Fix #1685

src/main/java/com/fasterxml/jackson/databind/ObjectMapper.java

@@ -3840,7 +3840,7 @@ public void acceptJsonFormatVisitor(Class<?> type, JsonFormatVisitorWrapper visi
     {
         acceptJsonFormatVisitor(_typeFactory.constructType(type), visitor);
     }
-    
+
     /**
      * Method for visiting type hierarchy for given type, using specified visitor.
      * Visitation uses <code>Serializer</code> hierarchy and related properties
@@ -3861,7 +3861,7 @@ public void acceptJsonFormatVisitor(JavaType type, JsonFormatVisitorWrapper visi
         }
         _serializerProvider(getSerializationConfig()).acceptJsonFormatVisitor(type, visitor);
     }
-    
+
     /*
     /**********************************************************
     /* Internal methods for serialization, overridable

src/main/java/com/fasterxml/jackson/databind/SerializerProvider.java

@@ -1422,7 +1422,7 @@ protected final DateFormat _dateFormat()
         if (_dateFormat != null) {
             return _dateFormat;
         }
-        /* At this point, all timezone configuration should have occured, with respect
+        /* At this point, all timezone configuration should have occurred, with respect
          * to default dateformat configuration. But we still better clone
          * an instance as formatters are stateful, not thread-safe.
          */

src/main/java/com/fasterxml/jackson/databind/ser/std/CalendarSerializer.java

@@ -37,19 +37,12 @@ protected long _timestamp(Calendar value) {
     }
 
     @Override
-    public void serialize(Calendar value, JsonGenerator jgen, SerializerProvider provider) throws IOException
+    public void serialize(Calendar value, JsonGenerator g, SerializerProvider provider) throws IOException
     {
         if (_asTimestamp(provider)) {
-            jgen.writeNumber(_timestamp(value));
-        } else if (_customFormat != null) {
-            // 21-Feb-2011, tatu: not optimal, but better than alternatives:
-            synchronized (_customFormat) {
-                // _customformat cannot parse Calendar, so Date should be passed
-                jgen.writeString(_customFormat.format(value.getTime()));
-            }
-        } else {
-            provider.defaultSerializeDateValue(value.getTime(), jgen);
+            g.writeNumber(_timestamp(value));
+            return;
         }
+        _serializeAsString(value.getTime(), g, provider);
     }
-
 }

src/main/java/com/fasterxml/jackson/databind/ser/std/DateSerializer.java

@@ -42,17 +42,12 @@ protected long _timestamp(Date value) {
     }
 
     @Override
-    public void serialize(Date value, JsonGenerator gen, SerializerProvider provider) throws IOException
+    public void serialize(Date value, JsonGenerator g, SerializerProvider provider) throws IOException
     {
         if (_asTimestamp(provider)) {
-            gen.writeNumber(_timestamp(value));
-        } else if (_customFormat != null) {
-            // 21-Feb-2011, tatu: not optimal, but better than alternatives:
-            synchronized (_customFormat) {
-                gen.writeString(_customFormat.format(value));
-            }
-        } else {
-            provider.defaultSerializeDateValue(value, gen);
+            g.writeNumber(_timestamp(value));
+            return;
         }
+        _serializeAsString(value, g, provider);
     }
 }

src/main/java/com/fasterxml/jackson/databind/ser/std/DateTimeSerializerBase.java

@@ -4,8 +4,10 @@
 import java.lang.reflect.Type;
 import java.text.DateFormat;
 import java.text.SimpleDateFormat;
+import java.util.Date;
 import java.util.Locale;
 import java.util.TimeZone;
+import java.util.concurrent.atomic.AtomicReference;
 
 import com.fasterxml.jackson.annotation.JsonFormat;
 
@@ -35,12 +37,23 @@ public abstract class DateTimeSerializerBase<T>
      */
     protected final DateFormat _customFormat;
 
+    /**
+     * If {@link #_customFormat} is used, we will try to reuse instances in simplest
+     * possible form; thread-safe, but without overhead of <code>ThreadLocal</code>
+     * (not from code, but wrt retaining of possibly large number of format instances
+     * over all threads, properties with custom formats).
+     *
+     * @since 2.9
+     */
+    protected final AtomicReference<DateFormat> _reusedCustomFormat;
+
     protected DateTimeSerializerBase(Class<T> type,
             Boolean useTimestamp, DateFormat customFormat)
     {
         super(type);
         _useTimestamp = useTimestamp;
         _customFormat = customFormat;
+        _reusedCustomFormat = (customFormat == null) ? null : new AtomicReference<DateFormat>();
     }
 
     public abstract DateTimeSerializerBase<T> withFormat(Boolean timestamp, DateFormat customFormat);
@@ -189,4 +202,29 @@ protected void _acceptJsonFormatVisitor(JsonFormatVisitorWrapper visitor, JavaTy
             visitStringFormat(visitor, typeHint, JsonValueFormat.DATE_TIME);
         }
     }
+
+    /**
+     * @since 2.9
+     */
+    protected void _serializeAsString(Date value, JsonGenerator g, SerializerProvider provider) throws IOException
+    {
+        if (_customFormat == null) {
+            provider.defaultSerializeDateValue(value, g);
+            return;
+        }
+
+        // 19-Jul-2017, tatu: Here we will try a simple but (hopefully) effective mechanism for
+        //    reusing formatter instance. This is our second attempt, after initially trying simple
+        //    synchronization (which turned out to be bottleneck for some users in production...).
+        //    While `ThreadLocal` could alternatively be used, it is likely that it would lead to
+        //    higher memory footprint, but without much upside -- if we can not reuse, we'll just
+        //    clone(), which has some overhead but not drastic one.
+
+        DateFormat f = _reusedCustomFormat.getAndSet(null);
+        if (f == null) {
+            f = (DateFormat) _customFormat.clone();
+        }
+        g.writeString(f.format(value));
+        _reusedCustomFormat.compareAndSet(null, f);
+    }
 }

src/main/java/com/fasterxml/jackson/databind/ser/std/SqlDateSerializer.java

@@ -40,34 +40,21 @@ protected long _timestamp(java.sql.Date value) {
     }
 
     @Override
-    public void serialize(java.sql.Date value, JsonGenerator gen, SerializerProvider provider)
+    public void serialize(java.sql.Date value, JsonGenerator g, SerializerProvider provider)
         throws IOException
     {
         if (_asTimestamp(provider)) {
-            gen.writeNumber(_timestamp(value));
-        } else if (_customFormat != null) {
-            // 11-Oct-2016, tatu: As per [databind#219], same as with `java.util.Date`
-            synchronized (_customFormat) {
-                gen.writeString(_customFormat.format(value));
-            }
-        } else {
+            g.writeNumber(_timestamp(value));
+            return;
+        }
+        // Alas, can't just call `_serializeAsString()`....
+        if (_customFormat == null) {
             // 11-Oct-2016, tatu: For backwards-compatibility purposes, we shall just use
             //    the awful standard JDK serialization via `sqlDate.toString()`... this
             //    is problematic in multiple ways (including using arbitrary timezone...)
-            gen.writeString(value.toString());
+            g.writeString(value.toString());
+            return;
         }
-    }
-
-    @Override
-    public JsonNode getSchema(SerializerProvider provider, Type typeHint)
-    {
-        //todo: (ryan) add a format for the date in the schema?
-        return createSchemaNode("string", true);
-    }
-
-    @Override
-    public void acceptJsonFormatVisitor(JsonFormatVisitorWrapper visitor, JavaType typeHint) throws JsonMappingException
-    {
-        _acceptJsonFormatVisitor(visitor, typeHint, _useTimestamp);
+        _serializeAsString(value, g, provider);
     }
 }