Java NIO分散/聚集或向量I/O

在Java NIO中，通道提供了称为分散/聚集或向量I/O的重要功能。 这是一种简单但功能强大的技术，通过这种技术，使用单个write()函数将字节从一组缓冲区写入流，并且可以使用单个read()函数将字节从流读取到一组缓冲区中。

Java NIO已经内置了分散/聚集支持。它可以用于从频道读取和写入频道。

分散读取

“分散读取”用于将数据从单个通道读取多个缓冲区中的数据。

下面来看看分散原理的说明：

下面是执行分射读取操作的代码示例：

public interface ScatteringByteChannel extends ReadableByteChannel  
{  
    public long read (ByteBuffer [] argv) throws IOException;  
    public long read (ByteBuffer [] argv, int length, int offset) throws IOException;  
}

聚集写入

“聚集写入”用于将数据从多个缓冲区写入单个通道。

下面来看看聚集原则的简单说明：

下面来看看看执行聚集写入操作的代码示例：

public interface GatheringByteChannel extends WritableByteChannel  
{  
    public long write(ByteBuffer[] argv) throws IOException;  
    public long write(ByteBuffer[] argv, int length, int offset) throws IOException;  
}

基本散点/聚集示例

下面来看看两个缓冲区的简单例子。 第一个缓冲区保存随机数，第二个缓冲区使用分散/聚集机制保存写入的数据：

package com.zaixian;

import java.io.FileOutputStream;
import java.io.FileInputStream;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.ScatteringByteChannel;
import java.nio.channels.GatheringByteChannel;

public class ScatterGatherIO {
    public static void main(String params[]) {
        String data = "Scattering and Gathering example shown in xuhuhu.com";
        gatherBytes(data);
        scatterBytes();
    }

    /*
     * gatherBytes() is used for reading the bytes from the buffers and write it
     * to a file channel.
     */
    public static void gatherBytes(String data) {
        String relativelyPath = System.getProperty("user.dir");
        // The First Buffer is used for holding a random number
        ByteBuffer buffer1 = ByteBuffer.allocate(8);
        // The Second Buffer is used for holding a data that we want to write
        ByteBuffer buffer2 = ByteBuffer.allocate(400);
        buffer1.asIntBuffer().put(420);
        buffer2.asCharBuffer().put(data);
        GatheringByteChannel gatherer = createChannelInstance(relativelyPath+"/testout.txt", true);
        // Write the data into file
        try {
            gatherer.write(new ByteBuffer[] { buffer1, buffer2 });
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    /*
     * scatterBytes() is used for reading the bytes from a file channel into a
     * set of buffers.
     */
    public static void scatterBytes() {
        String relativelyPath = System.getProperty("user.dir");
        // The First Buffer is used for holding a random number
        ByteBuffer buffer1 = ByteBuffer.allocate(8);
        // The Second Buffer is used for holding a data that we want to write
        ByteBuffer buffer2 = ByteBuffer.allocate(400);
        ScatteringByteChannel scatter = createChannelInstance(relativelyPath+"/testout.txt", false);
        // Reading a data from the channel
        try {
            scatter.read(new ByteBuffer[] { buffer1, buffer2 });
        } catch (Exception e) {
            e.printStackTrace();
        }
        // Read the two buffers seperately
        buffer1.rewind();
        buffer2.rewind();

        int bufferOne = buffer1.asIntBuffer().get();
        String bufferTwo = buffer2.asCharBuffer().toString();
        // Verification of content
        System.out.println(bufferOne);
        System.out.println(bufferTwo);
    }

    public static FileChannel createChannelInstance(String file, boolean isOutput) {
        FileChannel FChannel = null;
        try {
            if (isOutput) {
                FChannel = new FileOutputStream(file).getChannel();
            } else {
                FChannel = new FileInputStream(file).getChannel();
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return FChannel;
    }
}

在上述程序中，第一个缓冲区在控制台上打印随机输出，第二个缓冲区在控制台上打印“Scattering and Gathering example shown in xuhuhu.com”。

它还用“Scattering and Gathering example shown in xuhuhu.com”替换testout.txt文件的内容。

420
Scattering and Gathering example shown in xuhuhu.com