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Field, Method, and Constructor-that reflect class and
interface members and constructors. These classes provide:
Class that provide for the construction of new
instances of the Field, Method, and Constructor classes.
Array-that provides methods to dynamically construct
and access Java arrays.
Modifier-that helps decode Java language modifier
information about classes and their members.
java.lang package that support
reflection. These additions are:
Byte and Short. These new classes are subclasses of the
class Number, and are similar to the class Integer. Instances of these new
classes serve as object wrappers for primitive values of type byte and
short, respectively.
Class, to represent the primitive Java
types boolean, byte, char, short, int, long, float, and double, and the
keyword void, at run-time.
Void-to hold a reference to the
Class object representing the keyword void.
One category is comprised of applications that need to discover and use all of
the public members of a target object based on its run-time class. These
applications require run-time access to all the public fields, methods, and
constructors of an object. Examples in this category are services such as Java(TM)
Beans[1], and lightweight tools, such as object inspectors. These applications
use the instances of the classes Field, Method, and Constructor obtained
through the methods getField, getMethod, getConstructor, getFields,
getMethods, and getConstructors of class Class.
The second category consists of sophisticated applications that need to
discover and use the members declared by a given class. These applications
need run-time access to the implementation of a class at the level provided by
a class file. Examples in this category are development tools, such as
debuggers, interpreters, inspectors, and class browsers, and run-time services,
such as Java(TM) Object Serialization[2]. These applications use instances of the
classes Field, Method, and Constructor obtained through the methods
getDeclaredField, getDeclaredMethod, getDeclaredConstructor,
getDeclaredFields, getDeclaredMethods, and getDeclaredConstructors of
class Class.
Field, Method, and Constructor are final. Only the Java
Virtual Machine may create instances of these classes; these objects are used to
manipulate the underlying objects; that is, to:
final uninstantiable class Array provides static methods that permit
creating new arrays, and getting and setting the elements of arrays.
Field, Method and Constructor implement the Member interface.
The methods of Member are used to query a reflected member for basic
identifying information. Identifying information consists of the class or
interface that declared the member, the name of the member itself, and the Java
language modifiers (such as public, protected, abstract, synchronized, and
so on) for the member.
Field object represents a reflected field. The underlying field may be a class
variable (a static field) or an instance variable (a non-static field). Methods
of class Field are used to obtain the type of the underlying field, and to get
and set the underlying field's value on objects.
Method object represents a reflected method. The underlying method may be
an abstract method, an instance method, or a class (static) method.
Methods of class Method are used to obtain the formal parameter types, the
return type, and the checked exception types of the underlying method. In
addition, the invoke method of class Method is used to invoke the underlying
method on target objects. Instance and abstract method invocation uses
dynamic method resolution based on the target object's run-time class and the
reflected method's declaring class, name, and formal parameter types. (Thus, it
is permissible to invoke a reflected interface method on an object that is an
instance of a class that implements the interface.) Static method invocation
uses the underlying static method of the method's declaring class.
Constructor object represents a reflected constructor. Methods of class
Constructor are used to obtain the formal parameter types and the checked
exception types of the underlying constructor. In addition, the newInstance
method of class Constructor is used to create and initialize a new instance of
the class that declares the constructor, provided the class is instantiable.
Array class is an uninstantiable class that exports class methods to create
Java arrays with primitive or class component types. Methods of class Array
are also used to get and set array component values.
The Modifier class is an uninstantiable class that exports class methods to
decode Java language modifiers for classes and members. The language
modifiers are encoded in an integer, and use the encoding constants defined by
The Java Virtual Machine Specification.
Class objects that are used to represent the eight
primitive Java types and void at run-time. (Note that these are Class objects,
not classes.) The Core Reflection API uses these objects to identify the
following:
Class objects. They have the same
names as the types that they represent. The Class objects may only be
referenced via the following public final static variables:
java.lang.Boolean.TYPE
java.lang.Character.TYPE
java.lang.Byte.TYPE
java.lang.Short.TYPE
java.lang.Integer.TYPE
java.lang.Long.TYPE
java.lang.Float.TYPE
java.lang.Double.TYPE
java.lang.Void.TYPE
In particular, these Class objects are not accessible via the forName method of
class Class.
Class that give reflective access to a member or a
set of members of a class are the only source for instances of Field, Method,
and Constructor. These methods first delegate security checking to the
system security manager (if installed), which throws a SecurityException
should the request for reflective access be denied.
protected, default (package) access, and private classes and
members-will normally occur when the individual reflected members are
used to operate on the underlying members of objects,that is, to get or set
field values, to invoke methods, or to create and initialize new objects.
Unrestricted access, which overrides standard language access control rules,
may be granted to privileged code (such as debugger code)-a future
version of this specification will define the interface by which this may be
accomplished.
SecurityManager, the checkMemberAccess method
Thevoid checkMemberAccess(Class,int) throws SecurityException
Class parameter identifies the class or interface whose members need to
be accessed. The int parameter identifies the set of members to be
accessed-either Member.PUBLIC or Member.DECLARED.
If the requested access to the specified set of members of the specified class is
denied, the method should throw a SecurityException. If the requested
access to the set is granted, the method should return.
As stated earler, standard Java language access control will be enforced when a reflected member from this set is used to operate on an underlying object, that is, when:
Field is used to get or set a field value
Method is used to invoke a method
Constructor is used to create and initialize a new instance of a class
IllegalAccessException.
public
members and constructors) of any class it may link against. Application code
that gains reflective access to a member or constructor may only use the
reflected member or constructor with standard Java language access control.
AppletSecurity implements the following policy:
public members of all public classes loaded by the same class loader
as the untrusted code
public members of public system classes
public) members of all classes loaded by the
same class loader as the untrusted code
CLASSPATH, is additionally
granted access to all classes loaded by all class loaders.
This policy is conservative with respect to untrusted code-it is more
restrictive than the linker for the Java Virtual Machine. For example, an
untrusted class cannot, by itself, access a protected member of a system
superclass via reflection, although it can via the linker. (However, system code
may access such members and pass them to untrusted code.)
The JDK security policy is expected to evolve with the security framework for Java.
There are two types of automatic data conversions. Wrapping conversions convert from values of primitive types to objects of class types. Unwrapping conversions convert objects of class types to values of primitive types. The rules for these conversions are defined in "Wrapping and Unwrapping Conversions."
Additionally, field access and method invocation permit widening conversions on primitive and reference types. These conversions are documented in The Java Language Specification, section 5, and are detailed in "Widening Conversions."
Field.get or Array.get, or when it is returned by a method invoked via
Method.invoke.
Similarly, an object value is automatically unwrapped when supplied as a parameter in a context that requires a value of a primitive type. These contexts are:
Field.set, where the underlying field has a primitive type
Array.set, where the underlying array has a primitive element type
Method.invoke or Constructor.newInstance, where the corresponding
formal parameter of the underlying method or constructor has a primitive
type
boolean
|
java.lang.Boolean
|
char
|
java.lang.Character
|
byte
|
java.lang.Byte
|
short
|
java.lang.Short
|
int
|
java.lang.Integer
|
long
|
java.lang.Long
|
float
|
java.lang.Float
|
double
|
java.lang.Double
|
A method that is declared void returns the special reference null when it is
invoked via Method.invoke.
Widening conversions are performed at run-time:
Field
and Array
Field and
Array
Method.invoke or Constructor.newInstance
byte to short, int, long, float, or double
short to int, long, float, or double
char to int, long, float, or double
int to long, float, or double
long to float or double
float to double.
java.lang named
java.lang.reflect. This avoids compatibility problems caused by Java's
default package importation rules.