Simple summary of reflection

Reflection: obtain the relevant information of the Class to which the Object belongs through the Class Object of the Object, so as to realize various operations on the Class.

1, Method to get Class object

1. Class. Forname (fully qualified class name)

Fully qualified class name=Package name+Don't write the class name wrong here, otherwise it will be reported ClassNotFoundException
 For example: com.company.day23_reflect.Student
Class personCls = Class.forName("com.company.reflectdemo.demo1.Person");

2. Class name.class

Class personCls2 = Person.class;

3. Object. getClass()

Person person = new Person();
Class personCls3 = person.getClass();

2, Use of reflection

Reflection is often used with configuration files. Here is a brief introduction to the use of the Properties class.
Member method

Return value	method	explain
void	load(InputStream inStream)	Read the attribute list (key value pair) from the input stream.
void	load(Reader reader)	Read the attribute list (key value pair) from the input character stream in a simple line oriented format.
String	getProperty(String key)	Search for attributes in this attribute list with the specified key.

// Create Properties object
Properties properties = new Properties();
//There are two loading methods for loading the properties configuration file. Generally, there is no Chinese. The first parameter is the load method of InputStream type.
//If there are non ASCII characters in the configuration file, such as Chinese, you need to use the load method of the second Reader type parameter to specify the read character set
properties.load(new InputStreamReader(new FileInputStream("dict.properties"),"gbk"));

Three aspects of information can be obtained through Class objects
1. Construction method

method	effect
getConstructor(Class<?>... parameterTypes)	Gets a public constructor object
getConstructors()	Get all public constructor objects
getDeclaredConstructor(Class<?>... parameterTypes)	Gets a constructor object
getDeclaredConstructors()	Get all constructor objects

2. Member variables

method	effect
getField(String name)	Gets a public member variable object
getFields()	Get all public member variable objects
getDeclaredField(String name)	Get a member variable object (including private)
getDeclaredFields()	Get all member variable objects (including private)

3. Membership method

method	effect
getMethod(String methodName, Class<?>... parameterTypes)	Get a public method
getMethods()	Get all public methods
getDeclaredMethod(String methodName, Class<?>... parameterTypes)	Get a method
getDeclaredMethod()	Get all methods

Setaccessible (true) is the method to release private permission restrictions
Simple summary

• Get type to get a public type object
• gets type to get all public type objects
• getDeclared type, get a type object (can be private)
• Getdeclarations type, get all types of objects (including private)

Simple to use: the method of executing objects by reading configuration files

//configuration file
className=com.company.day23_reflect.Student
methodName=doSomething

public class Work2 {
    /**
     *This method creates a new object instance by reading the content of the configuration file, so as to execute the ordinary parameterless method of the object
     * @param configFilePath Represents the configuration file path
     */
    public void callTargetMethod(String configFilePath) throws Exception {
        Properties properties = new Properties();
        properties.load(new FileInputStream(configFilePath));
        //Get the method name and object name. The class name in the configuration file needs to write the fully qualified class name
        String className = properties.getProperty("className");
        String methodName = properties.getProperty("methodName");
        //Get Class objects, default parameterless construction methods, and specify normal parameterless methods (methods should preferably be set to be accessible)
        Class clazz = Class.forName(className);
        Constructor constructor = clazz.getConstructor();
        Method method = clazz.getMethod(methodName);
        method.setAccessible(true);
        Object o = constructor.newInstance();
        method.invoke(o);
    }
}

//Test class
//Try to write relative paths to reduce trouble
public class Test2 {
    public static void main(String[] args) throws Exception{
        new Work2().callTargetMethod("src\\com\\company\\reflectdemo\\dict.properties");
    }
}