NAME¶

javac - Reads Java class and interface definitions and compiles them into bytecode and class files.

SYNOPSIS¶

javac [ options ] [ sourcefiles ] [ classes] [ @argfiles ]

Arguments can be in any order:

options

Command-line options. See Options.

sourcefiles

One or more source files to be compiled (such as MyClass.java).

classes

One or more classes to be processed for annotations (such as MyPackage.MyClass).

@argfiles

One or more files that list options and source files. The -J options are not allowed in these files. See Command-Line Argument Files.

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, list the file names on the command line.

•

For a large number of source files, list the file names in a file that is separated by blanks or line breaks. Use the list file name preceded by an at sign (@) with the javac command.

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.

Arrange source files in a directory tree that reflects their package tree. For example, 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.

OPTIONS¶

The compiler has a set of standard options that are supported on the current development environment. An additional set of nonstandard options are specific to the current virtual machine and compiler implementations and are subject to change in the future. Nonstandard options begin with the -X option.

•

See also Cross-Compilation Options

•

See also Nonstandard Options

ENABLE OR DISABLE WARNINGS WITH THE -XLINT OPTION¶

Enable warning name with the -Xlint:name option, where name is one of the following warning names. Note that you can disable a warning with the -Xlint:-name: option.

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 do not have a @Deprecated annotation, for example:

/**

  * @deprecated As of Java SE 7, replaced by {@link #newMethod()}

  */

public static void deprecatedMethood() { }

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 ifstatements, 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 does not include a break statement, allowing code execution to fall through from that case to the next case. For example, the code following the case 1 label in this switch block does not 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 cannot 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.

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 cannot be suppressed with the @SuppressWarnings annotation, for example:

javac -Xlint:path -classpath /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 cannot 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 does not 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 does not 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 is not 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 cannot determine whether l refers to a List<String> type. In this case, l does not 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 do not 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);

    }

  }

}

Note: A non-reifiable type is a type whose type information is not 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 does not 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.

COMMAND-LINE ARGUMENT FILES¶

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.

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 (*) are not allowed in these lists (such as for specifying *.java). Use of the at sign (@) to recursively interpret files is not supported. The -J options are not supported because they are passed to the launcher, which does not 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.

Example 1 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

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:

-d classes

-g

-sourcepath /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

Example 3 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

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.

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. 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 java.applet.Applet, you are also using the ancestor classes of Applet: java.awt.Panel, 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.

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.

PROGRAMMATIC INTERFACE¶

The javac command supports the new Java Compiler API defined by the classes and interfaces in the javax.tools package.

JavaCompiler javac = ToolProvider.getSystemJavaCompiler();

public static int compile(String[] args);

public static int compile(String[] args, PrintWriter out);

EXAMPLES¶

Example 1 Compile a Simple Program

This example shows how to compile the Hello.java source file in the greetings directory. The class defined in Hello.java is called greetings.Hello. The greetings directory is the package directory both for the source file and the class file and is underneath the current directory. This makes it possible to use the default user class path. It also makes it unnecessary to specify a separate destination directory with the -d option.

The source code in 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]);

        }

    }

}

Compile greetings.Hello:

javac greetings/Hello.java

Run greetings.Hello:

java greetings.Hello World Universe Everyone

Hello World

Hello Universe

Hello Everyone

Example 2 Compile Multiple Source Files

This example compiles the Aloha.java, GutenTag.java, Hello.java, and Hi.java source files in the greetings package.

% javac greetings/*.java

% ls greetings

Aloha.class         GutenTag.class      Hello.class         Hi.class

Aloha.java          GutenTag.java       Hello.java          Hi.java

Example 3 Specify a User Class Path

After changing one of the source files in the previous example, recompile it:

pwd

/examples

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.

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.

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.

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

Example 4 Separate Source Files and Class Files

The following example uses javac to compile code that runs on JVM 1.7.

javac -source 1.7 -target 1.7 -bootclasspath jdk1.7.0/lib/rt.jar \ 

-extdirs "" OldCode.java

The -source 1.7 option specifies that release 1.7 (or 7) of the Java programming language be used to compile OldCode.java. The option -target 1.7 option ensures that the generated class files are compatible with JVM 1.7. Note that in most cases, the value of the -target option is the value of the -source option; in this example, you can omit the -target option.

You must specify the -bootclasspath option to specify the correct version of the bootstrap classes (the rt.jar library). If not, then the compiler generates a warning:

javac -source 1.7 OldCode.java

warning: [options] bootstrap class path not set in conjunction with 

-source 1.7

If you do not specify the correct version of bootstrap classes, then the compiler uses the old language rules (in this example, it uses version 1.7 of the Java programming language) combined with the new bootstrap classes, which can result in class files that do not work on the older platform (in this case, Java SE 7) because reference to nonexistent methods can get included.

Example 5 Cross Compile

This example uses javac to compile code that runs on JVM 1.7.

javac -source 1.7 -target 1.7 -bootclasspath jdk1.7.0/lib/rt.jar \

            -extdirs "" OldCode.java

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.

You must specify the -bootclasspath option to specify the correct version of the bootstrap classes (the rt.jar library). If not, then the compiler generates a warning:

javac -source 1.7 OldCode.java

warning: [options] bootstrap class path not set in conjunction with -source 1.7

If you do not specify the correct version of bootstrap classes, then the compiler uses the old language rules combined with the new bootstrap classes. This combination can result in class files that do not work on the older platform (in this case, Java SE 7) because reference to nonexistent methods can get included. In this example, the compiler uses release 1.7 of the Java programming language.

SEE ALSO¶

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java(1)

•

jdb(1)

•

javah(1)

•

javadoc(1)

•

jar(1)

•

jdb(1)