DynamicOps/1.17
The DynamicOps class is part of mojang's DataFixerUpper serialization library; DynamicOps are used alongside codecs to convert java objects to a serialized format and back. While Codecs describe how a java object is to be serialized, DynamicOps describe the format the object is to be serialized to.
The DataFixerUpper library includes several DynamicOps for serializing to json; vanilla minecraft also includes a DynamicOps for serializing to minecraft's NBT format.
Builtin DynamicOps
JsonOps
The JsonOps DynamicOps are used to serialize and deserialize json data. These are instances of DynamicOps<JsonElement>, meaning that they are used to convert java objects to and from JsonElement instances. These can used in conjunction with codecs to serialize/deserialize objects from assets and datapacks.
There are two public instances of JsonOps: JsonOps.INSTANCE and JsonOps.COMPRESSED. Compressed data is represented as a single string to read/write. However, this is never used within vanilla itself.
NbtOps
The NbtOps.INSTANCE is an instance of DynamicOps<Tag>, meaning it is used to convert java objects to and from Tag instances. This can be used for serializing data into packets to send across networks, as well as serializing persistent data for entities and similar objects.
There is only one public instance of NbtOps: NbtOps.INSTANCE.
Using DynamicOps
Serializing and Deserializing
By combining a Codec<SomeJavaType> with a DynamicOps<SomeSerializedFormat>, the proper java object can be converted to the serialized form and back. See codecs for details on this subject.
Format Conversion
By using the DynamicOps#convertTo instance method with a second DynamicOps instance, data can be converted between two different serialized formats, such as JsonElement to Tag and back.
// converting Tag to JsonElement JsonElement someJsonElement = NbtOps.INSTANCE.convertTo(JsonOps.INSTANCE, someTag); // converting JsonElement to Tag Tag someTag = JsonOps.INSTANCE.convertTo(NbtOps.INSTANCE, someJsonElement);
Depending on the implementation of the DynamicOps used, this may throw an exception or return an empty object if the two formats are incompatible.
In particular, it is not safe to convert lists of numbers to NBT in this manner, due to the NBT format's strong typing and the way the NbtOps attempts to create lists of numbers.