Replace Object metadata with a Map of metadata properties

[jakzal]

Serialization has not worked properly with Object(s). The problem has resurfaced when trying to implement Jackson-based serialization. In past, we discussed replacing the Object with a Map of properties.

I have added the Map of properties and deprecated the Object to be removed in future versions.

[VaughnVernon]

@jakzal Must be certain that reading from storage initializes the empty properties. Also, what if the object is set? Should it be put into properties with a key = "object", or...?

[jakzal]

Serializing just any objects in metadata never worked as they'd be deserialized as maps:

  class Foo {
    public String item = "value";
  }
 
  @Test
  public void testSerializedMetadataObject() {
    final Object object = new Foo();
    final String serialized = JsonSerialization.serialized(Metadata.withObject(object));
    final Metadata metadata = JsonSerialization.deserialized(serialized, Metadata.class);
    assertEquals("com.google.gson.internal.LinkedTreeMap", metadata.object.getClass().getName());
    assertEquals(Collections.singletonMap("item", "value"), metadata.object);
    // NOT: assertEquals(object, metadata.object);
  }

Serializing lists and maps worked as expected:

  @Test
  public void testSerializedMetadataList() {
    final Object object = Collections.singletonList("item1");
    final String serialized = JsonSerialization.serialized(Metadata.withObject(object));
    final Metadata metadata = JsonSerialization.deserialized(serialized, Metadata.class);
    assertEquals("java.util.ArrayList", metadata.object.getClass().getName());
    assertEquals(object, metadata.object);
  }

  @Test
  public void testSerializedMetadataMap() {
    final Object object = Collections.singletonMap("key1", "value1");
    final String serialized = JsonSerialization.serialized(Metadata.withObject(object));
    final Metadata metadata = JsonSerialization.deserialized(serialized, Metadata.class);
    assertEquals("com.google.gson.internal.LinkedTreeMap", metadata.object.getClass().getName());
    assertEquals(object, metadata.object);
  }

Also, what if the object is set? Should it be put into properties with a key = "object", or...?

I haven't considered migrating it to the map. The type I chose for the properties map is Map<String,String>. Too constrainted?

I thought to put it on end users to migrate their code as I can't predict what might've been put into the object and they'll eventually need to replace calls to the object with calls to properties.

Must be certain that reading from storage initializes the empty properties.

Yes.

[VaughnVernon]

@jakzal OK. Merge when ready.