Allow specifying 0 as the default number of threads when instantiating an EventLoopGroup

- Fixes #1426
- We already allow a user instantiate an EventLoopGroup with the default number of threads via the default constructor, so I think it's OK although it's not always optimal.

transport/src/main/java/io/netty/channel/AbstractChannel.java

@@ -18,8 +18,8 @@ package io.netty.channel;
 import io.netty.buffer.ByteBuf;
 import io.netty.buffer.ByteBufAllocator;
 import io.netty.buffer.ByteBufHolder;
-import io.netty.buffer.ByteBufUtil;
 import io.netty.util.DefaultAttributeMap;
+import io.netty.util.ReferenceCountUtil;
 import io.netty.util.internal.PlatformDependent;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
@@ -715,7 +715,7 @@ public abstract class AbstractChannel extends DefaultAttributeMap implements Cha
                     if (promise.isCancelled()) {
                         // If cancelled, release all unwritten messages and recycle.
                         for (int i = messageIndex; i < messageCount; i ++) {
-                            ByteBufUtil.release(messages.get(i));
+                            ReferenceCountUtil.release(messages.get(i));
                         }
                         messages.recycle();
                         if (!outboundBuffer.next()) {

transport/src/main/java/io/netty/channel/AbstractServerChannel.java

@@ -15,7 +15,7 @@
  */
 package io.netty.channel;
 
-import io.netty.buffer.ByteBufUtil;
+import io.netty.util.ReferenceCountUtil;
 
 import java.net.SocketAddress;
 
@@ -81,7 +81,7 @@ public abstract class AbstractServerChannel extends AbstractChannel implements S
             reject(promise);
             int size = msgs.size();
             for (int i = 0; i < size; i ++) {
-                ByteBufUtil.release(msgs.get(i));
+                ReferenceCountUtil.release(msgs.get(i));
             }
         }
 

transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java

@@ -19,7 +19,7 @@
  */
 package io.netty.channel;
 
-import io.netty.buffer.ByteBufUtil;
+import io.netty.util.ReferenceCountUtil;
 import io.netty.util.internal.logging.InternalLogger;
 import io.netty.util.internal.logging.InternalLoggerFactory;
 
@@ -228,7 +228,7 @@ final class ChannelOutboundBuffer {
             try {
                 for (int i = currentMessageIndex; i < currentMessages.size(); i++) {
                     Object msg = currentMessages.get(i);
-                    ByteBufUtil.release(msg);
+                    ReferenceCountUtil.release(msg);
                 }
             } finally {
                 currentMessages.recycle();

transport/src/main/java/io/netty/channel/DefaultChannelPipeline.java

@@ -15,8 +15,8 @@
  */
 package io.netty.channel;
 
-import io.netty.buffer.ByteBufUtil;
 import io.netty.channel.Channel.Unsafe;
+import io.netty.util.ReferenceCountUtil;
 import io.netty.util.concurrent.EventExecutor;
 import io.netty.util.concurrent.EventExecutorGroup;
 import io.netty.util.internal.PlatformDependent;
@@ -970,7 +970,7 @@ final class DefaultChannelPipeline implements ChannelPipeline {
                         "Discarded inbound message {} that reached at the tail of the pipeline. " +
                                 "Please check your pipeline configuration.", m);
 
-                ByteBufUtil.release(m);
+                ReferenceCountUtil.release(m);
             }
 
             if (length != 1) {

transport/src/main/java/io/netty/channel/MultithreadEventLoopGroup.java

@@ -31,7 +31,7 @@ public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutor
 
     private static final InternalLogger logger = InternalLoggerFactory.getInstance(MultithreadEventLoopGroup.class);
 
-    public static final int DEFAULT_EVENT_LOOP_THREADS;
+    private static final int DEFAULT_EVENT_LOOP_THREADS;
 
     static {
         DEFAULT_EVENT_LOOP_THREADS = Math.max(1, SystemPropertyUtil.getInt(
@@ -43,7 +43,7 @@ public abstract class MultithreadEventLoopGroup extends MultithreadEventExecutor
     }
 
     protected MultithreadEventLoopGroup(int nThreads, ThreadFactory threadFactory, Object... args) {
-        super(nThreads, threadFactory, args);
+        super(nThreads == 0? DEFAULT_EVENT_LOOP_THREADS : nThreads, threadFactory, args);
     }
 
     @Override

transport/src/main/java/io/netty/channel/local/LocalEventLoopGroup.java

@@ -26,10 +26,10 @@ import java.util.concurrent.ThreadFactory;
 public class LocalEventLoopGroup extends MultithreadEventLoopGroup {
 
     /**
-     * Create a new instance which used {@link #DEFAULT_EVENT_LOOP_THREADS} number of Threads
+     * Create a new instance with the default number of threads.
      */
     public LocalEventLoopGroup() {
-        this(DEFAULT_EVENT_LOOP_THREADS);
+        this(0);
     }
 
     /**

transport/src/main/java/io/netty/channel/nio/NioEventLoopGroup.java

@@ -29,15 +29,15 @@ import java.util.concurrent.ThreadFactory;
 public class NioEventLoopGroup extends MultithreadEventLoopGroup {
 
     /**
-     * Create a new instance using {@link #DEFAULT_EVENT_LOOP_THREADS} number of threads, the default
+     * Create a new instance using the default number of threads, the default {@link ThreadFactory} and
-     * {@link ThreadFactory} and  the {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
+     * the {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
      */
     public NioEventLoopGroup() {
-        this(DEFAULT_EVENT_LOOP_THREADS);
+        this(0);
     }
 
     /**
-     * Create a new instance using nThreads number of threads, {@link ThreadFactory} and the
+     * Create a new instance using the specified number of threads, {@link ThreadFactory} and the
      * {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
      */
     public NioEventLoopGroup(int nThreads) {
@@ -45,7 +45,7 @@ public class NioEventLoopGroup extends MultithreadEventLoopGroup {
     }
 
     /**
-     * Create a new instance using nThreads number of threads, the given {@link ThreadFactory} and the
+     * Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the
      * {@link SelectorProvider} which is returned by {@link SelectorProvider#provider()}.
      */
     public NioEventLoopGroup(int nThreads, ThreadFactory threadFactory) {
@@ -53,7 +53,7 @@ public class NioEventLoopGroup extends MultithreadEventLoopGroup {
     }
 
     /**
-     * Create a new instance using nThreads number of threads, the given {@link ThreadFactory} and the given
+     * Create a new instance using the specified number of threads, the given {@link ThreadFactory} and the given
      * {@link SelectorProvider}.
      */
     public NioEventLoopGroup(