Module java.base
Package java.lang

Class Package

java.lang.Object
java.lang.Package
All Implemented Interfaces:
AnnotatedElement

public class Package
extends Object
implements AnnotatedElement
Represents metadata about a run-time package associated with a class loader. Metadata includes annotations, versioning, and sealing.

Annotations for the run-time package are read from package-info.class at the same code source as classes in the run-time package.

The set of classes that make up the run-time package may implement a particular specification. The specification title, version, and vendor (indicating the owner/maintainer of the specification) can be provided when the Package is defined. An application can ask if the Package is compatible with a particular specification version by using the Package.isCompatibleWith(String) method. In addition, information about the actual classes that make up the run-time package can be provided when the Package is defined. This information consists of an implementation title, version, and vendor (indicating the supplier of the classes).

A Package may be explicitly defined with the ClassLoader.definePackage(String, String, String, String, String, String, String, URL) method. The caller supplies the specification and implementation titles, versions, and vendors. The caller also indicates whether the package is sealed. If a Package is not explicitly defined for a run-time package when a class in that run-time package is defined, then a Package is automatically defined by the class's defining class loader, as follows.

A Package automatically defined for classes in a named module has the following properties:

  • The name of the package is derived from the binary names of the classes. Since classes in a named module must be in a named package, the derived name is never empty.
  • The package is sealed with the module location as the code source, if known.
  • The specification and implementation titles, versions, and vendors are unspecified.
  • Any annotations on the package are read from package-info.class as specified above.

A Package automatically defined for classes in an unnamed module has the following properties:

  • The name of the package is either "" (for classes in an unnamed package) or derived from the binary names of the classes (for classes in a named package).
  • The package is not sealed.
  • The specification and implementation titles, versions, and vendors are unspecified.
  • Any annotations on the package are read from package-info.class as specified above.

A Package can be obtained with the Package.getPackage(String) and ClassLoader.getDefinedPackage(String) methods. Every Package defined by a class loader can be obtained with the Package.getPackages() and ClassLoader.getDefinedPackages() methods.

Implementation Note:
The builtin class loaders do not explicitly define Package objects for packages in named modules. Instead those packages are automatically defined and have no specification and implementation versioning information.
See Java Virtual Machine Specification:
5.3 Creation and Loading
Since:
1.2
See Also:
The JAR File Specification: Package Sealing, ClassLoader.definePackage(String, String, String, String, String, String, String, URL)
  • Method Details

    • getName

      public String getName()
      Return the name of this package.
      Returns:
      The fully-qualified name of this package as defined in section 6.5.3 of The Java Language Specification, for example, java.lang
    • getSpecificationTitle

      public String getSpecificationTitle()
      Return the title of the specification that this package implements.
      Returns:
      the specification title, null is returned if it is not known.
    • getSpecificationVersion

      public String getSpecificationVersion()
      Returns the version number of the specification that this package implements. This version string must be a sequence of non-negative decimal integers separated by "."'s and may have leading zeros. When version strings are compared the most significant numbers are compared.

      Specification version numbers use a syntax that consists of non-negative decimal integers separated by periods ".", for example "2.0" or "1.2.3.4.5.6.7". This allows an extensible number to be used to represent major, minor, micro, etc. versions. The version specification is described by the following formal grammar:

      SpecificationVersion:
      Digits RefinedVersionopt
      RefinedVersion:
      . Digits
      . Digits RefinedVersion
      Digits:
      Digit
      Digits
      Digit:
      any character for which Character.isDigit(char) returns true, e.g. 0, 1, 2, ...

      Returns:
      the specification version, null is returned if it is not known.
    • getSpecificationVendor

      public String getSpecificationVendor()
      Return the name of the organization, vendor, or company that owns and maintains the specification of the classes that implement this package.
      Returns:
      the specification vendor, null is returned if it is not known.
    • getImplementationTitle

      public String getImplementationTitle()
      Return the title of this package.
      Returns:
      the title of the implementation, null is returned if it is not known.
    • getImplementationVersion

      public String getImplementationVersion()
      Return the version of this implementation. It consists of any string assigned by the vendor of this implementation and does not have any particular syntax specified or expected by the Java runtime. It may be compared for equality with other package version strings used for this implementation by this vendor for this package.
      Returns:
      the version of the implementation, null is returned if it is not known.
    • getImplementationVendor

      public String getImplementationVendor()
      Returns the vendor that implemented this package, null is returned if it is not known.
      Returns:
      the vendor that implemented this package, null is returned if it is not known.
    • isSealed

      public boolean isSealed()
      Returns true if this package is sealed.
      Returns:
      true if the package is sealed, false otherwise
    • isSealed

      public boolean isSealed​(URL url)
      Returns true if this package is sealed with respect to the specified code source url.
      Parameters:
      url - the code source URL
      Returns:
      true if this package is sealed with respect to the given url
    • isCompatibleWith

      public boolean isCompatibleWith​(String desired) throws NumberFormatException
      Compare this package's specification version with a desired version. It returns true if this packages specification version number is greater than or equal to the desired version number.

      Version numbers are compared by sequentially comparing corresponding components of the desired and specification strings. Each component is converted as a decimal integer and the values compared. If the specification value is greater than the desired value true is returned. If the value is less false is returned. If the values are equal the period is skipped and the next pair of components is compared.

      Parameters:
      desired - the version string of the desired version.
      Returns:
      true if this package's version number is greater than or equal to the desired version number
      Throws:
      NumberFormatException - if the current version is not known or the desired or current version is not of the correct dotted form.
    • getPackage

      @Deprecated(since="9") public static Package getPackage​(String name)
      Deprecated.
      If multiple class loaders delegate to each other and define classes with the same package name, and one such loader relies on the lookup behavior of getPackage to return a Package from a parent loader, then the properties exposed by the Package may not be as expected in the rest of the program. For example, the Package will only expose annotations from the package-info.class file defined by the parent loader, even if annotations exist in a package-info.class file defined by a child loader. A more robust approach is to use the ClassLoader.getDefinedPackage(java.lang.String) method which returns a Package for the specified class loader.
      Finds a package by name in the caller's class loader and its ancestors.

      If the caller's class loader defines a Package of the given name, the Package is returned. Otherwise, the ancestors of the caller's class loader are searched recursively (parent by parent) for a Package of the given name.

      Calling this method is equivalent to calling ClassLoader.getPackage(java.lang.String) on a ClassLoader instance which is the caller's class loader.

      Parameters:
      name - A package name, such as "java.lang".
      Returns:
      The Package of the given name defined by the caller's class loader or its ancestors, or null if not found.
      Throws:
      NullPointerException - if name is null.
      See Also:
      ClassLoader.getDefinedPackage(java.lang.String)
    • getPackages

      public static Package[] getPackages()
      Returns all of the Packages defined by the caller's class loader and its ancestors. The returned array may contain more than one Package object of the same package name, each defined by a different class loader in the class loader hierarchy.

      Calling this method is equivalent to calling ClassLoader.getPackages() on a ClassLoader instance which is the caller's class loader.

      Returns:
      The array of Package objects defined by this class loader and its ancestors
      See Also:
      ClassLoader.getDefinedPackages()
    • hashCode

      public int hashCode()
      Return the hash code computed from the package name.
      Overrides:
      hashCode in class Object
      Returns:
      the hash code computed from the package name.
      See Also:
      Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)
    • toString

      public String toString()
      Returns the string representation of this Package. Its value is the string "package " and the package name. If the package title is defined it is appended. If the package version is defined it is appended.
      Overrides:
      toString in class Object
      Returns:
      the string representation of the package.
    • getAnnotation

      public <A extends Annotation> A getAnnotation​(Class<A> annotationClass)
      Returns this element's annotation for the specified type if such an annotation is present, else null.

      Note that any annotation returned by this method is a declaration annotation.

      Specified by:
      getAnnotation in interface AnnotatedElement
      Type Parameters:
      A - the type of the annotation to query for and return if present
      Parameters:
      annotationClass - the Class object corresponding to the annotation type
      Returns:
      this element's annotation for the specified annotation type if present on this element, else null
      Throws:
      NullPointerException - if the given annotation class is null
      Since:
      1.5
    • isAnnotationPresent

      public boolean isAnnotationPresent​(Class<? extends Annotation> annotationClass)
      Returns true if an annotation for the specified type is present on this element, else false. This method is designed primarily for convenient access to marker annotations.

      The truth value returned by this method is equivalent to: getAnnotation(annotationClass) != null

      Specified by:
      isAnnotationPresent in interface AnnotatedElement
      Parameters:
      annotationClass - the Class object corresponding to the annotation type
      Returns:
      true if an annotation for the specified annotation type is present on this element, else false
      Throws:
      NullPointerException - if the given annotation class is null
      Since:
      1.5
    • getAnnotationsByType

      public <A extends Annotation> A[] getAnnotationsByType​(Class<A> annotationClass)
      Returns annotations that are associated with this element. If there are no annotations associated with this element, the return value is an array of length 0. The difference between this method and AnnotatedElement.getAnnotation(Class) is that this method detects if its argument is a repeatable annotation type (JLS 9.6), and if so, attempts to find one or more annotations of that type by "looking through" a container annotation. The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.

      Note that any annotations returned by this method are declaration annotations.

      Specified by:
      getAnnotationsByType in interface AnnotatedElement
      Type Parameters:
      A - the type of the annotation to query for and return if present
      Parameters:
      annotationClass - the Class object corresponding to the annotation type
      Returns:
      all this element's annotations for the specified annotation type if associated with this element, else an array of length zero
      Throws:
      NullPointerException - if the given annotation class is null
      Since:
      1.8
    • getAnnotations

      public Annotation[] getAnnotations()
      Returns annotations that are present on this element. If there are no annotations present on this element, the return value is an array of length 0. The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.

      Note that any annotations returned by this method are declaration annotations.

      Specified by:
      getAnnotations in interface AnnotatedElement
      Returns:
      annotations present on this element
      Since:
      1.5
    • getDeclaredAnnotation

      public <A extends Annotation> A getDeclaredAnnotation​(Class<A> annotationClass)
      Returns this element's annotation for the specified type if such an annotation is directly present, else null. This method ignores inherited annotations. (Returns null if no annotations are directly present on this element.)

      Note that any annotation returned by this method is a declaration annotation.

      Specified by:
      getDeclaredAnnotation in interface AnnotatedElement
      Type Parameters:
      A - the type of the annotation to query for and return if directly present
      Parameters:
      annotationClass - the Class object corresponding to the annotation type
      Returns:
      this element's annotation for the specified annotation type if directly present on this element, else null
      Throws:
      NullPointerException - if the given annotation class is null
      Since:
      1.8
    • getDeclaredAnnotationsByType

      public <A extends Annotation> A[] getDeclaredAnnotationsByType​(Class<A> annotationClass)
      Description copied from interface: AnnotatedElement
      Returns this element's annotation(s) for the specified type if such annotations are either directly present or indirectly present. This method ignores inherited annotations. If there are no specified annotations directly or indirectly present on this element, the return value is an array of length 0. The difference between this method and AnnotatedElement.getDeclaredAnnotation(Class) is that this method detects if its argument is a repeatable annotation type (JLS 9.6), and if so, attempts to find one or more annotations of that type by "looking through" a container annotation if one is present. The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.
      Specified by:
      getDeclaredAnnotationsByType in interface AnnotatedElement
      Type Parameters:
      A - the type of the annotation to query for and return if directly or indirectly present
      Parameters:
      annotationClass - the Class object corresponding to the annotation type
      Returns:
      all this element's annotations for the specified annotation type if directly or indirectly present on this element, else an array of length zero
      Throws:
      NullPointerException - if the given annotation class is null
      Since:
      1.8
    • getDeclaredAnnotations

      public Annotation[] getDeclaredAnnotations()
      Returns annotations that are directly present on this element. This method ignores inherited annotations. If there are no annotations directly present on this element, the return value is an array of length 0. The caller of this method is free to modify the returned array; it will have no effect on the arrays returned to other callers.

      Note that any annotations returned by this method are declaration annotations.

      Specified by:
      getDeclaredAnnotations in interface AnnotatedElement
      Returns:
      annotations directly present on this element
      Since:
      1.5