初识反射

通过反射可以获取类的Class对象, 一个类在内存中只有一个Class对象,Class对象不能开发者自己创建, 只能获取, 一个类被加载后类的整个结构都将被封装在Class对象中

获得Class对象的几种方法

  1. public class GetClass {
  2.     public static void main(String[] args) throws ClassNotFoundException {
  4.         //1.通过 对象.getClass() 获得
  5.         Teacher william = new Teacher("william");
  6.         Class c1 = william.getClass();
  7.         System.out.println(c1);
  8.         System.out.println("c1.hashCode() = " + c1.hashCode());
  10.         //2.Class.forName(String className) 获得
  11.         Class c2 = Class.forName("com.cqu.william.AnnotationAndReflection.Student");
  12.         System.out.println("c2.hashCode() = " + c2.hashCode());
  14.         //3.通过 类名.class 获得
  15.         Class c3 = Student.class;
  16.         System.out.println(c3);
  17.         System.out.println("c3.hashCode() = " + c3.hashCode());  //同一个类的Class的hashcode相同
  19.         //4.基本内置类型的包装类都有一个TYPE属性
  20.         Class<Integer> c4 = Integer.TYPE;
  21.         System.out.println("c4 = " + c4);
  23.         //5.通过Class.getSuperclass()可以获得其父类的Class对象
  24.         Class c5 = c2.getSuperclass();
  25.         System.out.println("c5 = " + c5);
  26.     }
  27. }
  29. class Person {
  30.     public String name;
  32.     public Person() {
  33.     }
  35.     public Person(String name) {
  36.         this.name = name;
  37.     }
  38. }
  40. class Student extends Person {
  41.     public Student(String name) {
  42.         super(name);
  43.     }
  44. }
  46. class Teacher extends Person {
  47.     public Teacher(String name) {
  48.         super(name);
  49.     }
  50. }

Class.getxxx()

  1. public class GetName {
  2.     public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
  3.         Class c1 = Class.forName("com.cqu.william.AnnotationAndReflection.User");
  5.         //获得类的名字
  6.         System.out.println("c1.getName() = " + c1.getName());//获得包名+类名
  7.         System.out.println("c1.getSimpleName() = " + c1.getSimpleName());//获得类名
  9.         //获得类的属性
  10.         Field[] fields = c1.getFields(); //只能获得public属性的数组
  11.         for (Field field : fields) {
  12.             System.out.println(field);
  13.         }
  14.         System.out.println("======================================");
  16.         fields = c1.getDeclaredFields(); //获得所有属性的数组
  17.         for (Field field : fields) {
  18.             System.out.println(field);
  19.         }
  20.         System.out.println("======================================");
  22.         Field name = c1.getDeclaredField("name");
  23.         System.out.println(name);//获得指定属性
  24.         System.out.println("======================================");
  26.         //  获得方法
  27.         Method[] methods = c1.getMethods();//获得本类及父类的所有public方法的数组
  28.         for (Method method : methods) {
  29.             System.out.println(method);
  30.         }
  31.         System.out.println("======================================");
  34.         Field[] declaredFields = c1.getDeclaredFields();//获得本类的所有方法的数组
  35.         for (Field declaredField : declaredFields) {
  36.             System.out.println(declaredField);
  37.         }
  38.         System.out.println("======================================");
  40.         //获得所有public构造器的数组
  41.         Constructor[] constructors = c1.getConstructors();
  42.         for (Constructor constructor : constructors) {
  43.             System.out.println(constructor);
  44.         }
  45.         System.out.println("======================================");
  47.         //获得所有public构造器的数组
  48.         Constructor[] declaredConstructors = c1.getDeclaredConstructors();
  49.         for (Constructor declaredConstructor : declaredConstructors) {
  50.             System.out.println(declaredConstructor);
  51.         }
  52.     }
  53. }

动态创造对象

  1. public class Dyn {
  2.     private static Object william;
  4.     public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
  6.         Class c1 = Class.forName("com.cqu.william.AnnotationAndReflection.User");
  8.         //使用newInstance()动态构造对象
  9.         User user = (User) c1.newInstance();
  10.         System.out.println(user);
  12.         //使用getDeclaredConstructor(params)获取构造器, 通过构造器创建对象
  13.         Constructor c = c1.getDeclaredConstructor(String.class, int.class, int.class);
  14.         Object william = c.newInstance("william", 20193818, 19);
  16.         //使用getDeclaredMethod()获取指定方法
  17.         Method setName = c1.getDeclaredMethod("setName", String.class);
  18.         //使用invoke调用william对象的setName方法,参数为"ABC"
  19.         setName.invoke(william, "ABC");
  21.         //使用getDeclaredField()获得字段
  22.         Field name = c1.getDeclaredField("name");
  23.         //setAccessible(true)关闭反射的检测
  24.         name.setAccessible(true);
  25.         name.set(william, "william");
  26.     }
  27. }

反射的执行表现

反射使得执行效率大大降低, 通过关闭检测setAccessible(true)能稍微提升

  1. // 分别使用三种方式执行10亿次循环, 记录所用的时间, 比较执行效率
  3. public class Performance {
  4.     public static void main(String[] args) throws IllegalAccessException, NoSuchMethodException, InvocationTargetException {
  5.         test01();
  6.         test02();
  7.         test03();
  8.     }
  10.     //普通方式调用getName()
  11.     static void test01() {
  12.         User user = new User();
  14.         long startTime = System.currentTimeMillis();
  15.         for (int i = 0; i < 1000000000; i++) {
  16.             user.getName();
  17.         }
  18.         long endTime = System.currentTimeMillis();
  20.         System.out.println("普通方式执行循环体10亿次 " + (endTime - startTime) + "ms");
  21.     }
  23.     //反射方式调用getName()
  24.     static void test02() throws InvocationTargetException, IllegalAccessException, NoSuchMethodException {
  25.         User user = new User();
  26.         //通过getClass()得到user的Class对象
  27.         Class c1 = user.getClass();
  28.         // getDeclaredMethod("getName")得到User.getName()方法
  29.         Method getName = c1.getDeclaredMethod("getName");
  31.         long startTime = System.currentTimeMillis();
  32.         for (int i = 0; i < 1000000000; i++) {
  33.             // getName.invoke(user)表示对user对象使用getName方法
  34.             getName.invoke(user);
  35.         }
  36.         long endTime = System.currentTimeMillis();
  38.         System.out.println("反射方式执行循环体10亿次 " + (endTime - startTime) + "ms");
  39.     }
  42.     //反射方式调用getName()并关闭检测
  43.     static void test03() throws NoSuchMethodException {
  44.         User user = new User();
  45.         Class c3 = user.getClass();
  46.         Method getName = c3.getDeclaredMethod("getName");
  48.         //setAccessible(true)关闭检测
  49.         getName.setAccessible(true);
  51.         long startTime = System.currentTimeMillis();
  52.         for (int i = 0; i < 1000000000; i++) {
  53.             user.getName();
  54.         }
  55.         long endTime = System.currentTimeMillis();
  57.         System.out.println("关闭检测执行循环体10亿次 " + (endTime - startTime) + "ms");
  58.     }
  59. }