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236 lines
9.0 KiB
ReStructuredText
236 lines
9.0 KiB
ReStructuredText
.. highlight:: kotlin
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.. raw:: html
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<script type="text/javascript" src="_static/jquery.js"></script>
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<script type="text/javascript" src="_static/codesets.js"></script>
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API: Contracts
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==============
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.. note:: Before reading this page, you should be familiar with the key concepts of :doc:`key-concepts-contracts`.
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.. contents::
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Contract
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--------
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Contracts are classes that implement the ``Contract`` interface. The ``Contract`` interface is defined as follows:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
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:language: kotlin
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:start-after: DOCSTART 5
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:end-before: DOCEND 5
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``Contract`` has a single method, ``verify``, which takes a ``LedgerTransaction`` as input and returns
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nothing. This function is used to check whether a transaction proposal is valid, as follows:
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* We gather together the contracts of each of the transaction's input and output states
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* We call each contract's ``verify`` function, passing in the transaction as an input
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* The proposal is only valid if none of the ``verify`` calls throw an exception
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``verify`` is executed in a sandbox:
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* It does not have access to the enclosing scope
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* The libraries available to it are whitelisted to disallow:
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* Network access
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* I/O such as disk or database access
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* Sources of randomness such as the current time or random number generators
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This means that ``verify`` only has access to the properties defined on ``LedgerTransaction`` when deciding whether a
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transaction is valid.
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Here are the two simplest ``verify`` functions:
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* A ``verify`` that **accepts** all possible transactions:
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.. container:: codeset
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.. sourcecode:: kotlin
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override fun verify(tx: LedgerTransaction) {
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// Always accepts!
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}
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.. sourcecode:: java
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@Override
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public void verify(LedgerTransaction tx) {
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// Always accepts!
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}
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* A ``verify`` that **rejects** all possible transactions:
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.. container:: codeset
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.. sourcecode:: kotlin
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override fun verify(tx: LedgerTransaction) {
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throw IllegalArgumentException("Always rejects!")
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}
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.. sourcecode:: java
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@Override
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public void verify(LedgerTransaction tx) {
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throw new IllegalArgumentException("Always rejects!");
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}
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LedgerTransaction
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-----------------
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The ``LedgerTransaction`` object passed into ``verify`` has the following properties:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/transactions/LedgerTransaction.kt
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:language: kotlin
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:start-after: DOCSTART 1
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:end-before: DOCEND 1
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Where:
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* ``inputs`` are the transaction's inputs as ``List<StateAndRef<ContractState>>``
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* ``outputs`` are the transaction's outputs as ``List<TransactionState<ContractState>>``
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* ``commands`` are the transaction's commands and associated signers, as ``List<CommandWithParties<CommandData>>``
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* ``attachments`` are the transaction's attachments as ``List<Attachment>``
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* ``notary`` is the transaction's notary. This must match the notary of all the inputs
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* ``timeWindow`` defines the window during which the transaction can be notarised
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``LedgerTransaction`` exposes a large number of utility methods to access the transaction's contents:
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* ``inputStates`` extracts the input ``ContractState`` objects from the list of ``StateAndRef``
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* ``getInput``/``getOutput``/``getCommand``/``getAttachment`` extracts a component by index
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* ``getAttachment`` extracts an attachment by ID
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* ``inputsOfType``/``inRefsOfType``/``outputsOfType``/``outRefsOfType``/``commandsOfType`` extracts components based on
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their generic type
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* ``filterInputs``/``filterInRefs``/``filterOutputs``/``filterOutRefs``/``filterCommands`` extracts components based on
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a predicate
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* ``findInput``/``findInRef``/``findOutput``/``findOutRef``/``findCommand`` extracts the single component that matches
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a predicate, or throws an exception if there are multiple matches
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requireThat
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-----------
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``verify`` can be written to manually throw an exception for each constraint:
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.. container:: codeset
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.. sourcecode:: kotlin
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override fun verify(tx: LedgerTransaction) {
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if (tx.inputs.size > 0)
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throw IllegalArgumentException("No inputs should be consumed when issuing an X.")
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if (tx.outputs.size != 1)
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throw IllegalArgumentException("Only one output state should be created.")
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}
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.. sourcecode:: java
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public void verify(LedgerTransaction tx) {
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if (tx.getInputs().size() > 0)
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throw new IllegalArgumentException("No inputs should be consumed when issuing an X.");
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if (tx.getOutputs().size() != 1)
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throw new IllegalArgumentException("Only one output state should be created.");
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}
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However, this is verbose. To impose a series of constraints, we can use ``requireThat`` instead:
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.. container:: codeset
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.. sourcecode:: kotlin
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requireThat {
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"No inputs should be consumed when issuing an X." using (tx.inputs.isEmpty())
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"Only one output state should be created." using (tx.outputs.size == 1)
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val out = tx.outputs.single() as XState
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"The sender and the recipient cannot be the same entity." using (out.sender != out.recipient)
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"All of the participants must be signers." using (command.signers.containsAll(out.participants))
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"The X's value must be non-negative." using (out.x.value > 0)
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}
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.. sourcecode:: java
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requireThat(require -> {
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require.using("No inputs should be consumed when issuing an X.", tx.getInputs().isEmpty());
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require.using("Only one output state should be created.", tx.getOutputs().size() == 1);
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final XState out = (XState) tx.getOutputs().get(0);
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require.using("The sender and the recipient cannot be the same entity.", out.getSender() != out.getRecipient());
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require.using("All of the participants must be signers.", command.getSigners().containsAll(out.getParticipants()));
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require.using("The X's value must be non-negative.", out.getX().getValue() > 0);
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return null;
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});
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For each <``String``, ``Boolean``> pair within ``requireThat``, if the boolean condition is false, an
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``IllegalArgumentException`` is thrown with the corresponding string as the exception message. In turn, this
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exception will cause the transaction to be rejected.
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Commands
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--------
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``LedgerTransaction`` contains the commands as a list of ``CommandWithParties`` instances. ``CommandWithParties`` pairs
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a ``CommandData`` with a list of required signers for the transaction:
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.. container:: codeset
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.. literalinclude:: ../../core/src/main/kotlin/net/corda/core/contracts/Structures.kt
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:language: kotlin
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:start-after: DOCSTART 6
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:end-before: DOCEND 6
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Where:
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* ``signers`` is the list of each signer's ``PublicKey``
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* ``signingParties`` is the list of the signer's identities, if known
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* ``value`` is the object being signed (a command, in this case)
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Branching verify with commands
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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Generally, we will want to impose different constraints on a transaction based on its commands. For example, we will
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want to impose different constraints on a cash issuance transaction to on a cash transfer transaction.
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We can achieve this by extracting the command and using standard branching logic within ``verify``. Here, we extract
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the single command of type ``XContract.Commands`` from the transaction, and branch ``verify`` accordingly:
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.. container:: codeset
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.. sourcecode:: kotlin
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class XContract : Contract {
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interface Commands : CommandData {
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class Issue : TypeOnlyCommandData(), Commands
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class Transfer : TypeOnlyCommandData(), Commands
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}
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override fun verify(tx: LedgerTransaction) {
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val command = tx.findCommand<Commands> { true }
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when (command) {
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is Commands.Issue -> {
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// Issuance verification logic.
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}
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is Commands.Transfer -> {
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// Transfer verification logic.
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}
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}
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}
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}
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.. sourcecode:: java
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public class XContract implements Contract {
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public interface Commands extends CommandData {
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class Issue extends TypeOnlyCommandData implements Commands {}
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class Transfer extends TypeOnlyCommandData implements Commands {}
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}
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@Override
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public void verify(LedgerTransaction tx) {
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final Command<Commands> command = tx.findCommand(Commands.class, cmd -> true);
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if (command instanceof Commands.Issue) {
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// Issuance verification logic.
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} else if (command instanceof Commands.Transfer) {
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// Transfer verification logic.
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}
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}
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} |