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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 methodThevoid 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 valueMethod is used to invoke a methodConstructor is used to create and initialize a new instance of a classIllegalAccessException.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 codepublic members of public system classespublic) members of all classes loaded by the same class loader as the untrusted codeCLASSPATH, 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 typeArray.set, where the underlying array has a primitive element typeMethod.invoke or Constructor.newInstance, where the corresponding formal parameter of the underlying method or constructor has a primitive type
boolean
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java.lang.Boolean
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char
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java.lang.Character
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byte
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java.lang.Byte
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short
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java.lang.Short
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int
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java.lang.Integer
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long
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java.lang.Long
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float
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java.lang.Float
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double
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java.lang.Double
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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 ArrayField and ArrayMethod.invoke or Constructor.newInstance
byte to short, int, long, float, or doubleshort to int, long, float, or doublechar to int, long, float, or doubleint to long, float, or doublelong to float or doublefloat to double.
java.lang named java.lang.reflect. This avoids compatibility problems caused by Java's default package importation rules.