ObjectOutputStream

ObjectOutputStream is a Java I/O class in the java.io package that writes primitive data and object graphs to an OutputStream using Java’s built-in serialization mechanism. It enables persisting objects to files, sending objects over networks, or otherwise encoding data for later deserialization.

Serialization with ObjectOutputStream requires that the classes of the objects being written implement the Serializable interface. If a non-serializable object is encountered, a NotSerializableException is thrown. The stream writes a header and maintains an internal handle table to track object references, preserving object graphs, including shared and cyclic references, across serialization and deserialization.

Core functionality is exposed through methods such as writeObject(Object), which serializes an object, and various write methods for primitive types (writeInt, writeBoolean, etc.). Additional features include writeUnshared to avoid caching references, enableReplaceObject to substitute objects during serialization, and replaceObject to customize substitutions. The class also provides control over the stream format via writeStreamHeader and related methods, and supports operations like flush and close.

Usage typically involves wrapping an OutputStream, such as a FileOutputStream or ByteArrayOutputStream, with a new ObjectOutputStream. Deserialization is performed with ObjectInputStream. While convenient for Java-to-Java data transfer, the Java serialization format is Java-specific and can pose security risks when deserializing untrusted data; it may also be brittle with evolving class definitions. Consequently, many applications favor alternative serialization frameworks (for example JSON, XML, or protocol buffers) for interoperability, performance, and security considerations, while still relying on ObjectOutputStream for legacy Java serialization needs.