Minor improvement wrt #90: avoid exception, serialize as empty Object

guava/src/main/java/com/fasterxml/jackson/datatype/guava/GuavaSerializers.java

@@ -18,13 +18,14 @@
 import com.fasterxml.jackson.databind.ser.std.StdDelegatingSerializer;
 import com.fasterxml.jackson.databind.util.StdConverter;
 import com.fasterxml.jackson.datatype.guava.ser.RangeSetSerializer;
-
+import com.fasterxml.jackson.datatype.guava.ser.CacheSerializer;
 import com.fasterxml.jackson.datatype.guava.ser.GuavaOptionalSerializer;
 import com.fasterxml.jackson.datatype.guava.ser.MultimapSerializer;
 import com.fasterxml.jackson.datatype.guava.ser.RangeSerializer;
 import com.fasterxml.jackson.datatype.guava.ser.TableSerializer;
 
 import com.google.common.base.Optional;
+import com.google.common.cache.Cache;
 import com.google.common.cache.CacheBuilder;
 import com.google.common.cache.CacheBuilderSpec;
 import com.google.common.collect.FluentIterable;
@@ -96,6 +97,12 @@ public ValueSerializer<?> findSerializer(SerializationConfig config, JavaType ty
             JavaType iterableType = _findDeclared(type, Iterable.class);
             return new StdDelegatingSerializer(FluentConverter.instance, iterableType, null, null);
         }
+        // [datatypes-collections#90]: add bogus "serialize as empty" serializer to avoid
+        // error on "no properties". If proper serialization (and deserialization) needed,
+        // would need to resolve type parameters here
+        if (Cache.class.isAssignableFrom(raw)) {
+            return new CacheSerializer();
+        }
         return ImmutablePrimitiveTypes.isAssignableFromImmutableArray(raw)
                 .transform(ImmutablePrimitiveTypes.ImmutablePrimitiveArrays::newSerializer)
                 .orNull();

guava/src/main/java/com/fasterxml/jackson/datatype/guava/ser/CacheSerializer.java

@@ -0,0 +1,51 @@
+package com.fasterxml.jackson.datatype.guava.ser;
+
+import com.fasterxml.jackson.core.JacksonException;
+import com.fasterxml.jackson.core.JsonGenerator;
+import com.fasterxml.jackson.core.JsonToken;
+import com.fasterxml.jackson.core.type.WritableTypeId;
+import com.fasterxml.jackson.databind.SerializerProvider;
+import com.fasterxml.jackson.databind.jsontype.TypeSerializer;
+import com.fasterxml.jackson.databind.ser.std.StdSerializer;
+
+import com.google.common.cache.Cache;
+
+public class CacheSerializer extends StdSerializer<Cache<?, ?>>
+{
+    public CacheSerializer() {
+        super(Cache.class);
+    }
+
+    @Override
+    public boolean isEmpty(SerializerProvider prov, Cache<?, ?> value) {
+        // Since we serialize all as empty, let's claim we are always empty
+        return true;
+    }
+
+    @Override
+    public void serialize(Cache<?, ?> value, JsonGenerator gen, SerializerProvider provider)
+        throws JacksonException
+    {
+        gen.writeStartObject(value);
+        _writeContents(value, gen, provider);
+        gen.writeEndObject();
+    }
+
+    @Override
+    public void serializeWithType(Cache<?, ?> value, JsonGenerator gen, SerializerProvider ctxt,
+            TypeSerializer typeSer)
+        throws JacksonException
+    {
+        gen.assignCurrentValue(value);
+        WritableTypeId typeIdDef = typeSer.writeTypePrefix(gen, ctxt,
+                typeSer.typeId(value, JsonToken.START_OBJECT));
+        _writeContents(value, gen, ctxt);
+        typeSer.writeTypeSuffix(gen, ctxt, typeIdDef);
+    }
+
+    // Just a stub in case we have time to implement proper (if optional) serialization
+    protected void _writeContents(Cache<?, ?> value, JsonGenerator g, SerializerProvider provider)
+        throws JacksonException
+    {
+    }    
+}

guava/src/test/java/com/fasterxml/jackson/datatype/guava/CacheTypesTest.java

@@ -0,0 +1,20 @@
+package com.fasterxml.jackson.datatype.guava;
+
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.common.cache.Cache;
+import com.google.common.cache.CacheBuilder;
+
+public class CacheTypesTest extends ModuleTestBase
+{
+    private final ObjectMapper MAPPER = mapperWithModule();
+
+    // [datatypes-collections#90]: only ensure we can serialize caches as
+    //   empty, to begin with
+    public void testSerializabilityOfCacheAsEmpty() throws Exception
+    {
+        Cache<String, String> cache = CacheBuilder.newBuilder().build();
+        cache.put("key", "value");
+
+        assertEquals("{}", MAPPER.writeValueAsString(cache));
+    }
+}

release-notes/VERSION-2.x

@@ -19,6 +19,7 @@ Active Maintainers:
 2.13.0 (not yet released)
 
 #85: (eclipse-collections) Update eclipse-collections to version 10 and implement Triple deserialization. Still compatible with older EC versions
+- (guava) Serialize `Cache` instances as empty Objects (see #90)
 
 2.12.3 (12-Apr-2021)