NiO knowledge points

One 、NIO brief introduction

1）Java BIO ： Synchronize and block ( Traditional obstructive type ), The server implements a pattern of one thread per connection , That is, when the client has a connection request, the server needs to start a thread for processing , If this connection doesn't do anything, it will cause unnecessary thread overhead .

2）Java NIO ： Synchronous nonblocking , The server implementation mode deals with multiple requests for one thread ( Connect ), That is, the connection requests sent by the client will be registered on the multiplexer , The multiplexer polls the connection to have I/O The request is processed .

Two 、 Several concepts of operating system

1、 Kernel state and user state

Kernel mode ：cpu Access to all data in memory , Including peripherals , For example, hard disk , network card ,CUP You can also switch from one program to another .

User mode ： Only limited access to memory , And access to peripherals is not allowed , Occupy CPU The ability to be deprived of .

Why do you want to use user state and kernel state ？

Because it is necessary to restrict the access ability between different programs , To prevent other programs from accessing memory , Or get peripheral data , And send it to the network , CPU There are two levels of authority – User mode and kernel mode .

When will the switch between kernel mode and user mode occur ？

【 When the user mode needs to apply for external resources, it will switch to the kernel mode 】. For example, execute system calls 、 It's happening