Java中的反射机制示例详解

public class reflect {

    public static void main(String[] args) throws ClassNotFoundException {

        Apple apple = new Apple();

        // 使用对象的getClass()方法
        Class a1 = apple.getClass();

        // 使用Class类的静态方法forName()
        Class a2 = Class.forName("p1.apple");

        // 运用.class的方式来获取Class实例
        Class a3 = Apple.class;

        System.out.printf("a1: %s\na2: %s\na3: %s", a1, a2, a3);

    }

}

class Apple {

    private Integer weight;

    private String color;
    
}

public class reflect {

    public static void main(String[] args) throws InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {

        // 通过 Class 对象的 newInstance() 方法
        Class temp1 = Apple.class;
        Apple apple1 = (Apple) temp1.newInstance();
        
        // 通过 Constructor 对象的 newInstance() 方法
        Class temp2 = Apple.class;
        Constructor constructor1 = temp2.getConstructor();
        Apple apple2 = (Apple)constructor1.newInstance();

        // 通过 Constructor 对象创建类对象可以选择特定构造方法，而通过 Class 对象则只能使用默认的无参数构造方法。
        Class temp3 = Apple.class;
        Constructor constructor2 = temp3.getConstructor(Integer.class,String.class);
        Apple apple = (Apple)constructor2.newInstance(2, "Red");
        System.out.println(apple);

    }

}

class Apple {

    private Integer weight;

    private String color;

    // 无参构造器
    public Apple() {
        System.out.println("我是无参构造！");
    }

    // 有参构造器
    public Apple(Integer weight,String color) {
        this.weight = weight;
        this.color = color;
    }

    // 重写方法 方便打印显示对象内容
    @Override
    public String toString() {
        return "Apple{" +
                "weight=" + weight +
                ", color='" + color + '\'' +
                '}';
    }

}

public class reflect {

    public static void main(String[] args) throws NoSuchFieldException {

        // 返回一个类中所有可访问的公共字段，包括该类的公共字段和其继承的类的公共字段
        Field[] fields1 = Apple.class.getFields();
        System.out.println("getFields结果");
        Arrays.stream(fields1).forEach(System.out::println);

        // 返回一个类中全部字段，但只包括该类的字段
        Field[] fields2 = Apple.class.getDeclaredFields();
        System.out.println("getDeclaredFields结果");
        Arrays.stream(fields2).forEach(System.out::println);

        // 根据字段名返回一个公开字段
        Field field1 = Apple.class.getField("noThing");
        System.out.println("getField结果");
        System.out.println(field1);

        // 根据字段名返回一个字段
        Field field2 = Apple.class.getDeclaredField("color");
        System.out.println("getDeclaredField结果");
        System.out.println(field2);

        // 同Field Method也有四种获取方式
        // 这里举其中一个例子
        Method[] methods = Apple.class.getDeclaredMethods();
        System.out.println("getDeclaredMethods结果");
        Arrays.stream(methods).forEach(System.out::println);

        // 同上 举一个获取构造器的例子
        Constructor[] constructors = Apple.class.getDeclaredConstructors();
        System.out.println("getDeclaredConstructors结果");
        Arrays.stream(constructors).forEach(System.out::println);

    }

}

class Apple {

    private Integer weight;

    private String color;

    public String noThing;

    public Apple() {

    }

    public Apple(Integer weight, String color) {
        this.weight = weight;
        this.color = color;
    }

    @Override
    public String toString() {
        return "Apple{" +
                "weight=" + weight +
                ", color='" + color + '\'' +
                '}';
    }

}

public class reflect {

    public static void main(String[] args) throws NoSuchFieldException {

        // 实例化一个Apple
        Apple apple = new Apple();

        // 获取所有字段 并统一设定为公有属性
        Field[] fields = Apple.class.getDeclaredFields();
        Arrays.stream(fields).forEach( field -> {
            field.setAccessible(true);
            // 打印结果
            System.out.println(field);
            try {
                if (field.getType() == Integer.class) {
                    field.set(apple, 5);
                } else if (field.getType() == String.class) {
                    field.set(apple, "Red");
                }
            } catch (IllegalAccessException e) {
                e.printStackTrace();
            }
        });
        // 查看apple结果
        System.out.println(apple);
      
    }

}

class Apple {

    private Integer weight;

    private String color;

    @Override
    public String toString() {
        return "Apple{" +
                "weight=" + weight +
                ", color='" + color + '\'' +
                '}';
    }

}