Java的反射

public class ClassLoaderDemo1 {
    public static void main(String[] args) {
        //static ClassLoader getSystemClassLoader():返回用于委派的系统类加载器
        ClassLoader c=ClassLoader.getSystemClassLoader();
        System.out.println(c);//AppClassLoader

        //ClassLoader getParent():返回父类加载器进行委派
        ClassLoader  c2=c.getParent();
        System.out.println(c2);//ExtClassLoader

        ClassLoader parent = c2.getParent();
        System.out.println(parent);//null
    }
}

public class ReflectDemo1 {
    public static void main(String[] args) throws ClassNotFoundException {
        //使用类的class属性来获取该类对应的Class对象
        Class studentClass=Student.class;
        System.out.println(studentClass);//class demo11.Student

        Class studentClass1=Student.class;
        System.out.println(studentClass==studentClass1);//true
        System.out.println("--------------------");

        //调用对象的getClass()方法 返回该对象所属类对应的Class对象
        Student s=new Student();
        Class studentClass2=s.getClass();
        System.out.println(studentClass2==studentClass1);//true
        System.out.println("--------------------");

        //调用Class类中的静态方法forName(String className)
        Class studentClass3 = Class.forName("demo11.Student");
        System.out.println(studentClass3==studentClass1);//true
    }
}

public class ReflectDemo2 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException {
        //获取Class对象
        Class aClass = Class.forName("demo11.Student");

        
        Constructor[] constructors=aClass.getConstructors();
        for (Constructor con: constructors) {
            System.out.println(con);
        }

        //getDeclaredConstructor():获取当前所有运行时类中声明的所有构造器
        Constructor declaredConstructor = aClass.getDeclaredConstructor();
        System.out.println(declaredConstructor);
    }
}

我的Student类是个空类，感兴趣的可以自己加构造器玩玩

现在在Student类中添加属性(private的name属性，public的age属性)

public class ReflectDemo3 {
    public static void main(String[] args) throws NoSuchFieldException {
        Class studentClass = Student.class;

        //Field[] getFields():获取所有的"公有字段"
        //Field[] getDeclaredFields():获取所有字段，包括：私有、受保护、默认、公有

//        Field field1 = studentClass.getField();
        Field[] declaredFields = studentClass.getDeclaredFields();
        for (Field field : declaredFields){
            System.out.println(field);
        }
        System.out.println("-----------");

        
        Field age = studentClass.getField("age");
        System.out.println(age);
        System.out.println("-----------");

        Field name = studentClass.getDeclaredField("name");
        System.out.println(name);
    }
}

在Student类中添加两个方法(public的walk方法 private的eat方法)

public class ReflectDemo4 {
    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException, InstantiationException {
        Class studentClass = Student.class;


        Method[] method = studentClass.getMethods();
        for (Method me : method) {
            System.out.println(me);
        }
        System.out.println("----------------");

        Method[] declaredMethods = studentClass.getDeclaredMethods();
        for (Method d: declaredMethods) {
            System.out.println(d);
        }
        System.out.println("----------------");

        Method walk = studentClass.getMethod("walk");
        System.out.println(walk);
        System.out.println("----------------");

        //获取无参构造方法创建对象
        Constructor constructor = studentClass.getConstructor();
        Object obj = constructor.newInstance();

        walk.invoke(obj);
    }
}

• newInstance实例化对象只能调用无参构造方法
• newInstance创建类是这个类必须已经加载过且已经连接（Class.forName(“package”)这个过程），new创建类是则不需要这个类加载过
• 创建对象方式，是实用类的加载机制
• new是直接创造一个类