Introduction to reflection

1. What is reflection ？

Reflection is the soul of frame design , frame : It's a semi-finished product , It can be used , Add your own business code . Improve development efficiency .

Reflection is the process of extracting members from a class into other classes . This is reflection .

2. How to get reflection class objects --Class

There are three kinds of :

(1) adopt Class.forName Get the reflection object .

Class.forName(" The full path ")
--spring It is the mode used <bean class=" The full path ">

（2） By class name .class obtain

 Class name .class;
--- proxy class --->SqlSession.getMapper(StudentDao.class)

(3) Through object .getClass() Method to get

 object .getClass();
--- When you know the object, you can get the reflection object in this way 

public class Test01 {
    public static void main(String[] args) throws ClassNotFoundException {
        //1. adopt Class.forName To get reflection class objects .
        Class aClass = Class.forName("demo.People");
        //2. Called by the class name .class Get the reflection class object 
        Class aClass1 = People.class;
        //3. Get reflection class objects through objects 
        People p=new People();
        Class aClass2 = p.getClass();
​
        // reflection : Whether the reference addresses of the above three reflection objects are consistent !  It's consistent .  A class will only be added to memory once .
        System.out.println(aClass==aClass1);
        System.out.println(aClass2==aClass1);
    }
}

3. Get the corresponding class object through the reflection class .

public class Test02 {
    public static void main(String[] args) throws IllegalAccessException, InstantiationException {
        //1. Get the reflection class object 
        Class<People> aClass = People.class;
        //2. Create class objects from reflection classes --- Call a parameterless constructor 
        People people = aClass.newInstance();
        People people2 = aClass.newInstance();

        //3. Are their addresses the same 
        System.out.println(people==people2);
    }
}

4. Get the corresponding... Through reflection Field Property object .

public class Test03 {
    public static void main(String[] args) throws Exception {
        Class<?> aClass = Class.forName("demo01.People");
        // Get the attributes specified in this class and its parent class --- It has to be for public Embellished .
        Field sex = aClass.getField("sex");
        System.out.println(sex);
        // Get the property object specified in this class 
        Field name = aClass.getDeclaredField("name");
        System.out.println(name);
        // Get all in this class and its parent class public Decorated property object 
        Field[] fields = aClass.getFields();
        for (Field field:
             fields) {
            System.out.println(field);
        }
        // Get all property objects in this class 
        Field[] declaredFields = aClass.getDeclaredFields();
        for (Field field:
             declaredFields) {
            System.out.println(field);
        }

    }
}

4.2Field Common methods in attribute objects .

public class Test04 {
    public static void main(String[] args) throws  Exception{
        Class<People> peopleClass = People.class;
        // Gets the name of the property 
        Field nameFiled = peopleClass.getDeclaredField("name");

        String name = nameFiled.getName();
        System.out.println(name);
        // Assign values to properties 
        People people = peopleClass.newInstance();
        System.out.println(people);

        nameFiled.setAccessible(true);
        nameFiled.set(people," Wang Wu ");
        System.out.println(people);
        // Get attribute value 
        Object o = nameFiled.get(people);
        System.out.println(o);
    }
}

5. Get the corresponding... Through reflection Method Method object

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

         // Get the method object with the specified name in this class and its parent class 
        Method setName = peopleClass.getMethod("setName", String.class);
        System.out.println(setName);
        System.out.println("*******");
        // Get all in this class and parent class public The method of decoration 
        Method[] methods = peopleClass.getMethods();
        for (Method m :
             methods) {
            System.out.println(m);
        }
        System.out.println("+++++++");
       // The method object specified in this class 
        Method fun = peopleClass.getDeclaredMethod("fun");
        System.out.println(fun);

        System.out.println("=========");
        // Get all the methods in this class .
        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 Class

        //method.invoke( object , Method parameter value );
         System.out.println("-------");
        People people = peopleClass.newInstance();
        setName.invoke(people,"zhangsan");
        System.out.println(people);

6. Get the corresponding annotation object

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;
    }
}