diff --git a/java/org/apache/tomcat/util/net/NioEndpoint.java b/java/org/apache/tomcat/util/net/NioEndpoint.java
index 0831273d641b..a6f0f62a5e30 100644
--- a/java/org/apache/tomcat/util/net/NioEndpoint.java
+++ b/java/org/apache/tomcat/util/net/NioEndpoint.java
@@ -1503,11 +1503,32 @@ public void run() {
                                 return;
                             }
                             if (read) {
-                                nBytes = getSocket().read(buffers, offset, length);
-                                updateLastRead();
+                                // Read from main buffer first
+                                if (!socketBufferHandler.isReadBufferEmpty()) {
+                                    // There is still data inside the main read buffer, it needs to be read first
+                                    socketBufferHandler.configureReadBufferForRead();
+                                    for (int i = 0; i < length && !socketBufferHandler.isReadBufferEmpty(); i++) {
+                                        nBytes += transfer(socketBufferHandler.getReadBuffer(), buffers[offset + i]);
+                                    }
+                                }
+                                if (nBytes == 0) {
+                                    nBytes = getSocket().read(buffers, offset, length);
+                                    updateLastRead();
+                                }
                             } else {
-                                nBytes = getSocket().write(buffers, offset, length);
-                                updateLastWrite();
+                                boolean doWrite = true;
+                                // Write from main buffer first
+                                if (!socketBufferHandler.isWriteBufferEmpty()) {
+                                    // There is still data inside the main write buffer, it needs to be written first
+                                    doWrite(false);
+                                    if (!socketBufferHandler.isWriteBufferEmpty()) {
+                                        doWrite = false;
+                                    }
+                                }
+                                if (doWrite) {
+                                    nBytes = getSocket().write(buffers, offset, length);
+                                    updateLastWrite();
+                                }
                             }
                             if (nBytes != 0) {
                                 completionDone = false;
@@ -1543,90 +1564,37 @@ public void run() {
         public <A> CompletionState read(ByteBuffer[] dsts, int offset, int length,
                 BlockingMode block, long timeout, TimeUnit unit, A attachment,
                 CompletionCheck check, CompletionHandler<Long, ? super A> handler) {
-            IOException ioe = getError();
-            if (ioe != null) {
-                handler.failed(ioe, attachment);
-                return CompletionState.ERROR;
-            }
-            if (timeout == -1) {
-                timeout = toTimeout(getReadTimeout());
-            } else if (unit.toMillis(timeout) != getReadTimeout()) {
-                setReadTimeout(unit.toMillis(timeout));
-            }
-            if (block == BlockingMode.BLOCK || block == BlockingMode.SEMI_BLOCK) {
-                try {
-                    if (!readPending.tryAcquire(timeout, unit)) {
-                        handler.failed(new SocketTimeoutException(), attachment);
-                        return CompletionState.ERROR;
-                    }
-                } catch (InterruptedException e) {
-                    handler.failed(e, attachment);
-                    return CompletionState.ERROR;
-                }
-            } else {
-                if (!readPending.tryAcquire()) {
-                    if (block == BlockingMode.NON_BLOCK) {
-                        return CompletionState.NOT_DONE;
-                    } else {
-                        handler.failed(new ReadPendingException(), attachment);
-                        return CompletionState.ERROR;
-                    }
-                }
-            }
-            VectoredIOCompletionHandler<A> completion = new VectoredIOCompletionHandler<>();
-            OperationState<A> state = new OperationState<>(true, dsts, offset, length, block,
-                    attachment, check, handler, readPending, completion);
-            readOperation = state;
-            long nBytes = 0;
-            if (!socketBufferHandler.isReadBufferEmpty()) {
-                // There is still data inside the main read buffer, use it to fill out the destination buffers
-                // Note: It is not necessary to put this code in the completion handler
-                socketBufferHandler.configureReadBufferForRead();
-                for (int i = 0; i < length && !socketBufferHandler.isReadBufferEmpty(); i++) {
-                    nBytes += transfer(socketBufferHandler.getReadBuffer(), dsts[offset + i]);
-                }
-                if (nBytes > 0) {
-                    completion.completed(Long.valueOf(nBytes), state);
-                }
-            }
-            if (nBytes == 0) {
-                state.run();
-            }
-            if (block == BlockingMode.BLOCK) {
-                synchronized (state) {
-                    if (state.state == CompletionState.PENDING) {
-                        try {
-                            state.wait(unit.toMillis(timeout));
-                            if (state.state == CompletionState.PENDING) {
-                                return CompletionState.ERROR;
-                            }
-                        } catch (InterruptedException e) {
-                            handler.failed(new SocketTimeoutException(), attachment);
-                            return CompletionState.ERROR;
-                        }
-                    }
-                }
-            }
-            return state.state;
+            return readOrWrite(true, dsts, offset, length, block, timeout, unit, attachment, check, handler);
         }
 
         @Override
         public <A> CompletionState write(ByteBuffer[] srcs, int offset, int length,
                 BlockingMode block, long timeout, TimeUnit unit, A attachment,
                 CompletionCheck check, CompletionHandler<Long, ? super A> handler) {
+            return readOrWrite(false, srcs, offset, length, block, timeout, unit, attachment, check, handler);
+        }
+
+        private <A> CompletionState readOrWrite(boolean read,
+                ByteBuffer[] buffers, int offset, int length,
+                BlockingMode block, long timeout, TimeUnit unit, A attachment,
+                CompletionCheck check, CompletionHandler<Long, ? super A> handler) {
             IOException ioe = getError();
             if (ioe != null) {
                 handler.failed(ioe, attachment);
                 return CompletionState.ERROR;
             }
             if (timeout == -1) {
-                timeout = toTimeout(getWriteTimeout());
-            } else if (unit.toMillis(timeout) != getWriteTimeout()) {
-                setWriteTimeout(unit.toMillis(timeout));
+                timeout = toTimeout(read ? getReadTimeout() : getWriteTimeout());
+            } else if (unit.toMillis(timeout) != (read ? getReadTimeout() : getWriteTimeout())) {
+                if (read) {
+                    setReadTimeout(unit.toMillis(timeout));
+                } else {
+                    setWriteTimeout(unit.toMillis(timeout));
+                }
             }
             if (block == BlockingMode.BLOCK || block == BlockingMode.SEMI_BLOCK) {
                 try {
-                    if (!writePending.tryAcquire(timeout, unit)) {
+                    if (read ? !readPending.tryAcquire(timeout, unit) : !writePending.tryAcquire(timeout, unit)) {
                         handler.failed(new SocketTimeoutException(), attachment);
                         return CompletionState.ERROR;
                     }
@@ -1635,29 +1603,23 @@ public <A> CompletionState write(ByteBuffer[] srcs, int offset, int length,
                     return CompletionState.ERROR;
                 }
             } else {
-                if (!writePending.tryAcquire()) {
+                if (read ? !readPending.tryAcquire() : !writePending.tryAcquire()) {
                     if (block == BlockingMode.NON_BLOCK) {
                         return CompletionState.NOT_DONE;
                     } else {
-                        handler.failed(new WritePendingException(), attachment);
+                        handler.failed(read ? new ReadPendingException() : new WritePendingException(), attachment);
                         return CompletionState.ERROR;
                     }
                 }
             }
-            if (!socketBufferHandler.isWriteBufferEmpty()) {
-                // First flush the main buffer as needed
-                try {
-                    doWrite(true);
-                } catch (IOException e) {
-                    handler.failed(e, attachment);
-                    return CompletionState.ERROR;
-                }
-            }
             VectoredIOCompletionHandler<A> completion = new VectoredIOCompletionHandler<>();
-            OperationState<A> state = new OperationState<>(false, srcs, offset, length, block,
-                    attachment, check, handler, writePending, completion);
-            writeOperation = state;
-            // It should be less necessary to check the buffer state as it is easy to flush before
+            OperationState<A> state = new OperationState<>(read, buffers, offset, length, block,
+                    attachment, check, handler, read ? readPending : writePending, completion);
+            if (read) {
+                readOperation = state;
+            } else {
+                writeOperation = state;
+            }
             state.run();
             if (block == BlockingMode.BLOCK) {
                 synchronized (state) {
diff --git a/webapps/docs/changelog.xml b/webapps/docs/changelog.xml
index c8f851517c56..92f2aa0da705 100644
--- a/webapps/docs/changelog.xml
+++ b/webapps/docs/changelog.xml
@@ -100,6 +100,10 @@
         Add async IO for APR connector for consistency, but disable it by
         default due to low performance. (remm)
       </add>
+      <fix>
+        Avoid blocking write of internal buffer for NIO when using async IO.
+        (remm)
+      </fix>
     </changelog>
   </subsection>
   <subsection name="Other">