概念
- 反射是Java被视为动态语言的关键，反射机制允许程序在执行期借助于Reflection API取得任何类的内部信息，并能直接操作任意对象的内部属性及方法。
- 加载完类之后，在堆内存的方法区中就产生了一个Class类型的对象（一个类只有一个Class对象），这个对象就包含了完整的类的结构信息。我们通过这个对象看到类的结构。
代码演示：
package ReflectionStudy;
//什么是反射
public class demo01 {
public static void main(String[] args) throws ClassNotFoundException {
//通过反射获取类的Class对象
Class c1 = Class.forName("ReflectionStudy.user");
System.out.println(c1);//class ReflectionStudy.user
//一个类中在内存中只有一个Class对象
//一个类被加载后，类的整个结构都会被封装在Class对象中
Class c2 = Class.forName("ReflectionStudy.user");
Class c3 = Class.forName("ReflectionStudy.user");
Class c4 = Class.forName("ReflectionStudy.user");
Class c5 = Class.forName("ReflectionStudy.user");
System.out.println(c2.hashCode());//460141958
System.out.println(c3.hashCode());//460141958
System.out.println(c4.hashCode());//460141958
System.out.println(c5.hashCode());//460141958
}
}
//实体类
class user{
private String name;
private int age;
private int id;
public user() {
}
public user(String name, int age, int id) {
this.name = name;
this.age = age;
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
}
- 正常方式与反射方式的对比：
Class类
- 获得class对象的方法：
package ReflectionStudy;
//获得class对象
public class demo02 {
public static void main(String[] args) throws ClassNotFoundException {
Person person = new Student();
System.out.println("这个人是："+person.name);//这个人是：学生
//方式一：通过对象获得
Class c1 = person.getClass();
System.out.println(c1.hashCode());//460141958
//方式二：通过forname获得
Class c2 = Class.forName("ReflectionStudy.Student");
System.out.println(c2.hashCode());//460141958
//方式三：通过 类名.class 获得
Class c3 = Student.class;
System.out.println(c3);//class ReflectionStudy.Student
//方式四 基本内置类型的包装类都有一个Type属性
Class c4 = Integer.TYPE;
System.out.println(c4);//int
//获得父类类型
Class c5 = c1.getSuperclass();
System.out.println(c5);//class ReflectionStudy.Person
}
}
class Person{
public String name;
public Person() {
}
public Person(String name) {
this.name = name;
}
@Override
public String toString() {
return "Teacher{" +
"name='" + name + '\'' +
'}';
}
}
class Student extends Person{
public Student() {
this.name="学生";
}
}
class Teacher extends Person{
public Teacher() {
this.name="老师";
}
}
- 所有类型的class
package ReflectionStudy;
import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;
//所有类型的class
public class demo03 {
public static void main(String[] args) {
Class c1 = Object.class;//类
Class c2 = Comparable.class;//接口
Class c3 = String[].class;//一维数组
Class c4 = int[][].class;//二维数组
Class c5 = Annotation.class;//注解
Class c6 = ElementType.class;//枚举
Class c7 = Integer.class;//基本数据类型
Class c8 = void.class;//void
Class c9 = Class.class;//Class
System.out.println(c1);//class java.lang.Object
System.out.println(c2);//interface java.lang.Comparable
System.out.println(c3);//class [Ljava.lang.String;
System.out.println(c4);//class [[I
System.out.println(c5);//interface java.lang.annotation.Annotation
System.out.println(c6);//class java.lang.annotation.ElementType
System.out.println(c7);//class java.lang.Integer
System.out.println(c8);//void
System.out.println(c9);//class java.lang.Class
//只要元素的类型与维度一样，就是同一个Class
int[] a = new int[5];
int[] b = new int[50];
System.out.println(a.getClass().hashCode());//460141958
System.out.println(b.getClass().hashCode());//460141958
}
}
- Java的内存分析：
代码演示
package ReflectionStudy;
public class demo04 {
public static void main(String[] args) {
A a = new A();
System.out.println(A.m);
}
}
class A{
static {
System.out.println("静态代码块");
m = 30;
}
static int m = 100;
public A() {
System.out.println("无参构造");
}
}
代码运行结果：
- 获得类的运行时结构
直接代码演示：
package ReflectionStudy;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
//获得类的运行时结构
public class demo05 {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
Class c = Class.forName("ReflectionStudy.user");
System.out.println("=============获得类的名字==================");
System.out.println("getName()方法获得的类名：" + c.getName());
System.out.println("getSimpleName()方法获得的类名：" + c.getSimpleName());
System.out.println("=============获得类的属性==================");
Field[] fields = c.getFields();//只能获得本类的public属性
for (Field field : fields) {
System.out.println("getFields()方法获得的属性：" + fields);
}
fields = c.getDeclaredFields();
for (Field field : fields) {
System.out.println("getDeclaredFields()方法获得的属性：" + fields);
}
System.out.println("=============获得类的指定属性===============");
Field field = c.getDeclaredField("name");
System.out.println("获得类的指定属性" + field);
System.out.println("=============获得类的方法==================");
Method[] methods = c.getMethods();//找到本类及父类的全部public方法
for (Method method : methods) {
System.out.println("getMethods()方法获得的方法：" + method);
}
methods = c.getDeclaredMethods();//找到本类的所有方法
for (Method method : methods) {
System.out.println("getDeclaredMethods()方法获得的方法：" + method);
}
System.out.println("=============获得类的指定方法===============");
Method method = null;
method = c.getDeclaredMethod("getName", null);
System.out.println(method);
method = c.getDeclaredMethod("setName", String.class);
System.out.println(method);
System.out.println("=============获得构造器====================");
Constructor[] constructors = c.getConstructors();
for (Constructor constructor : constructors) {
System.out.println("getConstructors()获得的构造器："+constructor);
}
constructors = c.getDeclaredConstructors();
for (Constructor constructor : constructors) {
System.out.println("getDeclaredConstructors()获得的构造器："+constructor);
}
System.out.println("=============获得指定的构造器===============");
Constructor declaredConstructor = c.getDeclaredConstructor(String.class, int.class, int.class);
System.out.println("获得指定的构造器:"+declaredConstructor);
}
}
运行结果：
=============获得类的名字==================
getName()方法获得的类名：ReflectionStudy.user
getSimpleName()方法获得的类名：user
=============获得类的属性==================
getDeclaredFields()方法获得的属性：[Ljava.lang.reflect.Field;@74a14482
getDeclaredFields()方法获得的属性：[Ljava.lang.reflect.Field;@74a14482
getDeclaredFields()方法获得的属性：[Ljava.lang.reflect.Field;@74a14482
=============获得类的指定属性===============
获得类的指定属性private java.lang.String ReflectionStudy.user.name
=============获得类的方法==================
getMethods()方法获得的方法：public java.lang.String ReflectionStudy.user.getName()
getMethods()方法获得的方法：public int ReflectionStudy.user.getId()
getMethods()方法获得的方法：public void ReflectionStudy.user.setName(java.lang.String)
getMethods()方法获得的方法：public void ReflectionStudy.user.setId(int)
getMethods()方法获得的方法：public int ReflectionStudy.user.getAge()
getMethods()方法获得的方法：public void ReflectionStudy.user.setAge(int)
getMethods()方法获得的方法：public final void java.lang.Object.wait() throws java.lang.InterruptedException
getMethods()方法获得的方法：public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
getMethods()方法获得的方法：public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
getMethods()方法获得的方法：public boolean java.lang.Object.equals(java.lang.Object)
getMethods()方法获得的方法：public java.lang.String java.lang.Object.toString()
getMethods()方法获得的方法：public native int java.lang.Object.hashCode()
getMethods()方法获得的方法：public final native java.lang.Class java.lang.Object.getClass()
getMethods()方法获得的方法：public final native void java.lang.Object.notify()
getMethods()方法获得的方法：public final native void java.lang.Object.notifyAll()
getDeclaredMethods()方法获得的方法：public java.lang.String ReflectionStudy.user.getName()
getDeclaredMethods()方法获得的方法：public int ReflectionStudy.user.getId()
getDeclaredMethods()方法获得的方法：public void ReflectionStudy.user.setName(java.lang.String)
getDeclaredMethods()方法获得的方法：public void ReflectionStudy.user.setId(int)
getDeclaredMethods()方法获得的方法：public int ReflectionStudy.user.getAge()
getDeclaredMethods()方法获得的方法：public void ReflectionStudy.user.setAge(int)
=============获得类的指定方法===============
public java.lang.String ReflectionStudy.user.getName()
public void ReflectionStudy.user.setName(java.lang.String)
=============获得构造器====================
getConstructors()获得的构造器：public ReflectionStudy.user()
getConstructors()获得的构造器：public ReflectionStudy.user(java.lang.String,int,int)
getDeclaredConstructors()获得的构造器：public ReflectionStudy.user()
getDeclaredConstructors()获得的构造器：public ReflectionStudy.user(java.lang.String,int,int)
=============获得指定的构造器===============
获得指定的构造器:public ReflectionStudy.user(java.lang.String,int,int)
