netty(六) buffer 源码分析

问题 ：

• netty的 ByteBuff 和传统的ByteBuff的区别是什么？
• HeapByteBuf 和 DirectByteBuf 的区别 ？
  HeapByteBuf ： 使用堆内存，缺点 ，socket 传输的时候由于需要复制的原因，慢一点
  DirectByteBuf ： 堆外内存，可以使用零拷贝

概述

netty ByteBuf 存在两个指针，分成三个区域: 已读区（可丢弃），未读区（未读），可写区 。不像之前JDK 的 ByteBuffer 中只有一个position 指针。例如以下示例 ：

    public static void main(String[] args){
        ByteBuffer buffer = ByteBuffer.allocate(88);
        String value = "Netty~~";
        buffer.put(value.getBytes());
        //注意这个flip()方法，要是不调用，将读取到不正确的位置
        buffer.flip();
        byte[] vArray = new byte[buffer.remaining()];
        buffer.get(vArray);
        String result = new String(vArray);
        System.out.println(result);
    }

概述一下netty ByteBuff 的特点 ：

• 丰富API，存在readIndex 和 writeIndex 两个指针，方便读写
• 动态扩容
• 提供兼容 JDK ByteBuffer的方法

分类

内存池，循环利用创建的 ByteBuf 对象提升内存使用效率，降低由于高负载导致的频繁 GC .
PooledByteBuf 抽象类的子类 ：

• PooledDirectByteBuf
• PooledHeapByteBuf
• PooledUnsafeDirectByteBuf
  看一下类的结构图

源码分析

AbstractByteBuf 源码分析

    @Override
    public ByteBuf readBytes(ByteBuf dst, int dstIndex, int length) {
        checkReadableBytes(length);
        //抽象方法交由子类实现
        getBytes(readerIndex, dst, dstIndex, length);
        readerIndex += length;
        return this;
    }

看一下写操作

    @Override
    public ByteBuf writeBytes(byte[] src, int srcIndex, int length) {
        ensureWritable(length);
        setBytes(writerIndex, src, srcIndex, length);
        writerIndex += length;
        return this;
    }

    @Override
    public ByteBuf ensureWritable(int minWritableBytes) {
        if (minWritableBytes < 0) {
            throw new IllegalArgumentException(String.format(
                    "minWritableBytes: %d (expected: >= 0)", minWritableBytes));
        }

        if (minWritableBytes <= writableBytes()) {
            return this;
        }

        if (minWritableBytes > maxCapacity - writerIndex) {
            throw new IndexOutOfBoundsException(String.format(
                    "writerIndex(%d) + minWritableBytes(%d) exceeds maxCapacity(%d): %s",
                    writerIndex, minWritableBytes, maxCapacity, this));
        }

        // Normalize the current capacity to the power of 2.
        int newCapacity = calculateNewCapacity(writerIndex + minWritableBytes);

        // Adjust to the new capacity.
        capacity(newCapacity);
        return this;
    }

   /**
    * 计算新容量并没有一下子就增加一倍这样的简单思路，而是一点点地增加。
    *
    */
    private int calculateNewCapacity(int minNewCapacity) {
        final int maxCapacity = this.maxCapacity;
        final int threshold = 1048576 * 4; // 4 MiB page

        if (minNewCapacity == threshold) {
            return threshold;
        }

        // If over threshold, do not double but just increase by threshold.
        if (minNewCapacity > threshold) {
            int newCapacity = minNewCapacity / threshold * threshold;
            if (newCapacity > maxCapacity - threshold) {
                newCapacity = maxCapacity;
            } else {
                newCapacity += threshold;
            }
            return newCapacity;
        }

        // Not over threshold. Double up to 4 MiB, starting from 64.
        int newCapacity = 64;
        while (newCapacity < minNewCapacity) {
            newCapacity <<= 1;
        }

        return Math.min(newCapacity, maxCapacity);
    }

 丢弃已读区域，复用缓冲区

    @Override
    public ByteBuf discardReadBytes() {
        ensureAccessible();
        if (readerIndex == 0) {
            return this;
        }

        if (readerIndex != writerIndex) {
        	//子类实现，字节数组进行复制，读写区域进行前移
            setBytes(0, this, readerIndex, writerIndex - readerIndex);
            writerIndex -= readerIndex;
            adjustMarkers(readerIndex);
            readerIndex = 0;
        } else {
            adjustMarkers(readerIndex);
            writerIndex = readerIndex = 0;
        }
        return this;
    }

    protected final void adjustMarkers(int decrement) {
        int markedReaderIndex = this.markedReaderIndex;
        if (markedReaderIndex <= decrement) {
            this.markedReaderIndex = 0;
            int markedWriterIndex = this.markedWriterIndex;
            if (markedWriterIndex <= decrement) {
                this.markedWriterIndex = 0;
            } else {
                this.markedWriterIndex = markedWriterIndex - decrement;
            }
        } else {
            this.markedReaderIndex = markedReaderIndex - decrement;
            markedWriterIndex -= decrement;
        }
    }

AbstractReferenceCountedByteBuf 源码分析

 从名字看出该类主要对引用进行计数，类似于JVM 内存回收的对象引用计数器，用于跟踪对象的分配和销毁，做自动内存回收。

public abstract class AbstractReferenceCountedByteBuf extends AbstractByteBuf {
	//利用原子类进行CAS 操作，保证了线程安全
    private static final AtomicIntegerFieldUpdater<AbstractReferenceCountedByteBuf> refCntUpdater =
            AtomicIntegerFieldUpdater.newUpdater(AbstractReferenceCountedByteBuf.class, "refCnt");

    private static final long REFCNT_FIELD_OFFSET;

    static {
        long refCntFieldOffset = -1;
        try {
            if (PlatformDependent.hasUnsafe()) {
                refCntFieldOffset = PlatformDependent.objectFieldOffset(
                        AbstractReferenceCountedByteBuf.class.getDeclaredField("refCnt"));
            }
        } catch (Throwable t) {
            // Ignored
        }

        REFCNT_FIELD_OFFSET = refCntFieldOffset;
    }

    @SuppressWarnings("FieldMayBeFinal")
    private volatile int refCnt = 1;

    @Override
    public final boolean release() {
        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalReferenceCountException(0, -1);
            }

            if (refCntUpdater.compareAndSet(this, refCnt, refCnt - 1)) {
                if (refCnt == 1) {
                    deallocate();
                    return true;
                }
                return false;
            }
        }
    }

    @Override
    public ByteBuf retain() {
        for (;;) {
            int refCnt = this.refCnt;
            if (refCnt == 0) {
                throw new IllegalReferenceCountException(0, 1);
            }
            if (refCnt == Integer.MAX_VALUE) {
                throw new IllegalReferenceCountException(Integer.MAX_VALUE, 1);
            }
            //CAS 操作
            if (refCntUpdater.compareAndSet(this, refCnt, refCnt + 1)) {
                break;
            }
        }
        return this;
    }    

    ....    

这个类有三个重要的字段，一个原子类用于多线程操作，保证线程安全。REFCNT_FIELD_OFFSET 是一个内存偏移量，用于标识 refCnt字段在AbstractReferenceCountedByteBuf这个类
的内存地址，最后一个refCnt 是用 volatile 修饰的变量，保存对象应用次数。

UnpooledHeapByteBuf 非内存池堆内存ByteBuf源码分析

public class UnpooledHeapByteBuf extends AbstractReferenceCountedByteBuf {

	//内存分配
    private final ByteBufAllocator alloc;
    //字节缓存区
    private byte[] array;
    //作用和 JDK ByteBuffer 的转化
    private ByteBuffer tmpNioBuf;

    ...

    private int getBytes(int index, GatheringByteChannel out, int length, boolean internal) throws IOException {
        ensureAccessible();
        ByteBuffer tmpBuf;
        //使用自身字段 tmpNioBuf 进行操作返回，所以 UnpooledHeapByteBuf 是在 JDK ByteBuff 的基础上进行扩展的。
        if (internal) {
            tmpBuf = internalNioBuffer();
        } else {
            tmpBuf = ByteBuffer.wrap(array);
        }
        return out.write((ByteBuffer) tmpBuf.clear().position(index).limit(index + length));
    }

    @Override
    public int readBytes(GatheringByteChannel out, int length) throws IOException {
        checkReadableBytes(length);
        int readBytes = getBytes(readerIndex, out, length, true);
        readerIndex += readBytes;
        return readBytes;
    }

总结

文章主要介绍netty buffer 相关的知识，主要是父类方法和 unpooled 相关的实现。

参考资料

• 《netty 权威指南》