Java Reflection Mechanism Learning notes and example code

API for Java Reflection

Several classes that are often used in reflection are as follows:

In the Java.lang package:

--class class: Represents a Class

In Java, regardless of how many objects of a class are generated, these objects will correspond to the same class object

In the Java.lang.reflect package:

--field class: A member variable representing a class (a property of a class)

--method class: A method that represents a class, a method that corresponds to an object

--constructor class: A method of constructing a representative class

--array class: Provides a static method for dynamically creating an array, and for accessing the elements of an array

To invoke a method in a class by using the reflection mechanism:

1. Get the Class object

There are three common methods:

A) obtained through the static method of class forname (parameter: Package name + class name):

class<?> ClassType = Class.forName ("java.lang.String");

b) obtained by means of the class name + point class:

class<?> C = string.class;

Obtained by means of the getclass of the object of the class:

String str = "Test";

class<?> Typeclass = Str.getclass ();

2. Instantiate an object of this class

Two methods:

A) created by the Newinstance method of the Class object (this method applies only to the parameterless construction method):

class<?> ClassType = Class.forName ("java.lang.String");

Object obj = = classtype.newinstance ();

b) By getting the object of the Constructor constructor class, and then by the object's Newinstance (Object ... Initargs) method, obtain:

class<?> ClassType = Cus.getclass ();

Constructor Con

= Classtype.getconstructor (New Class[]{string.class,int.class});

Object Object

= Con.newinstance (New Object[]{cus.getname (), Cus.getage ()});

The method that writes two parameters as class[]{} and new object[]{} is equivalent to a)

3. Get the method object for the target methods

Method method = Classtype.getmethod (methodname,new class[]{parameter type. Class});

4. Use the method object to invoke the target approach:

Object result = Method.invoke (object,new object[]{});

Where object is the one that owns the method, object[]{} is the parameter of the method

5. Get the property value in the class, get value, set assignment

Get Property List

Field fields[] = Classtype.getdeclaredfields ();

Field field = Classtype.getdeclaredfields (attribute name);

Gets the property name corresponding to the Field object

Field.set (Object of class, value);

Assigning a new value to a property

Example: Accessing private-modified methods and properties

Use the reflection mechanism to access the following classes:

 Public class private{    Private" Zhang San ";         Private String getName ()    {        return  name;}    }

Change the name value of the private modifier in the class to "John Doe" and call the GetName method with the reflection mechanism to return the modified value.

Package Com.wangzhuo.reflect;import Java.lang.reflect.constructor;import java.lang.reflect.field;import Java.lang.reflect.Method; Public classprivatetest{ Public Static voidMain (string[] args) throws Exception {//get the class object for the private class&nbsp; class<?> ClassType = Class.forName ("com.wangzhuo.reflect.Private"); &nbsp;//gets the constructor object that corresponds to its construction methodConstructor con = classtype.getdeclaredconstructor (Newclass[]{}); &nbsp;//Create a Private objectObjectObject=con.newinstance (Newobject[]{});&nbsp;//Gets the Field object that corresponds to the Name property in the private classField field = Classtype.getdeclaredfield ("name");&nbsp;//Set bypass Java access control detectionField.setaccessible (true); &nbsp;//gets the value before the modificationObject str = field.Get(Object); &nbsp; System. out. println ("Modify the value of the previous name:"+(String) str); &nbsp;//Assigning a value to the Name propertyField.Set(Object,"John Doe"); &nbsp;//gets the method object corresponding to the GetName methodsMethod Getnamemethod = Classtype.getdeclaredmethod ("GetName",Newclass[]{}); &nbsp;//Set bypass Java access control detectionGetnamemethod.setaccessible (true); &nbsp;//call method, return valueObject o = Getnamemethod.invoke (Object,Newobject[]{}); System. out. println ("the value of name after modification:"+(String) o); }}

Java Reflection Mechanism Learning notes and example code