Build a CorDapp

This article covers the steps you need to take to build a CorDapp from scratch. You’ll learn how to:

• Set dependencies
• Create and install CorDapp JAR files
• Configure your CorDapp.
• Set minimum and target platform versions.
• Separate CorDapp contracts, flows, and services.
• Attach contracts.

Code samples guide you at every step.

Before you start

You will need to:

• Know what a CorDapp is.

• Set up your development environment.

• Run a sample CorDapp to see Corda in action (optional).

• Install the CorDapp gradle plugin. To ensure you are using the correct version of Gradle, use the Gradle wrapper provided. Copy across the following folder and files from the Kotlin CorDapp Template or the Java CorDapp Template to your project’s root directory:

  • gradle/
  • gradlew
  • gradlew.bat

Set dependencies

Your CorDapp will have certain dependencies, or requirements that must be met for the CorDapp to run correctly. You need to specify:

• Which versions of Corda, Quasar, and Kotlin your CorDapp requires.
• Any requirements for the user’s Corda installation.
• If your CorDapp depends on other CorDapps.
• Any other dependences, such as on apache-commons.

Define Corda, Quasar and Kotlin versions

A CorDapp’s build.gradle file uses several ext variables to define version numbers. These match the version of Corda you’re developing against:

• ext.corda_release_distribution defines the Corda artifact group ID.
• ext.corda_core_release_distribution defines the artifact group ID for the open source core dependencies.
• ext.corda_release_version defines the Corda version.
• ext.corda_core_release_version defines the open source version for the core dependencies.
• 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.quasar_classifier is used with quasar_version to set the version and classifier of the Quasar agent that the quasar-utils plugin will use. If quasar_classifier is not set as shown below, Gradle may not be able to resolve Quasar-related dependencies correctly.
• ext.kotlin_version defines the version of Kotlin (if using Kotlin to write your CorDapp).

Current versions:

ext.corda_release_distribution = 'com.r3.corda'
ext.corda_core_release_distribution = 'net.corda'
ext.corda_release_version = '4.11'
ext.corda_core_release_version = '4.11'
ext.corda_gradle_plugins_version = '5.0.12'
ext.quasar_version = '0.7.15_r3'
ext.quasar_classifier=''
ext.kotlin_version = '1.2.71'

repositories {
    // ... other dependencies
    maven { url "https://download.corda.net/maven/corda-dependencies" } // access to the patched Quasar and Caffeine version
}

Set Corda dependencies

There are a few guidelines to keep in mind when you set Corda dependencies.

• Always include $corda_core_release_distribution:corda-core:$corda_core_release_version as a cordaCompile dependency, and $corda_release_distribution:corda:$corda_release_version as a cordaRuntime dependency
• When building an RPC client that communicates with a node (e.g. a webserver), you should include $corda_release_distribution:corda-rpc:$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 $corda_release_distribution:corda-node-api:$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 $corda_release_distribution:corda-test-utils:$corda_release_version as a testCompile dependency. Never include corda-test-utils as a compile or cordaCompile dependency.
• Include any other Corda dependencies you need as cordaCompile dependencies.

Dependency list

• 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-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 fit your purpose.
• corda-node - The Corda node. Do not depend on this. Used only by the Corda fat JAR and indirectly in testing frameworks. (If your CorDapp must depend on this for some reason then you can use the compileOnly configuration here - however, this is not best practice).
• 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-shell - Used by the Corda node. Never depend on this 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-network-bootstrapper - The Network Builder tool. Useful in build scripts.

Any modules marked with (*) are part of the open core and must be pulled in from the matching Corda Community Edition distribution (using $corda_core_release_distribution and $corda_core_release_version.

The cordapp plugin adds additional 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. Use these configurations for any Corda dependency (for example, corda-core, corda-node) 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.

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 for all other cases.

The cordapp Gradle configuration serves two purposes:

• Indicates that the JAR should be included on your node as a CorDapp when using the cordformation Gradle plugin.
• Prevents the dependency from being included in the CorDapp JAR.

Other dependencies

You can specify any additional external dependencies using the same process you would for any Kotlin/Java dependencies in Gradle. See the example below, specifically the apache-commons include.

For further information about managing dependencies, see the Gradle docs.

Example

Below is a sample CorDapp Gradle dependencies block. When building your own CorDapp, use the build.gradle file of the Kotlin CorDapp Template or the Java CorDapp Template as a starting point.

dependencies {
    // Corda integration dependencies
    cordaCompile "$corda_core_release_distribution:corda-core:$corda_core_release_version"
    cordaCompile "$corda_core_release_distribution:corda-finance-contracts:$corda_core_release_version"
    cordaCompile "$corda_release_distribution:corda-finance-workflows:$corda_release_version"
    cordaCompile "$corda_release_distribution:corda-jackson:$corda_release_version"
    cordaCompile "$corda_release_distribution:corda-rpc:$corda_release_version"
    cordaCompile "$corda_release_distribution:corda-node-api:$corda_release_version"
    cordaRuntime "$corda_release_distribution:corda:$corda_release_version"
    cordaRuntime "$corda_release_distribution:corda-testserver:$corda_release_version"
    testCompile "$corda_release_distribution:corda-test-utils:$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 "$corda_release_distribution: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"
}

Create the CorDapp JAR

After you have set your dependencies, build your CorDapp JAR(s) using the Gradle jar task:

• Unix/macOS: ./gradlew jar
• Windows: gradlew.bat jar

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

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 Versioning).

Sign the CorDapp

The cordapp plugin can sign the generated CorDapp JAR file using the JAR signing and verification tool. Signing the CorDapp enables its contract classes to use signature constraints instead of other types of constraints. See Contract Constraints for more information. The JAR file is signed by the Corda development certificate by default.

You can disable the signing process or configure it to use an external keystore. The signing entry may contain the following parameters:

• enabled: The control flag to enable signing process. This is set to true by default. Set to false to disable signing.
• options: Any relevant parameters of SignJar ANT task. By default the JAR file is signed with Corda development key. You can specify the external keystore. The minimum options are listed below. Find additional options in the Apache manual.
  • keystore: The path to the keystore file. cordadevcakeys.jks keystore ships with the plugin by default.
  • 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 is different than the password for the keystore. The default value is cordacadevkeypass.
  • storetype the keystore type, the default value is JKS.

You can also set parameters by passing system properties to Gradle during the build process. Name the system properties using the relevant option name prefixed with ‘signing.’. For example, a value for alias can be taken from the signing.alias system property.

You can use:

• signing.enabled
• signing.keystore
• signing.alias
• signing.storepass
• signing.keypass
• signing.storetype

The resolution order of a configuration value is:

1. The signing process takes a value specified in the signing entry. The empty string “” is also accepted.
2. If you have not set an option, the signing process tries the system property named signing.option.
3. If you have not set a system property, the value defaults to the configuration of the Corda development certificate.

Here’s an example of the cordapp plugin with thesigning configuration:

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

Auto-sign the CorDapp

CorDapp auto-signing lets you use signature constraints for contracts from that CorDapp without the need to create a keystore or configuring the cordapp plugin. For a production deployment, make sure you sign the CorDapp with your own certificate.

You could sign the CorDapp automatically by:

• Setting system properties to point to an external keystore, or
• Disabling signing in the cordapp plugin and signing the CorDapp JAR downstream in your build pipeline.

Run development and production modes

Nodes only accept CorDapps signed by Corda development certificates when running in development mode. If you need to run a CorDapp signed by the (default) development key in the production mode (for example, for testing), add the cordappSignerKeyFingerprintBlacklist = [] property set to an empty list. See Configuring a node).

You can use one build.gradle file for both a development build (defaulting to the Corda development keystore) and for a production build (using an external keystore) by contexually overwriting signing options using system properties.

An example of the system properties setup for a build process that overrides signing options:

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

If you do not provide the system properties, the build signs the CorDapp with the default Corda development keystore:

./gradlew

You can disable CorDapp signing for a build:

./gradlew -Dsigning.enabled=false

To explicitly assign other system properties to options, call System.getProperty in the cordapp plugin configuration. The configuration below sets the specific signing algorithm when a system property is available. Otherwise, it defaults to an empty string:

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

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

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

To check if the CorDapp is signed, use the JAR signing and verification tool:

jarsigner --verify path/to/cordapp.jar

The Cordformation plugin can also sign CorDapp JARs when deploying a set of nodes.

If your build system post-processes the Cordapp JAR, then the modified JAR content may be out of date or missing a signature file. In this case, sign the Cordapp as a separate step and disable automatic signing by the cordapp plugin.

The cordapp plugin contains the standalone task signJar, which uses the same signing configuration. The task has two parameters:

• inputJars - to pass JAR files to be signed.
• An optional postfix, which is added to the name of signed JARs (it defaults to “-signed”).

The signed JARs are returned as the outputJars property.

For example, to sign a JAR modified by the modifyCordapp task:

1. Create an instance of the net.corda.plugins.SignJar task (named sign in the example below).
2. The output of the modifyCordapp task is passed to inputJars.
3. The sign task runs after modifyCordapp.
4. The task creates a new JAR file named -signed.jar for use later in your build/publishing process.

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

CorDapp configuration files

Place the CorDapp configuration files in <node_dir>/cordapps/config. Match the filename to the name of the CorDapp JAR (for example, if your CorDapp is called hello-0.1.jar the config should be config/hello-0.1.conf).

Config files are only available in the Typesafe/Lightbend config format. These files are loaded during node startup.

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

Using CorDapp configuration with the deployNodes task

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

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")
        }
    }
}

You can find an example project that demonstrates this in the samples folder of the Corda Git repository, cordapp-configuration . You can also refer to the API documentation.

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:

'Min-Platform-Version': 5
'Target-Platform-Version': 13

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:

cordapp {
    targetPlatformVersion 10
    minimumPlatformVersion 5
}

Separate CorDapp contracts, flows, and services

Package Contract code (states, commands, verification logic) separately from business flows (and associated services). This allows contracts to evolve independently from the flows and services that use them. Contracts may even be specified and implemented by different providers. For example, Corda ships with a cash financial contract, which in turn is used in many other flows and many other CorDapps.

CorDapps can explicitly differentiate their type by specifying the following attributes in the JAR manifest:

'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) you can specify contract-related attributes:

• Cordapp-Contract-Version (mandatory if Cordapp-Contract-Name is used). 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) you can specify workflow-related attributes:

• Cordapp-Workflow-Version (mandatory if Cordapp-Workflow-Name is used). 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.

You can specify your defaults using the Gradle cordapp plugin. For a contract-only CorDapp, specify the contract tag:

cordapp {
    targetPlatformVersion 10
    minimumPlatformVersion 5
    contract {
        name "my contract name"
        versionId 1
        vendor "my company"
        licence "my licence"
    }
}

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

cordapp {
    targetPlatformVersion 10
    minimumPlatformVersion 5
    workflow {
        name "my workflow name"
        versionId 1
        vendor "my company"
        licence "my licence"
    }
}

CorDapp Contract Attachments

CorDapp Contract JARs must be installed on a node by a trusted uploader, either by:

• Installing manually as per Installing the CorDapp JAR and re-starting the node.
• Uploading the attachment JAR to the node via RPC, either programmatically (see Connecting to a node via RPC) or via the shell using the command: >>> run uploadAttachment jar: path/to/the/file.jar.

Contract attachments received over the p2p network are untrusted and throw a UntrustedAttachmentsException exception if they are processed by a listening flow that cannot resolve the attachment with its local attachment storage. The flow will be suspended and sent to the node’s node-flow-hospital for recovery and retry. The untrusted attachment JAR is stored in the node’s local attachment store for review by a node operator. You can download it using a CRaSH shell command:

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

If the node operator decides to trust the attachment, they can issue a CRaSH shell command to reload it and retry the failed flow. This requires a node restart.

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

Install the CorDapp