Java Serialization

Article

The code refers to the following articles:

• English: Serialization in Java: Serializable, transient, serialVersionUID
• German: Serialisierung in Java: Serializable, transient, serialVersionUID

Contents

The examples work on a small order model: the classes Order and Customer and the record OrderItem. Chapters that need a variant of a class – one that is not serializable, has no serialVersionUID, or validates in readObject() – keep it in a package of its own (graph, transients, versioning, inheritance, custom, special, filter).

Every class with a main() method is one chapter of the article and prints the outputs shown there. Run one with java -cp target/classes eu.happycoders.serialization.<ClassName> after mvn compile. The examples that write files write them into the current directory, with the extension .ser.

• Intro – writes an order into a file and reads it back
• Ch1Writing – ObjectOutputStream: writing into a byte array, the first bytes of the stream, buffering
• Ch2Reading – ObjectInputStream: pattern matching on the result, several objects in one stream, buffering, a file that is not a serialization stream
• Ch3ObjectGraph – NotSerializableException, shared references, the same object written twice, reset() and writeUnshared()
• Ch4Transient – transient fields after deserialization and how readObject() restores them
• Ch5Versioning – the computed serialVersionUID and what happens when a class changes between writing and reading
• Ch5Misdeclaration – a misdeclared serialVersionUID, for the JFR event jdk.SerializationMisdeclaration (the command is in the class comment)
• Ch6Inheritance – a serializable class with a non-serializable superclass
• Ch7Custom – validation in readObject(), writeObject() for the superclass state, readResolve(), the serialization proxy
• Ch8Special – records, enums (including a renamed constant), lambdas, and inner classes
• Ch9Collections – ArrayList capacity, List.of(), views such as keySet() and subList()
• Ch10Filters – deserialization filters from patterns, the missing !*, and the limits maxdepth (against Joshua Bloch’s deserialization bomb) and maxarray
• Ch10FilterFactory – a filter factory that applies a filter within a ScopedValue scope
• Ch10GlobalFilter – a JVM-wide filter with -Djdk.serialFilter (the command is in the class comment; add -Djava.util.logging.config.file=serialfilter-logging.properties to log rejected classes)
• Ch11Size – the size of an order with ObjectOutputStream and with DataOutputStream

The class PackageSwitchingObjectInputStream in eu.happycoders.serialization.util reads a stream written by one version of a class with another version of it: the chapters keep both versions side by side, e.g. versioning.explicit.v1.Customer and versioning.explicit.v2.Customer. Serialization compares class names without their package, so this simulates a class that changed between writing and reading.

Additional Resources

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