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Building and installing a CorDapp
=================================

.. contents::

CorDapps run on the Corda platform and integrate with it and each other. This article explains how to build CorDapps.

Applications on Corda (CorDapps) can be written in any language targeting the JVM. However, Corda itself and most of the samples are written in Kotlin. Kotlin is an `official Android language <https://developer.android.com/kotlin/index.html>`_, and you can read more about why Kotlin is a strong successor to Java `here <https://medium.com/@octskyward/why-kotlin-is-my-next-programming-language-c25c001e26e3>`_. If you're unfamiliar with Kotlin, there is an official `getting started guide <https://kotlinlang.org/docs/tutorials/>`_, and a series of `Kotlin Koans <https://kotlinlang.org/docs/tutorials/koans.html>`_

To learn what a CorDapp is, please read :doc:`cordapp-overview`.

CorDapp format
--------------
A CorDapp is a semi-fat JAR that contains all of the CorDapp's dependencies *except* the Corda core libraries and any
other CorDapps it depends on.

For example, if a Cordapp depends on ``corda-core``, ``your-other-cordapp`` and ``apache-commons``, then the Cordapp
JAR will contain:

* All classes and resources from the ``apache-commons`` JAR and its dependencies
* *Nothing* from the other two JARs

Build tools
-----------
In the instructions that follow, we assume you are using Gradle and the ``cordapp`` plugin to build your
CorDapp. You can find examples of building a CorDapp using these tools in the
`Kotlin CorDapp Template <https://github.com/corda/cordapp-template-kotlin>`_ and the
`Java CorDapp Template <https://github.com/corda/cordapp-template-java>`_.

To ensure you are using the correct version of Gradle, you should use the provided Gradle Wrapper by copying across
the following folder and files from the `Kotlin CorDapp Template <https://github.com/corda/cordapp-template-kotlin>`_ or the
`Java CorDapp Template <https://github.com/corda/cordapp-template-java>`_ to the root of your project:

* ``gradle/``
* ``gradlew``
* ``gradlew.bat``

.. _cordapp_dependencies_ref:

Setting your dependencies
-------------------------

Choosing your Corda, Quasar and Kotlin versions
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Several ``ext`` variables are used in a CorDapp's ``build.gradle`` file to define version numbers that should match the version of
Corda you're developing against:

* ``ext.corda_release_version`` defines the version of Corda itself
* ``ext.corda_gradle_plugins_version`` defines the version of the Corda Gradle Plugins
* ``ext.quasar_version`` defines the version of Quasar, a library that we use to implement the flow framework
* ``ext.kotlin_version`` defines the version of Kotlin (if using Kotlin to write your CorDapp)

The current versions used are as follows:

.. code::

    ext.corda_release_version = '|corda_version|'
    ext.corda_gradle_plugins_version = '|gradle_plugins_version|'
    ext.quasar_version = '|quasar_version|'
    ext.kotlin_version = '|kotlin_version|'

In certain cases, you may also wish to build against the unstable Master branch. See :doc:`building-against-master`.

Corda dependencies
^^^^^^^^^^^^^^^^^^
The ``cordapp`` plugin adds three new gradle configurations:

* ``cordaCompile``, which extends ``compile``
* ``cordaRuntime``, which extends ``runtime``
* ``cordapp``, which extends ``compile``

``cordaCompile`` and ``cordaRuntime`` indicate dependencies that should not be included in the CorDapp JAR. These
configurations should be used for any Corda dependency (e.g. ``corda-core``, ``corda-node``) in order to prevent a
dependency from being included twice (once in the CorDapp JAR and once in the Corda JARs). The ``cordapp`` dependency
is for declaring a compile-time dependency on a "semi-fat" CorDapp JAR in the same way as ``cordaCompile``, except
that ``Cordformation`` will only deploy CorDapps contained within the ``cordapp`` configuration.

Here are some guidelines for Corda dependencies:

* When building a CorDapp, you should always include ``net.corda:corda-core:$corda_release_version`` as a
  ``cordaCompile`` dependency, and ``net.corda:corda:$corda_release_version`` as a ``cordaRuntime`` dependency

* When building an RPC client that communicates with a node (e.g. a webserver), you should include
  ``net.corda:corda-rpc:$corda_release_version`` as a ``cordaCompile`` dependency.

* When you need to use the network bootstrapper to bootstrap a local network (e.g. when using ``Cordformation``), you
  should include ``net.corda:corda-node-api:$corda_release_version`` as either a ``cordaRuntime`` or a ``runtimeOnly``
  dependency. You may also wish to include an implementation of SLF4J as a ``runtimeOnly`` dependency for the network
  bootstrapper to use.

* To use Corda's test frameworks, add ``net.corda:corda-test-utils:$corda_release_version`` as a ``testCompile``
  dependency. Never include ``corda-test-utils`` as a ``compile`` or ``cordaCompile`` dependency.

* Any other Corda dependencies you need should be included as ``cordaCompile`` dependencies.

Here is an overview of the various Corda dependencies:

* ``corda`` - The Corda fat JAR. Do not use as a compile dependency. Required as a ``cordaRuntime`` dependency when
  using ``Cordformation``
* ``corda-confidential-identities`` - A part of the core Corda libraries. Automatically pulled in by other libraries
* ``corda-core`` - Usually automatically included by another dependency, contains core Corda utilities, model, and
  functionality. Include manually if the utilities are useful or you are writing a library for Corda
* ``corda-core-deterministic`` - Used by the Corda node for deterministic contracts. Not likely to be used externally
* ``corda-djvm`` - Used by the Corda node for deterministic contracts. Not likely to be used externally
* ``corda-finance-contracts``, ``corda-finance-workflows`` and deprecated ``corda-finance``. Corda finance CorDapp, use contracts and flows parts respectively.
  Only include as a ``cordaCompile`` dependency if using as a dependent Cordapp or if you need access to the Corda finance types.
  Use as a ``cordapp`` dependency if using as a CorDapp dependency (see below)
* ``corda-jackson`` - Corda Jackson support. Use if you plan to serialise Corda objects to and/or from JSON
* ``corda-jfx`` - JavaFX utilities with some Corda-specific models and utilities. Only use with JavaFX apps
* ``corda-mock`` - A small library of useful mocks. Use if the classes are useful to you
* ``corda-node`` - The Corda node. Do not depend on. Used only by the Corda fat JAR and indirectly in testing
  frameworks. (If your CorDapp _must_ depend on this for some reason then it should use the ``compileOnly``
  configuration here - but please don't do this if you can possibly avoid it!)
* ``corda-node-api`` - The node API. Required to bootstrap a local network
* ``corda-node-driver`` - Testing utility for programmatically starting nodes from JVM languages. Use for tests
* ``corda-rpc`` - The Corda RPC client library. Used when writing an RPC client
* ``corda-serialization`` - The Corda core serialization library. Automatically included by other dependencies
* ``corda-serialization-deterministic`` - The Corda core serialization library. Automatically included by other
  dependencies
* ``corda-shell`` - Used by the Corda node. Never depend on directly
* ``corda-test-common`` - A common test library. Automatically included by other test libraries
* ``corda-test-utils`` - Used when writing tests against Corda/Cordapps
* ``corda-tools-explorer`` - The Node Explorer tool. Do not depend on
* ``corda-tools-network-bootstrapper`` - The Network Builder tool. Useful in build scripts
* ``corda-tools-shell-cli`` - The Shell CLI tool. Useful in build scripts

Dependencies on other CorDapps
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
Your CorDapp may also depend on classes defined in another CorDapp, such as states, contracts and flows. There are two
ways to add another CorDapp as a dependency in your CorDapp's ``build.gradle`` file:

* ``cordapp project(":another-cordapp")`` (use this if the other CorDapp is defined in a module in the same project)
* ``cordapp "net.corda:another-cordapp:1.0"`` (use this otherwise)

The ``cordapp`` gradle configuration serves two purposes:

* When using the ``cordformation`` Gradle plugin, the ``cordapp`` configuration indicates that this JAR should be
  included on your node as a CorDapp
* When using the ``cordapp`` Gradle plugin, the ``cordapp`` configuration prevents the dependency from being included
  in the CorDapp JAR

Note that the ``cordformation`` and ``cordapp`` Gradle plugins can be used together.

Other dependencies
^^^^^^^^^^^^^^^^^^
If your CorDapps have any additional external dependencies, they can be specified like normal Kotlin/Java dependencies
in Gradle. See the example below, specifically the ``apache-commons`` include.

For further information about managing dependencies, see
`the Gradle docs <https://docs.gradle.org/current/userguide/dependency_management.html>`_.

.. _cordapp_build_system_signing_cordapp_jar_ref:

Signing the CorDapp JAR
^^^^^^^^^^^^^^^^^^^^^^^
The ``cordapp`` plugin can sign the generated CorDapp JAR file using `JAR signing and verification tool <https://docs.oracle.com/javase/tutorial/deployment/jar/signing.html>`__.
Signing the CorDapp enables its contract classes to use signature constraints instead of other types of the constraints,
for constraints explanation refer to :doc:`api-contract-constraints`.
By default the JAR file is signed by Corda development certificate.
The signing process can be disabled or configured to use an external keystore.
The ``signing`` entry may contain the following parameters:

 * ``enabled`` the control flag to enable signing process, by default is set to ``true``, set to ``false`` to disable signing
 * ``options`` any relevant parameters of `SignJar ANT task <https://ant.apache.org/manual/Tasks/signjar.html>`_,
   by default the JAR file is signed with Corda development key, the external keystore can be specified,
   the minimal list of required options is shown below, for other options referer to `SignJar task <https://ant.apache.org/manual/Tasks/signjar.html>`_:

   * ``keystore`` the path to the keystore file, by default *cordadevcakeys.jks* keystore is shipped with the plugin
   * ``alias`` the alias to sign under, the default value is *cordaintermediateca*
   * ``storepass`` the keystore password, the default value is *cordacadevpass*
   * ``keypass`` the private key password if it's different than the password for the keystore, the default value is *cordacadevkeypass*
   * ``storetype`` the keystore type, the default value is *JKS*

The parameters can be also set by system properties passed to Gradle build process.
The system properties should be named as the relevant option name prefixed with '*signing.*', e.g.
a value for ``alias`` can be taken from the ``signing.alias`` system property. The following system properties can be used:
``signing.enabled``, ``signing.keystore``, ``signing.alias``, ``signing.storepass``, ``signing.keypass``, ``signing.storetype``.
The resolution order of a configuration value is as follows: the signing process takes a value specified in the ``signing`` entry first,
the empty string *""* is also considered as the correct value.
If the option is not set, the relevant system property named *signing.option* is tried.
If the system property is not set then the value defaults to the configuration of the Corda development certificate.

The example ``cordapp`` plugin with plugin ``signing`` configuration:

.. sourcecode:: groovy

    cordapp {
        signing {
            enabled true
            options {
                keystore "/path/to/jarSignKeystore.p12"
                alias "cordapp-signer"
                storepass "secret1!"
                keypass "secret1!"
                storetype "PKCS12"
            }
        }
        //...

CorDapp auto-signing allows to use signature constraints for contracts from the CorDapp without need to create a
keystore and configure the ``cordapp`` plugin. For production deployment ensure to sign the CorDapp using your own
certificate e.g. by setting system properties to point to an external keystore or by disabling signing in ``cordapp``
plugin and signing the CordDapp JAR downstream in your build pipeline. CorDapp signed by Corda development certificate
is accepted by Corda node only when running in the development mode. In case CordDapp signed by the (default)
development key is run on node in the production mode (e.g. for testing), the node may be set to accept the development
key by adding the ``cordappSignerKeyFingerprintBlacklist = []`` property set to empty list (see
:ref:`Configuring a node <corda_configuration_file_signer_blacklist>`).

Signing options can be contextually overwritten by the relevant system properties as described above. This allows the
single ``build.gradle`` file to be used for a development build (defaulting to the Corda development keystore) and for
a production build (using an external keystore). The example system properties setup for the build process which
overrides signing options:

.. sourcecode:: shell

    ./gradlew -Dsigning.keystore="/path/to/keystore.jks" -Dsigning.alias="alias" -Dsigning.storepass="password" -Dsigning.keypass="password"

Without providing the system properties, the build will sign the CorDapp with the default Corda development keystore:

.. sourcecode:: shell

    ./gradlew

CorDapp signing can be disabled for a build:

.. sourcecode:: shell

    ./gradlew -Dsigning.enabled=false

Other system properties can be explicitly assigned to options by calling ``System.getProperty`` in ``cordapp`` plugin
configuration. For example the below configuration sets the specific signing algorithm when a system property is
available otherwise defaults to an empty string:

.. sourcecode:: groovy

    cordapp {
        signing {
            options {
                sigalg System.getProperty('custom.sigalg','')
            }
        }
        //...

Then the build process can set the value for *custom.sigalg* system property and other system properties recognized by
``cordapp`` plugin:

.. sourcecode:: shell

    ./gradlew -Dcustom.sigalg="SHA256withECDSA" -Dsigning.keystore="/path/to/keystore.jks" -Dsigning.alias="alias" -Dsigning.storepass="password" -Dsigning.keypass="password"

To check if CorDapp is signed use `JAR signing and verification tool <https://docs.oracle.com/javase/tutorial/deployment/jar/verify.html>`__:

.. sourcecode:: shell

   jarsigner --verify path/to/cordapp.jar

Cordformation plugin can also sign CorDapps JARs, when deploying set of nodes, see :doc:`generating-a-node`.

If your build system post-processes the Cordapp JAR, then the modified JAR content may be out-of-date or not complete
with regards to a signature file. In this case you can sign the Cordapp as a separate step and disable the automatic signing by the ``cordapp`` plugin.
The ``cordapp`` plugin contains a standalone task ``signJar`` which uses the same ``signing`` configuration.
The task has two parameters: ``inputJars`` - to pass JAR files to be signed
and an optional ``postfix`` which is added to the name of signed JARs (it defaults to "-signed").
The signed JARs are returned as  ``outputJars`` property.

For example in order to sign a JAR modified by *modifyCordapp* task,
create an instance of the ``net.corda.plugins.SignJar`` task (below named as *sign*).
The output of *modifyCordapp* task is passed to *inputJars* and the *sign* task is run after *modifyCordapp* one:

.. sourcecode:: groovy

    task sign(type: net.corda.plugins.SignJar) {
        inputJars modifyCordapp
    }
    modifyCordapp.finalizedBy sign
    cordapp {
        signing {
            enabled false
        }
        //..
    }

The task creates a new JAR file named *\*-signed.jar* which should be used further in your build/publishing process.
Also the best practice is to disable signing by the ``cordapp`` plugin as shown in the example.

Example
^^^^^^^
Below is a sample CorDapp Gradle dependencies block. When building your own CorDapp, use the ``build.gradle`` file of the
`Kotlin CorDapp Template <https://github.com/corda/cordapp-template-kotlin>`_ or the
`Java CorDapp Template <https://github.com/corda/cordapp-template-java>`_ as a starting point.

.. container:: codeset

    .. sourcecode:: groovy

        dependencies {
            // Corda integration dependencies
            cordaCompile "net.corda:corda-core:$corda_release_version"
            cordaCompile "net.corda:corda-finance-contracts:$corda_release_version"
            cordaCompile "net.corda:corda-finance-workflows:$corda_release_version"
            cordaCompile "net.corda:corda-jackson:$corda_release_version"
            cordaCompile "net.corda:corda-rpc:$corda_release_version"
            cordaCompile "net.corda:corda-node-api:$corda_release_version"
            cordaCompile "net.corda:corda-webserver-impl:$corda_release_version"
            cordaRuntime "net.corda:corda:$corda_release_version"
            cordaRuntime "net.corda:corda-testserver:$corda_release_version"
            testCompile "net.corda:corda-test-utils:$corda_release_version"

            // Corda Plugins: dependent flows and services
            // Identifying a CorDapp by its module in the same project.
            cordapp project(":cordapp-contracts-states")
            // Identifying a CorDapp by its fully-qualified name.
            cordapp "net.corda:bank-of-corda-demo:1.0"

            // Some other dependencies
            compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
            testCompile "org.jetbrains.kotlin:kotlin-test:$kotlin_version"
            testCompile "junit:junit:$junit_version"

            compile "org.apache.commons:commons-lang3:3.6"
        }

Creating the CorDapp JAR
------------------------
Once your dependencies are set correctly, you can build your CorDapp JAR(s) using the Gradle ``jar`` task

* Unix/Mac OSX: ``./gradlew jar``

* Windows: ``gradlew.bat jar``

Each of the project's modules will be compiled into its own CorDapp JAR. You can find these CorDapp JARs in the ``build/libs``
folders of each of the project's modules.

.. warning:: The hash of the generated CorDapp JAR is not deterministic, as it depends on variables such as the
   timestamp at creation. Nodes running the same CorDapp must therefore ensure they are using the exact same CorDapp
   JAR, and not different versions of the JAR created from identical sources.

The filename of the JAR must include a unique identifier to deduplicate it from other releases of the same CorDapp.
This is typically done by appending the version string to the CorDapp's name. This unique identifier should not change
once the JAR has been deployed on a node. If it does, make sure no one is relying on ``FlowContext.appName`` in their
flows (see :doc:`versioning`).


.. _cordapp_install_ref:

Installing the CorDapp JAR
--------------------------

.. note:: Before installing a CorDapp, you must create one or more nodes to install it on. For instructions, please see
   :doc:`generating-a-node`.

At start-up, nodes will load any CorDapps present in their ``cordapps`` folder. In order to install a CorDapp on a node, the
CorDapp JAR must be added to the ``<node_dir>/cordapps/`` folder (where ``node_dir`` is the folder in which the node's JAR
and configuration files are stored) and the node restarted.

.. _cordapp_configuration_files_ref:

CorDapp configuration files
---------------------------

CorDapp configuration files should be placed in ``<node_dir>/cordapps/config``. The name of the file should match the
name of the JAR of the CorDapp (eg; if your CorDapp is called ``hello-0.1.jar`` the config should be ``config/hello-0.1.conf``).

Config files are currently only available in the `Typesafe/Lightbend <https://github.com/lightbend/config>`_ config format.
These files are loaded during node startup.

CorDapp configuration can be accessed from ``CordappContext::config`` whenever a ``CordappContext`` is available. For example:

.. literalinclude:: ../../samples/cordapp-configuration/workflows/src/main/kotlin/net/corda/configsample/GetStringConfigFlow.kt
    :language: kotlin
    :start-after: DOCSTART 1
    :end-before: DOCEND 1

Using CorDapp configuration with the deployNodes task
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If you want to generate CorDapp configuration when using the ``deployNodes`` Gradle task, then you can use the ``cordapp`` or ``projectCordapp``
properties on the node. For example:

.. sourcecode:: groovy

    task deployNodes(type: net.corda.plugins.Cordform, dependsOn: ['jar']) {
        nodeDefaults {
            // this external CorDapp will be included in each project
            cordapp("$corda_release_group:corda-finance-contracts:$corda_release_version")
            // this external CorDapp will be included in each project with the given config
            cordapp("$corda_release_group:corda-finance-workflows:$corda_release_version") {
                config "issuableCurrencies = [ USD ]"
            }
        }
        node {
            name "O=Bank A,L=London,C=GB"c
            ...
            // This adds configuration for another CorDapp project within the build
            cordapp (project(':my-project:workflow-cordapp')) {
                config "someStringValue=test"
            }
            cordapp(project(':my-project:another-cordapp')) {
                // Use a multiline string for complex configuration
                config '''
                    someStringValue=test
                    anotherStringValue=10
                 '''
            }
        }
        node {
            name "O=Bank B,L=New York,C=US"
            ...
            // This adds configuration for the default CorDapp for this project
            projectCordapp {
                config project.file("src/config.conf")
            }
        }
    }

There is an example project that demonstrates this in the ``samples`` folder of the Corda Git repository, called ``cordapp-configuration`` .
API documentation can be found at `<api/kotlin/corda/net.corda.core.cordapp/index.html>`_.

Minimum and target platform version
-----------------------------------

CorDapps can advertise their minimum and target platform version. The minimum platform version indicates that a node has to run at least this
version in order to be able to run this CorDapp. The target platform version indicates that a CorDapp was tested with this version of the Corda
Platform and should be run at this API level if possible. It provides a means of maintaining behavioural compatibility for the cases where the
platform's behaviour has changed. These attributes are specified in the JAR manifest of the CorDapp, for example:

.. sourcecode:: groovy

    'Min-Platform-Version': 4
    'Target-Platform-Version': 4

**Defaults**
    - ``Target-Platform-Version`` (mandatory) is a whole number and must comply with the rules mentioned above.
    - ``Min-Platform-Version`` (optional) will default to 1 if not specified.

Using the `cordapp` Gradle plugin, this can be achieved by putting this in your CorDapp's `build.gradle`:

.. container:: codeset

    .. sourcecode:: groovy

        cordapp {
            targetPlatformVersion 4
            minimumPlatformVersion 4
        }

.. _cordapp_separation_ref:

Separation of CorDapp contracts, flows and services
---------------------------------------------------
It is recommended that **contract** code (states, commands, verification logic) be packaged separately from **business flows** (and associated services).
This decoupling enables *contracts* to evolve independently from the *flows* and *services* that use them. Contracts may even be specified and implemented by different
providers (eg. Corda currently ships with a cash financial contract which in turn is used in many other flows and many other CorDapps).

As of Corda 4, CorDapps can explicitly differentiate their type by specifying the following attributes in the JAR manifest:

.. sourcecode:: groovy

    'Cordapp-Contract-Name'
    'Cordapp-Contract-Version'
    'Cordapp-Contract-Vendor'
    'Cordapp-Contract-Licence'

    'Cordapp-Workflow-Name'
    'Cordapp-Workflow-Version'
    'Cordapp-Workflow-Vendor'
    'Cordapp-Workflow-Licence'

**Defaults**

``Cordapp-Contract-Name`` (optional) if specified, the following Contract related attributes are also used:

    - ``Cordapp-Contract-Version`` (mandatory), must be a whole number starting from 1.
    - ``Cordapp-Contract-Vendor`` (optional), defaults to UNKNOWN if not specified.
    - ``Cordapp-Contract-Licence`` (optional), defaults to UNKNOWN if not specified.

``Cordapp-Workflow-Name`` (optional) if specified, the following Workflow related attributes are also used:

    - ``Cordapp-Workflow-Version`` (mandatory), must be a whole number starting from 1.
    - ``Cordapp-Workflow-Vendor`` (optional), defaults to UNKNOWN if not specified.
    - ``Cordapp-Workflow-Licence`` (optional), defaults to UNKNOWN if not specified.

As with the general CorDapp attributes (minimum and target platform version), these can be specified using the Gradle `cordapp` plugin as follows:

For a contract only CorDapp we specify the `contract` tag:

.. container:: codeset

    .. sourcecode:: groovy

        cordapp {
            targetPlatformVersion 4
            minimumPlatformVersion 3
            contract {
                name "my contract name"
                versionId 1
                vendor "my company"
                licence "my licence"
            }
        }

For a CorDapp that contains flows and/or services we specify the `workflow` tag:

.. container:: codeset

    .. sourcecode:: groovy

        cordapp {
            targetPlatformVersion 4
            minimumPlatformVersion 3
            workflow {
                name "my workflow name"
                versionId 1
                vendor "my company"
                licence "my licence"
            }
        }

.. note:: It is possible, but *not recommended*, to include everything in a single CorDapp jar and use both the ``contract`` and ``workflow`` Gradle plugin tags.

.. warning:: Contract states may optionally specify a custom schema mapping (by implementing the ``Queryable`` interface) in its *contracts* JAR.
  However, any associated database schema definition scripts (eg. Liquibase change set XML files) must currently be packaged in the *flows* JAR.
  This is because the node requires access to these schema definitions upon start-up (*contract* JARs are now loaded in a separate attachments classloader).
  This split also caters for scenarios where the same *contract* CorDapp may wish to target different database providers (and thus, the associated schema DDL may vary
  to use native features of a particular database). The finance CorDapp provides an illustration of this packaging convention.
  Future versions of Corda will de-couple this custom schema dependency to remove this anomaly.

.. _cordapp_contract_attachments_ref:

CorDapp Contract Attachments
----------------------------

As of Corda 4, CorDapp Contract JARs must be installed on a node by a trusted uploader, either by

- installing manually as per :ref:`Installing the CorDapp JAR <cordapp_install_ref>` and re-starting the node.

- uploading the attachment JAR to the node via RPC, either programmatically (see :ref:`Connecting to a node via RPC <clientrpc_connect_ref>`)
  or via the :doc:`shell` by issuing the following command:

``>>> run uploadAttachment jar: path/to/the/file.jar``

Contract attachments that are received from a peer over the p2p network are considered **untrusted** and will throw a `UntrustedAttachmentsException` exception
when processed by a listening flow that cannot resolve that attachment from its local attachment storage. The flow will be aborted and sent to the nodes flow hospital for recovery and retry.
The untrusted attachment JAR will be stored in the nodes local attachment store for review by a node operator. It can be downloaded for viewing using the following CRaSH shell command:

``>>> run openAttachment id: <hash of untrusted attachment given by `UntrustedAttachmentsException` exception``

Should the node operator deem the attachment trustworthy, they may then issue the following CRaSH shell command to reload it as trusted:

``>>> run uploadAttachment jar: path/to/the/trusted-file.jar``

and subsequently retry the failed flow (currently this requires a node re-start).

.. note:: this behaviour is to protect the node from executing contract code that was not vetted. It is a temporary precaution until the
   Deterministic JVM is integrated into Corda whereby execution takes place in a sandboxed environment which protects the node from malicious code.
