一.组合模式作用以及场景
1.当对象之间具有部分和整体结构时，比如目录与子目录，总类和子类，父节点和子节点。
2.当子对象操作可以统一处理时
二.复合缓冲区 Composite Buffer
Composite Buffer是Netty特有的缓冲区。本质上类似于提供一个或多个ByteBuf的组合视图，
可以根据需要添加和删除不同类型的ByteBuf。它是一个组合视图。它提供一种访问方式让使用者自
由的组合多个ByteBuf，避免了拷贝和分配新的缓冲区。
Composite Buffer不支持访问其支撑数组。因此如果要访问，需要先将内容拷贝到堆内存
中，再进行访问
下图是将多个ByteBuf组合在一起，没有进行任何复制过程。仅仅创建了一个视
图
三.用法
private static ByteBuf newCompositeBuffer(ByteBufAllocator alloc) {
CompositeByteBuf compositeByteBuf = alloc.compositeBuffer();
compositeByteBuf.addComponent(true, alloc.directBuffer(4).writeInt(100));
compositeByteBuf.addComponent(true, alloc.directBuffer(8).writeLong(123));
compositeByteBuf.addComponent(true, alloc.directBuffer(8).writeLong(456));
assertEquals(EXPECTED_BYTES, compositeByteBuf.readableBytes());
return compositeByteBuf;
}
public static void testSingleCompositeBufferWrite(ServerBootstrap sb, Bootstrap cb) throws Throwable {
Channel serverChannel = null;
Channel clientChannel = null;
try {
final CountDownLatch latch = new CountDownLatch(1);
final AtomicReference<Object> clientReceived = new AtomicReference<Object>();
sb.childHandler(new ChannelInitializer<Channel>() {
@Override
protected void initChannel(Channel ch) throws Exception {
ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
@Override
public void channelActive(ChannelHandlerContext ctx) throws Exception {
ctx.writeAndFlush(newCompositeBuffer(ctx.alloc()))
.addListener(ChannelFutureListener.CLOSE);
}
});
}
});
cb.handler(new ChannelInitializer<Channel>() {
@Override
protected void initChannel(Channel ch) throws Exception {
ch.pipeline().addLast(new ChannelInboundHandlerAdapter() {
private ByteBuf aggregator;
@Override
public void handlerAdded(ChannelHandlerContext ctx) {
aggregator = ctx.alloc().buffer(EXPECTED_BYTES);
}
@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) {
try {
if (msg instanceof ByteBuf) {
aggregator.writeBytes((ByteBuf) msg);
}
} finally {
ReferenceCountUtil.release(msg);
}
}
@Override
public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
// IOException is fine as it will also close the channel and may just be a connection reset.
if (!(cause instanceof IOException)) {
clientReceived.set(cause);
latch.countDown();
}
}
@Override
public void channelInactive(ChannelHandlerContext ctx) throws Exception {
if (clientReceived.compareAndSet(null, aggregator)) {
try {
assertEquals(EXPECTED_BYTES, aggregator.readableBytes());
} catch (Throwable cause) {
aggregator.release();
aggregator = null;
clientReceived.set(cause);
} finally {
latch.countDown();
}
}
}
});
}
});
serverChannel = sb.bind().syncUninterruptibly().channel();
clientChannel = cb.connect(serverChannel.localAddress()).syncUninterruptibly().channel();
ByteBuf expected = newCompositeBuffer(clientChannel.alloc());
latch.await();
Object received = clientReceived.get();
if (received instanceof ByteBuf) {
ByteBuf actual = (ByteBuf) received;
assertEquals(expected, actual);
expected.release();
actual.release();
} else {
expected.release();
throw (Throwable) received;
}
} finally {
if (clientChannel != null) {
clientChannel.close().sync();
}
if (serverChannel != null) {
serverChannel.close().sync();
}
}
}
四.源码
内部类 CompositeByteBuf.Component
private static final class Component {
final ByteBuf srcBuf; // the originally added buffer
final ByteBuf buf; // srcBuf unwrapped zero or more times
int srcAdjustment; // index of the start of this CompositeByteBuf relative to srcBuf
int adjustment; // index of the start of this CompositeByteBuf relative to buf
int offset; // offset of this component within this CompositeByteBuf
int endOffset; // end offset of this component within this CompositeByteBuf
private ByteBuf slice; // cached slice, may be null
Component(ByteBuf srcBuf, int srcOffset, ByteBuf buf, int bufOffset,
int offset, int len, ByteBuf slice) {
this.srcBuf = srcBuf;
this.srcAdjustment = srcOffset - offset;
this.buf = buf;
this.adjustment = bufOffset - offset;
this.offset = offset;
this.endOffset = offset + len;
this.slice = slice;
}
int srcIdx(int index) {
return index + srcAdjustment;
}
int idx(int index) {
return index + adjustment;
}
int length() {
return endOffset - offset;
}
void reposition(int newOffset) {
int move = newOffset - offset;
endOffset += move;
srcAdjustment -= move;
adjustment -= move;
offset = newOffset;
}
// copy then release
void transferTo(ByteBuf dst) {
dst.writeBytes(buf, idx(offset), length());
free();
}
ByteBuf slice() {
ByteBuf s = slice;
if (s == null) {
slice = s = srcBuf.slice(srcIdx(offset), length());
}
return s;
}
ByteBuf duplicate() {
return srcBuf.duplicate();
}
ByteBuffer internalNioBuffer(int index, int length) {
// Some buffers override this so we must use srcBuf
return srcBuf.internalNioBuffer(srcIdx(index), length);
}
void free() {
slice = null;
// Release the original buffer since it may have a different
// refcount to the unwrapped buf (e.g. if PooledSlicedByteBuf)
srcBuf.release();
}
}
public class CompositeByteBuf extends AbstractReferenceCountedByteBuf implements Iterable<ByteBuf> {
private static final ByteBuffer EMPTY_NIO_BUFFER = Unpooled.EMPTY_BUFFER.nioBuffer();
private static final Iterator<ByteBuf> EMPTY_ITERATOR = Collections.<ByteBuf>emptyList().iterator();
private final ByteBufAllocator alloc;
private final boolean direct;
private final int maxNumComponents;
private int componentCount;
private Component[] components; // resized when needed
private boolean freed;
private CompositeByteBuf(ByteBufAllocator alloc, boolean direct, int maxNumComponents, int initSize) {
super(AbstractByteBufAllocator.DEFAULT_MAX_CAPACITY);
this.alloc = ObjectUtil.checkNotNull(alloc, "alloc");
if (maxNumComponents < 1) {
throw new IllegalArgumentException(
"maxNumComponents: " + maxNumComponents + " (expected: >= 1)");
}
this.direct = direct;
this.maxNumComponents = maxNumComponents;
components = newCompArray(initSize, maxNumComponents);
}
public CompositeByteBuf(ByteBufAllocator alloc, boolean direct, int maxNumComponents) {
this(alloc, direct, maxNumComponents, 0);
}
public CompositeByteBuf(ByteBufAllocator alloc, boolean direct, int maxNumComponents, ByteBuf... buffers) {
this(alloc, direct, maxNumComponents, buffers, 0);
}
CompositeByteBuf(ByteBufAllocator alloc, boolean direct, int maxNumComponents,
ByteBuf[] buffers, int offset) {
this(alloc, direct, maxNumComponents, buffers.length - offset);
addComponents0(false, 0, buffers, offset);
consolidateIfNeeded();
setIndex0(0, capacity());
}
public CompositeByteBuf(
ByteBufAllocator alloc, boolean direct, int maxNumComponents, Iterable<ByteBuf> buffers) {
this(alloc, direct, maxNumComponents,
buffers instanceof Collection ? ((Collection<ByteBuf>) buffers).size() : 0);
addComponents(false, 0, buffers);
setIndex(0, capacity());
}
//......
public CompositeByteBuf addComponent(ByteBuf buffer) {
return addComponent(false, buffer);
}
public CompositeByteBuf addComponents(ByteBuf... buffers) {
return addComponents(false, buffers);
}
public CompositeByteBuf addComponent(boolean increaseWriterIndex, ByteBuf buffer) {
return addComponent(increaseWriterIndex, componentCount, buffer);
}
public CompositeByteBuf addComponent(boolean increaseWriterIndex, int cIndex, ByteBuf buffer) {
checkNotNull(buffer, "buffer");
addComponent0(increaseWriterIndex, cIndex, buffer);
consolidateIfNeeded();
return this;
}
private int addComponent0(boolean increaseWriterIndex, int cIndex, ByteBuf buffer) {
assert buffer != null;
boolean wasAdded = false;
try {
checkComponentIndex(cIndex);
// No need to consolidate - just add a component to the list.
Component c = newComponent(ensureAccessible(buffer), 0);
int readableBytes = c.length();
// Check if we would overflow.
// See https://github.com/netty/netty/issues/10194
checkForOverflow(capacity(), readableBytes);
addComp(cIndex, c);
wasAdded = true;
if (readableBytes > 0 && cIndex < componentCount - 1) {
updateComponentOffsets(cIndex);
} else if (cIndex > 0) {
c.reposition(components[cIndex - 1].endOffset);
}
if (increaseWriterIndex) {
writerIndex += readableBytes;
}
return cIndex;
} finally {
if (!wasAdded) {
buffer.release();
}
}
}
//......
}
五.总结
通过组合模式将不同的ByteBuf合并成一个ByteBuf，并且不产生额外空间，将数据的合并封装并统一处理，方便了管理和调用。