arguments
To compile as though providing command-line arguments, use the following syntax:
JavaCompiler javac = ToolProvider.getSystemJavaCompiler();
The example writes diagnostics to the standard output stream and returns the exit code that javac
command would give when called from the command line.
You can use other methods in the javax.tools.JavaCompiler
interface to handle diagnostics, control where files are read from and written to, and more.
Old Interface
Note:
This API is retained for backward compatibility only. All new code should use the Java Compiler API.
The com.sun.tools.javac.Main
class provides two static methods to call the compiler from a program:
public static int compile(String[] args); public static int compile(String[] args, PrintWriter out);
The args
parameter represents any of the command-line arguments that would typically be passed to the compiler.
The out
parameter indicates where the compiler diagnostic output is directed.
The return
value is equivalent to the exit
value from javac
.
Note:
All other classes and methods found in a package with names that start with com.sun.tools.javac
(subpackages of com.sun.tools.javac
) are strictly internal and subject to change at any time.
Example of Compiling Multiple Source Files
This example compiles the Aloha.java
, GutenTag.java
, Hello.java
, and Hi.java
source files in the greetings
package.
Oracle Solaris, Linux, and OS X::
% javac greetings/*.java % ls greetings Aloha.class GutenTag.class Hello.class Hi.class Aloha.java GutenTag.java Hello.java Hi.java
Windows:
C:\>javac greetings\*.java C:\>dir greetings Aloha.class GutenTag.class Hello.class Hi.class Aloha.java GutenTag.java Hello.java Hi.java
Example of Specifying a User Class Path
After changing one of the source files in the previous example, recompile it:
Oracle Solaris, Linux, and OS X::
pwd /examples javac greetings/Hi.java
Windows:
C:\>cd \examples C:\>javac greetings\Hi.java
Because greetings.Hi
refers to other classes in the greetings
package, the compiler needs to find these other classes. The previous example works because the default user class path is the directory that contains the package directory. If you want to recompile this file without concern for which directory you are in, then add the examples directory to the user class path by setting CLASSPATH
. This example uses the -classpath
option.
Oracle Solaris, Linux, and OS X::
javac -classpath /examples /examples/greetings/Hi.java
Windows:
C:\>javac -classpath \examples \examples\greetings\Hi.java
If you change greetings.Hi
to use a banner utility, then that utility also needs to be accessible through the user class path.
Oracle Solaris, Linux, and OS X::
javac -classpath /examples:/lib/Banners.jar \ /examples/greetings/Hi.java
Windows:
C:\>javac -classpath \examples;\lib\Banners.jar ^ \examples\greetings\Hi.java
To execute a class in the greetings
package, the program needs access to the greetings
package, and to the classes that the greetings
classes use.
Oracle Solaris, Linux, and OS X::
java -classpath /examples:/lib/Banners.jar greetings.Hi
Windows:
C:\>java -classpath \examples;\lib\Banners.jar greetings.Hi
The -source 1.7
option specifies that release 1.7 (or 7) of the Java programming language must be used to compile OldCode.java. The -target 1.7
option ensures that the generated class files are compatible with JVM 1.7.
The javac
command provides direct support for annotation processing, superseding the need for the separate annotation processing command, apt
.
The API for annotation processors is defined in the javax.annotation.processing
and javax.lang.model
packages and subpackages.
How Annotation Processing Works
Unless annotation processing is disabled with the -proc:none
option, the compiler searches for any annotation processors that are available. The search path can be specified with the -processorpath
option. If no path is specified, then the user class path is used. Processors are located by means of service provider-configuration files named META-INF/services/javax.annotation.processing
. Processor on the search path. Such files should contain the names of any annotation processors to be used, listed one per line. Alternatively, processors can be specified explicitly, using the -processor
option.
After scanning the source files and classes on the command line to determine what annotations are present, the compiler queries the processors to determine what annotations they process. When a match is found, the processor is called. A processor can claim the annotations it processes, in which case no further attempt is made to find any processors for those annotations. After all of the annotations are claimed, the compiler does not search for additional processors.
If any processors generate new source files, then another round of annotation processing occurs: Any newly generated source files are scanned, and the annotations processed as before. Any processors called on previous rounds are also called on all subsequent rounds. This continues until no new source files are generated.
After a round occurs where no new source files are generated, the annotation processors are called one last time, to give them a chance to complete any remaining work. Finally, unless the -proc:only
option is used, the compiler compiles the original and all generated source files.
To compile a source file, the compiler often needs information about a type, but the type definition is not in the source files specified on the command line.
To compile a source file, the compiler often needs information about a type, but the type definition is not in the source files specified on the command line. 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 that provide information through inheritance.
For example, when you create a subclass of java.awt.Window
, you are also using the ancestor classes of Window
: java.awt.Container
, java.awt.Component
, and java.lang.Object
.
When the compiler needs type information, it searches for a source file or class file that defines the type. The compiler searches for class files first in the bootstrap and extension classes, then in the user class path (which by default is the current directory). The user class path is defined by setting the CLASSPATH
environment variable or by using the -classpath
option.
If you set the -sourcepath
option, then the compiler searches the indicated path for source files. Otherwise, the compiler searches the user class path for both class files and source files.
You can specify different bootstrap or extension classes with the -bootclasspath
and the -extdirs
options. See Cross-Compilation Options for javac.
A successful type search may produce a class file, a source file, or both. If both are found, then you can use the -Xprefer
option to instruct the compiler which to use. If newer
is specified, then the compiler uses the newer of the two files. If source
is specified, the compiler uses the source file. The default is newer
.
If a type search finds a source file for a required type, either by itself, or as a result of the setting for the -Xprefer
option, then the compiler reads the source file to get the information it needs. By default the compiler also compiles the source file. You can use the -implicit
option to specify the behavior. If none
is specified, then no class files are generated for the source file. If class
is specified, then class files are generated for the source file.
The compiler might not discover the need for some type information until after annotation processing completes. When the type information is found in a source file and no -implicit
option is specified, the compiler gives a warning that the file is being compiled without being subject to annotation processing. To disable the warning, either specify the file on the command line (so that it will be subject to annotation processing) or use the -implicit
option to specify whether or not class files should be generated for such source files.