Java反射

反射（Reflection）

• 加载类，并允许以编程的方式解剖类中的各种成分

反射学什么？

1.反射第一步：加载类,获取类的字节码：Class对象

• 获取Class对象的3种方法：

public class Test1Class {public static void main(String[] args) throws ClassNotFoundException {Class<Student> c1 = Student.class;System.out.println(c1.getName()); //全类名System.out.println(c1.getSimpleName()); //简明:StudentClass<?> c2 = Class.forName("thread.Student");System.out.println(c1 == c2);Student s = new Student();Class c3 = s.getClass();System.out.println(c3 == c2);}}

2.获取类的构造器：Constructor对象

• Class提供了从类中获取构造器的方法

public class Test2Constructor {@Testpublic void testGetConstructors(){//1.反射第一步:必须先得到这个类的Class对象Class<Cat> c = Cat.class;//2.获取全部构造器
//        Constructor<?>[] constructors = c.getConstructors();Constructor<?>[] constructors = c.getDeclaredConstructors());//3.遍历数组中的每个构造器对象for (Constructor<?> constructor : constructors) {System.out.println(constructor.getName() + "--->"+ constructor.getParameterCount());}}@Testpublic void testGetConstructor() throws NoSuchMethodException {//1.反射第一步:必须先得到这个类的Class对象Class<Cat> c = Cat.class;//2.获取某个构造器:无参构造器
//        Constructor<Cat> constructor = c.getConstructor();Constructor<Cat> constructor = c.getDeclaredConstructor();System.out.println(constructor.getName() + "--->"+ constructor.getParameterCount());//3.获取有参数构造器Constructor<Cat> constructor2 =c.getDeclaredConstructor(String.class, int.class);System.out.println(constructor.getName() + "--->"+ constructor.getParameterCount());}
}

• 获取类构造器的作用：初始化对象返回 

//无参构造器constructor1.setAccessible(true); //禁止检查访问权限Cat cat = constructor1.newInstance();System.out.println(cat);//有参构造器constructor2.setAccessible(true);Cat cat2 = constructor2.newInstance("喜洋洋", 3);System.out.println(cat2);

3.获取类的成员变量

public class Test3Field {@Testpublic void testGetFields() throws NoSuchFieldException, IllegalAccessException {//1.反射第一步:必须是先得到类的Class对象Class<Cat> c = Cat.class;//2.获取类的全部成员变量Field[] fields = c.getDeclaredFields();//3.遍历这个成员变量数组for (Field field : fields) {System.out.println(field.getName() + "--->" + field.getType());}//4.定位某个成员变量Field fName = c.getDeclaredField("name");System.out.println(fName.getName() + "--->" + fName.getType());Field fAge = c.getDeclaredField("age");System.out.println(fAge.getName() + "--->" + fAge.getType());//赋值Cat cat = new Cat();fName.setAccessible(true);fName.set(cat,"加菲猫");System.out.println(cat);//取值String name = (String) fName.get(cat);System.out.println(name);}
}

4.获取类的成员方法：Method对象

• public class Test3Method {@Testpublic void testGetMethods() throws NoSuchFieldException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {Class<Cat> c = Cat.class;//2.获取全部成员方法Method[] methods = c.getDeclaredMethods();//3.遍历for (Method method : methods) {System.out.println(method.getName() + "--->"+ method.getParameterCount() + "-->"+ method.getReturnType());}//4.获取某个方法对象Method run = c.getDeclaredMethod("run");System.out.println(run.getName() + "--->"+ run.getParameterCount() + "-->"+ run.getReturnType());Method eat = c.getDeclaredMethod("eat", String.class);System.out.println(eat.getName() + "--->"+ eat.getParameterCount() + "-->"+ eat.getReturnType());Cat cat = new Cat();run.setAccessible(true);Object rs = run.invoke(cat);//调用无参数的run方法,用cat对象触发调用的System.out.println(rs);eat.setAccessible(true);String rs2 = (String) eat.invoke(cat, "鱼儿");System.out.println(rs2);}
  }
  

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反射的作用？

• 最重要的用途是：适合做Java的框架,基本上,主流的框架都会基于反射设计出一些通用的功能

使用反射做一个简易版的框架

需求：

• 对于任意一个对象,该框架都可以把对象的字段名和对应的值，保存到文件中去

public class ObjectFrame {public static void saveObject(Object obj) throws IllegalAccessException, FileNotFoundException {PrintStream ps = new PrintStream(new FileOutputStream("D:\\java\\src\\date.txt", true));//obj是任意对象,到底有多少个字段要保存Class<?> c = obj.getClass();String cName = c.getSimpleName();ps.println("-----------" + cName + "-------------");//从这个类中提取它的全部成员变量Field[] fields = c.getDeclaredFields();//遍历for (Field field : fields) {//拿到变量名字String name = field.getName();//拿到这个成员变量在对象中的数据field.setAccessible(true);String value = field.get(obj) + "";ps.println(name + "=" + value);}ps.close();}
}

public class Test5Frame {@Testpublic void save() throws FileNotFoundException, IllegalAccessException {Student s1 = new Student("wyf",35,'男',185,"篮球");Teacher t1 = new Teacher("xxx",4000);ObjectFrame.saveObject(s1);ObjectFrame.saveObject(t1);}
}