Java中Class类详解、用法以及泛化

在前面我们将类的加载的时候，类加载一共有5步，加载，验证，准备，解析和初始化。其中加载阶段，除了将字节码加载到方法区，还生成了这个了的Java.lang.Class对象。那么这个Class对象到底有什么用呢？

前面的关于反射的文章，我们多次都用到了Class类，可以用这个类创建对应类的实例对象，这可以看做是Class类的一个作用。但是这个Class类和对应类本身又有什么联系和区别呢？比如new A（）和A.class.newInstance()，联系和区别？

1.什么是Class类&Class类的作用

Class类说变了也是一个类，只不过是一个比较特殊的类。特殊在哪里呢？特殊在这是一个在类加载过程中由虚拟机生成的，由于表示被加载类的类型信息的对象。简单地说，我们创建一个int变量，那么这个int变量是个整数类型，那么我们怎么知道这个类型是整数类型呢？就是通过这个Class类来直到的。

java是面向对象编程的，java中几乎所有的数据都是对象，那么是对象，就必须知道自己到底是哪一种类型的对象。于是Class类便顺势而生了。

Class类的作用放在这边讲有点不太合适，要想了解Class类的作用建议读完下面的内容。我自己理解的话，Class类的作用，本质上讲，就是前面所说的，它代表着一个类的类型信息。正是因为这个特殊作用的存在，Class类能够实现它所代表的这个类的所有功能，包括创建这个类的实例，获得所有的构造函数，方法，字段值等等，可以说无所不能。

2.Class类里面存储了什么

既然我们知道Class类是表示类型的一个特殊的类，那么它是怎么表示某个类或者对象的类型呢？或者说，它里面存储了什么呢？通过反射机制我们知道，Class里面存储了对应类的几乎所有的信息，当然这些信息是未初始化的信息，比如所有的方法，所有的构造函数，所有的字段（成员属性）等等。

那这些信息是怎么存储进去的呢？好吧，这个应该去深入的参考java虚拟机规范吧，毕竟Class这个类的构造函数是私有的，可能是底层的C++代码实现的。

3.class类的静态方法forName()

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.

上述两个方法摘自javadoc，详细的内容可以去javadoc看。最常用的是第一个，注意这个className一定是全限定名，而不能是简单名称。也就是包名.类名

4.class类的所有方法（包括静态方法）

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 extendsAnnotation> 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 extendsAnnotation> 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 extendsAnnotation> 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 extendsAnnotation> 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 Classobject.
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.

顺便说一下这里的newInstace（）方法，因为这个方法比较常用，但是这个会调用对应类的默认无参数构造函数，如果要使用调用有参数的构造器创建一个对象，那么，就必须获得这个构造器，然后再使用构造器调用了。具体的方式向下面这样

//假设A的含参构造器含有4个参数，分别是int，int，string，和类B

Constructor cons = A.class.getConstructor(int.class, int.class, String.class, B.class);

B b = new B();

//获得A的实例。

A a  = cons.newInstance(5,5,"Jack",b);