Java基础之反射入门

Reflection（反射）是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。

public class Person {
    private String name;
    public int age;

    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 Person(String name, int age) {
        this.name = name;
        this.age = age;
    }
    private Person(String name){
        this.name = name;
    }

    public Person() {
        System.out.println("调用person的空参构造");
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + ''' +
                ", age=" + age +
                '}';
    }

    public void show(){
        System.out.println("我是一个人");
    }

    private String showNation(String nation){
        System.out.println("我的国籍是：" + nation);
        return nation;
    }
}

public class PersonTest {
    public static void main(String[] args) throws Exception{
        // 1. 实例化对象
        Person person = new Person("tom",12);
        // 通过对象调用属性方法
        person.age = 10;
        person.show();
        System.out.println(person.toString());

        System.out.println("************反射*************");
        // 通过反射来创建对象(调用私有的方法)
        Class clazz = Person.class;
        Constructor conn = clazz.getDeclaredConstructor(String.class);
        conn.setAccessible(true);
        Person hong = (Person)conn.newInstance("hong");
        System.out.println(hong.toString());
        // 通过反射，调取对象的属性，方法
        Field name = clazz.getDeclaredField("name");
        name.setAccessible(true);
        name.set(hong,"tbc");
        System.out.println(hong);

        Method show = clazz.getDeclaredMethod("showNation", String.class);
        show.setAccessible(true);
        String result = (String)show.invoke(hong, "中国");

        System.out.println(result);


        
        
    }
}

public class ClassTest {
    public static void main(String[] args) throws Exception{
        
        // 1. 类.class：调用运行时类的属性
        Class clazz1 = Person.class;
        System.out.println(clazz1);

        // 2. 通过运行时类的对象
        Person p1 = new Person();
        Class clazz2 = p1.getClass();
        System.out.println(clazz2);

        // 3. 调用Class的forName方法  这种使用得多
        Class clazz3 = Class.forName("tbc.demo12.Person");
        System.out.println(clazz3);

        System.out.println(clazz1 == clazz2);
        System.out.println(clazz1 == clazz3);


        // 4. 使用类加载器
        ClassLoader classLoader = ClassTest.class.getClassLoader();
        Class clazz4 = classLoader.loadClass("tbc.demo12.Person");
        System.out.println(clazz4);

        System.out.println(clazz1 == clazz4);
    }
}

public class ClassLoaderDemo2 {
    public static void main(String[] args) throws Exception{
        Properties pro = new Properties();
        // 读取配置文件方式一：读取当前的module下的
        

        // 读取配置文件方式二：读取src下的
        ClassLoader classLoader = ClassLoaderDemo2.class.getClassLoader();
        InputStream resourceAsStream = classLoader.getResourceAsStream("jdbc1.properties");
        pro.load(resourceAsStream);

        String username = pro.getProperty("username");
        String password = pro.getProperty("password");
        System.out.println(username);
        System.out.println(password);
    }
}

public class ClassLoaderDemo {
    public static void main(String[] args) {
        // 对于自定义类，使用系统类加载器加载
        ClassLoader classLoader = ClassLoaderDemo.class.getClassLoader();
        System.out.println(classLoader);// 这是系统类加载器
        ClassLoader parent1 = classLoader.getParent();
        System.out.println(parent1); // 扩展类加载器
        ClassLoader parent2 = parent1.getParent();
        // 引导类加载器加载java的核心类库，无法加载自定义类
        System.out.println(parent2);// 获取不到引导类加载器
    }
}

public class ClassDemo1 {
    public static void main(String[] args) {
        Class c1 = Object.class; // 类
        Class c2 = Comparable.class; // 接口
        Class c3 = String[].class; // String数组
        Class c4 = int[][].class; // int数组
        Class c5 = ElementType.class; // 枚举类型
        Class c6 = Override.class; // 注解
        Class c7 = int.class; // 基本数据类型
        Class c8 = void.class; // void
        Class c9 = Class.class; // Class

        int[] a = new int[10];
        int[] b = new int[100];
        Class c10 = a.getClass(); // void
        Class c11 = b.getClass(); // Class
        // 只要数组的元素类型和纬度一样，就是同一个Class
        System.out.println(c10 == c11);

    }
}