反射的介绍

1.什么是反射？

反射是框架设计的灵魂，框架:它是一个半成品，可以拿来使用，添加上自己的业务代码。提高开发效率。

反射就是把类中成员抽取成其他类的过程。这就是反射。

2.如何获取反射类对象--Class

有三种:

(1) 通过Class.forName获取反射对象.

Class.forName("全路径")
--spring它就是使用的该模式<bean class="全路径">

（2）通过类名.class获取

类名.class;
---代理类--->SqlSession.getMapper(StudentDao.class)

(3) 通过对象.getClass()方法获取

对象.getClass();
---当知道对象时可以通过这种方式获取反射对象

public class Test01 {
    public static void main(String[] args) throws ClassNotFoundException {
        //1.通过Class.forName来获取反射类对象。
        Class aClass = Class.forName("demo.People");
        //2.通过类名调用.class获取反射类对象
        Class aClass1 = People.class;
        //3.通过对象获取反射类对象
        People p=new People();
        Class aClass2 = p.getClass();
​
        //思考:上面三个反射对象的引用地址是否一致! 是一致的。 一个类只会被加在到内存中一次。
        System.out.println(aClass==aClass1);
        System.out.println(aClass2==aClass1);
    }
}

3.通过反射类获取对应的类对象。

public class Test02 {
    public static void main(String[] args) throws IllegalAccessException, InstantiationException {
        //1.获取反射类对象
        Class<People> aClass = People.class;
        //2.由反射类创建类对象---调用为无参构造函数
        People people = aClass.newInstance();
        People people2 = aClass.newInstance();

        //3.他们的地址是否相同
        System.out.println(people==people2);
    }
}

4.通过反射获取对应的Field属性对象。

public class Test03 {
    public static void main(String[] args) throws Exception {
        Class<?> aClass = Class.forName("demo01.People");
        //获取本类以及父类中指定的属性---必须为public修饰的。
        Field sex = aClass.getField("sex");
        System.out.println(sex);
        //获取本类中指定的属性对象
        Field name = aClass.getDeclaredField("name");
        System.out.println(name);
        //获取本类以及父类中所有public修饰的属性对象
        Field[] fields = aClass.getFields();
        for (Field field:
             fields) {
            System.out.println(field);
        }
        //获取本类中所有的属性对象
        Field[] declaredFields = aClass.getDeclaredFields();
        for (Field field:
             declaredFields) {
            System.out.println(field);
        }

    }
}

4.2Field属性对象中常见的方法。

public class Test04 {
    public static void main(String[] args) throws  Exception{
        Class<People> peopleClass = People.class;
        //获取属性的名称
        Field nameFiled = peopleClass.getDeclaredField("name");

        String name = nameFiled.getName();
        System.out.println(name);
        //为属性赋值
        People people = peopleClass.newInstance();
        System.out.println(people);

        nameFiled.setAccessible(true);
        nameFiled.set(people,"王五");
        System.out.println(people);
        //获取属性值
        Object o = nameFiled.get(people);
        System.out.println(o);
    }
}

5.通过反射获取对应的Method方法对象

public class Test05 {
   public static void main(String[] args) throws  Exception{
        Class<People> peopleClass = People.class;

         //获取本类以及父类中指定名称的方法对象
        Method setName = peopleClass.getMethod("setName", String.class);
        System.out.println(setName);
        System.out.println("*******");
        //获取本类和父类中所有public修饰的方法
        Method[] methods = peopleClass.getMethods();
        for (Method m :
             methods) {
            System.out.println(m);
        }
        System.out.println("+++++++");
       //本类中指定的方法对象
        Method fun = peopleClass.getDeclaredMethod("fun");
        System.out.println(fun);

        System.out.println("=========");
        //获取本类中所有的方法。
        Method[] declaredMethods = peopleClass.getDeclaredMethods();
        for (Method m:declaredMethods
             ) {
            System.out.println(m);
        }
        System.out.println("-------");
        People people = peopleClass.newInstance();
        setName.invoke(people,"zhangsan");
        System.out.println(people);

    }
}

5.2 Method类中常见的方法

        //method.invoke(对象,方法参数值);
         System.out.println("-------");
        People people = peopleClass.newInstance();
        setName.invoke(people,"zhangsan");
        System.out.println(people);

6.获取相应注解对象

TableFiled tableField = declaredField.getAnnotation(TableFiled.class);

 public int insert(T t){
        try{
            StringBuffer sb = new StringBuffer("insert into ");
            Class<?> aClass = t.getClass();

            TableName annotation = aClass.getAnnotation(TableName.class);
            String tableName ="";
            if(annotation!=null){
                tableName = annotation.value();
            }else {
                tableName = aClass.getSimpleName();
            }
            sb.append(tableName);
            System.out.println(sb);
            Field[] declaredFields = aClass.getDeclaredFields();
            List<String> columns = new ArrayList<>();
            List<String> values = new ArrayList<>();
            for (Field declaredField:declaredFields){
                declaredField.setAccessible(true);
                TableFiled tableField = declaredField.getAnnotation(TableFiled.class);
            if(tableField!=null){
                columns.add(tableField.value());
            }else {
                columns.add(declaredField.getName());
            }
            values.add("'"+declaredField.get(t)+"'");
            }
        sb.append(columns.toString().replace("[","(").replace("]",")"));
        sb.append(" values ");
        sb.append(values.toString().replace("[","(").replace("]",")"));
            System.out.println(sb);
            Connection conn = DBUtils.getConn();
            PreparedStatement ps = conn.prepareStatement(sb.toString());
            int i = ps.executeUpdate();
            return i;
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            DBUtils.closeAll(rs,ps,connection);
        }
        return 0;
    }
}