UnpooledHeapByteBuf 是基于堆内存进行内存分配的字节缓冲区，没有基于对象池技术实现，这意味着每次I/O的读写都会创建一个新的UnpooledHeapByteBuf，频繁进行大块内存的分配和回收对性能会造成一定的影响，但是对比与堆外内存的申请和释放，它的成本会低一些。
相对与PooledHeapByteBuf,UnpooledHeapByteBuf 的实现原理更加简单，也不容易出现内存管理方面的问题，在满足性能的情况下，尽量使用UnpooledHeapByteBuf 。
1.成员变量
private final ByteBufAllocator alloc;//聚合一个ByteBufAllocator，用于UnpooledHeapByteBuf的内存分配private byte[] array;//byte 数组作为缓冲区private ByteBuffer tmpNioBuf;//用于实现Netty ByteBuf 到 JDK ByteBuffer 的转换
事实上，如果使用JDK 的ByteBuffer替换byte数组也是可行的 ，直接使用byte数组的根本原因是提升性能和更加便捷的进行位操作。JDK 的ByteBuffer底层实现也是byte数组，如下。
public abstract class ByteBuffer ???extends Buffer ???implements Comparable<ByteBuffer>{ ???// These fields are declared here rather than in Heap-X-Buffer in order to ???// reduce the number of virtual method invocations needed to access these ???// values, which is especially costly when coding small buffers. ???// ???final byte[] hb; ?????????????????// Non-null only for heap buffers ???final int offset; ???boolean isReadOnly; ????????????????// Valid only for heap buffers2.动态扩展缓冲区
@Overridepublic ByteBuf capacity(int newCapacity) { ???ensureAccessible(); ???if (newCapacity < 0 || newCapacity > maxCapacity()) { ???????throw new IllegalArgumentException("newCapacity: " + newCapacity); ???} ???int oldCapacity = array.length; ???if (newCapacity > oldCapacity) { ???????byte[] newArray = new byte[newCapacity]; ???????System.arraycopy(array, 0, newArray, 0, array.length); ???????setArray(newArray); ???} else if (newCapacity < oldCapacity) { ???????byte[] newArray = new byte[newCapacity]; ???????int readerIndex = readerIndex(); ???????if (readerIndex < newCapacity) { ???????????int writerIndex = writerIndex(); ???????????if (writerIndex > newCapacity) { ???????????????writerIndex(writerIndex = newCapacity); ???????????} ???????????System.arraycopy(array, readerIndex, newArray, readerIndex, writerIndex - readerIndex); ???????} else { ???????????setIndex(newCapacity, newCapacity); ???????} ???????setArray(newArray); ???} ???return this;}/** * Should be called by every method that tries to access the buffers content to check * if the buffer was released before. */protected final void ensureAccessible() { ???if (refCnt() == 0) { ???????throw new IllegalReferenceCountException(0); ???}}方法 的入口首先对新容量进行合法性校验，如果大于容量上限或者小于0，则抛出IllegalArgumentException异常。
判断新的容量值是否大于当前的缓冲区容量，如果大于则需要动态扩展，通过 byte[] newArray = new byte[newCapacity];创建新的缓冲区字节数组，然后通过 System.arraycopy进行内存复制，将旧的字节数组复制到新创建的字节数组中，最后调用setArray(newArray);替换旧的字节数组。
private void setArray(byte[] initialArray) { ???array = initialArray; ???tmpNioBuf = null;}动态扩容完成后，需要将原来的视图tmpNioBuf设置为空。
如果新的容量小于当前缓冲区容量不需要动态扩展，但是需要截取当前缓冲区创建一个新的子缓冲区。先判断下读索引是否小于新的容量值，如果小于进一步判断写索引是否大于新的容量值，如果大于则将写索引设置为新的容量值（防止越界）。更新完写索引之后，通过 System.arraycopy将当前可读的字节数组复制到新创建的子缓冲区中， System.arraycopy(array, readerIndex, newArray, readerIndex, writerIndex - readerIndex);
如果新的容量值小于读索引，说明没有可读的字节数组需要复制到新创建的缓冲区中，将读写索引为新的容量值即可。最后调用setArray方法替换原来的字节数组。
3.字节数组复制
@Overridepublic ByteBuf setBytes(int index, byte[] src, int srcIndex, int length) { ???checkSrcIndex(index, length, srcIndex, src.length); ???System.arraycopy(src, srcIndex, array, index, length); ???return this;}首先做合法性校验。
protected final void checkSrcIndex(int index, int length, int srcIndex, int srcCapacity) { ???checkIndex(index, length); ???if (srcIndex < 0 || srcIndex > srcCapacity - length) { ???????throw new IndexOutOfBoundsException(String.format( ???????????????"srcIndex: %d, length: %d (expected: range(0, %d))", srcIndex, length, srcCapacity)); ???}}校验index，length的值，如果小于0，抛出IllegalArgumentException异常，然后对两者之和进行判断，如果大于缓冲区的容量，则抛出IndexOutOfBoundsException异常。srcIndex和srcCapacity，与之类似。校验通过之后，调用 System.arraycopy(src, srcIndex, array, index, length)方法进行字节数组的复制。
protected final void checkIndex(int index, int fieldLength) { ???ensureAccessible(); ???if (fieldLength < 0) { ???????throw new IllegalArgumentException("length: " + fieldLength + " (expected: >= 0)"); ???} ???if (index < 0 || index > capacity() - fieldLength) { ???????throw new IndexOutOfBoundsException(String.format( ???????????????"index: %d, length: %d (expected: range(0, %d))", index, fieldLength, capacity())); ???}}注意： ButeBuf以set和get开头读写缓冲区的方法不会修改读写索引。
4.转换为JDK ByteBuffer
ByteBuffer 是基于byte数组实现，NIO的ByteBuffer提供wrap方法，可以将byte数组转换成ByteBuffer对象。
/** ????* Wraps a byte array into a buffer. ????* ????* <p> The new buffer will be backed by the given byte array; ????* that is, modifications to the buffer will cause the array to be modified ????* and vice versa. ?The new buffer‘s capacity will be ????* <tt>array.length</tt>, its position will be <tt>offset</tt>, its limit ????* will be <tt>offset + length</tt>, and its mark will be undefined. ?Its ????* {@link #array backing array} will be the given array, and ????* its {@link #arrayOffset array offset} will be zero. ?</p> ????* ????* @param ?array ????* ????????The array that will back the new buffer ????* ????* @param ?offset ????* ????????The offset of the subarray to be used; must be non-negative and ????* ????????no larger than <tt>array.length</tt>. ?The new buffer‘s position ????* ????????will be set to this value. ????* ????* @param ?length ????* ????????The length of the subarray to be used; ????* ????????must be non-negative and no larger than ????* ????????<tt>array.length - offset</tt>. ????* ????????The new buffer‘s limit will be set to <tt>offset + length</tt>. ????* ????* @return ?The new byte buffer ????* ????* @throws ?IndexOutOfBoundsException ????* ?????????If the preconditions on the <tt>offset</tt> and <tt>length</tt> ????* ?????????parameters do not hold ????*/ ???public static ByteBuffer wrap(byte[] array, ???????????????????????????????????int offset, int length) ???{ ???????try { ???????????return new HeapByteBuffer(array, offset, length); ???????} catch (IllegalArgumentException x) { ???????????throw new IndexOutOfBoundsException(); ???????} ???}UnpooledHeapByteBuf.java
@Overridepublic ByteBuffer nioBuffer(int index, int length) { ???ensureAccessible(); ???return ByteBuffer.wrap(array, index, length).slice();}调用了ByteBuffer的slice方法，由于每次调用nioBuffer都会创建一个新的ByteBuffer，因此此处的slice方法起不到重用缓冲区的效果，只能保证读写索引的独立性。
5.与子类相关的方法
isDirect方法：如果基础堆内存实现的ByteBuf，返回false，
@Overridepublic boolean isDirect() { ???return false;}hasArray方法：因为UnpooledHeapByteBuf是基于字节数组实现，返回true
@Overridepublic boolean hasArray() { ???return true;}array方法：因为UnpooledHeapByteBuf是基于字节数组实现，所以返回值是内部的字节数组成员变量。调用array方法之前，可以先使用hasArray方法进行判断，如果返回false说明当前ByteBuf不支持array方法。
@Overridepublic byte[] array() { ???ensureAccessible(); ???return array;}netty UnpooledHeapByteBuf 源码分析
原文地址：https://www.cnblogs.com/zwb1234/p/9577757.html