準備一個實體類：

package test.reflect;
/**
 * 反射使用的實體類
 *
 * @author liulei on 2018/11/28
 */
public class User {
    //欄位
    private String username;
    private String pwd;
    private String uid="1001";
    //構造方法
    public User(){

    }

    public User(String username){
        this.username=username;
    }
    public User(String username,String pwd){
        this.username=username;
        this.pwd=pwd;
    }
    //例項方法
    public void function1(){
        System.out.println("function1");
    }

    public void function2(String username){
        System.out.println("function2");
    }

    public String function3(String username,String pwd){
        System.out.println("function3");
        return username;
    }

    public String getUid(){
        return  uid;
    }
}

測試類：

package test.reflect;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * 反射的測試類
 *
 * @author liulei on 2018/11/28
 */
public class ReflectDemo {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException, NoSuchFieldException {
        //獲取一個類物件
        Class<?> c = Class.forName("test.reflect.User");
        //獲取一個無引數的構造器
        Constructor<?> constructor = c.getConstructor();
        //利用構造方法建立物件
        Object o = constructor.newInstance();
        //因為欄位是私有的，所以不能這樣獲取
        // Field field = c.getField(null);
 

        Field[] declaredFields = c.getDeclaredFields();
        //迴圈輸出欄位
        for(int i=0;i<declaredFields.length;i++){
            //設定私有欄位可以訪問，如果不設定這一步，就會報private錯誤
            declaredFields[i].setAccessible(true);
            System.out.println("ziduan:"+declaredFields[i].getName());
           // System.out.println(declaredFields[i].toString());
        }

//獲取構造方法
        System.out.println(c.getDeclaredConstructor(null));
        System.out.println(c.getConstructor(String.class));
        System.out.println(c.getConstructor(String.class,String.class));

        //獲取例項方法的名稱
        System.out.println(c.getMethod("function1").getName());
        System.out.println(c.getMethod("function2", String.class).getName());
        System.out.println(c.getMethod("function3", String.class, String.class).getName());
        //method和invoke；invoke是呼叫例項方法
        Method function1 = c.getDeclaredMethod("function1");
        function1.invoke(o);

        //獲取實體類中的常量值
        Method id= c.getDeclaredMethod("getUid");
        //invoke方法是對有引數的方法進行呼叫
        Object invoke = id.invoke(o);
        System.out.println(invoke);

        Method function3 = c.getDeclaredMethod("function3", String.class, String.class);
        String name=function3.invoke(o,"zhangsan","lisi").toString();
        System.out.println("1111111111");
        System.out.println(name);
    }
}