HTTP状态码 | Baeldung中文网

1. Introduction

In this tutorial, we’ll be exploring something that we deal with daily when we browse the internet. HTTP status codes are part of the HTTP standard as well as how computers communicate with each other over the World Wide Web. We’ll learn about the different categories of HTTP status codes, and also take a deeper look into the meaning of some of the most commonly used ones.

2. First, What Is HTTP?

Before we talk about HTTP status codes, let’s get a quick recap of what HTTP is. HTTP stands for Hypertext Transfer Protocol and is a standard that details how communication occurs over the web. The HTTP protocol was initially invented by Tim Berners-Lee and later on developed by the HTTP Working Group in the Internet Engineering Task Force (IETF). Versions of the HTTP protocol are published as RFC (Request For Comment) documents and we can find more details by searching on the RFC Editor website.

HTTP takes the form of requests and responses between a client (e.g. a browser) and a server. When we click on a link, type a URL in our browser, or submit a form, the browser is initiating an HTTP request to a server telling that server what we would like it to do. For example, a typical HTTP request/response communication between two computers could look like this:

Client Request:

GET / HTTP/1.1 
Host: www.google.com

Server Response:

HTTP/1.1 200 OK 
Date: Sat, 12 Sep 2020 14:26:56 GMT 
Expires: -1 
Cache-Control: private, max-age=0 
Content-Type: text/html; charset=ISO-8859-1 
Set-Cookie: ...truncated... 
Accept-Ranges: none 
Vary: Accept-Encoding 
Transfer-Encoding: chunked 

(html)

This is what happens behind the scenes when we type https://www.google.com into our browsers. The first line of response is the status line, which contains the HTTP version being used, an HTTP status code, and its associated reason phrase. In this case, the status code is 200 and the reason phrase is OK. We’ll learn about what this means in the following section.

3. HTTP Status Codes

According to the IETF, an HTTP status code is a three-digit integer code giving the result of the server’s attempt to understand and satisfy the request. Status codes are not meant for the human user; instead, they’re intended for the client to interpret and act accordingly. A client can be a browser, another server, or any application making an HTTP request.

HTTP status codes are divided into 5 classes based on the first digit of the status code:

1xx – informational – the request was received, continuing process
2xx – successful – the request was received successfully and accepted
3xx – redirection – further action needs to be taken to complete the request
4xx – client error – request contains bad syntax or cannot be fulfilled
5xx – server error – the server failed to fulfill an apparently valid request

Using this understanding, we can immediately deduce that the example request above was a request which was processed successfully by the server.

Next, let’s take a look at some of the most commonly used HTTP status codes that we might encounter.

3.1. 200 OK

A 200 status code indicates that the request was successful and could be the result of any of the following operations: GET, POST, PUT, PATCH, and DELETE. When a server responds with a 200, the response must also contain a payload that is appropriate for the action. For example, the payload of a 200 response to a GET request would typically contain the requested resource, like the requested Html page.

3.2. 201 Created

A 201 response indicates that a request was successful and resulted in a new resource being created. An example of a request which could result in a 201 status code is when we submit a form to create a new resource.

3.3. 301 Found

A 301 response tells us that the resource we’re requesting has moved permanently. A 301 response must contain a Location header with the URI of the new location. This Location can then be used by the client to repeat the request successfully at the new location.

3.4. 400 Bad Request

This status code means that the HTTP request sent over to the server has incorrect syntax. For example, if we have submitted a form with incorrectly formatted data or the wrong data type, this could result in a 400 response.

3.5. 401 Unauthorized

This means that the user who is trying to access the resource by sending the request has not been properly authenticated. According to the HTTP standards, when a server responds with a 401 it must also send a WWW-Authenticate header with a list of the authentication schemes that the server uses.

3.6. 403 Forbidden

A 403 response is commonly confused with 401; however, the 403 response means that the server is refusing our request due to insufficient access or permissions. Even if we’re authenticated, we may not be allowed to make a certain request.

3.7. 404 Not Found

This status code is returned when the requested resource is not found. For instance, if we entered a URL for a page that does not exist on a website or was deleted for some reason, it would result in a 404 response.

Take a look at this example where we try to access a page that does not exist:

POST /test HTTP/1.1 
Host: www.google.com

This will result in a 404 status code from Google:

HTTP/1.1 404 Not Found 
Content-Type: text/html; charset=UTF-8 
Referrer-Policy: no-referrer 
Content-Length: 1565 
Date: Sat, 12 Sep 2020 16:38:02 GMT 

(html)

3.8. 405 Method Not Allowed

A 405 response means that the server does not support the method used in the request. For example, if an endpoint only accepts GET requests and we try to send a POST, it will result in a 405 response code. If this is the case, servers must also send back an Allow header field with the list of the allowed methods.

For example, the following request to Google:

DELETE / HTTP/1.1 
Host: www.google.com

Will result in the response:

HTTP/1.1 405 Method Not Allowed 
Allow: GET, HEAD 
Date: Sat, 12 Sep 2020 15:39:35 GMT 
Content-Type: text/html; charset=UTF-8 
Server: gws 
Content-Length: 1591 
X-XSS-Protection: 0 
X-Frame-Options: SAMEORIGIN 

(html)

3.9. 408 Request Timeout

This status code is an indication from the server that the request message was not completed within the time that the server was prepared to wait. If a client is too slow in sending its HTTP request, this could happen. This could be due to many reasons, including a slow internet connection.

3.10. 413 Request Entity Too Large

This status code tells us that the server is unable or unwilling to process the request because the request payload is too large. This could happen if we try to upload a file that is too large and exceeds the server limits.

3.11. 500 Internal Server Error

An internal server error means that for some reason the server could not process the request due to an error. This is generally due to a bug or an unhandled exception on the server.

3.12. 429 Too Many Requests

Occasionally, services may want to limit the number of requests being made to their resources. The 429 response code indicates to the client that the number of allowed requests has been exceeded. This is useful for rate-limiting users and even for monetizing APIs.

3.13. 418 I’m a Teapot

The 418 status code was published as an April Fools joke in April 1998. It has not been registered as an official response code but has been adopted by many frameworks and libraries such as Node.js and ASP.Net as an “easter egg” joke. In fact, we could even visit https://www.google.com/teapot to see this in action.

4. Unassigned HTTP Status Codes

HTTP status codes are extensible, and the ones we listed above are only a small part of the many status codes in use. A complete list of status codes is maintained by IANA and can be seen in their HTTP Status Code Registry. To support as many applications as possible, it’s advised to make use of the officially registered status codes; however, there is a range of status codes which are unassigned and can be used by services to expand on the registered status codes:

5. Conclusion

In this article, we learned about HTTP status codes and how they are an essential part of the HTTP standard used to communicate on the web. We examined some of the most commonly used status codes and briefly touched on the range of unassigned status codes that are yet to be officially used.

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