Make sure successful SslHandshakeCompletionEvent is a singleton

- Related: #1373
- This commit is an amendment to 2a7bea2ad3

handler/src/main/java/io/netty/handler/ssl/SslHandler.java

@@ -189,8 +189,6 @@ public class SslHandler
     private volatile long handshakeTimeoutMillis = 10000;
     private volatile long closeNotifyTimeoutMillis = 3000;
 
-    private static final SslHandshakeCompletionEvent HANDSHAKE_SUCCESS_EVENT = new SslHandshakeCompletionEvent(null);
-
     /**
      * Creates a new instance.
      *
@@ -908,7 +906,7 @@ public class SslHandler
      */
     private void setHandshakeSuccess() {
         if (handshakePromise.trySuccess(ctx.channel())) {
-            ctx.fireUserEventTriggered(HANDSHAKE_SUCCESS_EVENT);
+            ctx.fireUserEventTriggered(SslHandshakeCompletionEvent.SUCCESS);
         }
     }
 

handler/src/main/java/io/netty/handler/ssl/SslHandshakeCompletionEvent.java

@@ -22,9 +22,25 @@ package io.netty.handler.ssl;
  */
 public final class SslHandshakeCompletionEvent {
 
+    public static final SslHandshakeCompletionEvent SUCCESS = new SslHandshakeCompletionEvent();
+
     private final Throwable cause;
 
+    /**
+     * Creates a new event that indicates a successful handshake.
+     */
+    private SslHandshakeCompletionEvent() {
+        cause = null;
+    }
+
+    /**
+     * Creates a new event that indicates an unsuccessful handshake.
+     * Use {@link #SUCCESS} to indicate a successful handshake.
+     */
     public SslHandshakeCompletionEvent(Throwable cause) {
+        if (cause == null) {
+            throw new NullPointerException("cause");
+        }
         this.cause = cause;
     }