反射

一、類是怎麼加載到內存中的

1.類的加載：

   就是將class文件讀入內存，並創建一個class對象
   任何類在使用時系統都會創建一個class對象
2.類的加載時機：
  (1).創建類的實例
  (2).訪問類的靜態成員
  (3).通過反射創建對象
  (4).初始化某個子類
  (5).運行main方法
3.加載器：負責將.class加載到內存，並生成Class對象
  (1).BootStrap ClassLoader(根類加載器)：負責java核心類的加載（jdk\jre\lib\rt.jar,API）
  (2).Extension ClassLoader(擴展類加載器): 負責JRE的擴展目錄中的jar包的加載(jdk\jre\lib\ext目錄中)
  (3).System ClassLoader(系統類加載器):負責字JVM啓動時加載來自java命令的class文件，以及classpath環境變量指定的jar包和類路徑
二、反射(reflect)
  定義：在運行過程中“動態的獲取對象的信息”和“動態的調用對象的方法”的功能就是反射
  實現：就是獲取Class對象，操作Class對象
三、獲取Class對象
  第一種：Object的getClass()方法返回Object的運行時類
  第二種：通過任意對象的class靜態屬性

  第三種：將類名作爲字符串參數傳遞給Class類的forName()方法，參數使用類的帶包路徑

package com.et.example;
public class ReflectTest {
    public static void main(String[] args) throws ClassNotFoundException {
        //對象的getClass()方法返回對象的運行時類
        Animal a = new Animal();
        Class<? extends Animal> c1 = a.getClass();
        //任意對象的class靜態屬性
        Class<? extends Animal> c2 = Animal.class;
        //類名作爲字符串參數傳遞給Class類的forName()方法，參數使用類的帶包路徑
        Class<? extends Object> c3 = Class.forName("com.et.example.Animal");
        System.out.println(c1==c2);
        System.out.println(c2==c3);
    }
}
class Animal {
    private int age;
    private String name;
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Animal other = (Animal) obj;
        if (age != other.age)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    @Override
    public String toString() {
        return "Animal [age=" + age + ", name=" + name + "]";
    }
}

四、通過反射獲取類的構造方法

package com.et.example;

import java.lang.reflect.Constructor;

public class ReflectTest {
    public static void main(String[] args) throws Exception{
        Class<? extends Object> c = Class.forName("com.et.example.Animal");
        //可獲取訪問權限內的構造方法
        Constructor<? extends Object> cst1 = c.getConstructor(int.class,String.class);
        //可獲取類的所有構造方法
        Constructor<? extends Object> cst2 = c.getDeclaredConstructor();
        //取消對Java語言的訪問檢查
        cst2.setAccessible(true);
        Animal a1 = (Animal) cst1.newInstance(10,"小提莫");
        Animal a2 = (Animal) cst2.newInstance();
        System.out.println(a1);
        System.out.println(a2);
    }
}
class Animal {
    private int age;
    private String name;
    private Animal(){
    }
    public Animal(int age,String name) {
        this.age = age;
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Animal other = (Animal) obj;
        if (age != other.age)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    @Override
    public String toString() {
        return "Animal [age=" + age + ", name=" + name + "]";
    }
}

5.通過反射獲取成員變量

package com.et.example;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;

public class ReflectTest {
    public static void main(String[] args) throws Exception{
        Class<? extends Object> c = Class.forName("com.et.example.Animal");
        Constructor<? extends Object> cst = c.getDeclaredConstructor();
        cst.setAccessible(true);
        Animal animal = (Animal) cst.newInstance();
        Field age = c.getDeclaredField("age");
        Field name = c.getDeclaredField("name");
        age.setAccessible(true);
        name.setAccessible(true);
        age.set(animal,20);
        name.set(animal,"大提莫");
        System.out.println(animal);
    }
}
class Animal {
    private int age;
    private String name;
    private Animal(){
    }
    public Animal(int age,String name) {
        this.age = age;
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Animal other = (Animal) obj;
        if (age != other.age)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    @Override
    public String toString() {
        return "Animal [age=" + age + ", name=" + name + "]";
    }
}

6.通過反射獲取普通方法

package com.et.example;

import java.lang.reflect.Method;

public class ReflectTest {
    public static void main(String[] args) throws Exception{
        Class<? extends Object> c = Class.forName("com.et.example.Animal");
        Method[] methods = c.getDeclaredMethods();
        for(Method method:methods) {
            //獲取方法的權限類編號
            int modifiers = method.getModifiers();
            //獲取方法的參數類型
            System.out.print(modifiers+"--");
            Class<?>[] parameterTypes=method.getParameterTypes();
            for(Class parameterType:parameterTypes){
                System.out.print(parameterType+"--");
            }
            System.out.println(method.getName());
        }

    }
}
class Animal {
    private int age;
    private String name;
    private Animal(){
    }
    public Animal(int age,String name) {
        this.age = age;
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + age;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        return result;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Animal other = (Animal) obj;
        if (age != other.age)
            return false;
        if (name == null) {
            if (other.name != null)
                return false;
        } else if (!name.equals(other.name))
            return false;
        return true;
    }
    @Override
    public String toString() {
        return "Animal [age=" + age + ", name=" + name + "]";
    }
}