summary

Reflection is a mechanism , Using this mechanism, we can dissect the class and manipulate all the members of the class during the running process of the program ( Member variables , Member method , Construction method );
The premise of using reflection is to obtain this kind of bytecode file object , Namely Class object .

1、class Object

*  The way 1:  By class name .class get  
*  The way 2： By object name .getClass() Methods to get 
*  The way 3： adopt Class Class to get ： static Class forName(" Class full name ") 
 Be careful ：  Of each class Class There's only one object .
 Code example ：
public class Demo01 {
    
    public static void main(String[] args) throws ClassNotFoundException {
    
        //  get Girls Class Class object 
        Class c1 = Girls.class;

        //  Create a girl object 
        Girls g = new Girls();
        //  adopt getClass Method 
        Class c2 = g.getClass();
        System.out.println(c1 == c2);

        //  adopt Class Class to get ： static Class forName(" Class full name ")
        Class c3 = Class.forName("com.girl.Girls");
        System.out.println(c1 == c3);
        System.out.println(c2 == c3);
    }
}

class Common methods of objects ：

* String getSimpleName();  Get the class name string ： Class name 
* String getName();   Get full class name ： Package name + Class name 
* T newInstance() ;   Get an instance object 
 Code example ：
public class Demo02 {
    
    public static void main(String[] args) throws Exception {
    
        //  get Class object 
        Class c = Girls.class;
        //  Get the class name string ： Class name 
        System.out.println(c.getSimpleName());
        //  Get full class name ： Package name + Class name 
        System.out.println(c.getName());
        //  Create objects 
        Student stu = (Student) c.newInstance();
        System.out.println(stu);
    }
}

2、 Get the construction method through reflection

class Object gets the construction method 
1. Constructor getConstructor(Class... parameterTypes)
        *  Get the corresponding... According to the parameter type Constructor object .
        *  You can only get public Modifies the constructor 
2. Constructor getDeclaredConstructor(Class... parameterTypes)
         *  Get the corresponding... According to the parameter type Constructor object , Include private
3. Constructor[] getConstructors()
         Get all constructor objects in the class , You can only get public Of 
4. Constructor[] getDeclaredConstructors()
         Get all constructor objects in the class , Include private Embellished 

Common methods for constructing method objects ：

1. T newInstance(Object... initargs)
 	 Creates an object based on the specified parameters 
2. void setAccessible(true)
    Set whether to cancel permission check ,true Cancel permission check ( Violent reflex ),false Means not to cancel 
 Code example ：
public class Demo03 {
    

    @Test
    public void test1() throws Exception {
    
        //  get Class object 
        Class c = Girls.class;
        //  Get two parameters to construct the method object 
        Constructor con = c.getDeclaredConstructor(String.class,String.class);
        //  Cancel permission check ( Violent reflex )
        con.setAccessible(true);
        //  Create objects based on construction methods 
        Object obj = con.newInstance(" Zhang San "," Woman ");
        System.out.println(obj);
    }

    @Test
    public void test2() throws Exception {
    
        //  get Class object 
        Class c = Student.class;
        //  Get the parameterless constructor object 
        Constructor con = c.getConstructor();
        //  Create objects based on construction methods 
        Object obj = con.newInstance();
        System.out.println(obj);
        //  Get the constructor object with parameters 
        Constructor con2 = c.getConstructor(String.class, String.class,int.class);
        //  Create objects   The order of parameters is consistent with the order of attributes in the class 
        Object obj2 = con2.newInstance("jack", " male ",18);
        System.out.println(obj2);
    }
}

3、 Get member methods through reflection

Every member method is a Method Class object , therefore , When we get class Object time , Get by reflection method object , You can call the corresponding member method .

* Method getMethod(String name,Class...args);
    *  Obtain the corresponding construction method object according to the method name and parameter type , You can only get public Of 

* Method getDeclaredMethod(String name,Class...args);
    *  Obtain the corresponding construction method object according to the method name and parameter type , Include private Of 

* Method[] getMethods();
    *  Get all member method objects in the class , Returns an array of , You can only get public Decorated and containing a parent class 

* Method[] getDeclaredMethods();
    *  Get all member method objects in the class , Returns an array of , Only get this kind of , contain private Embellished 

method Common methods of objects ：

*  Object invoke(Object obj, Object... args)
    *  Call the specified object obj The method of 
    * args： Parameters passed when calling methods 
*  void setAccessible(true)
     Set whether to cancel permission check ,true Cancel permission check ( Violent reflex ),false Means not to cancel 

Code example

public class Demo04 {
    

    //  Reflection operation static method 
    @Test
    public void test01() throws Exception {
    
        //  get Class object 
        Class c = Girls.class;
        //  Get the corresponding member method object according to the method name 
        Method method = c.getDeclaredMethod("work",String.class);
        //  adopt method Execute the corresponding method 
        method.invoke(null," Write code ");
    }

    /* * Method[] getMethods(); *  Get all member method objects in the class , Returns an array of , You can only get public Decorated and containing a parent class  * Method[] getDeclaredMethods(); *  Get all member method objects in the class , Returns an array of , Only get this kind of , contain private Embellished  */
    @Test
    public void test02() throws Exception {
    
        //  get Class object 
        Class c = Student.class;
        //  Get all member method objects in the class , Returns an array of , You can only get public Decorated and containing a parent class 
        // Method[] methods = c.getMethods();
        //  Get all member method objects in the class , Returns an array of , Only get this kind of , contain private Embellished 
        Method[] methods = c.getDeclaredMethods();
        for (Method m: methods) {
    
            System.out.println(m);
        }

    }

    /* Method getDeclaredMethod(String name,Class...args); *  Obtain the corresponding construction method object according to the method name and parameter type , */
    @Test
    public void test03() throws Exception {
    
        //  get Class object 
        Class c = Girls.class;
        Girls girl = (Girls) c.newInstance();
        //  get sleep Method corresponding to Method object 
        Method m =  c.getDeclaredMethod("sleep");
        //  Violent reflex 
        m.setAccessible(true);
        m.invoke(girl);
    }

    @Test
    public void test04() throws Exception {
    
        //  get Class object 
        Class c = Girls.class;
        Girls girl = (Girls) c.newInstance();
        Method m =  c.getMethod("study");
        //  adopt m Object to perform stuy Method 
        m.invoke(girl);
        ///  get study Method corresponding to Method object 
        Method m2  = c.getMethod("study", int.class);
        //  adopt m2 Object to perform stuy Method 
        m2.invoke(girl,8);
    }
}

4、 Reflection get member variable

 The purpose of reflecting the operation of member variables 
    *  adopt Field Object to assign and value the corresponding member variable 

Field Class Overview 
    *  Every member variable is a Field Class object .

class Object and Field Related methods

* Field getField(String name);
    *   Get the corresponding value according to the member variable name Field object , You can only get public modification 
* Field getDeclaredField(String name);
    *   Get the corresponding value according to the member variable name Field object , contain private Embellished 
* Field[] getFields();
    *  Get the corresponding values of all member variables Field object , You can only get public Of 
* Field[] getDeclaredFields();
    *  Get the corresponding values of all member variables Field object , contain private Of 

field Common methods of objects

void  set(Object obj, Object value) 
void setInt(Object obj, int i) 	
void setLong(Object obj, long l)
void setBoolean(Object obj, boolean z) 
void setDouble(Object obj, double d) 

Object get(Object obj)  
int	getInt(Object obj) 
long getLong(Object obj) 
boolean getBoolean(Object ob)
double getDouble(Object obj)

Code example ：

public class Demo05 {
    
    @Test
    public void test01() throws Exception {
    
        //  get Class object 
        Class c  = Girls.class;
        //  Get the corresponding values of all member variables Field object 
        // Field[] fields = c.getFields();
        //  Get the corresponding values of all member variables Field object , Include private
        Field[] fields = c.getDeclaredFields();
        for (Field f: fields) {
    
            System.out.println(f);
        }
    }

    @Test
    public void test02() throws Exception {
    
        //  get Class object 
        Class c  = Girls.class;
        //  Create objects 
        Object obj = c.newInstance();
        //  Get member variable name Corresponding Field object 
        Field f = c.getField("name");
        //  Give member variables name assignment 
        //  Assign objects to obj Of name Attribute is assigned wisdom 
        f.set(obj," wisdom ");
        System.out.println(f.get(obj)); //  wisdom 
        //  Get the name of the member variable 
        System.out.println(f.getName()); // name
        //  Give member variables gender assignment 
        //  Get member variable gender Corresponding Field object 
        Field f1 = c.getDeclaredField("gender");
        //  Violent reflex 
        f1.setAccessible(true);
        //  Assign objects to obj Of gender Attribute assignment is male 
        f1.set(obj," male ");
        System.out.println(obj);

    }
}

5、 The effect of reflection

I talked so much about the use of reflection , that , What is the function of reflection ？
You can get all the components of a class at run time and then operate .
Can destroy encapsulation . You can also break the constraints of generics .
A more important use is suitable for ： do Java Advanced framework , Basically, mainstream frameworks will design some general technical functions based on reflection .
such as ：
Mybatis frame ：
If you give any object data, I can directly help you parse the fields and save the corresponding data .
Student ( register , Store all the information fields )
Teacher ( register , Store all the information fields )
Manager ( register , Store all the information fields )
To make a long story short , Reflection is suitable for the bottom implementation of general technology framework , We often see the shadow of reflection in the underlying source code of the framework .