Reflection Learning Summary--preparing for understanding the SPRINGMVC bottom

What is reflection?

Popular Understanding-X-ray.

Java: A class is like an X-ray in front of reflection, clearly and plainly.

Application: In our IDE, we are able to "." You know that all the methods in a class are implemented by reflection.

The class full path dynamically generated classes configured in XML. Framework

The program creates objects dynamically during the run.

--As long as you know the name of the class, you can use his bytecode object to create an object of that class.

What is a class object:

Helloworld.java

by Javac Helloworld.java----->helloworld.class (bytecode file)---> java HelloWorld JVM loads helloworld.class into memory and becomes class pair Like

　　

About the loading of class:

public static void Main (string[] args) {person        p = new Person ();        Class Pclassclass = P.getclass ();        System.out.println (pclassclass);        person person = new person ("AA", "+", "USA");        Class Personclass  = Person.getclass ();        System.out.println (personclass);        System.out.println (Pclassclass = = Personclass); True bytecode files are loaded into memory only once during program run    }

　　

Three ways to get class: P.getclass (), Person.class, Class.forName ("Class name full path")

 Public Static voidMain (string[] args)throwsclassnotfoundexception {person P=NewPerson (); Class P1=P.getclass (); Person Person=NewPerson ("AA", "+", "USA"); Class P2=Person.getclass (); System.out.println (P1==p2); Class<Person> P3 = person.class; SYSTEM.OUT.PRINTLN (P3==p1); Class<?> P4 = Class.forName ("Com.java.demo.clazz.Person"); System.out.println (P4==p3); }

Either way, there's only one byte code

Use class

First look at the relationship of class and bytecode objects

　　

About Construction methods:

Mode one: Using the Newinstance () method

Class.newinstance (), the newinstance () layer needs to call the parameterless constructor, so we need to have an argument-free constructor in our reflected class.

     Public Static void throws illegalaccessexception, Instantiationexception, classnotfoundexception {        // via byte-code object, Create an object        class<?> aclass = Class.forName ("Com.java.demo.clazz.Person");         = (person) aclass.newinstance ();    }

Way two: With constructor, this time, the method of constructing the null parameter is not necessary.

     Public Static void throws classnotfoundexception, Nosuchmethodexception, Illegalaccessexception, InvocationTargetException, instantiationexception {        Class<?> aclass = Class.forName ("Com.java.demo.clazz.Person");        Constructor<?> Constructor = Aclass.getconstructor (String.  Class, String. class, String. class );         = (person) constructor.newinstance ("AA", "+", "USA");    

Enhanced version of Mode two: When using the Getdeclaredconstructor () method, Setaccessible (true), even if his constructor is private, he can create the object (he can get everything, including public private).

     Public Static voidMain (string[] args)throwsclassnotfoundexception, Nosuchmethodexception, Illegalaccessexception, InvocationTargetException, instantiationexception {Class<?> AClass = Class.forName ("Com.java.demo.clazz.Person"); Constructor<?> constructor = Aclass.getdeclaredconstructor (String.class, String.class, String.class); Constructor.setaccessible (true);//violent visit to Illegalaccessexceptionperson instance = (person) constructor.newinstance ("AA", "+", "USA")); }

Get field (member variable)

As long as it's declared, you can get all the declared variables.

 Public Static void Main (string[] args) {        Class<Person> personclass = person.  Class;         // This can only be obtained in the public         for (Field f:fields) {            System.out.println (f);        }         // This can be obtained to all the         for (Field f:declaredfields) {            System.out.println (f);        }    }

Get Method (Methods in Class)

As long as it's declared, you can get all the way to the declaration.

 Public Static void Main (string[] args) {        Class<Person> personclass = person.  Class;         = personclass.getdeclaredmethods ();          for (method Method:declaredmethods) {            System.out.println (method);        }    }

Invocation of Method methods

     Public Static voidMain (string[] args)throwsillegalaccessexception, Instantiationexception, InvocationTargetException, nosuchmethodexception {Class<Person> Personclass = person.class; Person Person=personclass.newinstance (); Method Spark= Personclass.getdeclaredmethod ("Spark", String.class); //execution method, invoke, two parameters//the first parameter method belongs to that object, and the second object methods parameterSpark.setaccessible (true); Spark.invoke (person,"HeLLO Gril"); }

Reflection Learning Summary--preparing for understanding the SPRINGMVC bottom