What you need to know about HTTP protocol

Today we will analyze the HTTP protocol, which is essential for Web development. For Web containers such as Tomcat and Jetty, the HTTP protocol is a foundation, and the difference between HTTP and HTML is the key starting point for understanding this protocol.

In this article, I will lead you to gradually understand the working mechanism of the HTTP protocol and further understand its principles through relevant source code snippets. Through this study, you will not only deepen your understanding of HTTP, but also lay a solid foundation for understanding the working principles of Web containers.

1. Differences between HTTP and HTML

In the eyes of many novice web developers, HTTP and HTML are easily confused, but in fact their functions and positioning are very different.

HTML (Hypertext Markup Language) is a markup language used to define the structure of web page content.
HTTP (Hypertext Transfer Protocol) is a network transmission protocol used to transmit data between the client and the server.

Simply put, HTML is the content, and HTTP is the means of transmitting the content. The browser obtains the HTML file from the server through HTTP request, and then renders and presents the page.

2. HTTP Protocol Overview

HTTP is a stateless protocol based on the request-response model. Statelessness means that the server does not remember the status of each request, so each request is independent. This feature brings higher scalability, but also requires developers to manage user sessions themselves (such as through Cookie or Session).

2.1 HTTP request structure

HTTP request consists of three parts: request line, request header, and request body. The following is an example of a typical HTTP request:

 GET /index.html HTTP/1.1 Host: www.example.com User-Agent: Mozilla/5.0 Accept: text/html

Request line: contains HTTP method, requested URI, and HTTP version.
Request header: includes request metadata, such as host name, user agent, data type, etc.
Request body: used to transfer data (usually used to transfer form data in POST requests).

2.2 HTTP Response Structure

HTTP response consists of three parts: status line, response header, and response body. The following is an example of an HTTP response:

 HTTP/1.1 200 OK Content-Type: text/html Content-Length: 123 <html> <head><title>Example</title></head> <body><p>Sample Page</p></body> </html>

Status line: contains HTTP version, status code and status description.
Response header: contains information such as content type and content length.
Response body: The actual returned content, such as an HTML document or other resources.

2.3 Common HTTP Methods

HTTP defines a series of methods for requesting operations:

GET: Request data, not including the request body. GET requests are idempotent.
POST: Submits data, usually used for form submission, including the request body. POST requests are not necessarily idempotent.
PUT: uploads resources, usually used to update resources, idempotent.
DELETE: delete resources, idempotent.
HEAD: Similar to GET, but does not return the request body. It is used to obtain metadata about resources.
OPTIONS: Used to query the server's supported functions.

3. Key concepts and source code analysis of HTTP protocol

Understanding the implementation of the HTTP protocol is inseparable from its implementation in Java. Next, we will analyze the HTTP request processing based on some of the Tomcat source code.

3.1 Request Processing Flow

In Tomcat, the HTTP request processing flow is as follows:

Receiving request: Tomcat receives the client's request data (byte stream).
Parsing request: Tomcat parses the byte stream into an HTTP request object.
Distribute requests: The requests are distributed to the corresponding Servlet for processing.
Generate response: Servlet generates response content, Tomcat encapsulates the response and returns it to the client.

3.2 Request parsing source code in Tomcat

In Tomcat, the Http11Processor class is responsible for parsing HTTP requests. The following is the key code for Tomcat to parse the request line:

 // Http11Processor.java protected boolean parseRequestLine() { // 从Socket中读取请求行数据if (!inputBuffer.parseRequestLine()) { return false; } // 提取HTTP方法、URI和协议版本ByteChunk methodBC = inputBuffer.getMethod(); request.method().setBytes(methodBC.getBytes(), methodBC.getStart(), methodBC.getLength()); ByteChunk uriBC = inputBuffer.getUri(); request.requestURI().setBytes(uriBC.getBytes(), uriBC.getStart(), uriBC.getLength()); ByteChunk protocolBC = inputBuffer.getProtocol(); request.protocol().setBytes(protocolBC.getBytes(), protocolBC.getStart(), protocolBC.getLength()); return true; }

Code analysis:

inputBuffer.parseRequestLine() reads the request line data from the Socket buffer.
Then parse the HTTP method, URI and protocol version respectively, and set them to the request object for subsequent processing.

3.3 Parsing the request header

After the request line is parsed, the next step is to parse the request header. Tomcat uses the parseHeaders() method to parse the HTTP request header. The following is the core code:

 // Http11Processor.java protected boolean parseHeaders() { while (true) { MimeHeaders headers = request.getMimeHeaders(); if (!inputBuffer.parseHeader(headers)) { break; } } return true; }

Code analysis:

inputBuffer.parseHeader() will loop through each request header field and add it to the MimeHeaders object for easy subsequent retrieval.

3.4 Generating Responses

Tomcat's response generation process also uses buffer objects. The following code shows how to generate a simple response header:

 // Http11Processor.java protected void prepareResponse() { response.setStatus(200); response.setHeader("Content-Type", "text/html"); response.setHeader("Content-Length", "123"); outputBuffer.write("HTTP/1.1 200 OK\r\n"); outputBuffer.write("Content-Type: text/html\r\n"); outputBuffer.write("Content-Length: 123\r\n\r\n"); }

Code analysis:

response.setStatus(200) sets the response status code.
response.setHeader() is used to set the response header.
Finally, the response data is written to the Socket through outputBuffer.write() and returned to the client.

IV. Evolution of HTTP: From 1.0 to 2.0 to 3.0

4.1 HTTP/1.1 Optimization

HTTP/1.1 has made many improvements based on HTTP/1.0:

Persistent connection: Keep-Alive was introduced in HTTP/1.1, allowing multiple requests to be sent in the same TCP connection, reducing handshake overhead.
Chunked transfer encoding: allows the server to start sending response data before the data is fully generated, improving transmission efficiency.

4.2 Features of HTTP/2

HTTP/2 has made significant improvements over HTTP/1.1:

Binary framing: HTTP/2 uses binary frame transmission to solve the serial problem in HTTP/1.x.
Multiplexing: allows multiple requests to be sent simultaneously in one TCP connection.
Header compression: Reduce duplicate request headers and improve transmission efficiency.

4.3 Innovations in HTTP/3

HTTP/3 is based on the QUIC protocol and further improves performance:

Reduced connection establishment time, enabling faster handshakes over UDP.
Supports connection migration to avoid interruptions caused by network changes.

5. Common Problems and Best Practices of HTTP Protocol

5.1 Problem 1: Session Management Due to Statelessness

The statelessness means that the server cannot remember the user's status. Cookies, Sessions, or Tokens can be used to manage sessions.

5.2 Problem 2: Security risks of HTTP plain text transmission

HTTP plain text transmission is vulnerable to eavesdropping, so data can be transmitted encrypted via HTTPS. HTTPS combined with SSL/TLS ensures data integrity and security.

5.3 Question 3: HTTP performance optimization

Use HTTP/2 multiplexing and header compression to reduce request latency.
Use caching and compression for static resources.
Properly configure HTTP headers, such as enabling GZIP compression, setting cache control, etc.

Summarize

The HTTP protocol is not only the foundation of Web development, it also determines the performance and user experience of Web applications. In this article, we explored the basic principles of the HTTP protocol and the implementation source code in Tomcat, and analyzed the version evolution and common problems of HTTP. With this knowledge, we have the ability to understand and optimize Web applications.

I hope that through today’s content, everyone can have a deeper understanding of the HTTP protocol and lay a solid foundation for future Web development and tuning.

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