Java ByteBuffer allocateDirect()方法及示例

java.nio.ByteBuffer 类的 allocateDirect() 方法是用来分配一个新的直接字节缓冲区。

新的缓冲区的位置将是零，它的极限是它的容量，它的标记将是未定义的，它的每个元素将被初始化为零。它是否有一个支持数组是未指定的。

这个方法比allocate()方法快25%-75%。

语法

public static ByteBuffer allocateDirect(int capacity)

参数： 该方法以字节为单位接收容量，作为参数。

返回值： 该方法返回新的字节缓冲区。

异常： 如果容量是一个负整数，该方法会抛出 IllegalArgumentException 。

下面是一些例子来说明allocateDirect()方法。

例子 1 :

// Java program to demonstrate
// allocateDirect() method
  
import java.nio.*;
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // Declaring the capacity of the ByteBuffer
        int capacity = 4;
  
        // Creating the ByteBuffer
        try {
  
            // creating object of ByteBuffer
            // and allocating size capacity
            ByteBuffer bb = ByteBuffer.allocateDirect(capacity);
  
            // creating byte array of size capacity
            byte[] value = { 20, 30, 40, 50 };
  
            // wrap the byte array into ByteBuffer
            bb = ByteBuffer.wrap(value);
  
            // print the ByteBuffer
            System.out.println("Direct ByteBuffer:  "
                               + Arrays.toString(bb.array()));
  
            // print the state of the buffer
            System.out.print("\nState of the ByteBuffer : ");
            System.out.println(bb.toString());
        }
  
        catch (IllegalArgumentException e) {
            System.out.println("Exception thrown : " + e);
        }
  
        catch (ReadOnlyBufferException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

Direct ByteBuffer:  [20, 30, 40, 50]

State of the ByteBuffer : java.nio.HeapByteBuffer[pos=0 lim=4 cap=4]

示例2： 显示IllegalArgumentException

// Java program to demonstrate
// allocateDirect() method
  
import java.nio.*;
import java.util.*;
  
public class GFG {
    public static void main(String[] args)
    {
  
        // Declaring the capacity
        // with negative value
        int capacity = -4;
  
        // Creating the ByteBuffer
        try {
  
            // creating object of ByteBuffer
            // and allocating size capacity
  
            System.out.println("Trying to allocate"
                               + " negative value in ByteBuffer");
            ByteBuffer bb = ByteBuffer.allocateDirect(capacity);
  
            // creating byte array of size capacity
            byte[] value = { 20, 30, 40, 50 };
  
            // wrap the byte array into ByteBuffer
            bb = ByteBuffer.wrap(value);
  
            // print the ByteBuffer
            System.out.println("Direct ByteBuffer:  "
                               + Arrays.toString(bb.array()));
  
            // print the state of the buffer
            System.out.print("\nState of the ByteBuffer : ");
            System.out.println(bb.toString());
        }
  
        catch (IllegalArgumentException e) {
            System.out.println("Exception thrown : " + e);
        }
  
        catch (ReadOnlyBufferException e) {
            System.out.println("Exception thrown : " + e);
        }
    }
}

输出。

Trying to allocate negative value in ByteBuffer
Exception thrown : java.lang.IllegalArgumentException: Negative capacity: -4