Redirection and request forwarding details

   Abstract ： This note focuses on the difference between redirection and request forwarding 、 Their characteristics .

1. Request forwarding

   Request forwarding itself is a server behavior , Its essence is that after the client request reaches the server , The server forwards the request information and saves the object to be forwarded , At this point, if we use a browser , You'll find... Above the browser URL The path in the path bar is unchanged . This is an internal behavior of the server , It's a servlet Forward a request to another server inside the server servlet act , This jump can only occur inside the server , In other words, the request can only be forwarded to a resource located inside the server , It cannot be forwarded to the outside , This is related to the storage method of request information inside the server . Next, we use diagrams to describe the process of request forwarding ：

   We send a request to the server in the client , First you need to use URL Conduct a path construction , After the connection is established, we can send our request , This is a computer network related knowledge , Before we communicate online , First, a connection will be established in some way , Then carry out data transmission , The main body in data transmission is the request we send , There is actually no standardized storage format for our requests outside the server , They are electrical signals or optical signals on the network cable , At this time, they are in the physical layer , The data of the physical layer cannot be directly recognized by the application of the application layer , However, they can be received by the server specified by the link layer protocol , After being received by the server , Naturally, there is a need for driver and operating system processing , Then send it to Tomcat 了 , After receiving the message Tomcat These request information will be converted into HttpServlet The format is saved in its own process , And send according to its request target , As shown above, it was sent to ServletA,ServletA It's a Servlet, stay Servlet in , The request is converted to HttpServletRequest The format of , Therefore, there is also a format conversion , and ServletA It is found that there is an operation requesting forwarding , Then execute the relevant code ,Tomcat The request object will be formatted , First convert to HttpServlet The format of , Then forward it to another Servlet. In the process ,HttpRequest Cannot be transformed into Request, But only in HttpServletRequest And HttpRequest Transition between two states , because Tomcat It is not provided to change the internal format back to Request Format function , therefore , Request forwarding cannot jump to external resources , And it is a request from the client to the server .

   Request forwarding is a server-side behavior , It's not about users , Completely implemented by the server .

2. Redirect

   Redirection and request forwarding can realize the function of page Jump , But when we learn from them, we stop here , At best, I know that redirection will make URL Change , The request for resources can no longer use the relative path of the previous page , The request can only be made again with the current page as the starting point . Why is this , Let's look at the principle of focusing on orientation in a graphical way ：

   In redirection , Another object is used ,Response object , This object is specially used to return information to the client , When using redirection ,Servlet No more jumping down , But use HttpServletResponse Object returned to the client , It will ServletB The address of is returned to the client , Then let the client access the address again , So the client sends another request , This request is made directly to ServletB Request . This explains why when we use redirection ,URL It will change , So we resend the request , Redirection is a secondary request , It's different from request forwarding , meanwhile ,ServletA The returned address is not necessarily the internal resource address of the server , You can also make an off-site address , In this way, the jump of resources outside the station can be realized .

   Request forwarding is not a simple server-side behavior , It's a server-side guided client-side behavior .

3. summary

   When using the browser to observe, we can find that the request forwarding is only one request , Redirection has a second request , Both have advantages and disadvantages , Redirection takes longer , And need to relocate resources , The request forwarding time is slightly shorter , But you can only access the resources in the station , Usually, we don't use redirection when writing website page Jump , This is to avoid the trouble caused by relocating resources , Of course, the way we use absolute positioning for resource access is not an obstacle , It can only be said that the choice is made according to the specific situation .