Interpretation of Java underlying technology: how to implement reflection and dynamic proxy

Interpretation of Java underlying technology: How to implement reflection and dynamic proxy

Introduction:
Java is an object-oriented programming language. During the development process, we It is often necessary to use some underlying technologies, such as reflection and dynamic proxies. This article will introduce the principles of reflection and dynamic proxy, and give specific code examples to help readers better understand and use these two underlying technologies.

1. The principle of reflection
Reflection is a powerful and flexible feature in Java, which allows us to dynamically obtain and operate the methods, properties, and constructors of a class at runtime. wait. The principle of reflection mainly involves three classes: Class, Field and Method.

1. Class class
   The Class class is the core of Java reflection, which represents a Java class or interface. We can obtain all the information of a class through an instance of the Class class.

For example, assuming we have a class named "Person", we can get the Class object of this class using the following method:

Class<Person> personClass = Person.class;

2. Field class
   The Field class represents the attributes of a class. We can get and set the property value of an object through an instance of the Field class.

For example, assuming we have a class named "Person", which has a property named "name", we can get and set the value of the property using the following method:

Person person = new Person();
Field nameField = personClass.getDeclaredField("name");

// 设置name属性值
nameField.setAccessible(true); // 取消访问限制
nameField.set(person, "John");

// 获取name属性值
String name = (String) nameField.get(person);

3. Method class
   The Method class represents the method of a class. We can call an object's method through an instance of the Method class.

For example, suppose we have a class named "Person", which has a method named "sayHello", we can call the method using:

Person person = new Person();
Method sayHelloMethod = personClass.getDeclaredMethod("sayHello");

// 调用sayHello方法
sayHelloMethod.invoke(person);

2. The principle of dynamic proxy (Dynamic Proxy)
Dynamic proxy is another powerful feature in Java. It allows us to create a proxy class that implements a specified interface at runtime. Dynamic proxy mainly involves two classes: Proxy and InvocationHandler.

1. Proxy class
   The Proxy class is the key to implementing dynamic proxy in Java. It creates a proxy object by calling the static method Proxy.newProxyInstance().

For example, assuming we have an interface named "IHello", we can create a proxy object that implements the interface using the following method:

IHello helloProxy = (IHello) Proxy.newProxyInstance(
    IHello.class.getClassLoader(),
    new Class<?>[]{IHello.class},
    new HelloInvocationHandler()
);

2. InvocationHandler class
   The InvocationHandler interface is the actual processing class of dynamic proxy. It defines a method invoke() for calling methods on the proxy object.

For example, assuming we have an interface named "IHello", and the interface has a method named "sayHello", we can implement the InvocationHandler class in the following way:

public class HelloInvocationHandler implements InvocationHandler {
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        if (method.getName().equals("sayHello")) {
            System.out.println("Hello, World!");
        }
        return null;
    }
}

When using a dynamic proxy to call the "sayHello" method, the invoke() method of the HelloInvocationHandler class will be called to implement the custom behavior of the proxy object.

Summary:
This article introduces the principles of reflection and dynamic proxy in Java's underlying technology, and gives specific code examples. Through reflection, we can obtain and operate the methods, properties, constructors, etc. of a class at runtime; through dynamic proxy, we can create a proxy object that implements the specified interface at runtime. These two underlying technologies provide greater flexibility and scalability for Java development, and readers can flexibly use them according to their actual needs.

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