.. highlight:: kotlin .. raw:: html Object serialization ==================== .. contents:: Introduction ------------ 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. Corda pervasively uses a custom form of type safe binary serialisation. This stands in contrast to some other systems that use weakly or untyped string-based serialisation schemes like JSON or XML. The primary drivers for this were: * A desire to have a schema describing what has been serialized alongside the actual data: #. To assist with versioning, both in terms of being able to interpret data archived long ago (e.g. trades from a decade ago, long after the code has changed) and between differing code versions. #. To make it easier to write generic code e.g. user interfaces that can navigate the serialized form of data. #. To support cross platform (non-JVM) interaction, where the format of a class file is not so easily interpreted. * 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. * A desire to support open-ended polymorphism, where the number of subclasses of a superclass can expand over time and the subclasses do not need to be defined in the schema *upfront*. This is key to many Corda concepts, such as states. * Increased security by constructing deserialized objects through supported constructors, rather than having data inserted directly into their fields without an opportunity to validate consistency or intercept attempts to manipulate supposed invariants. * Binary formats work better with digital signatures than text based formats, as there's much less scope for changes that modify syntax but not semantics. 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: #. 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. #. By implementing the ``SerializationWhitelist`` interface and specifying a list of `whitelist` classes. #. 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 :doc:`tutorial-clientrpc-api`. It's reproduced here as an example of both ways you can do this for a couple of example classes. .. literalinclude:: example-code/src/main/kotlin/net/corda/docs/ClientRpcTutorial.kt :language: kotlin :start-after: START 7 :end-before: END 7 .. 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 c = (Callable & Serializable) () -> "Hello World";``. .. warning:: We will be replacing the use of Kryo in the serialization framework and so additional changes here are likely. .. _amqp_ref: AMQP ---- Corda uses an extended form of AMQP 1.0 as its binary wire protocol. Corda serialisation is currently used for: #. Peer-to-peer networking. #. Persisted messages, like signed transactions and states. .. note:: At present, the Kryo-based format is still used by the RPC framework on both the client and server side. However, it is planned that the RPC framework will move to the AMQP framework soon. For the checkpointing of flows Corda uses a private scheme that is subject to change. It is currently based on the Kryo framework, but this may not be true in future. This separation of serialization schemes into different contexts allows us to use the most suitable framework for that context rather than attempting to force a one-size-fits-all approach. Kryo is more suited to the serialization of a program's stack frames, as it is more flexible than our AMQP framework in what it can construct and serialize. However, that flexibility makes it exceptionally difficult to make secure. Conversely, our AMQP framework allows us to concentrate on a secure framework that can be reasoned about and thus made safer, with far fewer security holes. Selection of serialization context should, for the most part, be opaque to CorDapp developers, the Corda framework selecting the correct context as configured. This document describes what is currently and what will be supported in the Corda AMQP format from the perspective of CorDapp developers, to allow CorDapps to take into consideration the future state. The AMQP serialization format will continue to apply the whitelisting functionality that is already in place and described in :doc:`serialization`. Core Types ---------- This section describes the classes and interfaces that the AMQP serialization format supports. 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. For example, if you use a Guava implementation of a collection, it will deserialize as the primitive collection type. The declared types of properties should only use these types, and not any concrete implementation types (e.g. Guava implementations). Collections must specify their generic type, the generic type parameters will be included in the schema, and the element's type will be 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, as a convenience, we explicitly support the concrete implementation types below, and they can be used as the declared types of properties. :: 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 `````` Arrays of any type are supported, primitive or otherwise. JDK Types ````````` The following JDK library types are supported: :: 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 Corda Types ``````````` Any classes and interfaces in the Corda codebase annotated with ``@CordaSerializable`` are 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. .. _amqp_custom_types_ref: Custom Types ------------ You own types must adhere to the following rules to be supported: Classes ``````` General Rules ''''''''''''' #. 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 using the ``-parameters`` command line option to ``javac`` .. note:: In circumstances where classes cannot be recompiled, such as when using a third-party library, a proxy serializer can be used to avoid this problem. Details on creating such an object can be found on the :doc:`cordapp-custom-serializers` page. #. The class must be annotated with ``@CordaSerializable`` #. The declared types of constructor arguments, getters, and setters 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 #. Any superclass must adhere to the same rules, but can be abstract #. Object graph cycles are not supported, so an object cannot refer to itself, directly or indirectly Constructor Instantiation ''''''''''''''''''''''''' The primary way Corda's AMQP serialization framework instantiates objects is via a specified constructor. This is used to first determine which properties of an object are to be serialised, then, on deserialization, it is used to instantiate the object with the serialized values. It is recommended that serializable objects in Corda adhere to the following rules, as they allow immutable state objects to be deserialised: #. A Java Bean getter for each of the properties in the constructor, with a name of the form ``getX``. For example, for a constructor parameter ``foo``, there must be a getter called ``getFoo()``. If ``foo`` is a boolean, the getter may optionally be called ``isFoo()`` (this is why the class must be compiled with parameter names turned on) #. 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 #. 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 In Kotlin, this maps cleanly to a data class where there getters are synthesized automatically. For example, suppose we have the following data class: .. container:: codeset .. sourcecode:: kotlin data class Example (val a: Int, val b: String) Properties ``a`` and ``b`` will be included in the serialised form. However, properties not mentioned in the constructor will not be serialised. For example, in the following code, property ``c`` will not be considered part of the serialised form: .. container:: codeset .. sourcecode:: kotlin data class Example (val a: Int, val b: String) { var c: Int = 20 } var e = Example (10, "hello") e.c = 100; val e2 = e.serialize().deserialize() // e2.c will be 20, not 100!!! Setter Instantiation '''''''''''''''''''' As an alternative to constructor-based initialisation, Corda can also determine the important elements of an object by inspecting the getter and setter methods present on the class. If a class has **only** a default constructor **and** properties then the serializable properties will be determined by the presence of both a getter and setter for that property that are both publicly visible (i.e. the class adheres to the classic *idiom* of mutable JavaBeans). On deserialization, a default instance will first be created, and then the setters will be invoked on that object to populate it with the correct values. For example: .. container:: codeset .. sourcecode:: kotlin class Example(var a: Int, var b: Int, var c: Int) .. sourcecode:: java class Example { private int a; private int b; private int c; public int getA() { return a; } public int getB() { return b; } public int getC() { return c; } public void setA(int a) { this.a = a; } public void setB(int b) { this.b = b; } public void setC(int c) { this.c = c; } } .. warning:: We do not recommend this pattern! Corda tries to use immutable data structures throughout, and if you rely heavily on mutable JavaBean style objects then you may sometimes find the API behaves in unintuitive ways. Inaccessible Private Properties ``````````````````````````````` Whilst the Corda AMQP serialization framework supports private object properties without publicly accessible getter methods, this development idiom is strongly discouraged. For example. .. container:: codeset .. sourcecode:: kotlin class C(val a: Int, private val b: Int) .. sourcecode:: java class C { public Integer a; private Integer b; public C(Integer a, Integer b) { this.a = a; this.b = b; } } When designing Corda states, it should be remembered that they are not, despite appearances, traditional OOP style objects. They are signed over, transformed, serialised, and relationally mapped. As such, all elements should be publicly accessible by design. .. warning:: IDEs will indicate erroneously that properties can be given something other than public visibility. Ignore this, as whilst it will work, as discussed above there are many reasons why this isn't a good idea. Providing a public getter, as per the following example, is acceptable: .. container:: codeset .. sourcecode:: kotlin class C(val a: Int, b: Int) { var b: Int = b private set } .. sourcecode:: java class C { public Integer a; private Integer b; C(Integer a, Integer b) { this.a = a; this.b = b; } public Integer getB() { return b; } } Mismatched Class Properties / Constructor Parameters ```````````````````````````````````````````````````` Consider an example where you wish to ensure that a property of class whose type is some form of container is always sorted using some specific criteria yet you wish to maintain the immutability of the class. This could be codified as follows: .. container:: codeset .. sourcecode:: kotlin @CordaSerializable class ConfirmRequest(statesToConsume: List, val transactionId: SecureHash) { companion object { private val stateRefComparator = compareBy({ it.txhash }, { it.index }) } private val states = statesToConsume.sortedWith(stateRefComparator) } The intention in the example is to always ensure that the states are stored in a specific order regardless of the ordering of the list used to initialise instances of the class. This is achieved by using the first constructor parameter as the basis for a private member. However, because that member is not mentioned in the constructor (whose parameters determine what is serializable as discussed above) it would not be serialized. In addition, as there is no provided mechanism to retrieve a value for ``statesToConsume`` we would fail to build a serializer for this Class. In this case a secondary constructor annotated with ``@ConstructorForDeserialization`` would not be a valid solution as the two signatures would be the same. Best practice is thus to provide a getter for the constructor parameter which explicitly associates it with the actual member variable. .. container:: codeset .. sourcecode:: kotlin @CordaSerializable class ConfirmRequest(statesToConsume: List, val transactionId: SecureHash) { companion object { private val stateRefComparator = compareBy({ it.txhash }, { it.index }) } private val states = statesToConsume.sortedWith(stateRefComparator) //Explicit "getter" for a property identified from the constructor parameters fun getStatesToConsume() = states } Enums ````` All enums are supported, provided they are annotated with ``@CordaSerializable``. Corda supports interoperability of enumerated type versions. This allows such types to be changed over time without breaking backward (or forward) compatibility. The rules and mechanisms for doing this are discussed in :doc:`serialization-enum-evolution`. Exceptions `````````` The following rules apply to supported ``Throwable`` implementations. #. If you wish for your exception to be serializable and transported type safely it should inherit from either ``CordaException`` or ``CordaRuntimeException`` #. 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 `````````````` Kotlin's non-anonymous ``object`` s (i.e. constructs like ``object foo : Contract {...}``) 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, which will deserialize to new instances of the class. This is hard to fix because there's no perfectly standard idiom for Java singletons. Kotlin's anonymous ``object`` s (i.e. constructs like ``object : Contract {...}``) are not currently supported and will not serialize correctly. They need to be re-written as an explicit class declaration. Class synthesis --------------- Corda serialization supports dynamically synthesising classes from the supplied schema when deserializing, without the supporting classes being present on the classpath. This can be useful where generic code might expect to be able to use reflection over the deserialized data, for scripting languages that run on the JVM, and also for ensuring classes not on the classpath can be deserialized without loading potentially malicious code. Possible future enhancements include: #. Java singleton support. We will add support for identifying classes which are singletons and identifying the static method responsible for returning the singleton instance #. 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()``) .. Type Evolution: Type Evolution -------------- Type evolution is the mechanism by which classes can be altered over time yet still remain serializable and deserializable across all versions of the class. This ensures an object serialized with an older idea of what the class "looked like" can be deserialized and a version of the current state of the class instantiated. More detail can be found in :doc:`serialization-default-evolution`.