nio: Working with Buffers

Buffers store fixed amount of data. In technical terms, a buffer can store fixed data of specific primitive type.

A Buffer can be stored in heap space of jvm, or off-heap space of the jvm.

'java.nio.Buffer' is the abstract super class for all the below primitive type buffers.

a.   ByteBuffer,

b.   CharBuffer,

c.    DoubleBuffer,

d.   FloatBuffer,

e.   IntBuffer,

f.     LongBuffer,

g.   ShortBuffer, and

h.   MappedByteBuffer

How to create a Buffer?

Buffer classes provide 'allocate' method to create buffer instances.

Example

ByteBuffer buffer = ByteBuffer.allocate(1024);

CharBuffer charBuffer = CharBuffer.allocate(1024);

Key properties of Buffer

capacity, limit and position are the essential properties associated with the buffers.

a. capacity: It represents the number of elements that a buffer can contain. This value never changes through the life cycle. This value is set at the time of buffer creation.

b. limit: It specifies the index of the first element that should not be read or written. It must not greater than the capacity. In simple terms, it represents the number of live elements in the buffer.

c. position: it specifies the index of the next element to be read (or) written. A buffer's position is never negative and is never greater than its limit.

Let’s see an example of Buffer class, before digging further.

testData.txt

If you can keep your head when all about you   
    Are losing theirs and blaming it on you,   
If you can trust yourself when all men doubt you,
    But make allowance for their doubting too;   

let’s see an example, how can we read the data using ByteBuffer.

Step 1: Define an instance of ByteBuffer.

ByteBuffer buffer = ByteBuffer.allocate(10);

Step 2: Get the FileChannel instance from gien file path.

Path path = Paths.get(FILE_PATH);

FileChannel fileChannel = FileChannel.open(path);

Step 3: Read the information from fileChannel to the buffer.

int noOfBytesRead = -1;

while ((noOfBytesRead = fileChannel.read(buffer)) != -1) {

         System.out.println(new String(buffer.array()));

         buffer.clear();

         noOfBytesRead = fileChannel.read(buffer);

}

Find the below working application.

Test.java

package com.sample.nio;

import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.Path;
import java.nio.file.Paths;

public class Test {

 private static final String FILE_PATH = "C:\\Users\\krishna\\testData.txt";

 public static void main(String args[]) throws Exception {
  /* Define new byte buffer of capacity 10 */
  ByteBuffer buffer = ByteBuffer.allocate(10);

  /* Get FileChannel instance from the file */
  Path path = Paths.get(FILE_PATH);

  try (FileChannel fileChannel = FileChannel.open(path)) {
   while ((fileChannel.read(buffer)) != -1) {
    System.out.println(new String(buffer.array()));
    buffer.clear();
   }
  }
 }
}

Output

If you can
 keep your
 head when
 all about
 you   
 
   Are los
ing theirs
 and blami
ng it on y
ou,   
If
 you can t
rust yours
elf when a
ll men dou
bt you,
 
   But mak
e allowanc
e for thei
r doubting
 too;   ng

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