反射【Java】

简介

Java的Class关系示意图

反射是Java语言的一种机制，可以在运行时获取操作对象的具体类型，而且通过类型可以获取类的构造，包括成员变量以及成员方法，并且可以动态的调用这些方法或者改变这些变量的值。

Java一般认为是一门静态语言。静态语言是指编译时就可以确定变量的数据类型的语言，大多数静态语言要求在使用变量之前必须声明数据的类型。常见的静态语言有C、C++、Golang等，Java通常也被认为是静态语言。

PHP、JavaScript、Python、Perl等语言的变量在使用前不需要声明数据类型，在运行时根据被赋值的值类型才确定数据的类型，此类语言称为动态语言。

而通过反射的能力，Java语言可以动态的改变对象的值，甚至可以动态改变数据类型，所以反射使得Java语言具备了动态语言的能力。

Java主要通过Class类来实现反射的能力，JVM在完成一个类的加载过程之后，在堆内存会构造一个Class类型的对象，该对象包含了类完整的构造信息，通过该类可以窥视到Java类的结构，通过Class类可以实例化类的对象，获取类的方法，修改方法的值。

原理

反射是一种“语义同像”，语义同像将程序的一些内部状态，例如对象的属性，对象的方法指针等暴露给程序的运行态，通过这种能力，程序可以在运行时动态修改对象的值。

反射需要存储或者可以关联对象的元信息，因为静态语言在编译时已经可以确定对象的类型，所以这些信息不是运行时的必须信息，为了执行效率，这些信息往往在编译后的信息中剥离，如果要支持反射，就需要存储这些相关的信息。

反射违反了面向对象编程中的封装性。

Java中反射的核心来自Class类。

Class类是描述对象类型信息的类，除了Class类自身之外，所有类都有一个Class类型的对象。

例如A.class经过ClassLoader加载到JVM之后，在方法区会产生一个A.class的对象，这个对象是Class类型的，在范型化也可以表示为Class<A>。

反射API

Class对象是反射实现的关键，通过Class对象可以获取对象的属性以及方法。

获取Class

Java语言中获取 Class 类对象的方法一般如下三种。

• 使用Class.forName可以加载ClassPath路径下的类，在加载jdbc驱动程序的时候经常采用该方式。

Class clazz = Class.forName("oracle.jdbc.driver.OracleDriver")

• 通过.class可以获得已经加载到JVM的类

Class clazz = String.class;

使用对象的getClass()方法可以获取该对象的Class，该方法继承自Object.class。

Class clazz = obj.getClass();

获取属性/方法/构造器

Modifier and Type	Method and Description
<U> Class<? extends U>	asSubclass(Class<U> clazz) Casts this Class object to represent a subclass of the class represented by the specified class object.
T	cast(Object obj) Casts an object to the class or interface represented by this Class object.
boolean	desiredAssertionStatus() Returns the assertion status that would be assigned to this class if it were to be initialized at the time this method is invoked.
static Class<?>	forName(String className) Returns the Class object associated with the class or interface with the given string name.
static Class<?>	forName(String name, boolean initialize, ClassLoader loader) Returns the Class object associated with the class or interface with the given string name, using the given class loader.
AnnotatedType[]	getAnnotatedInterfaces() Returns an array of AnnotatedType objects that represent the use of types to specify superinterfaces of the entity represented by this Class object.
AnnotatedType	getAnnotatedSuperclass() Returns an AnnotatedType object that represents the use of a type to specify the superclass of the entity represented by this Class object.
<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.
Annotation[]	getAnnotations() Returns annotations that are present on this element.
<A extends Annotation>A[]	getAnnotationsByType(Class<A> annotationClass) Returns annotations that are associated with this element.
String	getCanonicalName() Returns the canonical name of the underlying class as defined by the Java Language Specification.
Class<?>[]	getClasses() Returns an array containing Class objects representing all the public classes and interfaces that are members of the class represented by this Class object.
ClassLoader	getClassLoader() Returns the class loader for the class.
Class<?>	getComponentType() Returns the Class representing the component type of an array.
Constructor<T>	getConstructor(Class<?>... parameterTypes) Returns a Constructor object that reflects the specified public constructor of the class represented by this Class object.
Constructor<?>[]	getConstructors() Returns an array containing Constructor objects reflecting all the public constructors of the class represented by this Class object.
<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.
Annotation[]	getDeclaredAnnotations() Returns annotations that are directly present on this element.
<A extends Annotation>A[]	getDeclaredAnnotationsByType(Class<A> annotationClass) Returns this element's annotation(s) for the specified type if such annotations are either directly present or indirectly present.
Class<?>[]	getDeclaredClasses() Returns an array of Class objects reflecting all the classes and interfaces declared as members of the class represented by this Class object.
Constructor<T>	getDeclaredConstructor(Class<?>... parameterTypes) Returns a Constructor object that reflects the specified constructor of the class or interface represented by this Class object.
Constructor<?>[]	getDeclaredConstructors() Returns an array of Constructor objects reflecting all the constructors declared by the class represented by this Class object.
Field	getDeclaredField(String name) Returns a Field object that reflects the specified declared field of the class or interface represented by this Class object.
Field[]	getDeclaredFields() Returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object.
Method	getDeclaredMethod(String name, Class<?>... parameterTypes) Returns a Method object that reflects the specified declared method of the class or interface represented by this Class object.
Method[]	getDeclaredMethods() Returns an array containing Method objects reflecting all the declared methods of the class or interface represented by this Class object, including public, protected, default (package) access, and private methods, but excluding inherited methods.
Class<?>	getDeclaringClass() If the class or interface represented by this Class object is a member of another class, returns the Class object representing the class in which it was declared.
Class<?>	getEnclosingClass() Returns the immediately enclosing class of the underlying class.
Constructor<?>	getEnclosingConstructor() If this Class object represents a local or anonymous class within a constructor, returns a Constructor object representing the immediately enclosing constructor of the underlying class.
Method	getEnclosingMethod() If this Class object represents a local or anonymous class within a method, returns a Method object representing the immediately enclosing method of the underlying class.
T[]	getEnumConstants() Returns the elements of this enum class or null if this Class object does not represent an enum type.
Field	getField(String name) Returns a Field object that reflects the specified public member field of the class or interface represented by this Class object.
Field[]	getFields() Returns an array containing Field objects reflecting all the accessible public fields of the class or interface represented by this Class object.
Type[]	getGenericInterfaces() Returns the Types representing the interfaces directly implemented by the class or interface represented by this object.
Type	getGenericSuperclass() Returns the Type representing the direct superclass of the entity (class, interface, primitive type or void) represented by this Class.
Class<?>[]	getInterfaces() Determines the interfaces implemented by the class or interface represented by this object.
Method	getMethod(String name, Class<?>... parameterTypes) Returns a Method object that reflects the specified public member method of the class or interface represented by this Class object.
Method[]	getMethods() Returns an array containing Method objects reflecting all the public methods of the class or interface represented by this Class object, including those declared by the class or interface and those inherited from superclasses and superinterfaces.
int	getModifiers() Returns the Java language modifiers for this class or interface, encoded in an integer.
String	getName() Returns the name of the entity (class, interface, array class, primitive type, or void) represented by this Class object, as a String.
Package	getPackage() Gets the package for this class.
ProtectionDomain	getProtectionDomain() Returns the ProtectionDomain of this class.
URL	getResource(String name) Finds a resource with a given name.
InputStream	getResourceAsStream(String name) Finds a resource with a given name.
Object[]	getSigners() Gets the signers of this class.
String	getSimpleName() Returns the simple name of the underlying class as given in the source code.
Class<? super T>	getSuperclass() Returns the Class representing the superclass of the entity (class, interface, primitive type or void) represented by this Class.
String	getTypeName() Return an informative string for the name of this type.
TypeVariable<Class<T>>[]	getTypeParameters() Returns an array of TypeVariable objects that represent the type variables declared by the generic declaration represented by this GenericDeclaration object, in declaration order.
boolean	isAnnotation() Returns true if this Class object represents an annotation type.
boolean	isAnnotationPresent(Class<? extends Annotation> annotationClass) Returns true if an annotation for the specified type is present on this element, else false.
boolean	isAnonymousClass() Returns true if and only if the underlying class is an anonymous class.
boolean	isArray() Determines if this Class object represents an array class.
boolean	isAssignableFrom(Class<?> cls) Determines if the class or interface represented by this Class object is either the same as, or is a superclass or superinterface of, the class or interface represented by the specified Class parameter.
boolean	isEnum() Returns true if and only if this class was declared as an enum in the source code.
boolean	isInstance(Object obj) Determines if the specified Object is assignment-compatible with the object represented by this Class.
boolean	isInterface() Determines if the specified Class object represents an interface type.
boolean	isLocalClass() Returns true if and only if the underlying class is a local class.
boolean	isMemberClass() Returns true if and only if the underlying class is a member class.
boolean	isPrimitive() Determines if the specified Class object represents a primitive type.
boolean	isSynthetic() Returns true if this class is a synthetic class; returns false otherwise.
T	newInstance() Creates a new instance of the class represented by this Class object.
String	toGenericString() Returns a string describing this Class, including information about modifiers and type parameters.
String	toString() Converts the object to a string.