javac

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.

• See also Cross-Compilation Options

• See also Nonstandard Options

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.

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

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.

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.

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