corda/docs/source/writing-a-cordapp.rst

Structuring a CorDapp

Modules

The source code for a CorDapp is divided into one or more modules, each of which will be compiled into a separate JAR. Together, these JARs represent a single CorDapp. Typically, a Cordapp contains all the classes required for it to be used standalone. However, some Cordapps are only libraries for other Cordapps and cannot be run standalone.

A common pattern is to have:

• One module containing only the CorDapp's contracts and/or states, as well as any required dependencies
• A second module containing the remaining classes that depend on these contracts and/or states

This is because each time a contract is used in a transaction, the entire JAR containing the contract's definition is attached to the transaction. This is to ensure that the exact same contract and state definitions are used when verifying this transaction at a later date. Because of this, you will want to keep this module, and therefore the resulting JAR file, as small as possible to reduce the size of your transactions and keep your node performant.

However, this two-module structure is not prescriptive:

• A library CorDapp containing only contracts and states would only need a single module
• In a CorDapp with multiple sets of contracts and states that do not depend on each other, each independent set of contracts and states would go in a separate module to reduce transaction size
• In a CorDapp with multiple sets of contracts and states that do depend on each other, either keep them in the same module or create separate modules that depend on each other
• The module containing the flows and other classes can be structured in any way because it is not attached to transactions

Template CorDapps

You should base your project on one of the following templates:

• Java Template CorDapp (for CorDapps written in Java)
• Kotlin Template CorDapp (for CorDapps written in Kotlin)

Please use the branch of the template that corresponds to the major version of Corda you are using. For example, someone building a CorDapp on Corda 4.1 should use the release-V4 branch of the template.

Build system

The templates are built using Gradle. A Gradle wrapper is provided in the wrapper folder, and the dependencies are defined in the build.gradle files. See cordapp-build-systems for more information.

No templates are currently provided for Maven or other build systems.

Modules

The templates are split into two modules:

• A cordapp-contracts-states module containing the contracts and states
• A cordapp module containing the remaining classes that depends on the cordapp-contracts-states module

These modules will be compiled into two JARs - a cordapp-contracts-states JAR and a cordapp JAR - which together represent the Template CorDapp.

Module one - cordapp-contracts-states

Here is the structure of the src directory for the cordapp-contracts-states module of the Java template:

. └── main └── java └── com └── template ├── TemplateContract.java └── TemplateState.java

The directory only contains two class definitions:

• TemplateContract
• TemplateState

These are definitions for classes that we expect to have to send over the wire. They will be compiled into their own CorDapp.

Module two - cordapp

Here is the structure of the src directory for the cordapp module of the Java template:

. ├── main │   ├── java │   │   └── com │   │   └── template │   │   ├── TemplateApi.java │   │   ├── TemplateClient.java │   │   ├── TemplateFlow.java │   │   ├── TemplateSerializationWhitelist.java │   │   └── TemplateWebPlugin.java │   └── resources │   ├── META-INF │   │   └── services │   │   ├── net.corda.core.serialization.SerializationWhitelist │   │   └── net.corda.webserver.services.WebServerPluginRegistry │   ├── certificates │   └── templateWeb ├── test │ └── java │ └── com │ └── template │ ├── ContractTests.java │ ├── FlowTests.java │ └── NodeDriver.java └── integrationTest    └── java    └── com    └── template    └── DriverBasedTest.java

The src directory is structured as follows:

• main contains the source of the CorDapp
• test contains example unit tests, as well as a node driver for running the CorDapp from IntelliJ
• integrationTest contains an example integration test

Within main, we have the following directories:

• java, which contains the source-code for our CorDapp:

  • TemplateFlow.java, which contains a template FlowLogic subclass
  • TemplateState.java, which contains a template ContractState implementation
  • TemplateContract.java, which contains a template Contract implementation
  • TemplateSerializationWhitelist.java, which contains a template SerializationWhitelist implementation
  • TemplateApi.java, which contains a template API for the deprecated Corda webserver
  • TemplateWebPlugin.java, which registers the API and front-end for the deprecated Corda webserver
  • TemplateClient.java, which contains a template RPC client for interacting with our CorDapp

• resources/META-INF/services, which contains various registries:

  • net.corda.core.serialization.SerializationWhitelist, which registers the CorDapp's serialisation whitelists
  • net.corda.webserver.services.WebServerPluginRegistry, which registers the CorDapp's web plugins

• resources/templateWeb, which contains a template front-end

In a production CorDapp:

• We would remove the files related to the deprecated Corda webserver (TemplateApi.java, TemplateWebPlugin.java, resources/templateWeb, and net.corda.webserver.services.WebServerPluginRegistry) and replace them with a production-ready webserver
• We would also move TemplateClient.java into a separate module so that it is not included in the CorDapp