Java SDK Hyperledger Fabric Add commitListener

I'm trying to create a commitListener using the Java SDK to listen for commit events after submitting a transaction, although the listener is not responding.

I'm using the fabcar example.

// create a gateway connection
		try (Gateway gateway = builder.connect()) {

			// get the network and contract
			Network network = gateway.getNetwork("mychannel");
			Contract contract = network.getContract("fabcar");

			FabcarCommitListener listener = new FabcarCommitListener();
			network.addCommitListener(listener, network.getChannel().getPeers(), "createCar");
		}

The FabcarCommitListener:

public class FabcarCommitListener implements CommitListener {
    @Override
    public void acceptCommit(BlockEvent.TransactionEvent transactionEvent) {
        System.out.println("TX COMMITTED");
    }

    @Override
    public void acceptDisconnect(PeerDisconnectEvent peerDisconnectEvent) {
        System.out.println("peerDisconnected");
    }
}

Any ideas how a commitListener works using the Java SDK?

A commit listener receives events only for a specific transaction invocation, not for all invocations of a given transaction name. Every transaction invocation has its own unique transaction ID, which you can obtain from the Transaction object prior to submitting:

https://hyperledger.github.io/fabric-gateway-java/release-2.2/org/hyperledger/fabric/gateway/Transaction.html#getTransactionId--

By default, a transaction submit will also listen for the transaction to be committed by peers so there is no need for your code to listen for transaction commits. There are several built-in strategies for determining when a transaction has been successfully committed, which you can select either:

1. When connecting the Gateway: https://hyperledger.github.io/fabric-gateway-java/release-2.2/org/hyperledger/fabric/gateway/Gateway.Builder.html#commitHandler-org.hyperledger.fabric.gateway.spi.CommitHandlerFactory-
2. For a specific transaction invocation: https://hyperledger.github.io/fabric-gateway-java/release-2.2/org/hyperledger/fabric/gateway/Transaction.html#setCommitHandler-org.hyperledger.fabric.gateway.spi.CommitHandlerFactory-

If you want to implement your own custom logic for identifying whether a transaction has committed successfully, you can write your own custom commit handler implementation, and this implementation can use a commit listener to identify the commit and connection status of all the peers you care about. Here is a sample commit handler and factory implementation that make use of commit listeners:

• https://github.com/hyperledger/fabric-gateway-java/blob/release-2.2/src/test/java/org/hyperledger/fabric/gateway/sample/SampleCommitHandlerFactory.java
• https://github.com/hyperledger/fabric-gateway-java/blob/release-2.2/src/test/java/org/hyperledger/fabric/gateway/sample/SampleCommitHandler.java

If you want to look at all the transactions committed to the blockchain, even if only to pick out certain ones you care about, then use a block listener:

https://hyperledger.github.io/fabric-gateway-java/release-2.2/org/hyperledger/fabric/gateway/Network.html#addBlockListener-java.util.function.Consumer-

From the block event you can navigate down through all the transactions included in the block.

Having said all this, both block listeners and commit listeners really deal with the mechanics of Fabric blockchains. So inspecting the transactions that have operated on the ledger and checking whether they were successfully committed. If you want to orchestrate business processes around transactional events then probably you should actually be using a contract event listener.

If you want to trigger some business process when a new car is created, implement your createCar transaction function so that it emits an event when it is committed:

https://hyperledger.github.io/fabric-chaincode-java/release-2.2/api/org/hyperledger/fabric/shim/ChaincodeStub.html#setEvent-java.lang.String-byte:A-

In your client application, simply listen for this event using a contract event listener:

https://hyperledger.github.io/fabric-gateway-java/release-2.2/org/hyperledger/fabric/gateway/Contract.html#addContractListener-java.util.function.Consumer-java.lang.String-

You can use checkpointing to allow your client to resume listening for events at the last processed block position after a client application restart:

https://hyperledger.github.io/fabric-gateway-java/release-2.2/org/hyperledger/fabric/gateway/Contract.html#addContractListener-org.hyperledger.fabric.gateway.spi.Checkpointer-java.util.function.Consumer-java.lang.String-