GPT-4 is a master of programming, I'm really convinced!

Last week I sent you an introduction to GPT-4 teaching mathematics. Many people were shocked and felt that it might trigger a revolution in the education industry.

How does it perform in the programming field? Let’s not talk about whether it can replace programmers. This still needs more testing and feedback. I want to try first whether it can teach programming like mathematics.

I found a concept that is difficult to understand in Java programming: dynamic proxy and tested it.

The following is the complete conversation I had with it, where I pretended to be a newbie learning Java programming and didn’t know the proxy pattern to see how it taught me.

The content is a bit long. It is recommended to read it patiently and feel the capabilities of AI.

If you really can’t stand it, you can skip to the end and watch the highlights directly.

[Liu Xin]: Suppose you are a tutor who always answers questions in a Socratic style. You won't give me the answer directly, but will guide me to think, okay?

[GPT]: Of course. If you have any questions, let me guide you to think and discuss.

[Liu Xin]: I am learning Java, and there is a concept of dynamic proxy, which I can’t understand

[GPT]: Very good, let’s use the Socratic method together Explore Java dynamic proxies. First, we need to understand the proxy pattern. What do you know about the agency model? What is the purpose of proxy pattern?

[Liu Xin]: Does it mean that one person does things for another person?

[GPT]: Yes, in a sense, the proxy pattern is to implement similar concepts in software design. In this case, one object (the agent) performs a specific task on behalf of another object (the target object). Now, think about it, why might we need to use the proxy pattern in programming? In what situations would using proxy mode be helpful?

[Liu Xin]: I don’t know

[GPT]: It doesn’t matter, let’s explore some possible reasons together. The proxy pattern is useful in many scenarios, such as when we need to perform some additional operations before or after accessing an object. For example, we can use the proxy pattern in the following situations:

Control access to the target object (access control)

Add additional functionality before and after the target object performs operations (such as logging, caching, etc. )

Lazy loading, creating target objects only when really needed

These are just some application scenarios of the proxy pattern. So, in your opinion, how do dynamic proxies come into play in these scenarios? What is the difference between dynamic proxies and static proxies?

[Liu Xin] What does this mean, I don’t understand it

[GPT] It doesn’t matter, we can help you understand the proxy mode through a simple example.

Suppose you have an application that needs to connect to a remote server to get data. In this case, you can create a class called RemoteServerConnection, which is responsible for establishing a connection with the remote server and getting data.

Now, your application needs to perform some additional actions, such as checking the network status before connecting to the remote server and logging after fetching the data. At this time, you can use proxy mode.

You can create a class called RemoteServerConnectionProxy, which has the same interface as RemoteServerConnection, but adds the functionality of checking network status and logging. The application will use RemoteServerConnectionProxy to get data instead of using RemoteServerConnection directly. The RemoteServerConnectionProxy will handle additional operations and then communicate with the RemoteServerConnection on behalf of the application when needed.

In this example, RemoteServerConnectionProxy is a proxy that represents RemoteServerConnection (target object) to perform specific tasks.

Now that you have a basic understanding of the proxy model, let's look at dynamic proxy. In the above example, we created a separate proxy class (static proxy). Unlike static proxies, dynamic proxies are dynamically generated at runtime rather than written in advance. Dynamic agents can respond to different scenarios more flexibly.

In Java, dynamic proxies are usually implemented by implementing the InvocationHandler interface and using the Proxy class. So, think about it, why dynamic proxies might have advantages over static proxies in some situations?

[Liu Xin]: Wait a minute, let’s finish talking about the static proxy first. In your example

RemoteServerConnectionProxy adds some functions for checking network status and logging to the previous class, but why does it have the same interface as RemoteServerConnection?

[GPT]: Sorry I didn’t explain it clearly before. Let's review the purpose of the proxy pattern: one object (the proxy) performs a specific task on behalf of another object (the target object). In order for the proxy object to successfully perform tasks on behalf of the target object, they need to have the same interface. This way, the client code doesn't care whether it's interacting with the proxy object or the target object, because they have the same interface.

In our example, RemoteServerConnectionProxy (proxy object) and RemoteServerConnection (target object) have the same interface, because this way the application can use these two classes transparently. When we decide to use a proxy in our application, we don't need to change the client code because both the proxy object and the target object implement the same interface. This is why the proxy object and the target object need to have the same interface.

In addition, having the same interface means that the proxy object can call the target object's methods when performing tasks. This is important for implementing additional functionality like access control, logging, etc., as the proxy object needs to pass control to the target object at the appropriate time.

[Liu Xin]: I understand, this is the proxy mode. So what is a dynamic proxy?

[GPT]: Very good, now that you have understood the proxy mode, let's continue to discuss dynamic proxy.

The main difference between dynamic and static proxies is how they are created. Static proxies are pre-written proxy classes, which means that for each target class, you need to create a separate proxy class for it. However, dynamic proxies are dynamically generated at runtime, which allows you to create generic proxies for multiple classes without changing the source code. This way, you reduce code duplication and improve maintainability.

In Java, dynamic proxy is mainly implemented through the java.lang.reflect.Proxy class and the java.lang.reflect.InvocationHandler interface. The following are the basic steps of a dynamic proxy:

Define an InvocationHandler implementation class, which is responsible for processing method calls on the proxy object. In this class, you can add additional operations as needed, such as logging, access control, etc.

Use the Proxy.newProxyInstance() method to create a dynamic proxy. This method receives three parameters: class loader, interface array (the interface that the proxy class needs to implement) and InvocationHandler instance. This method returns a proxy object that implements the specified interface.

In client code, use the created dynamic proxy object to call methods. When you call a method on the proxy object, it forwards the call to the invoke() method of the InvocationHandler instance.

This way, you can create dynamic proxies for one or more classes at runtime without having to write a separate proxy class for each class.

Now that you understand the concept of dynamic proxies, please think about the advantages and limitations of dynamic proxies compared to static proxies.

[Liu Xin] looked confused

[GPT] It doesn’t matter, let us try to help you understand dynamic proxy through a simple example.

Suppose we have a Bank interface, which has a method transfer() to perform transfer operations. We want to add logging functionality to this interface, but we don't want to modify the implementation of the Bank interface. At this time, we can use dynamic proxy.

First, we need to implement an InvocationHandler. This class will be responsible for handling the method invocation of the proxy object and adding logging functionality:

public class LoggingHandler implements InvocationHandler {
private Object target;
public LoggingHandler(Object target) {
this.target = target;
}


@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("日志记录：开始执行 " + method.getName() + " 方法");
Object result = method.invoke(target, args);
System.out.println("日志记录：执行 " + method.getName() + " 方法完成");
return result;
}
}

Now, we can use Proxy. The newProxyInstance() method creates a dynamic proxy for the Bank interface. Note that we need to pass the implementation class instance of the Bank interface to LoggingHandler:

Bank realBank = new RealBank(); // 假设RealBank是Bank接口的一个实现
InvocationHandler handler = new LoggingHandler(realBank);




Bank proxyBank = (Bank) Proxy.newProxyInstance(
realBank.getClass().getClassLoader(),
realBank.getClass().getInterfaces(),
handler
);

Now, when we call the transfer() method using proxyBank, the logging function will be automatically added:

proxyBank.transfer();

Pass With this example, we can see the main advantage of dynamic proxies: instead of writing a separate proxy class for each class, a generic proxy can be created for one or more classes at runtime. At the same time, it also makes the code more flexible and easier to maintain.

However, dynamic proxies also have some limitations. For example, in Java, dynamic proxies can only create proxies for classes that implement interfaces. If the class does not implement any interface, dynamic proxies cannot be used.

Hope this example can help you understand the concept of dynamic proxy. If you still have any questions, please feel free to ask.

[Liu Xin]: In your example, we hope to add logging function to the Bank interface. Then I can just write a BankProxy and add the logging code? Why go to all the trouble to use InvocationHandler?

[GPT]: You raised a very good question. In some cases, using a static proxy (such as the BankProxy you mentioned) can indeed achieve the functionality we want. However, in more complex scenarios, dynamic proxies may bring some significant advantages:

Versatility: Dynamic proxies can create proxies for multiple classes and interfaces without having to write separate Agent class. If you need to add the same functionality to many interfaces, such as logging, access control, etc., dynamic proxies can help you reduce duplicate code and improve code reusability.

Easy to maintain: If you need to modify or add the functionality of the proxy in the future, it will be easier to use a dynamic proxy. For static proxies, you need to modify each proxy class one by one; for dynamic proxies, you only need to modify the InvocationHandler implementation. This way you can easily modify the agent's behavior without changing the original classes and interfaces.

Decoupling: Dynamic proxies help separate proxy logic from business logic. When you need to modify the proxy logic, there is no need to change the original class and interface. This makes the code easier to understand and maintain.

Still, dynamic proxies are not always the best solution. In some cases, a static proxy may suit your needs better. When choosing between using a dynamic proxy or a static proxy, you need to weigh the pros and cons based on specific scenarios and needs.

Hope this answer can help you better understand the difference between dynamic proxy and static proxy. If you have any further questions, please feel free to ask.

[Liu Xin]: I understand, but why do we need to create a new class through Proxy.newProxyInstance? Isn’t it good to modify the original RealBank class directly?

[GPT]: Your question involves the concepts of code design and decoupling. It is of course feasible to directly modify the RealBank class to add logging functionality, but this may lead to the following problems:

Single responsibility principle: According to the single responsibility principle, a class should only be responsible for one clear function. The main responsibility of the RealBank class is to implement the Bank interface and handle bank-related business logic. If we add logging functionality in the RealBank class, it will be responsible for both business logic and logging, which may violate the single responsibility principle. By using proxies, we can separate these two responsibilities so that each class focuses on a well-defined function.

Decoupling: If we add logging functionality directly to the RealBank class, we will need to change the implementation of the RealBank class when we need to modify or remove the logging functionality in the future. This may result in the RealBank class being tightly coupled with the logging functionality, reducing the maintainability of the code. By using a proxy, we can separate the logging functionality from the RealBank class so that if we need to modify or remove the logging functionality, we do not need to change the RealBank class.

Reusability: Suppose we want to add logging functionality to multiple classes. If we directly modify the implementation of each class, this will lead to a lot of duplication of code. By using proxies (especially dynamic proxies), we can provide common logging functions for multiple classes, thereby reducing code duplication and improving code reusability.

In short, by using proxies, we can follow the single responsibility principle, achieve decoupling, and improve code reusability. While directly modifying the original class is feasible in some cases, there are many scenarios where using a proxy can lead to better code design and maintainability.

Hope this answer is helpful to you. If you have any further questions, please feel free to ask.

[Liu Xin]: The parameters of Proxy.newProxyInstance are so complicated. I understand passing

InvocationHandler in. Why do we need to pass the other two parameters?

[GPT]: The Proxy.newProxyInstance method does require three parameters. Let me explain their functions in detail:

ClassLoader: The class loader is responsible for loading byte code into the Java virtual machine. The purpose of passing the class loader is to let the Proxy.newProxyInstance method know where to look for the class definition and how to load the proxy class into memory. Usually, we use the class loader of the target class because we want the proxy class to be loaded in the same class loader as the target class. This ensures that the proxy class has access to the target class's interface. In most cases, the class loader of the target class can be obtained through target.getClass().getClassLoader().

Interface array (Interface[]): The interface array defines the interface that the proxy class needs to implement. These interfaces are usually the same as those implemented by the target class so that the proxy class can transparently substitute for the target class. When the methods of the proxy object are called, the InvocationHandler will handle the methods defined in these interfaces. In most cases, the interfaces implemented by the target class can be obtained through target.getClass().getInterfaces().

InvocationHandler instance: This parameter is an instance of InvocationHandler implementation, which is responsible for processing method calls of proxy objects. When the proxy object's method is called, the InvocationHandler's invoke() method will be executed. We can customize the behavior of the proxy object by passing an InvocationHandler instance to the Proxy.newProxyInstance method.

Although the Proxy.newProxyInstance method requires three parameters, they are all to ensure that the proxy class can be loaded correctly, implement the interface of the target class, and customize the behavior of the proxy object. Hopefully this explanation helps you understand what these parameters do. If you have any further questions, please feel free to ask.

[Liu Xin]: Although your answer is a bit long-winded, I still understand it

[GPT]: I’m glad you understand. The reason why I explain it in detail is to ensure that you fully understand the parameters of this method and what it does. If you encounter other questions while studying, or need a more concise explanation, please feel free to ask. I will try my best to help you. I wish you good luck with your studies!

Highlights

In terms of teaching methods:

1. It starts from a simple concept (static proxy), very good!

2. When I don’t understand a concept, I will explain it with examples, which is very good!

3. When I deliberately express my incomprehension about what it says, it can explain further every time.

4. It is a bit eager to introduce dynamic agents, and it can go back to the basic concepts during my repeated questions.

Technically, it explains accurately:

1. Why there is a dynamic proxy and the difference between it and a dynamic proxy.

2. Why does Proxy need to be the same as the original class interface?

3. Why three parameters are needed to create a dynamic proxy.

4. Introduced some design principles (single responsibility), decoupling, reuse and other functions.

Whether it is the teaching method or the teaching of knowledge, GPT-4 is very good. I think even if there is an excellent lecturer, this is probably the case.

It just feels like it's a bit verbose. It would be better if it supports voice effects.

No matter how GPT-4 is done internally, it behaves like a programming master externally. Not only has it mastered the concept of dynamic proxy, it has a large number of examples to support it, and it can also explain it from simple to deep. , very powerful. I will try its design capabilities later and share it with you. ​

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