Java basics essay

1. Basic concepts

IO is the process of copying data from main memory and external devices (hard disk, terminal, network, etc.). IO is the bottom function implementation of the operating system, which is completed by I / O instructions.

All language runtime systems provide higher-level tools for performing I / O. (printfscanf of C, object-oriented encapsulation of Java)

2. Java standard IO review

Java standard IO class library is an abstraction of IO object-oriented. Based on the underlying implementation of local methods, we don't need to pay attention to the underlying implementation. InputStream \ OutputStream (byte stream): transfer one byte at a time. Reader \ writer (character stream): one character at a time.

3. NiO introduction

NiO is the abbreviation of javanewio, which is in jdk1 4. Sun's official features are as follows:

C provides buffer caching support for all primitive types.

C character set encoding and decoding solution.

Ccchannel: a new primitive I / O abstraction.

C supports file access interfaces for locks and memory mapped files.

C provides non blocking high scalability network I / O.

This article will study and introduce these features.

4.Buffer&Chanel

Channel and buffer are the two most basic data type abstractions of NiO.

Buffer:

C is a contiguous block of memory.

C is the transfer place for NiO data reading or writing.

Channel:

C source of data or destination of data

C is used to provide data to buffer or read buffer data. It is the only interface of buffer object.

C asynchronous I / O support

Example 1: copyfile java:

The internal structure of the buffer is as follows (the figure below is copied from the data):

Figure 2: buffer internal structure

A buffer is mainly controlled by three variables: position, limit and capacity. The meanings of these three variables are shown in the following table:

Common buffer methods:

Flip (): convert write mode to read mode

Rewind(): resets position to 0, which is generally used for repeated reading.

Clear (): clear the buffer to be written again (position becomes 0 and limit becomes capacity).

Compact(): copy the unread data to the header of the buffer.

Mark (), reset (): mark can mark a position, and reset can reset to that position.

Common types of buffer: ByteBuffer, mappedbytebuffer, charbuffer, doublebuffer, floatbuffer, intbuffer, longbuffer and shortbuffer.

Common types of channels: filechannel, datagram channel (UDP), socketchannel (TCP), serversocketchannel (TCP)

A simple performance test is done on this machine. My notebook has average performance. (see the attachment for the specific code. See the following example in nio.sample.filecopy package) the following is the reference data:

C scenario 1: copy a 370m file

C scenario 2: three threads copy at the same time, and each thread copies a 370m file

5.nio. charset

Character encoding and decoding: bytecode itself is just some numbers, which are correctly parsed in the correct context. When storing data in ByteBuffer, the encoding mode of character set needs to be considered. When reading and displaying ByteBuffer data, it involves decoding the character set.

Java. nio. Charset provides a set of coding and decoding solutions.

Taking our most common HTTP request as an example, the request must be encoded correctly. When the response is received, the response must be decoded correctly.

The following code sends a request to Baidu and obtains the results for display. The example demonstrates the use of charset.

Example 2baidureader java

6. Non blocking IO

Non blocking IO will be understood from what is blocking, what is non blocking, non blocking principle and asynchronous core API.

What is blocking?

A common network IO communication process is as follows:

Understand what is blocking from the network communication process:

In the above process, if the connection does not arrive, accept will block, the program will have to hang up, and the CPU will execute other threads instead.

In the above process, if the data is not ready, the read will also be blocked.

Characteristics of blocking network IO: multithreading handles multiple connections. Each thread has its own stack space and takes up some CPU time. Each thread will block when it is ready for external. The result of blocking is a large number of process context switches. And most process context switches may be meaningless. For example, suppose a thread listens to a port, and only a few requests come in a day, but the CPU has to make context switching attempts for the thread, and most of the switching ends in blocking.

What is non blocking?

Here is a metaphor:

On a bus from a to B, people may get off at many points on the road. The driver doesn't know which people will get off at which point. How to deal with those who need to get off better?

1. The driver regularly asks whether each passenger has arrived at the destination. If someone says so, the driver stops and the passengers get off. (similar to blocking type)

2. Everyone tells the conductor their destination and then goes to bed. The driver only interacts with the conductor. At a certain point, the conductor informs the passengers to get off. (similar to non blocking)

Obviously, everyone who wants to reach a destination can be regarded as a thread, and the driver can be regarded as the CPU. In blocking mode, each thread needs constant polling and context switching to find the destination. In the non blocking mode, each passenger (thread) is sleeping (sleeping) and wakes up only when the real external environment is ready. Such wakeup will certainly not block.

Non blocking principle

The whole process is switched to small tasks and completed through inter task cooperation.

A dedicated thread handles all IO events and is responsible for distribution.

Event driven mechanism: events are triggered when they arrive, rather than monitoring events synchronously.

Thread communication: threads communicate through wait, notify, etc. Ensure that each context switch is meaningful. Reduce unnecessary process switching.

The following is the structure of asynchronous IO:

Reactor is the conductor role metaphorized above. The processing flow of each thread is probably to read data, decode, calculate and process, encode and send response.

Asynchronous IO core API

Selector

The core class of asynchronous IO, which can detect events on one or more channels and distribute them.

Using a select thread, you can listen to events on multiple channels and trigger corresponding responses based on event driving. There is no need to allocate a thread for each channel.

SelectionKey

It contains the state information of the event and the binding of the channel corresponding to the time.

summary

The above is all about the basic knowledge of Java in this article. I hope it will be helpful to you. Interested friends can continue to refer to other related topics on this site. If there are deficiencies, please leave a message to point out. Thank you for your support!