Improve CallbackResultHolder exception message to aid debugging

JAVA-3038

Author: jyemin

config/checkstyle-exclude.xml

@@ -43,6 +43,9 @@
     <suppress checks="AvoidStarImportCheck" files=".*primer.*"/>
     <suppress checks="VisibilityModifierCheck" files=".*primer.*"/>
 
+    <!-- Allow printStackTrace in this file -->
+    <suppress checks="Regexp" files="CallbackResultHolder"/>
+
     <!--Do not check documentation tests classes -->
     <suppress checks="Javadoc*" files=".*documentation.*"/>
     <suppress checks="Regexp" files=".*documentation.*"/>

driver-core/src/test/unit/com/mongodb/async/CallbackResultHolder.java

@@ -16,6 +16,9 @@
 
 package com.mongodb.async;
 
+import java.io.PrintWriter;
+import java.io.StringWriter;
+
 /**
  * A SingleResultCallback implementation that saves the result of the callback.
  *
@@ -35,13 +38,21 @@ class CallbackResultHolder<T> implements SingleResultCallback<T> {
      */
     public void onResult(final T result, final Throwable error) {
         if (isDone) {
-            throw new IllegalStateException("The CallbackResult cannot be initialized multiple times.");
+            throw new IllegalStateException("The CallbackResult cannot be initialized multiple times.  The first time it was initialized "
+                    + "with " + (this.error != null ? getErrorString(this.error) : this.result) + "\n The second time it was initialized "
+                    + "with " + (error != null ? getErrorString(error) : result));
         }
         this.result = result;
         this.error = error;
         this.isDone = true;
     }
 
+    private String getErrorString(final Throwable error) {
+        StringWriter writer = new StringWriter();
+        error.printStackTrace(new PrintWriter(writer));
+        return writer.toString();
+    }
+
     /**
      * Returns the result of the callback or null.
      *
@@ -81,9 +92,9 @@ public boolean isDone() {
     @Override
     public String toString() {
         return "CallbackResultHolder{"
-               + "result=" + result
-               + ", error=" + error
-               + ", isDone=" + isDone
-               + '}';
+                + "result=" + result
+                + ", error=" + error
+                + ", isDone=" + isDone
+                + '}';
     }
 }