Chuanzhi Podcast Learning java reflection_javascript skills

In addition, Teacher Bi explained the reflection mechanism to us. Although we may not necessarily involve reflection in the development process in the future, reflection is indeed a powerful tool in Java. It allows us to create flexible code that can be executed at runtime. Assembly without source representative linking between components. For example, in the Tomcat class loader, this mechanism is used by many frameworks.
1. What is reflection
The concept of reflection was first proposed by Smith in 1982. It mainly refers to the ability of a program to access, detect and modify its own state behavior. Reflection is one of the features of the Java program development language. It also allows the running Java program to check itself, or "self-audit", and directly operate the internal properties of the program.
2. Classes required for java reflection
1.Class class: Its instances represent classes and interfaces in the running java application
2.Constructor class: Provides a single constructor method for a class information and access to him.
3.Method class: Provides information about a single method of a class or interface
4.Array class: Provides static methods for dynamically creating arrays or accessing arrays.
5.Field class: Provides information about the properties of a class or interface, as well as dynamic access rights to it.
3. The function of reflection
We have listed the relevant classes above, so what can we do specifically with these classes?
1) Get the Class object of the specified class
Method 1, through the object.getClass. (Example: Person p; Class c=p.getClass();)
Method 2, through the forName method of the Class class (Example: Class c=Class.forName(class name);)
Method 3: Obtain through type name.class (Class c = int.class)
2) Construct an object of any class at runtime, The steps are as follows:
① Create its object based on the class name
② Return the newly created object
l Using the no-argument constructor, you only need to use the newInstance method of the class object.
Class c=Class.forName(class name); Person p =(Person)c.newInstance;
l Use the parameterized constructor, the steps are as follows:
① Obtain the object of the specified class (same as above)
② Obtain the constructor class object that meets the requirements of the specified parameters through the Class object
Constructor cons = c.getConstructor(String.class)
③ Call the newInstance method of the specified Constructor object and pass in the corresponding parameters value to create the object.
Person p = (Person) cons.newInstance ("Zhang San");
3) Call the method of any object at runtime, and you can even call the private method through reflection
① Get the specified class Object (same as above)
② Create an instance as required
③ Obtain the instance of Method through getDeclaredMethods of the Class object (you can select different methods as needed), and call the method through the invoke method
Method m=c.getDeclaredMethods( "setName", String.class);
returnvalue = m.invoke(p,"zhangsan");
Note: If it is a private method, you can first call setAccessible(true) of the Method object corresponding to this method. Cancel the security check on this method
4) Call the properties of any object at runtime
① Obtain the object of the specified class (same as above)
② Create an instance as required
③ Pass the getDeclaredField of the Class object (You can choose different methods as needed) Obtain an instance of the Field class, modify the value of the specified object through the Set method, and get the value of the object.
Field f =c.getDeclaredField("name");
f.set(p,"Zhang San"); System.out.println(f.get(p));