1、概述

是在運(yùn)行狀態(tài)中，對(duì)于任意一個(gè)類，都能夠知道這個(gè)類的所有屬性和方法；對(duì)于任意一個(gè)對(duì)象，都能夠調(diào)用它的任意屬性和方法；這種動(dòng)態(tài)獲取信息以及動(dòng)態(tài)調(diào)用對(duì)象方法的功能稱為Java語(yǔ)言的反射機(jī)制。

2、獲取Class類對(duì)象的三種方式

• 類名.class屬性
• 對(duì)象名.getClass()方法

• Class.forName(全類名)方法

  
  
  獲取Class對(duì)象的三種方式.png

//1.Class類中的靜態(tài)方法forName("全類名")
//全類名:包名 + 類名
Class clazz = Class.forName("com.Student");
System.out.println(clazz);

//2.通過(guò)class屬性來(lái)獲取
Class clazz2 = Student.class;
System.out.println(clazz2);

//3.利用對(duì)象的getClass方法來(lái)獲取class對(duì)象
//getClass方法是定義在Object類中.
Student s = new Student();
Class clazz3 = s.getClass();
System.out.println(clazz3);

3反射獲取構(gòu)造方法

Class clazz = Student.class;

System.out.print("返回所有構(gòu)造方法對(duì)象的數(shù)組:\n");
Constructor[] constructors1 = clazz.getConstructors();
for (Constructor constructor : constructors1) {
    System.out.println(constructor);
}
System.out.printf("返回所有構(gòu)造方法對(duì)象的數(shù)組:\n");
Constructor[] constructors2 = clazz.getDeclaredConstructors();
for (Constructor constructor : constructors2) {
    System.out.println(constructor);
}

//小括號(hào)中,一定要跟構(gòu)造方法的形參保持一致.
System.out.printf("返回單個(gè)構(gòu)造方法對(duì)象:\n");
Constructor constructor = clazz.getDeclaredConstructor();
System.out.println(constructor);

System.out.printf(" 返回單個(gè)公共構(gòu)造方法對(duì)象:\n");
Constructor constructor1 = clazz.getConstructor();
System.out.println(constructor1);

static void test() throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
 //1.獲取class對(duì)象
 Class clazz = Student.class;

 //2.獲取一個(gè)私有化的構(gòu)造方法.
 Constructor constructor = clazz.getDeclaredConstructor(String.class);

 //被private修飾的成員,不能直接使用的
 //如果用反射強(qiáng)行獲取并使用,需要臨時(shí)取消訪問(wèn)檢查
 constructor.setAccessible(true);

 //3.直接創(chuàng)建對(duì)象
 Student student = (Student) constructor.newInstance("zhangsan");

 System.out.println(student);
}

4反射獲取成員變量

static void test() throws NoSuchFieldException {
    Class clazz = Student.class;

    System.out.println("公共成員變量對(duì)象的數(shù)組:");
    Field[] fields1 = clazz.getFields();
    for (Field field : fields1) {
        System.out.println(field);
    }

    System.out.println("返回所有成員變量對(duì)象的數(shù)組:");
    Field[] fields2 = clazz.getDeclaredFields();
    for (Field field : fields2) {
        System.out.println(field);
    }

    //想要獲取的成員變量必須是真實(shí)存在的
    //且必須是public修飾的.
    System.out.println("單個(gè)公共成員變量對(duì)象:");
    Field field1 = clazz.getField("name");
    System.out.println(field1);

    System.out.println("單個(gè)成員變量對(duì)象:");
    Field field2 = clazz.getDeclaredField("money");
    System.out.println(field2);
}

static void test() throws NoSuchFieldException, InstantiationException, IllegalAccessException {
    Class clazz = Student.class;

    //1先創(chuàng)建一個(gè)對(duì)象
    Student student = (Student) clazz.newInstance();
    //2.獲取成員變量Field的對(duì)象
    Field field = clazz.getField("name");
    //3有了對(duì)象才可以給指定對(duì)象進(jìn)行賦值
    field.set(student,"zhangsan");
    System.out.println(student);

    //1創(chuàng)建一個(gè)對(duì)象
    Student student2 = (Student) clazz.newInstance();
    //2.獲取成員變量Field的對(duì)象
    Field field2 = clazz.getDeclaredField("money");
    //3.取消一下訪問(wèn)檢查
    field2.setAccessible(true);
    //4獲取指定對(duì)象的money的值
    Object o = field.get(student);
    //5.打印一下
    System.out.println(o);
}

5、反射獲取成員方法并使用【應(yīng)用】

static void test() throws NoSuchMethodException {
    Class clazz = Student.class;

    System.out.println("返回所有公共成員方法對(duì)象的數(shù)組，包括繼承的:");
    Method[] methods1 = clazz.getMethods();
    for (Method method : methods1) {
        System.out.println(method);
    }

    System.out.println("返回所有成員方法對(duì)象的數(shù)組，不包括繼承的:");
    Method[] methods2 = clazz.getDeclaredMethods();
    for (Method method : methods2) {
        System.out.println(method);
    }

    System.out.println("返回單個(gè)公共成員方法對(duì)象:");
    Method method1 = clazz.getMethod("function1");
    System.out.println(method1);

    System.out.println("返回單個(gè)成員方法對(duì)象:");
    Method method = clazz.getDeclaredMethod("show");
    System.out.println(method);
}

• 運(yùn)行方法: Object invoke(Object obj, Object... args)
  參數(shù)一: 用obj對(duì)象調(diào)用該方法
  參數(shù)二: 調(diào)用方法的傳遞的參數(shù)(如果沒(méi)有就不寫)
  返回值: 方法的返回值(如果沒(méi)有就不寫)

static void test() throws NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
    //1.獲取class對(duì)象
    Class clazz = Student.class;
    //2.獲取里面的Method對(duì)象  function4
    Method method = clazz.getMethod("function4", String.class);
    //3.運(yùn)行function4方法就可以了
    //3.1創(chuàng)建一個(gè)Student對(duì)象,當(dāng)做方法的調(diào)用者
    Student student = (Student) clazz.newInstance();
    //3.2運(yùn)行方法
    Object result = method.invoke(student, "zhangsan");
    //4.打印一下返回值
    System.out.println(result);
}