Reflection mechanism of JavaSE review summary

reflection concept

The concept of reflection was first proposed by Smith in 1982. It mainly refers to the ability of a program to access, detect and modify its own state or behavior, and to adjust or modify the behavior described by the application according to the state and result of its own behavior. state and associated semantics.

How reflection works

When the program is running, the Java system will always perform so-called runtime type recognition on all objects, which records the class to which each object belongs. This information is accessible through specialized classes. The class used to save this information is the class class, which can manipulate bytecode files through the reflection mechanism in the Java language

Three ways to get Class

• The first: Class c = Class.forName("full class name");
• The second: Class c = object.getClass();
• The third type: Class c = any type.class;

What can I do with the Class

1. Instantiate the object

	//不使用反射机制创建对象
	User user = new User();
	System.out.println(user);
	//使用反射机制创建对象
	try {
    
    
		// 通过反射机制，获取Class，通过Class来实例化对象
		Class c = Class.forName("com.bjpowernode.java.bean.User");
		// newInstance()调用的是无参构造，必须保证无参构造是存在的！
		Object obj = c.newInstance();
		System.out.println(obj);
	} catch (Exception e) {
    
    
		e.printStackTrace();
	}

Second, get the properties of the class

return type	method name	describe
String	getName()	Get full class name
String	getSimpleName()	Get short class name
Field[]	getFields()	Get all public modified properties in a class
Field[]	getDeclaredFields()	Get all properties in a class
Field	getDeclaredField(String name)	Get property by property name

Common methods of the Filed class

return type	method name	describe
String	getName()	get attribute name
int	getModifiers()	The modifier returned is a number (the modifier's codename)
Class	getType()	Get the type of the property

public class ReflectTest {
    
    
    public static void main(String[] args) throws Exception{
    
    
        Class studentClass = Class.forName("com.why.java.bean.Student");
        String className = studentClass.getName();
        System.out.println("完整类名：" + className);
        String simpleName = studentClass.getSimpleName();
        System.out.println("简类名：" + simpleName);
        // 获取类中所有的public修饰的Field
        Field[] fields = studentClass.getFields();
        System.out.println(fields.length);
        // 取出这个Field
        Field f = fields[0];
        // 取出这个Field它的名字
        String fieldName = f.getName();
        System.out.println(fieldName);
        // 获取所有的Field
        Field[] fs = studentClass.getDeclaredFields();
        // 遍历
        for(Field field : fs){
    
    
            // 获取属性的修饰符列表
            int i = field.getModifiers(); 
            // 将这个“代号”数字转换成“字符串”吗
            String modifierString = Modifier.toString(i);
            System.out.print(modifierString+" ");
            // 获取属性的类型
            Class fieldType = field.getType();
            String fName = fieldType.getSimpleName();
            System.out.print(fName+" ");
            // 获取属性的名字
            System.out.println(field.getName());
        }
    }
}

【operation result】

完整类名：com.why.java.bean.Student
简类名：Student
private String name
protected int age
 boolean sex
public int no
public static final double MATH_PI

Process finished with exit code 0

3. Access object properties

Assign a value to a property set
to get the value of the property get

public class ReflectTest {
    
    
    public static void main(String[] args) throws Exception{
    
    

        // 不使用反射机制访问一个对象的属性
        Student s = new Student();
        s.no = 1111;
        System.out.println(s.no);

        // 使用反射机制访问一个对象的属性
        Class studentClass = Class.forName("com.why.java.bean.Student");
        Object obj = studentClass.newInstance(); 
        // 获取no属性（根据属性的名称来获取Field）
        Field noFiled = studentClass.getDeclaredField("no");
        // 给obj对象(Student对象)的no属性赋值
        noFiled.set(obj, 22222); // 给obj对象的no属性赋值2222
        // 读取属性的值
        System.out.println(noFiled.get(obj));
        Field nameField = studentClass.getDeclaredField("name");
        // 这样设置完之后，在外部也是可以访问private的。
        nameField.setAccessible(true);
        // 给name属性赋值
        nameField.set(obj, "jackson");
        // 获取name属性的值
        System.out.println(nameField.get(obj));
    }
}

Fourth, the method of obtaining the class

return type	method name	describe
Method[]	getDeclaredMethods()	Get all methods including private
Method	getMethod(String name, Class<?>… parameterTypes)	Get method based on method name and formal parameters
Class[]	getParameterTypes()	get parameter type
Constructor[]	getDeclaredConstructors()	Get all constructors

public class ReflectTest {
    
    
    public static void main(String[] args) throws Exception{
    
    
        Class userServiceClass = Class.forName("com.why.java.service.UserService");
        Method[] methods = userServiceClass.getDeclaredMethods();
        // 遍历Method
        for(Method method : methods){
    
    
            // 获取修饰符列表
            System.out.print(Modifier.toString(method.getModifiers())+" ");
            // 获取方法的返回值类型
            System.out.print(method.getReturnType().getSimpleName()+" ");
            // 获取方法名
            System.out.print(method.getName()+"(");
            // 方法的修饰符列表（一个方法的参数可能会有多个。）
            Class[] parameterTypes = method.getParameterTypes();
            for(Class parameterType : parameterTypes){
    
    
                System.out.print(parameterType.getSimpleName()+",");
            }
            System.out.println(")");
        }
    }
}

【operation result】

public void login(int,)
public boolean login(String,String,)
public void logout()

Process finished with exit code 0

Five, call the object method

public class ReflectTest10 {
    
    
    public static void main(String[] args) throws Exception{
    
    
        // 不使用反射机制调用方法
        UserService userService = new UserService();
        boolean loginSuccess = userService.login("admin","123");
        System.out.println(loginSuccess ? "登录成功" : "登录失败");

        // 使用反射机制来调用一个对象的方法
        Class userServiceClass = Class.forName("com.why.java.service.UserService");
        Object obj = userServiceClass.newInstance();
        // 获取Method
        Method loginMethod = userServiceClass.getDeclaredMethod("login", String.class, String.class);
        Object retValue = loginMethod.invoke(obj, "admin","123123");
        System.out.println(retValue);
    }
}

Six, through the reflection mechanism to call the constructor to instantiate the java object

public class ReflectTest12 {
    
    
    public static void main(String[] args) throws Exception{
    
    
        // 使用反射机制怎么创建对象
        Class c = Class.forName("com.bjpowernode.java.bean.Vip");
        // 调用无参数构造方法
        Object obj = c.newInstance();
        System.out.println(obj);
        // 调用有参数的构造方法
        // 第一步：先获取到这个有参数的构造方法
        Constructor con = c.getDeclaredConstructor(int.class, String.class, String.class,boolean.class);
        // 第二步：调用构造方法new对象
        Object newObj = con.newInstance(110, "jackson", "1990-10-11", true);
        System.out.println(newObj);

        // 获取无参数构造方法
        Constructor con2 = c.getDeclaredConstructor();
        Object newObj2 = con2.newInstance();
        System.out.println(newObj2);
    }
}

7. Get the parent class and the implemented interface

return type	method name	describe
Class	getSuperclass()	get parent class
Class[]	getInterfaces()	Get all implemented interfaces

public class ReflectTest13 {
    
    
    public static void main(String[] args) throws Exception{
    
    
        Class stringClass = Class.forName("java.lang.String");
        // 获取String的父类
        Class superClass = stringClass.getSuperclass();
        System.out.println(superClass.getName());
        // 获取String类实现的所有接口
        Class[] interfaces = stringClass.getInterfaces();
        for(Class in : interfaces){
    
    
            System.out.println(in.getName());
        }
    }
}

【operation result】

java.lang.Object
java.io.Serializable
java.lang.Comparable
java.lang.CharSequence
java.lang.constant.Constable
java.lang.constant.ConstantDesc

Process finished with exit code 0

resource binder

A resource binder is provided under the java.util package to facilitate obtaining the content in the property configuration file. When using the following method, the property configuration file xxx.properties must be placed in the classpath.

properties file

className=java.lang.String

public class ResourceBundleTest {
    
    
    public static void main(String[] args) {
    
    

        // 这个文件必须在类路径下。文件扩展名也必须是properties
        // 并且在写路径的时候，路径后面的扩展名不能写
        //ResourceBundle bundle = ResourceBundle.getBundle("db");

        ResourceBundle bundle = ResourceBundle.getBundle("com/why/java/bean/db");
        String className = bundle.getString("className");
        System.out.println(className);
    }
}

【result】

java.lang.String

Process finished with exit code 0