java.io.StreamCorruptedException: invalid stream header: 54657374

Clearly you aren’t sending the data with ObjectOutputStream: you are just writing the bytes.

• If you read with readObject() you must write with writeObject().
• If you read with readUTF() you must write with writeUTF().
• If you read with readXXX() you must write with writeXXX(), for most values of XXX.