How to Implement Ethereum Listener for Listening Transactions

Maven:

        
            org.web3j
            core
            3.0.2
        
 

Gradle:

compile ('org.web3j:core:3.0.2')

import javax.annotation.PostConstruct;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Service;
import org.web3j.protocol.Web3j;
import org.web3j.protocol.http.HttpService;

@Service
public class EthereumListenerService {

    @Value("${ethereum.service.url}")//added in application.properties file/or can be hardcoded here
    private String serverurl;

    private Web3j web3j;

    @PostConstruct
    void init() {
        web3j = Web3j.build(new HttpService(serverurl));  //web3j  object after connecting with server for transaction
    }

    public void startListener() {
       //Ethereum listener started here tx is web3j transaction object described below
        web3j.transactionObservable().subscribe(tx -> {
            try {
                isActive = true;
                logger.info("Listened the transaction for trx hash = {} and address = {} ", tx.getHash(), tx.getTo());
                if (tx.getTo() == null || tx.getTo().trim().isEmpty()) {
                    //Empty block
                } else {
                    //can add checks for your application wallets because it will listen all transactions happening on ethereum
                   //To Do
                    }
                }
            } catch (Exception e) {
               //To do
            }
        }, error -> {
           //error in transaction listen
           //To do

        });
 

package org.web3j.protocol.core.methods.response;

import java.math.BigInteger;

import org.web3j.utils.Numeric;

public class Transaction {
    private String hash;
    private String nonce;
    private String blockHash;
    private String blockNumber;
    private String transactionIndex;
    private String from;
    private String to;
    private String value;
    private String gasPrice;
    private String gas;
    private String input;
    private String creates;
    private String publicKey;
    private String raw;
    private String r;
    private String s;
    private int v;  // see https://github.com/web3j/web3j/issues/44
    }