What can I do with a Class object?

• Create Class object: call newInstance() method of Class object

  • Class must have a parameterless constructor
  • The constructor of the class should have sufficient access rights

  //Get Class object
  Class c1 = Class.forName("Annotation and reflection.User");
  //Construct an object
  User user = (User) c1.newInstance();
  System.out.println(user);
  

Can't you create an object without a parameterless constructor? As long as the constructor in the class is explicitly called during the operation and the parameters are passed in, the operation can be instantiated.

• The steps are as follows:

  • Get the specified parameter types of this Class through getdeclaraedconstructor (Class... paramerTypes) of Class class
  • Pass an object array into the formal parameters of the constructor, which contains all the parameters required by the constructor
  • Instantiate objects through Constructor

  //Get Class object
  Class c1 = Class.forName("Annotation and reflection.User");
  //Create an object through a constructor
  Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
  User user2 = (User) constructor.newInstance("Zhang San", 1, 18);
  System.out.println(user2);
  

Call the specified method

Through reflection, the Method in the class is called and completed through the Method class.

• Get a Method object through the getMethod(String name,Class... parameterTypes) Method of Class class and set the parameter type required for the operation of this Method
• Then use Object invoke(Object obj,Object[] args) to call and pass the parameter information of the obj object to be set to the method.

• Object invoke(Object obj,Object ... args)
  • object corresponds to the return value of the original method. If the original method has no return value, null is returned
  • If the original method is a static method, the formal parameter Object obj can be null at this time
  • If the original method parameter list is empty, Object[] args is null
  • If the original method is declared as private, you need to explicitly call the setAccessible(true) method of the method object before calling the invoke() method to access the private method.
• setAccessible
  • Method, Field and Constructor objects all have setAccessible() methods
  • setAccessible is a switch that enables and disables access security checks
  • If the parameter value is true, it indicates that the Java language access check should be cancelled when the reflected object is used
    • Improve reflection efficiency. If reflection must be used in the code and the code of this sentence needs to be called frequently, please set it to true
    • So that private members that cannot be accessed can also be accessed
  • If the parameter value is false, it indicates that the reflected object should implement Java language access check

Code demonstration:

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

//Create objects dynamically
public class Test05 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //Normal new an object
        //User user1 = new User();
        //System.out.println(user1);

        //Get Class object
        //Class c1 = Class.forName("annotation and reflection. User");
        //Construct an object
        //User user = (User) c1.newInstance();
        //System.out.println(user);

        //Create an object through a constructor
        //Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        //User user2 = (User) constructor.newInstance("Zhang San", 1, 18);
        //System.out.println(user2);

		System.out.println("---------------------------------------------");
        Class c1 = Class.forName("Annotation and reflection.User");
        User user3 = (User) c1.newInstance();

        //Call normal methods through reflection
        //Get a method by reflection
        Method setName = c1.getDeclaredMethod("setName", String.class);
        //Activate: invoke (object, "method value")
        setName.invoke(user3,"Li Si");
        System.out.println(user3.getName());

        //Operation properties by reflection
        User user4 = (User) c1.newInstance();
        Field name = c1.getDeclaredField("name");
        //The properties in User are private and inaccessible by default
        //setAccessible(): set accessibility
        //setAccessible(): set a method to true to access directly
        name.setAccessible(true);
        name.set(user4,"Wang Wu");
        System.out.println(user4.getName());
    }
}