针对每一种基本类型的 Buffer ，NIO 又根据 Buffer 背后的数据存储内存不同分为了：HeapBuffer，DirectBuffer，MappedBuffer。

HeapBuffer 顾名思义它背后的存储内存是在 JVM 堆中分配，在堆中分配一个数组用来存放 Buffer 中的数据。

public abstract class ByteBufferextends Bufferimplements Comparable<ByteBuffer>
{// Cached array base offsetprivate static final long ARRAY_BASE_OFFSET = UNSAFE.arrayBaseOffset(byte[].class);// 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.//// 在堆中使用一个数组存放Buffer数据final byte[] hb;                  // Non-null only for heap buffers
}

DirectBuffer 背后的存储内存是在堆外内存中分配，MappedBuffer 是通过内存文件映射将文件中的内容直接映射到堆外内存中，其本质也是一个 DirectBuffer 。

由于 DirectBuffer 和 MappedBuffer 背后的存储内存是在堆外内存中分配，不受 JVM 管理，所以不能用一个 Java 基本类型的数组表示，而是直接记录这段堆外内存的起始地址。

public abstract class Buffer {...// Used by heap byte buffers or direct buffers with Unsafe access// For heap byte buffers this field will be the address relative to the// array base address and offset into that array. The address might// not align on a word boundary for slices, nor align at a long word// (8 byte) boundary for byte[] allocations on 32-bit systems.// For direct buffers it is the start address of the memory region. The// address might not align on a word boundary for slices, nor when created// using JNI, see NewDirectByteBuffer(void*, long).// Should ideally be declared final// NOTE: hoisted here for speed in JNI GetDirectBufferAddress// //堆外内存地址long address;
}