java 反射

创建对象的方式

//创建对象1：new
//创建对象2：clone
//创建对象3：序列化(序列化会引发安全漏洞，未来将会被移除jdk)
//创建对象4：反射 动态性 字符串
public class CreateNewObj implements Cloneable{
    public void hello() {
        System.out.println("hello world");
 }
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
 }
    public static void main(String[] args) throws CloneNotSupportedException, ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException, InstantiationException {
         //new和克隆
         CreateNewObj test = new CreateNewObj();
         test.hello();
         CreateNewObj clone = (CreateNewObj) test.clone();//内存中位置不一样 深度克隆和浅克隆？
         clone.hello();
         //反射
         Class<?> cloneClass = Class.forName("reflect.CreateNewObj");
         Object o = cloneClass.newInstance();
         cloneClass.getMethod("hello").invoke(o);
    }
}

反射常用方法

public class ReflectTest {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException {
        //获取Class方法
        Class s1 = Class.forName("reflect.Student");
        Class s2 = Student.class;
        Student student = new Student("hello");
        Class s3 = student.getClass();
        //常用方法
        for (Field f : s3.getDeclaredFields()) {
            f.setAccessible(true); //private->public 临时的
        System.out.println(f.getName() + ":" + f.get(student));
        }
        System.out.println(Arrays.toString(s1.getConstructors())); //Construct.getParameterCount获取参数
        System.out.println(Arrays.toString(s1.getDeclaredFields()));//获取自身声明的变量,包括私有变量（getFields获取自身和父类public字段）
        System.out.println(Arrays.toString(s1.getDeclaredMethods()));//获取自身声明的方法（不包括父类） m.invoke（obj,args） 不是静态的就要传具体对象
        System.out.println(s1.getModifiers());//修饰符？
        System.out.println(s1.getSuperclass().getName());//父类
        System.out.println(Arrays.toString(s1.getInterfaces()));//实现的接口
        System.out.println(s1.getPackage().getName());//所属包
        System.out.println(Arrays.toString(s1.getAnnotations()));//获取注解
    }
}
class Student implements Cloneable  {
    private String name;
    private int id;
    public Student(String name) {
        this.name = name;
    }
    protected Object clone() throws CloneNotSupportedException {
        return super.clone();
    }
    public void hello() {
        System.out.println("hello world");
    }
}

反射的应用 - 数组长度拓展

public class ArrayLenExtend {
    public static Object goodCopy(Object oldArray, int newLength) {
//Array类型
         Class c = oldArray.getClass();
         System.out.println(c.getName());
//元素类型
         Class componentType = c.getComponentType();
         System.out.println(componentType.getName());
//创建新数组
         Object newArray = Array.newInstance(componentType, newLength);
//拷贝
        int oldLength = Array.getLength(oldArray);
        System.arraycopy(oldArray, 0, newArray, 0, oldLength);
//返回
        return newArray;
}
    public static void main(String[] args) {
        int[] a = {1, 2, 3, 4, 5};
         a = (int[]) goodCopy(a,10);
         System.out.println(a.length);
    }
}