Web3j how to get event args when parsing logs?

Question

I'm trying to get an event args in web3j version 4.0.1.
I use the simple syntax which is explained in the docs. I'm getting the log object but there is no args property or such thing in it. Here is my code:

EthFilter filter = new EthFilter(DefaultBlockParameterName.EARLIEST,
    DefaultBlockParameterName.LATEST, <contract-address>);
web3j.ethLogFlowable(filter).subscribe(log -> {
    System.out.println(log);
});

I expected to find args in the log it is not there. There is a data index and its value is some unreadable hex string. I must mention that I'm able to see the log arguments in remix and even in truffle test transaction receipt. But no success in web3j.

Answer 1

For the following event:

event MyEvent(address indexed _arg1, bytes32 indexed _arg2, uint8 _arg3); 

You can extract the event arguments from the log like this

// Event definition
public static final Event MY_EVENT = new Event("MyEvent", Arrays.<TypeReference<?>>asList(new TypeReference<Address>(true) {}, new TypeReference<Bytes32>(true) {}, new TypeReference<Uint8>(false) {}));

// Event definition hash
private static final String MY_EVENT_HASH = EventEncoder.encode(MY_EVENT);

// Filter
EthFilter filter = new EthFilter(DefaultBlockParameterName.EARLIEST, DefaultBlockParameterName.LATEST, <contract-address>);

// Pull all the events for this contract
web3j.ethLogFlowable(filter).subscribe(log -> {
    String eventHash = log.getTopics().get(0); // Index 0 is the event definition hash

    if(eventHash.equals(MY_EVENT_HASH)) { // Only MyEvent. You can also use filter.addSingleTopic(MY_EVENT_HASH) 
        // address indexed _arg1
        Address arg1 = (Address) FunctionReturnDecoder.decodeIndexedValue(log.getTopics().get(1), new TypeReference<Address>() {});
        // bytes32 indexed _arg2
        Bytes32 arg2 = (Bytes32) FunctionReturnDecoder.decodeIndexedValue(log.getTopics().get(2), new TypeReference<Bytes32>() {});
        // uint8 _arg3
        Uint8 arg3 = (Uint8) FunctionReturnDecoder.decodeIndexedValue(log.getTopics().get(3), new TypeReference<Uint8>() {});
    }

});

Answer 2

Generate contracts with web3j-cli:

web3j generate truffle --truffle ./artifacts/contracts/MyContract.sol/MyContract.json --outputDir web3j --package com.example.contracts

Check out event methods on generated contract:

get[EventName]Events(TransactionReceipt transactionReceipt) {}

These methods automatically create Class with parsed topic values.

The key-method there is extractEventParametersWithLog(Event event, Log log) which exists on every Contract

Answer 3

So I've been attempting to write a unit test for testing an Event in Kotlin, this might help someone (or help contribute to unit testing events here).

build.gradle:

plugins {
    id 'java'
    id 'org.jetbrains.kotlin.jvm' version '1.4.10'
    id 'application'
    id "com.github.johnrengelman.shadow" version "5.2.0"
    id 'org.web3j' version '4.8.4'
}
...
dependencies {
    implementation "org.web3j:core:$web3jVersion",
            "ch.qos.logback:logback-core:$logbackVersion",
            "ch.qos.logback:logback-classic:$logbackVersion"
    implementation "org.web3j:web3j-unit:$web3jVersion"
    implementation "org.web3j:web3j-evm:$web3jVersion"
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk8"
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.3.1'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.3.1'
}
...

Say I have a contract with the following:

MyContract.sol:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract MyContract {
    uint public someNumber;

    event NumberIncremented(uint someNumber);

    constructor() public {
        someNumber = 0;
    }

    function incrementNumber() public {
        someNumber = someNumber + 1;
        emit NumberIncremented(someNumber);
    }
}

MyContractTests.kt looks like the following:

@EVMTest(NodeType.EMBEDDED)
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
class MyContractTests {
  private lateinit var myContract: MyContract

  @BeforeAll
  fun deploy(
    web3j: Web3j,
    transactionManager: TransactionManager,
    contractGasProvider: ContractGasProvider
  ) {
    myContract = MyContract.deploy(web3j, transactionManager, contractGasProvider).send()
  }

  @Test
  fun `increment number and emit event`() {
    val transactionReceiptVal = myContract.incrementNumber().send()

    val response = myContract.getNumberIncrementedEvents(transactionReceiptVal)
    Assertions.assertEquals(1, response.size)
  }
}

I've tried Greg Jeanmart's method in the unit test, however it seems like using the EmbeddedWeb3JService throws an UnsupportedOperationException for eth_filter when using send().