Java basics: Reflection

Reflection is to map each component in the Java class into a java object. Using Reflection technology, you can dissect a class and map each component into objects. The biggest use of reflection is in framework learning.

 

How to obtain the class object corresponding to a class file:

1) class name. class

2) object. getClass ()

3) Class. forName ("Class Name") the Class name must use the fully qualified name of the Class.

// Class name. classClass c1 = String. class; // object. getClass () Class c2 = new String (). getClass (); // Class. forName () Class c3 = Class. forName ("java. lang. string ");

Constructor class:

The instance object of this class represents a constructor of the class.

Use the Constructor class to instantiate an object. The demo is as follows:

 

(1) web. java. reflect. Person class

package web.java.reflect;public class Person {private String name;private Person(){}private Person(String name) {super();this.name = name;}public String getName() {return name;}public void setName(String name) {this.name = name;}public void show(){System.out.println(this.getName()+"is a person");}}

(2) instantiate an object Using Reflection

Package web. java. reflect; import java. lang. reflect. constructor; public class Demo2 {public static void main (String [] args) throws Exception {Class clazz = Class. forName ("web. java. reflect. person "); // obtain the non-argument Constructor c = clazz. getDeclaredConstructor (null); // sets the access of non-public members. setAccessible (true); // create an instance object of this class and convert it to Person p = (Person) c. newInstance (null); // Method p in the execution class. show ();}}

In addition, for non-private constructor methods, you can use getConstructors () to obtain all or getConstructor (***) to obtain the number of specified parameters.

 

Field class:

It represents a member variable in a class.

Package web. java. reflect; import java. lang. reflect. field; public class Demo3 {public static void main (String [] args) throws Exception {Person p = new Person (); // The premise is: the preceding construction method without parameters is default Class c = Class. forName ("web. java. reflect. person "); // obtain the specified private Field f = c. getDeclaredField ("name"); f. setAccessible (true); f. set (p, "Tom"); // equivalent to p. setName ("Tom"); System. out. println (p. getName ());}}

Method class:

It represents a member method in a class.

public class Demo4 {public static void main(String[] args) throws Exception {Class c = Class.forName("cn.itcast.java.reflect.Person");Constructor con = c.getConstructor(null);Person p = (Person) con.newInstance(null);Method m = c.getDeclaredMethod("goodbye",null);m.setAccessible(true);String returnValue = (String) m.invoke(p,null);System.out.println(returnValue);//Method m = c.getMethod("show",String[].class,double.class);//m.invoke(p,new String[]{"sing","dance"},5000);//new Person().show()}}