java反射

含义：将类的字段和方法用对象来表示。

作用：JAVA反射机制是在运行状态中，对于任意一个类，都能够知道这个类的所有属性和方法；对于任意一个对象，都能够调用它的任意一个方法和属性；这种动态获取的信息以及动态调用对象的法的功能称为java语言的反射机制。

为测试创建User对象和Student对象

public class User {
	
	private String name;
	
	public String loginName;
	
	private User(String loginName){
		this.loginName = loginName;
	}
	
	public User(){
		
	}
}

public class Student extends User{
	private String num;
	public int classId;
	
	public Student(){
		
	}
	
	private Student(String num){
		this.num = num;
	}
}

每个类被加载之后，系统就会为该类生成一个对应的字节码对象，通过该字节码对象就可以访问到JVM中的对应的类。在Java中获得Class对象通常有三种方式

以Student类为例

		//方式一
Class clazz1 = Student.class;
		
		//方式二
Class clazz2 = (Class)Class.forName("com.test.Student");//参数为类的全限定名
		
		//方式三
Class clazz3 =  (Class) new Student().getClass();

方法

案例

public class Test {
	
	
	public static void main(String[] args) throws Exception {
		
		//获取类对象
		Class clazz1 = Student.class;
		

		System.out.println("------------getFields()-----------");
		Field[] fields = clazz1.getFields();
		for(Field field : fields){
			System.out.println(field.getName());   
		}
		
		System.out.println("------------getField(String name)-----------");
		
		System.out.println(clazz1.getField("loginName"));
		
		
		System.out.println("------------getDeclaredFields()-----------");
		
		
		fields = clazz1.getDeclaredFields();
		for(Field field : fields){
			System.out.println(field.getName());   
		}
		
		System.out.println("------------getDeclaredField(String name)-----------");
		Field field = clazz1.getDeclaredField("num");
		System.out.println(field);
		
		System.out.println("-----------------------设置、获取--------------------------");
		
		Student student = new Student();
		
		Field classIdField = clazz1.getField("classId");
		//常用操作
		    //设置
		    //获取
		
		//给student对象的classId设置值为1
		classIdField.set(student, 1);
		
		System.out.println(classIdField.get(student));
		System.out.println(student.classId);
		
		Field numField = clazz1.getDeclaredField("num");
		
		//设置强行访问
		numField.setAccessible(true);
		numField.set(student, "10");
		
		System.out.println(numField.get(student));
	}
}

方法

案例

public class Test2 {
	
	public static void main(String[] args) throws Exception {
		
		Class clazz = Student.class;
		
		
		System.out.println("-------------getMethods()-------------");
		Method methods [] = clazz.getMethods();
		for(Method method : methods){
			System.out.println(method.getName());
		}
		
		
		System.out.println("-------------getMethod(String name, Class... parameterTypes)-------------");
		Method method = clazz.getMethod("say");
		System.out.println(method);
		 
		
		
		method = clazz.getMethod("say",String.class);
		System.out.println(method);
		
		method = clazz.getMethod("say",String.class,int.class);
		System.out.println(method);
		
		
		
		System.out.println("-------------getDeclaredMethods()-------------");
		methods = clazz.getDeclaredMethods();
		for(Method methodTemp : methods){
			System.out.println(methodTemp.getName());
		}
		
		
		System.out.println("-------------getDeclaredMethod(String name, Class... parameterTypes)-------------");
	
		method = clazz.getDeclaredMethod("cal",int.class,int.class);
		System.out.println(method);
		
		System.out.println("------------------调用----------------------");
		
		
		
		Student student = new Student();
		method.setAccessible(true); 
		 
		int result = (int) method.invoke(student,1,2);
		System.out.println(result);
		
		//destroy方法
		Method staticMethod = clazz.getDeclaredMethod("destroy");
		staticMethod.setAccessible(true);
		staticMethod.invoke(null);
				
	}
}

方法

案例

public class Test3 {
	
	public static void main(String[] args) throws Exception {
		
		Class clazz = Student.class;
		
		
		//获取public的构造器
	 
		System.out.println("-------------getConstructors()-----------");
		Constructor [] constructors =	(Constructor[]) clazz.getConstructors();
		
		for( Constructor constructor : constructors ){
			System.out.println(constructor.getName());
		}
		
	
		//获取指定public的构造器
		//clazz.getConstructor(parameterTypes)
		
        //获取所有的构造器
		System.out.println("-------------getDeclaredConstructors()-----------");
        constructors =	(Constructor[]) clazz.getDeclaredConstructors();
		
		for( Constructor constructor : constructors ){
			System.out.println(constructor.getName());
		}
		
        //获取指定的构造器
		System.out.println("-------------getDeclaredConstructor(Class... parameterTypes)-----------");
		Constructor privateConstructor = clazz.getDeclaredConstructor(String.class);
		System.out.println(privateConstructor);
		
        //使用构造器创建对象
		System.out.println("--------------------创建对象-------------------");
		
		privateConstructor.setAccessible(true);
		Student student = privateConstructor.newInstance("110");
		System.out.println(student);
		
		System.out.println("无参构造器");
		Student.class.newInstance();
	}
}

案例

public class TestMain {
	public static void main(String[] args) {
		System.out.println("反射main");
	}
}

public class Test{
	
	
	public static void main(String[] args) throws Exception, SecurityException {
		
		Class clazz = TestMain.class;
		
		
		Method mainMethod = clazz.getMethod("main", String[].class);
		
		System.out.println(mainMethod);
		
		
		//调用
		mainMethod.invoke(null,new Object[]{new String[]{"a","b"}}); 
		
		//如下都是错误的
		//mainMethod.invoke(null, "a","b"); //可变参数？？
		//mainMethod.invoke(null,new String[]{"a","b"} ); //可变参数？？
	}
	
}