Implement internal memory access methods of CompositeByteBuf correctly

Motivation:

When a CompositeByteBuf is empty (i.e. has no component), its internal
memory access operations do not always behave as expected.

Modifications:

Check if the nunmber of components is zero. If so, return an empty
array or an empty NIO buffer, etc.

Result:

More robustness

buffer/src/main/java/io/netty/buffer/CompositeByteBuf.java

@@ -39,13 +39,13 @@ import java.util.ListIterator;
  */
 public class CompositeByteBuf extends AbstractReferenceCountedByteBuf {
 
+    private static final ByteBuffer EMPTY_NIO_BUFFER = Unpooled.EMPTY_BUFFER.nioBuffer();
+
     private final ResourceLeak leak;
     private final ByteBufAllocator alloc;
     private final boolean direct;
     private final List<Component> components = new ArrayList<Component>();
     private final int maxNumComponents;
-    private static final ByteBuffer FULL_BYTEBUFFER = (ByteBuffer) ByteBuffer.allocate(1).position(1);
-    private static final ByteBuffer EMPTY_BYTEBUFFER = ByteBuffer.allocateDirect(0);
 
     private boolean freed;
 
@@ -444,50 +444,71 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf {
 
     @Override
     public boolean hasArray() {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return true;
+        case 1:
             return components.get(0).buf.hasArray();
+        default:
+            return false;
         }
-        return false;
     }
 
     @Override
     public byte[] array() {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return EmptyArrays.EMPTY_BYTES;
+        case 1:
             return components.get(0).buf.array();
+        default:
+            throw new UnsupportedOperationException();
         }
-        throw new UnsupportedOperationException();
     }
 
     @Override
     public int arrayOffset() {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return 0;
+        case 1:
             return components.get(0).buf.arrayOffset();
+        default:
+            throw new UnsupportedOperationException();
         }
-        throw new UnsupportedOperationException();
     }
 
     @Override
     public boolean hasMemoryAddress() {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return Unpooled.EMPTY_BUFFER.hasMemoryAddress();
+        case 1:
             return components.get(0).buf.hasMemoryAddress();
+        default:
+            return false;
         }
-        return false;
     }
 
     @Override
     public long memoryAddress() {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return Unpooled.EMPTY_BUFFER.memoryAddress();
+        case 1:
             return components.get(0).buf.memoryAddress();
+        default:
+            throw new UnsupportedOperationException();
         }
-        throw new UnsupportedOperationException();
     }
 
     @Override
     public int capacity() {
-        if (components.isEmpty()) {
+        final int numComponents = components.size();
+        if (numComponents == 0) {
             return 0;
         }
-        return components.get(components.size() - 1).endOffset;
+        return components.get(numComponents - 1).endOffset;
     }
 
     @Override
@@ -964,7 +985,7 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf {
     public int setBytes(int index, ScatteringByteChannel in, int length) throws IOException {
         checkIndex(index, length);
         if (length == 0) {
-            return in.read(FULL_BYTEBUFFER);
+            return in.read(EMPTY_NIO_BUFFER);
         }
 
         int i = toComponentIndex(index);
@@ -1093,9 +1114,12 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf {
 
     @Override
     public int nioBufferCount() {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return 1;
+        case 1:
             return components.get(0).buf.nioBufferCount();
-        } else {
+        default:
             int count = 0;
             int componentsCount = components.size();
             for (int i = 0; i < componentsCount; i++) {
@@ -1108,26 +1132,35 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf {
 
     @Override
     public ByteBuffer internalNioBuffer(int index, int length) {
-        if (components.size() == 1) {
+        switch (components.size()) {
+        case 0:
+            return EMPTY_NIO_BUFFER;
+        case 1:
             return components.get(0).buf.internalNioBuffer(index, length);
+        default:
+            throw new UnsupportedOperationException();
         }
-        throw new UnsupportedOperationException();
     }
 
     @Override
     public ByteBuffer nioBuffer(int index, int length) {
-        if (components.size() == 1) {
+        checkIndex(index, length);
+
+        switch (components.size()) {
+        case 0:
+            return EMPTY_NIO_BUFFER;
+        case 1:
             ByteBuf buf = components.get(0).buf;
             if (buf.nioBufferCount() == 1) {
                 return components.get(0).buf.nioBuffer(index, length);
             }
         }
+
         ByteBuffer merged = ByteBuffer.allocate(length).order(order());
         ByteBuffer[] buffers = nioBuffers(index, length);
 
-        //noinspection ForLoopReplaceableByForEach
+        for (ByteBuffer buf: buffers) {
-        for (int i = 0; i < buffers.length; i++) {
+            merged.put(buf);
-            merged.put(buffers[i]);
         }
 
         merged.flip();
@@ -1138,7 +1171,7 @@ public class CompositeByteBuf extends AbstractReferenceCountedByteBuf {
     public ByteBuffer[] nioBuffers(int index, int length) {
         checkIndex(index, length);
         if (length == 0) {
-            return new ByteBuffer[] { EMPTY_BYTEBUFFER };
+            return new ByteBuffer[] { EMPTY_NIO_BUFFER };
         }
 
         List<ByteBuffer> buffers = new ArrayList<ByteBuffer>(components.size());