Object serialization

What is serialization (and deserialization)?

Object serialization is the process of converting objects into a stream of bytes and, deserialization, the reverse process of creating objects from a stream of bytes. It takes place every time nodes pass objects to each other as messages, when objects are sent to or from RPC clients from the node, and when we store transactions in the database.

Whitelisting

In classic Java serialization, any class on the JVM classpath can be deserialized. This has shown to be a source of exploits and vulnerabilities by exploiting the large set of 3rd party libraries on the classpath as part of the dependencies of a JVM application and a carefully crafted stream of bytes to be deserialized. In Corda, we prevent just any class from being deserialized (and pro-actively during serialization) by insisting that each object’s class belongs on a whitelist of allowed classes.

Classes get onto the whitelist via one of three mechanisms:

  1. Via the @CordaSerializable annotation. In order to whitelist a class, this annotation can be present on the class itself, on any of the super classes or on any interface implemented by the class or super classes or any interface extended by an interface implemented by the class or superclasses.
  2. By implementing the SerializationWhitelist interface and specifying a list of whitelist classes. See Writing a CorDapp.
  3. Via the built in Corda whitelist (see the class DefaultWhitelist). Whilst this is not user editable, it does list common JDK classes that have been whitelisted for your convenience.

The annotation is the preferred method for whitelisting. An example is shown in Using the client RPC API. It’s reproduced here as an example of both ways you can do this for a couple of example classes.

// Not annotated, so need to whitelist manually.
data class ExampleRPCValue(val foo: String)

// Annotated, so no need to whitelist manually.
@CordaSerializable
data class ExampleRPCValue2(val bar: Int)

class ExampleRPCSerializationWhitelist : SerializationWhitelist {
    // Add classes like this.
    override val whitelist = listOf(ExampleRPCValue::class.java)
}

Note

Several of the core interfaces at the heart of Corda are already annotated and so any classes that implement them will automatically be whitelisted. This includes Contract, ContractState and CommandData.

Warning

Java 8 Lambda expressions are not serializable except in flow checkpoints, and then not by default. The syntax to declare a serializable Lambda expression that will work with Corda is Runnable r = (Runnable & Serializable) () -> System.out.println("Hello World");, or Callable<String> c = (Callable<String> & Serializable) () -> "Hello World";.

Warning

We will be replacing the use of Kryo in the serialization framework and so additional changes here are likely.

AMQP

Note

AMQP serialization is not currently live and will be turned on in a future release.

The long term goal is to migrate the current serialization format for everything except checkpoints away from the current Kryo-based format to a more sustainable, self-describing and controllable format based on AMQP 1.0. The primary drivers for that move are:

  1. A desire to have a schema describing what has been serialized along-side the actual data: #. To assist with versioning, both in terms of being able to interpret long ago archived data (e.g. trades from

    a decade ago, long after the code has changed) and between differing code versions.

    1. To make it easier to write user interfaces that can navigate the serialized form of data.
    2. To support cross platform (non-JVM) interaction, where the format of a class file is not so easily interpreted.

  2. A desire to use a documented and static wire format that is platform independent, and is not subject to change with 3rd party library upgrades etc.

  3. A desire to support open-ended polymorphism, where the number of subclasses of a superclass can expand over time and do not need to be defined in the schema upfront, which is key to many Corda concepts, such as contract states.

  4. Increased security from deserialized objects being constructed through supported constructors rather than having data poked directy into their fields without an opportunity to validate consistency or intercept attempts to manipulate supposed invariants.

Documentation on that format, and how JVM classes are translated to AMQP, will be linked here when it is available.

We describe here what is and will be supported in the Corda AMQP format from the perspective of CorDapp developers, to allow for CorDapps to take into consideration the future state. The AMQP serialization format will of course continue to apply the whitelisting functionality that is already in place and described in Object serialization.

Core Types

Here we describe the classes and interfaces that the AMQP serialization format will support.

Collection Types

The following collection types are supported. Any implementation of the following will be mapped to an implementation of the interface or class on the other end. e.g. If you, for example, use a Guava implementation of a collection it will deserialize as a different implementation, but will continue to adhere to the most specific of any of the following interfaces. You should use only these types as the declared types of fields and properties, and not the concrete implementation types. Collections must be used in their generic form, the generic type parameters will be included in the schema, and the elements type checked against the generic parameters when deserialized.

java.util.Collection
java.util.List
java.util.Set
java.util.SortedSet
java.util.NavigableSet
java.util.NonEmptySet
java.util.Map
java.util.SortedMap
java.util.NavigableMap

However, we will support the concrete implementation types below explicitly and also as the declared type of a field, as a convenience.

java.util.LinkedHashMap
java.util.TreeMap
java.util.EnumSet
java.util.EnumMap (but only if there is at least one entry)

JVM primitives

All the primitive types are supported.

boolean
byte
char
double
float
int
long
short

Arrays

We also support arrays of any supported type, primitive or otherwise.

JDK Types

The following types are supported from the JDK libraries.

java.io.InputStream

java.lang.Boolean
java.lang.Byte
java.lang.Character
java.lang.Class
java.lang.Double
java.lang.Float
java.lang.Integer
java.lang.Long
java.lang.Short
java.lang.StackTraceElement
java.lang.String
java.lang.StringBuffer

java.math.BigDecimal

java.security.PublicKey

java.time.DayOfWeek
java.time.Duration
java.time.Instant
java.time.LocalDate
java.time.LocalDateTime
java.time.LocalTime
java.time.Month
java.time.MonthDay
java.time.OffsetDateTime
java.time.OffsetTime
java.time.Period
java.time.YearMonth
java.time.Year
java.time.ZonedDateTime
java.time.ZonedId
java.time.ZoneOffset

java.util.BitSet
java.util.Currency
java.util.UUID

Third Party Types

The following 3rd party types are supported.

kotlin.Unit
kotlin.Pair

org.apache.activemq.artemis.api.core.SimpleString

org.bouncycastle.asn1.x500.X500Name
org.bouncycastle.cert.X509CertificateHolder

Corda Types

Classes and interfaces in the Corda codebase annotated with @CordaSerializable are of course supported.

All Corda exceptions that are expected to be serialized inherit from CordaThrowable via either CordaException, for checked exceptions, or CordaRuntimeException, for unchecked exceptions. Any Throwable that is serialized but does not conform to CordaThrowable will be converted to a CordaRuntimeException with the original exception type and other properties retained within it.

Custom Types

Here are the rules to adhere to for support of your own types:

Classes

  1. A constructor which takes all of the properties that you wish to record in the serialized form. This is required in order for the serialization framework to reconstruct an instance of your class.
  2. If more than one constructor is provided, the serialization framework needs to know which one to use. The @ConstructorForDeserialization annotation can be used to indicate which one. For a Kotlin class, without the @ConstructorForDeserialization annotation, the primary constructor will be selected.
  3. The class must be compiled with parameter names included in the .class file. This is the default in Kotlin but must be turned on in Java (-parameters command line option to javac).
  4. A Java Bean getter for each of the properties in the constructor, with the names matching up. For example, for a constructor parameter foo, there must be a getter called getFoo(). If the type of foo is boolean, the getter may optionally be called isFoo(). This is why the class must be compiled with parameter names turned on.
  5. The class is annotated with @CordaSerializable.
  6. The declared types of constructor arguments / getters must be supported, and where generics are used the generic parameter must be a supported type, an open wildcard (*), or a bounded wildcard which is currently widened to an open wildcard.
  7. Any superclass must adhere to the same rules, but can be abstract.
  8. Object graph cycles are not supported, so an object cannot refer to itself, directly or indirectly.

Enums

  1. All enums are supported, provided they are annotated with @CordaSerializable.

Warning

Use of enums in CorDapps requires potentially deeper consideration than in other application environments due to the challenges of simultaneously upgrading the code on all nodes. It is therefore important to consider the code evolution perspective, since an older version of the enum code cannot accommodate a newly added element of the enum in a new version of the enum code. See Type Evolution. Hence, enums are a good fit for genuinely static data that will never change. e.g. Days of the week is not going to be extended any time soon and is indeed an enum in the Java library. A Buy or Sell indicator is another. However, something like Trade Type or Currency Code is likely not, since who’s to say a new trade type or currency will not come along soon. For those it is better to choose another representation: perhaps just a string.

Exceptions

The following rules apply to supported Throwable implementations.

  1. If you wish for your exception to be serializable and transported type safely it should inherit from either CordaException or CordaRuntimeException.
  2. If not, the Throwable will deserialize to a CordaRuntimeException with the details of the original Throwable contained within it, including the class name of the original Throwable.

Kotlin Objects

  1. Kotlin object s are singletons and treated differently. They are recorded into the stream with no properties and deserialize back to the singleton instance.

Currently, the same is not true of Java singletons, and they will deserialize to new instances of the class.

The Carpenter

We will support a class carpenter that can dynamically manufacture classes from the supplied schema when deserializing in the JVM without the supporting classes on the classpath. This can be useful where other components might expect to be able to use reflection over the deserialized data, and also for ensuring classes not on the classpath can be deserialized without loading potentially malicious code dynamically without security review outside of a fully sandboxed environment. A more detailed discussion of the carpenter will be provided in a future update to the documentation.

Future Enhancements

  1. Java singleton support. We will add support for identifying classes which are singletons and identifying the static method responsible for returning the singleton instance.
  2. Instance internalizing support. We will add support for identifying classes that should be resolved against an instances map to avoid creating many duplicate instances that are equal. Similar to String.intern().
  3. Enum evolution support. We may introduce an annotation that can be applied to an enum element to indicate that if an unrecognised enum entry is deserialized from a newer version of the code, it should be converted to that element in the older version of the code. This is dependent on identifying a suitable use case, since it does mutate the data when transported to another node, which could be considered hazardous.

Type Evolution

When we move to AMQP as the serialization format, we will be adding explicit support for interoperability of different versions of the same code. We will describe here the rules and features for evolvability as part of a future update to the documentation.