Why org.springframework.http.ResponseEntity doesn't implements Serializable

I try to understand Serializable interface and it's utility.

I know now that objects that implement Serializable will be converted to Byte stream so they could be transferred over the network or persisted in database.

My question is why org.springframework.http.ResponseEntity doesn't implements Serializable interface since ResponseEntity is the spring RestController response?

PS: I ask the question and I know that maybe I am totaly missing the point and that I do not yet understand the concept Srializable and ResponseEntity.

java.io.Serializable uses a specific kind of serialization known as Java serialization. It was a major innovation when it was introduced as part of the core language platform (and still has some respectable features, like handling reference cycles), but it has some serious disadvantages, the biggest of which is that it's not compatible with other platforms (including the human eyeball).

Instead, modern applications tend to use other serialization mechanisms (primarily JSON, still plenty of XML, and some other options like Avro and Protocol Buffers). These don't use the built-in platform serialization features and instead use a library (such as Jackson) that inspects the objects and outputs the serialized form. This means that such objects have no need of implementing Serializable.

One major reason that ResponseEntity doesn't implement Serializable is that doing so requires all of its contents to also implement Serializable, or a surprise NotSerializableException can happen at runtime. Since there's no useful value, it's better to skip it.

The Java Serialization format is for objects that are intended to be converted back to java objects later, and usually only works if you have the same classes while decoding as while encoding.

A browser is usually not written in Java and should not know the inner workings of the org.springframework.http.ResponseEntity class. It only knows the HTTP protocol.

A ResponseEntity will therefor be transferred in the HTTP response format, which is very different from the Java Serialization format.

Have a look at Java Tutorial on Serialization
>To serialize an object means to convert its state to a byte stream so that the byte stream can be reverted back into a copy of the object.

Serialization is not about how to send byte-stream through network, but rather about how to decompose a Java object into bytes, and revert that decomposition back into exactly same Java object, like:

Object <--> Bytes <--> Object.

ResponseEntity<T> is nothing but a specially formatted String message, that gets sent from one machine to another, most likely along with Content-Type header, via HTTP protocol (TCP connection).

Receiver (let it be a browser) receives the data, converts that data back into human-understandable representational format, specified in Content-Type, and that is the result, which is not a Java object anymore.