Netty学习笔记 3.7 关于Buffer 和 Channel的注意事项和细节
ByteBuffer 支持类型化的put 和 get, put 放入的是什么数据类型,get就应该使用相应的数据类型来取出,否则可能有 BufferUnderflowException 异常。[举例说明]
package com.my.nio;
import java.nio.ByteBuffer;
public class NIOByteBufferPutGet {
public static void main(String[] args) {
//创建一个Buffer
ByteBuffer buffer = ByteBuffer.allocate(64);
//类型化方式放入数据
buffer.putInt(100);
buffer.putLong(9);
buffer.putChar('尚');
buffer.putShort((short) 4);
//取出
buffer.flip();
System.out.println();
System.out.println(buffer.getInt());
System.out.println(buffer.getLong());
System.out.println(buffer.getChar());
System.out.println(buffer.getShort());
}
}
可以将一个普通Buffer 转成只读Buffer [举例说明]
package com.my.nio;
import java.nio.ByteBuffer;
public class ReadOnlyBuffer {
public static void main(String[] args) {
//创建一个buffer
ByteBuffer buffer = ByteBuffer.allocate(64);
for(int i = 0; i < 64; i++) {
buffer.put((byte)i);
}
//读取
buffer.flip();
//得到一个只读的Buffer
ByteBuffer readOnlyBuffer = buffer.asReadOnlyBuffer();
System.out.println(readOnlyBuffer.getClass());
//读取
while (readOnlyBuffer.hasRemaining()) {
System.out.println(readOnlyBuffer.get());
}
readOnlyBuffer.put((byte)100); //ReadOnlyBufferException
}
}
NIO 还提供了 MappedByteBuffer, 可以让文件直接在内存(堆外的内存)中进行修改, 而如何同步到文件由NIO 来完成. [举例说明]
package com.atguigu.nio;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
/*
说明
1. MappedByteBuffer 可让文件直接在内存(堆外内存)修改, 操作系统不需要拷贝一次
*/
public class MappedByteBufferTest {
public static void main(String[] args) throws Exception {
RandomAccessFile randomAccessFile = new RandomAccessFile("1.txt", "rw");
//获取对应的通道
FileChannel channel = randomAccessFile.getChannel();
/**
* 参数1: FileChannel.MapMode.READ_WRITE 使用的读写模式
* 参数2: 0 : 可以直接修改的起始位置
* 参数3: 5: 是映射到内存的大小(不是索引位置) ,即将 1.txt 的多少个字节映射到内存
* 可以直接修改的范围就是 0-5
* 实际类型 DirectByteBuffer
*/
MappedByteBuffer mappedByteBuffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, 5);
mappedByteBuffer.put(0, (byte) 'H');
mappedByteBuffer.put(3, (byte) '9');
mappedByteBuffer.put(5, (byte) 'Y');//IndexOutOfBoundsException
randomAccessFile.close();
System.out.println("修改成功~~");
}
}
前面我们讲的读写操作,都是通过一个Buffer 完成的,NIO 还支持 通过多个Buffer (即 Buffer 数组) 完成读写操作,即 Scattering 和 Gathering 【举例说明】
package com.atguigu.nio;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Arrays;
/**
* Scattering:将数据写入到buffer时,可以采用buffer数组,依次写入 [分散]
* Gathering: 从buffer读取数据时,可以采用buffer数组,依次读
*/
public class ScatteringAndGatheringTest {
public static void main(String[] args) throws Exception {
//使用 ServerSocketChannel 和 SocketChannel 网络
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
InetSocketAddress inetSocketAddress = new InetSocketAddress(7000);
//绑定端口到socket ,并启动
serverSocketChannel.socket().bind(inetSocketAddress);
//创建buffer数组
ByteBuffer[] byteBuffers = new ByteBuffer[2];
byteBuffers[0] = ByteBuffer.allocate(5);
byteBuffers[1] = ByteBuffer.allocate(3);
//等客户端连接(telnet)
SocketChannel socketChannel = serverSocketChannel.accept();
int messageLength = 8; //假定从客户端接收8个字节
//循环的读取
while (true) {
int byteRead = 0;
while (byteRead < messageLength ) {
long l = socketChannel.read(byteBuffers);
byteRead += l; //累计读取的字节数
System.out.println("byteRead=" + byteRead);
//使用流打印, 看看当前的这个buffer的position 和 limit
Arrays.asList(byteBuffers).stream().map(buffer -> "postion=" + buffer.position() + ", limit=" + buffer.limit()).forEach(System.out::println);
}
//将所有的buffer进行flip
Arrays.asList(byteBuffers).forEach(buffer -> buffer.flip());
//将数据读出显示到客户端
long byteWirte = 0;
while (byteWirte < messageLength) {
long l = socketChannel.write(byteBuffers); //
byteWirte += l;
}
//将所有的buffer 进行clear
Arrays.asList(byteBuffers).forEach(buffer-> {
buffer.clear();
});
System.out.println("byteRead:=" + byteRead + " byteWrite=" + byteWirte + ", messagelength" + messageLength);
}
}
}