说明

• 通过io.netty.buffer.ByteBuf的maxFastWritableBytes()函数可以得到最大快速可写入字节数。最大快速可写入字节数表示没有内部再分配内存的情况下肯定可以写入的最大字节数。
• maxFastWritableBytes() = capacity（容量） - writerIndex
• writableBytes() <= maxFastWritableBytes() <= maxWritableBytes()。

示例

package com.thb;import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;public class Test {public static void main(String[] args) {// 创建一个ByteBuf，没有明确指明初始容量、最大容量ByteBuf buf = Unpooled.buffer();// 写入1个字节buf.writeByte(0x68);  // 当前的writerIndexSystem.out.println("buf.writerIndex: " + buf.writerIndex());// 当前的容量System.out.println("buf.capacity: " + buf.capacity());// 最大容量System.out.println("buf.maxCapacity: " + buf.maxCapacity());// 可写入字节数System.out.println("buf.writableBytes: " + buf.writableBytes());// 最大可写入字节数System.out.println("buf.maxWritableBytes: " + buf.maxWritableBytes());// 最大快速可写入字节数System.out.println("buf.maxFastWritableBytes: " + buf.maxFastWritableBytes());System.out.println("------------------------");// 创建一个ByteBuf，明确指明初始容量是3、最大容量是10ByteBuf buf2 = Unpooled.buffer(3, 10);// 写入1个字节buf2.writeByte(0x68);	System.out.println("buf2.writerIndex: " + buf2.writerIndex());	System.out.println("buf2.capacity: " + buf2.capacity());	System.out.println("buf2.maxCapacity: " + buf2.maxCapacity());		System.out.println("buf2.writableBytes: " + buf2.writableBytes());		System.out.println("buf2.maxWritableBytes: " + buf2.maxWritableBytes());		System.out.println("buf2.maxFastWritableBytes: " + buf2.maxFastWritableBytes());System.out.println("------------------------");// 创建一个ByteBuf，明确指明初始容量是3、最大容量是10ByteBuf buf3 = Unpooled.buffer(3, 10);// 写入3个字节，刚好把初始容量用完了for (int i = 0; i < 3; i++) {buf3.writeByte(0x68);}		System.out.println("buf3.writerIndex: " + buf3.writerIndex());System.out.println("buf3.capacity: " + buf3.capacity());System.out.println("buf3.maxCapacity: " + buf3.maxCapacity());System.out.println("buf3.writableBytes: " + buf3.writableBytes());	System.out.println("buf3.maxWritableBytes: " + buf3.maxWritableBytes());System.out.println("buf3.maxFastWritableBytes: " + buf3.maxFastWritableBytes());System.out.println("------------------------");// 创建一个ByteBuf，明确指明初始容量是3、最大容量是10ByteBuf buf4 = Unpooled.buffer(3, 10);// 写入4个字节。在写入第4个字节的时候，ByteBuf自动扩展了一次for (int i = 0; i < 4; i++) {buf4.writeByte(0x68);}		System.out.println("buf4.writerIndex: " + buf4.writerIndex());System.out.println("buf4.capacity: " + buf4.capacity());System.out.println("buf4.maxCapacity: " + buf4.maxCapacity());System.out.println("buf4.writableBytes: " + buf4.writableBytes());		System.out.println("buf4.maxWritableBytes: " + buf4.maxWritableBytes());System.out.println("buf4.maxFastWritableBytes: " + buf4.maxFastWritableBytes());}
}

运行输出：

buf.writerIndex: 1
buf.capacity: 256
buf.maxCapacity: 2147483647
buf.writableBytes: 255
buf.maxWritableBytes: 2147483646
buf.maxFastWritableBytes: 255
------------------------
buf2.writerIndex: 1
buf2.capacity: 3
buf2.maxCapacity: 10
buf2.writableBytes: 2
buf2.maxWritableBytes: 9
buf2.maxFastWritableBytes: 2
------------------------
buf3.writerIndex: 3
buf3.capacity: 3
buf3.maxCapacity: 10
buf3.writableBytes: 0
buf3.maxWritableBytes: 7
buf3.maxFastWritableBytes: 0
------------------------
buf4.writerIndex: 4
buf4.capacity: 10
buf4.maxCapacity: 10
buf4.writableBytes: 6
buf4.maxWritableBytes: 6
buf4.maxFastWritableBytes: 6