Introduction to Java reflection mechanism

Reflection: the soul of frame design

  • Framework: semi-finished software, which can be developed on the basis of framework and simplify code
  • Reflection: encapsulates the components of a class as other objects, which is the reflection mechanism
    • Benefits:
    1. These objects can be manipulated during program execution
    2. It can be decoupled to improve the scalability of the program

**Java code goes through three stages in the computer:
Source code stage, Class object stage, Runtime runtime stage

  • How to get the Class object:

    1. Class.forname: load the bytecode file into memory and return the class object
    • Most of them are used for configuration files, and the class name is defined in the configuration file. Read file, load class
    1. Class name. Class: obtained by the attribute class of class name
    • Mostly used for parameter passing
    1. Object. getClass(): the getClass() method defines the
    • How to get bytecode for objects
  • Conclusion:
    The same bytecode file (*. class) will only be loaded once during a program run, and the class object obtained in any way is the same.

  • Class object function (get function):

    1. Get member variable
      • Field[] getFields(): get all public decorated member variables
      • Field getField(String name): gets the public decorated member variable of the specified name
      • Field[] getDeclaredFields(): get all member variables without considering modifiers
      • Field getDeclaredField(String name): gets the member variable with the specified name
    2. Get construction method
      • Constructor<?>[] getConstructors()
      • Constructor getconstructor (class <? > parameterTypes)
      • Constructor<?>[] getDeclaredConstructors()
      • Constructor getdeclaredconstructor (class <? > parameterTypes)
    3. Get member method
      • Method[] getMethods()
      • Method getmethod(String name, class <? > parameterTypes)
      • Method[] getDeclaredMethods()
      • Method getDeclaredmethod(String name, class <? > parameterTypes)
    4. Get class name
      • String getName()
  • Field: member variable

    • Operation:
    1. Set value void set(Object obj,Object value)
    2. Get value get(Object obj)
    3. Ignore security check for access modifier
      • setAccessible(true): violent reflection
  • Constructor: construction method

    • Create object:
    • T newInstance(Object... initargs)
    • If you use the empty parameter construction method to create an object, the operation can be simplified:
      newInstance of Class object
  • Method: Method Object
    *Execution method:
    *Object invoke(Object obj, Object... args)

  • Get method name:

  • String getName: get method name

  • Case study:

    • Requirement: write a "framework" that can help us create objects of any class and execute any methods of any class without changing any code of the class
    • Realization:
    1. configuration file
    2. reflex
    • Steps:
    1. Define the full class name of the object to be created and the method to be executed in the configuration file
    2. Load read profile in program
    3. Using reflection technology to load class files into memory
    4. create object
    5. Execution method
  • Case code (get Class object function):

  1. Create a class
package product;

public class Person {
    public String name = "Zhang classmate";
    public void eat(){
        System.out.println("Person eating...");
  1. Create a Class that can get the function of Class object
package product;

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

public class ClassObject function {
    public static void main(String[] args) throws Exception {
        //Get the Class object of Person
        Class PersonClass = Person.class;
        Method[] b = PersonClass.getDeclaredMethods();
        for (Method e:b) {
        //Get the value of member variable name
        Field c = PersonClass.getField("name");
        Person p = new Person();
        Object d = c.get(p);

3. Test results:

  • Case code (Note: the profile must be at the same level as the package):
  1. Create a class to test
package product;
public class Person {
    public void eat(){
        System.out.println("Person eating...");
  1. Create a configuration file (file suffix is. properties)
className = product.Person   //Full class name of test class
methodName = eat	//Methods to be tested

3. Create a class that can load configuration files

package product;
import java.lang.reflect.Method;
import java.util.Properties;

public class reflectTest {
    public static void main(String[] args) throws Exception {
        //1 load profile
        //1.1 create Properties object
        Properties pro = new Properties();
        //1.2 loading configuration files, converting to a collection
        //1.2.1 get the configuration file under the class directory
        ClassLoader classLoader = reflectTest.class.getClassLoader() ;
        InputStream is = classLoader.getResourceAsStream("");
        //2 get the data defined in the configuration file
        String className = pro.getProperty("className");
        String methodName = pro.getProperty("methodName");
        //3 load this class and enter memory
        Class cls = Class.forName(className);
        //4 creating objects
        Object obj = cls.newInstance();
        //5 get method object
        Method method = cls.getMethod(methodName);
        //6 execution method
  1. Test results:
    Person eating...

Keywords: Java Attribute

Added by billcoker on Sun, 12 Apr 2020 12:08:37 +0300