Java Bindings Example Project

To try out the Java bindings library, use the examples on GitHub: PingPongReactive.

The example implements the PingPong application, which consists of:

  • a Daml model with two contract templates, Ping and Pong
  • two parties, Alice and Bob

The logic of the application goes like this:

  1. The application injects a contract of type Ping for Alice.
  2. Alice sees this contract and exercises the consuming choice RespondPong to create a contract of type Pong for Bob.
  3. Bob sees this contract and exercises the consuming choice RespondPing to create a contract of type Ping for Alice.
  4. Points 2 and 3 are repeated until the maximum number of contracts defined in the Daml is reached.

Set Up the Example Projects

To set up the example projects, clone the public GitHub repository at github.com/digital-asset/ex-java-bindings and follow the setup instruction in the README file.

This project contains two examples of the PingPong application, built directly with gRPC and using the RxJava2-based Java bindings.

Example Project

PingPongMain.java

The entry point for the Java code is the main class src/main/java/examples/pingpong/grpc/PingPongMain.java. Look at this class to see:

  • how to connect to and interact with a Daml Ledger via the Java bindings
  • how to use the Reactive layer to build an automation for both parties.

At high level, the code does the following steps:

  • creates an instance of DamlLedgerClient connecting to an existing Ledger

  • connect this instance to the Ledger with DamlLedgerClient.connect()

  • create two instances of PingPongProcessor, which contain the logic of the automation

    (This is where the application reacts to the new Ping or Pong contracts.)

  • run the PingPongProcessor forever by connecting them to the incoming transactions

  • inject some contracts for each party of both templates

  • wait until the application is done

PingPongProcessor.runIndefinitely()

The core of the application is the PingPongProcessor.runIndefinitely().

The PingPongProcessor queries the transactions first via the TransactionsClient of the DamlLedgerClient. Then, for each transaction, it produces Commands that will be sent to the Ledger via the CommandSubmissionClient of the DamlLedgerClient.

Output

The application prints statements similar to these:

Bob is exercising RespondPong on #1:0 in workflow Ping-Alice-1 at count 0
Alice is exercising RespondPing on #344:1 in workflow Ping-Alice-7 at count 9

The first line shows that:

  • Bob is exercising the RespondPong choice on the contract with ID #1:0 for the workflow Ping-Alice-1.
  • Count 0 means that this is the first choice after the initial Ping contract.
  • The workflow ID Ping-Alice-1 conveys that this is the workflow triggered by the second initial Ping contract that was created by Alice.

The second line is analogous to the first one.