Add configureAbsentsAsNulls config setting

This aligns this feature with the Guava module. However, the default is
'false' as compared to the Guava 'true' for backwards compatibility.

Author: Henning Schmiedehausen

src/main/java/com/fasterxml/jackson/datatype/jdk8/Jdk8BeanSerializerModifier.java

@@ -0,0 +1,30 @@
+package com.fasterxml.jackson.datatype.jdk8;
+
+import com.fasterxml.jackson.databind.BeanDescription;
+import com.fasterxml.jackson.databind.SerializationConfig;
+import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
+import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;
+
+import java.util.List;
+import java.util.Optional;
+
+/**
+ * {@link BeanSerializerModifier} needed to sneak in handler to exclude "absent"
+ * optional values iff handling of "absent as nulls" is enabled.
+ */
+public class Jdk8BeanSerializerModifier extends BeanSerializerModifier
+{
+    @Override
+    public List<BeanPropertyWriter> changeProperties(SerializationConfig config,
+            BeanDescription beanDesc,
+            List<BeanPropertyWriter> beanProperties)
+    {
+        for (int i = 0; i < beanProperties.size(); ++i) {
+            final BeanPropertyWriter writer = beanProperties.get(i);
+            if (Optional.class.isAssignableFrom(writer.getPropertyType())) {
+                beanProperties.set(i, new Jdk8OptionalBeanPropertyWriter(writer));
+            }
+        }
+        return beanProperties;
+    }
+}

src/main/java/com/fasterxml/jackson/datatype/jdk8/Jdk8Module.java

@@ -5,19 +5,57 @@
 
 public class Jdk8Module extends Module
 {
+    /**
+     * Configuration setting that determines whether `Optional.empty()` is
+     * considered "same as null" for serialization purposes; that is, to be
+     * filtered same as nulls are.
+     * If enabled, absent values are treated like nulls; if disabled, they are not.
+     * In either case, absent values are always considered "empty".
+     *<p>
+     * Default value is `false` for backwards compatibility (2.5 and prior
+     * only had this behavior).
+     *<p>
+     * Note that this setting MUST be changed BEFORE registering the module:
+     * changes after registration will have no effect.
+     */
+    protected boolean _cfgHandleAbsentAsNull = false;
+
     @Override
     public void setupModule(SetupContext context) {
         context.addSerializers(new Jdk8Serializers());
         context.addDeserializers(new Jdk8Deserializers());
         // And to fully support Optionals, need to modify type info:
         context.addTypeModifier(new Jdk8TypeModifier());
+
+        // Allow enabling "treat Optional.empty() like Java nulls"
+        if (_cfgHandleAbsentAsNull) {
+            context.addBeanSerializerModifier(new Jdk8BeanSerializerModifier());
+        }
     }
 
     @Override
     public Version version() {
         return PackageVersion.VERSION;
     }
 
+    /**
+     * Configuration method that may be used to change configuration setting
+     * {@link #_cfgHandleAbsentAsNull}: enabling means that `Optional.empty()` values
+     * are handled like Java nulls (wrt filtering on serialization); disabling that
+     * they are only treated as "empty" values, but not like native Java nulls.
+     * Recommended setting for this value is `false`. For compatibility with older versions
+     * of other "optional" values (like Guava optionals), it can be set to 'true'. The
+     * default is `false` for backwards compatibility.
+     *
+     * @return This module instance, useful for chaining calls
+     *
+     * @since 2.6
+     */
+    public Jdk8Module configureAbsentsAsNulls(boolean state) {
+        _cfgHandleAbsentAsNull = state;
+        return this;
+    }
+
     @Override
     public int hashCode() {
         return getClass().hashCode();

src/main/java/com/fasterxml/jackson/datatype/jdk8/Jdk8OptionalBeanPropertyWriter.java

@@ -0,0 +1,27 @@
+package com.fasterxml.jackson.datatype.jdk8;
+
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
+
+import java.util.Optional;
+
+public class Jdk8OptionalBeanPropertyWriter extends BeanPropertyWriter {
+
+    protected Jdk8OptionalBeanPropertyWriter(BeanPropertyWriter base) {
+        super(base);
+    }
+
+    @Override
+    public void serializeAsField(Object bean, JsonGenerator jgen, SerializerProvider prov) throws Exception
+    {
+        if (_nullSerializer == null) {
+            Object value = get(bean);
+            if (value == null || Optional.empty().equals(value)) {
+                return;
+            }
+        }
+        super.serializeAsField(bean, jgen, prov);
+    }
+
+}

src/test/java/com/fasterxml/jackson/datatype/jdk8/TestConfigureAbsentsAsNulls.java

@@ -0,0 +1,58 @@
+package com.fasterxml.jackson.datatype.jdk8;
+
+import java.util.Optional;
+
+import com.fasterxml.jackson.annotation.JsonAutoDetect;
+import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility;
+import com.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.databind.ObjectMapper;
+
+public class TestConfigureAbsentsAsNulls extends ModuleTestBase
+{
+    @JsonAutoDetect(fieldVisibility=Visibility.ANY)
+    public static final class OptionalData {
+        public Optional<String> myString = Optional.empty();
+    }
+
+    /*
+    /**********************************************************************
+    /* Test methods
+    /**********************************************************************
+     */
+
+    public void testConfigAbsentsAsNullsTrue() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.registerModule(new Jdk8Module().configureAbsentsAsNulls(true));
+
+        OptionalData data = new OptionalData();
+        String value = mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL).writeValueAsString(data);
+        assertEquals("{}", value);
+    }
+
+    public void testConfigAbsentsAsNullsFalse() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.registerModule(new Jdk8Module().configureAbsentsAsNulls(false));
+
+        OptionalData data = new OptionalData();
+        String value = mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL).writeValueAsString(data);
+        assertEquals("{\"myString\":null}", value);
+    }
+
+    public void testConfigNonAbsentAbsentsAsNullsTrue() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.registerModule(new Jdk8Module().configureAbsentsAsNulls(true));
+
+        OptionalData data = new OptionalData();
+        String value = mapper.setSerializationInclusion(JsonInclude.Include.NON_ABSENT).writeValueAsString(data);
+        assertEquals("{}", value);
+    }
+
+    public void testConfigNonAbsentAbsentsAsNullsFalse() throws Exception {
+        ObjectMapper mapper = new ObjectMapper();
+        mapper.registerModule(new Jdk8Module().configureAbsentsAsNulls(false));
+
+        OptionalData data = new OptionalData();
+        String value = mapper.setSerializationInclusion(JsonInclude.Include.NON_ABSENT).writeValueAsString(data);
+        assertEquals("{}", value);
+    }
+}