Pre-General Availability: 2017-05-24

javac

You can use the javac tools 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.

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 occurred and any implicitly loaded source files are compiled. The -implicit option provides a way to suppress the warning.

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 filenames from 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 enables 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

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:

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 -classpath, -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:

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, this option specifies the directories that contain the extension classes. See Cross-Compilation Options for javac.

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.

-g

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

lines

Line number debugging information.

vars

Local variable debugging information.

source

Source file debugging information.

-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

Print help on the 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 java 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 timestamps.

--module—path module-path or -p module-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
Specifies the version of modules that are being compiled.
-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 XXX API for a specific VM version. Supported release targets are 6, 7, 8, and 9.

XXX is one of the following choices:

  • public

  • supported

  • documented

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 API in the XXX 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, javac doesn’t support -source release settings less than or equal to 5. If settings less than or equal to 5 are used, javac behaves as if -source 6 was 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 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

This is the default value. The compiler accepts code with features introduced in Java SE 9.

--source-path sourcepath or -sourcepath sourcepath

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 release prior to JDK 9 classes were compiled against the bootstrap classes of the platform that shipped with javac . But javac also supports cross-compiling, in which classes are compiled against bootstrap classes of a different Java platform implementation. It is 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:

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:

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:

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:

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 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 will not check for HTML errors for classes and members that have access level package and higher (which includes package, protected and public):

-Xdoclint:all,-html/package
Xdoclint/package:[-]packages(,[-]package)*

Enable or disable 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)*

Warnings to enable or disable, separated by comma. Precede a key by a - to disable the specified warning.

Supported values for key are:

  • all — Enables all warnings.

  • auxiliaryclass — Warn about an auxiliary class that is hidden in a source file, and is used from other files.

  • cast — Warns about use of unnecessary casts.

  • classfile — Warns about issues related to classfile contents.

  • deprecation — Warns about use of deprecated items.

  • dep-ann — Warns about items marked as deprecated in javadoc but not using the @Deprecated annotation.

  • divzero — Warns about division by constant integer 0.

  • empty — Warns about empty statement after if.

  • exports — Warns about issues regarding module exports.

  • fallthrough — Warns about 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 module system related issues.

  • opens — Warns about issues regarding module opens.

  • options — Warns about issues relating to use of command line options.

  • overloads — Warns about issues regarding method overloads.

  • overrides — Warns about issues regarding method overrides.

  • path — Warns about invalid path elements on the command l ine.

  • processing — Warns about issues regarding annotation processing.

  • rawtypes — Warns about use of raw types.

  • removal — Warns about use of API that has been marked for removal.

  • serial — Warns about Serializable classes that don’t provide a serial version ID. Also warn about access to non-public members from a serializable element.

  • static — Warns about accessing a static member using an instance.

  • try — Warns about issues relating to use of try blocks ( i.e. try-with-resources).

  • unchecked — Warns about unchecked operations.

  • varargs — Warns about potentially unsafe vararg methods.

  • none — Disables all warnings

See Examples of Using -Xlint keys.

-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 javac generates package-info.class files from package-info.java files.

always

Always generate 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. Doesn't generate a package-info.class file if package-info.java only contains 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

Name and optional arguments for a plugin 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. See Searching for Types.

  • -Xprefer:newer — Reads the newer of the source or class file 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. 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 the location of the argument file. Wild cards (*) 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 are 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

Single Argument File

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

Two Argument Files

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
Argument Files with Paths

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 an @deprecated Javadoc comment, but don’t have a @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 complete 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 is 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 regarding 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, which 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

Warn about issues regarding 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 is strongly recommended that all serializable classes explicitly declare serialVersionUID values because the default process of computing serialVersionUID vales is highly sensitive to class details that can vary depending on compiler implementations, and 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, 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 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 usages 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 is 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 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 to be used to compile OldCode.java. The -target 1.7 option ensures that the generated class files are compatible with JVM 1.7.

Annotation Processing

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.

Searching for Types

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.