One 、 Basic concept description

1.1 I/O brief introduction

I/O I.e. input and output , It's an interface between the computer and the outside world .IO The actual theme of operation is the operating system . stay java Programming , It's usually handled in a stream way IO, be-all IO It's seen as a single byte move , adopt stream Objects move one byte at a time . flow IO Responsible for converting objects into bytes , And then convert to objects .

About Java IO Please refer to my other article for related knowledge :Java IO Detailed explanation

1.2 What is? NIO

NIO namely New IO, This library is in JDK1.4 It was introduced in China .NIO and IO Have the same function and purpose , But the implementation is different ,NIO The main use is block , therefore NIO Is more efficient than IO Much higher .

stay Java API Two sets of NIO, One is for Standard input and output NIO, The other is Network programming NIO, This article focuses on NIO, About network programming NIO Please see Java NIO Detailed explanation ( Two ).

1.3 Flow vs. block

NIO and IO The biggest difference is the way data is packaged and transmitted .IO In order to flow How to process data , and NIO In order to block How to process data .

Facing the flow Of IO Processing data one byte at a time , One input stream produces one byte , One output stream consumes one byte . It's very easy to create filters for streaming data , Link a few filters , So that the data processing is very convenient and simple , But stream oriented IO It's usually very slow .

Facing the block Of IO The system processes data in blocks . Each operation generates or consumes a block of data in one step . It's much faster by block than by stream , But block oriented IO Lack of stream oriented IO Elegance and simplicity .

Two 、NIO Basics

Buffer and Channel It's a standard. NIO The core object of ( The Internet NIO There's another one. Selector The core object , Please refer to Java NIO Detailed explanation ( Two )), Almost every IO They are used in all operations .

Channel It's right IO Simulation of midstream , Any source and destination data must pass through a Channel object . One Buffer It's essentially a container object , issue Channel All of the objects in must be placed in Buffer in ; alike , from Channel Any data that you read in must read Buffer in .

2.1 About Buffer

Buffer It's an object , It contains some data to be written or read out . stay NIO in , Data is put into buffer Object's , And in the IO in , Data is written or read directly to Stream Object's . Applications can't directly address Channel Read and write , And must pass Buffer To carry out , namely Channel It's through Buffer To read and write data .

stay NIO in , All the data is in Buffer To deal with the , It is NIO Transfer pool for reading and writing data .Buffer It's essentially an array , It's usually a byte of data , But it can also be other types of arrays . But a buffer is more than an array , What's important is that it provides structured access to data , It can also track the reading and writing process of the system .

Use Buffer Reading and writing data generally follows the following four steps :

  1. Write data to Buffer;
  2. call flip() Method ;
  3. from Buffer Read data from ;
  4. call clear() Methods or compact() Method .

Direction Buffer When writing data ,Buffer How much data has been written down . Once you want to read the data , Need to pass through flip() Methods will Buffer  Switch from write mode to read mode . In read mode , Can read before writing to Buffer All data for .

Once you've read all the data , You need to clear the buffer , Let it be written again . There are two ways to clear the buffer : call clear() or compact() Method .clear() Method will clear the entire buffer .compact() Method will only clear the data that has been read . Any unread data is moved to the beginning of the buffer , The newly written data will be placed behind the unread data in the buffer .

Buffer There are mainly the following :

2.3 About Channel

Channel It's an object , It can be used to read and write data . Think of it as IO Flow in . But it's a little bit different from streaming :

  1. Channel It's two-way , Can read and write , And the flow is unidirectional
  2. Channel It can read and write asynchronously
  3. Yes Channel You have to read and write through buffer object

As mentioned above , All data passed Buffer Object processing , therefore , You never write bytes directly to Channel in , contrary , You are writing data to Buffer in ; Again , And you won't come from Channel Read bytes in , It's about taking data from Channel Read in Buffer, Again from Buffer Get this byte .

because Channel It's two-way , therefore Channel It can better reflect the real situation of the underlying operating system than the flow . Especially in Unix In the model , The underlying operating system is usually bidirectional .

stay Java NIO in Channel There are mainly the following types :

  • FileChannel: Reading data from a file
  • DatagramChannel: Reading and writing UDP Network protocol data
  • SocketChannel: Reading and writing TCP Network protocol data
  • ServerSocketChannel: Can monitor TCP Connect

3、 ... and 、 From theory to practice :NIO Reading and writing in

IO Reading and writing in , Corresponding to data and Stream,NIO Reading and writing in , The corresponding is the channel and buffer .NIO Read from channel in : Create a buffer , Then let the channel read the data into the buffer .NIO Write data to channel : Create a buffer , Fill it with data , Then let the channel use the data to write .

3.1 Read from file

We already know , stay NIO In the system , Any time a read operation is performed , You are all from Channel Read from , And you're not directly from Channel Read data from , Because all the data has to be Buffer To encapsulate , So you should be from Channel Read data to Buffer.

therefore , If you read data from a file , There are three steps :

  1. from FileInputStream obtain Channel
  2. establish Buffer
  3. from Channel Read data to Buffer

Now let's take a look at the specific process : 
First step : Get access to

FileInputStream fin = new FileInputStream( "readandshow.txt" );
FileChannel fc = fin.getChannel();

The second step : Create buffer

ByteBuffer buffer = ByteBuffer.allocate( 1024 );

The third step : Read data from channel to buffer

fc.read( buffer );

3.2 Write data to file

It's like reading data from a file , 
First step : Get a channel

FileOutputStream fout = new FileOutputStream( "writesomebytes.txt" );
FileChannel fc = fout.getChannel();

The second step : Create buffer , Put the data into the buffer

ByteBuffer buffer = ByteBuffer.allocate( 1024 );

for (int i=0; i<message.length; ++i) {
buffer.put( message[i] );
}
buffer.flip();

The third step : Write buffer data to channel

fc.write( buffer );

3.3 Combination of reading and writing

CopyFile It's a very good example of a combination of reading and writing , We will pass CopyFile This strength makes you realize NIO Operation process .CopyFile Perform three basic operations : Create a Buffer, Then read the data from the source file to the buffer , Then write the buffer to the target file .

/**
* use java NIO api Copy files
* @param src
* @param dst
* @throws IOException
*/
public static void copyFileUseNIO(String src,String dst) throws IOException{
// Declare source and destination files
FileInputStream fi=new FileInputStream(new File(src));
FileOutputStream fo=new FileOutputStream(new File(dst));
// Get the transmission channel channel
FileChannel inChannel=fi.getChannel();
FileChannel outChannel=fo.getChannel();
// Get the container buffer
ByteBuffer buffer=ByteBuffer.allocate(1024);
while(true){
// Decide if you have read the document
int eof =inChannel.read(buffer);
if(eof==-1){
break;
}
// Reset it buffer Of position=0,limit=position
buffer.flip();
// Start writing
outChannel.write(buffer);
// Reset after writing buffer, reset position=0,limit=capacity
buffer.clear();
}
inChannel.close();
outChannel.close();
fi.close();
fo.close();
}

Four 、 Something to watch out for

There are three things to note in the above procedure

4.1 Check status

When there is no more data , Copy is complete , here read() Method will return -1 , We can judge whether we have finished reading according to this method .

int r= fcin.read( buffer );
if (r==-1) {
break;
}

4.2 Buffer Class flip、clear Method

control buffer Three variables of state

  • position: Track how much data has been written or read , It points to where the next byte comes from
  • limit: Represents how much data can be retrieved or how much space can be written , Its value is less than or equal to capacity.
  • capacity: Represents the maximum size of the buffer , When creating a new buffer ,limit The value of and capacity The default value of is equal .

flip、clear These two methods are used to set these values .

flip Method

Let's take a look first flip Source code :

public final Buffer flip() {
limit = position;
position = 0;
mark = -1;
return this;
}

Above FileCopy In the program , Before we write the data, we call buffer.flip(); Method , This method takes the current pointer position position Set up a limit, And then the current pointer position Point to the beginning of the data , We can now write data from the buffer to the channel . position Set to 0, This means that the next byte we get is the first byte . limit Has been set to the original position, This means that it includes all bytes read before , And not a single byte .

clear Method

Have a look first clear Source code :

 public final Buffer clear() {
position = 0;
limit = capacity;
mark = -1;
return this;
}

Above FileCopy In the program , After writing data, before reading data , We call  buffer.clear(); Method , This method resets the buffer to receive more bytes . The figure above shows calling clear() Status of the back buffer .

Reprint please explain the source , Link to the original text :http://blog.csdn.net/suifeng3051/article/details/48160753

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