Introduction to Blockchain with Java using Ethereum, web3j and Spring Boot

Blockchain is one of the buzzwords in IT world during some last months. This term is related to cryptocurrencies, and was created together with Bitcoins. It is decentralized, immutable data structure divided into blocks, which are linked and secured using cryptographic algorithms. Every single block in this structure typically contains a cryptographic hash of the previous block, a timestamp, and transaction data. Blockchain is managed by peer-to-peer network, and during inter-node communication every new block is validated before adding. This is short portion of theory about blockchain. In a nutshell, this is a technology which allows us to managed transactions between two parties in a decentralized way. Now, the question is how we can implement it in our system.
Here comes Ethereum. It is a decentralized platform created by Vitarik Buterin that provides scripting language for a development of applications. It is based on ideas from Bitcoin, and is driven by the new cryptocurrency called Ether. Today, Ether is the second largest cryptocurrency after Bitcoin. The heart of Ethereum technology is EVM (Ethereum Virtual Machine), which can be treated as something similar to JVM, but using a network of fully decentralized nodes. To implement transactions based Ethereum in Java world we use web3j library. This is a lightweight, reactive, type safe Java and Android library for integrating with nodes on Ethereum blockchains. More details can be found on its website

1. Running Ethereum locally

Although there are many articles on the Web about blockchain and ethereum it is not easy to find a solution describing how to run ready-for-use instance of Ethereum on the local machine. It is worth to mention that generally there are two most popular Ethereum clients we can use: Geth and Parity. It turns out we can easily run Geth node locally using Docker container. By default it connects the node to the Ethereum main network. Alternatively, you can connect it to test network or Rinkeby network. But the best option for beginning is just to run it in development mode by setting --dev parameter on Docker container running command.
Here’s the command that starts Docker container in development mode and exposes Ethereum RPC API on port 8545.

$ docker run -d --name ethereum -p 8545:8545 -p 30303:30303 ethereum/client-go --rpc --rpcaddr "" --rpcapi="db,eth,net,web3,personal" --rpccorsdomain "*" --dev

The one really good message when running that container in development mode is that you have plenty of Ethers on your default, test account. In that case, you don’t have to mine any Ethers to be able to start tests. Great! Now, let’s create some other test accounts and also check out some things. To achieve it we need to run Geth’s interactive JavaScript console inside Docker container.

$ docker exec -it ethereum geth attach ipc:/tmp/geth.ipc

2. Managing Ethereum node using JavaScript console

After running JavaScript console you can easily display default account (coinbase), the list of all available accounts and their balances. Here’s the screen illustrating results for my Ethereum node.
Now, we have to create some test accounts. We can do it by calling personal.newAccount(password) function. After creating required accounts, you can perform some test transactions using JavaScript console, and transfer some funds from base account to the newly created accounts. Here are the commands used for creating accounts and executing transactions.

3. System architecture

The architecture of our sample system is very simple. I don’t want to complicate anything, but just show you how to send transaction to Geth node and receive notifications. While transaction-service sends new transaction to Ethereum node, bonus-service observe node and listening for incoming transactions. Then it send bonus to the sender’s account once per 10 transactions received from his account. Here’s the diagram that illustrates an architecture of our sample system.

4. Enable Web3j for Spring Boot app

I think that now we have clarity what exactly we want to do. So, let’s proceed to the implementation. First, we should include all required dependencies in order to be able to use web3j library inside Spring Boot application. Fortunately, there is a starter that can be included.


Because we are running Ethereum Geth client on Docker container we need to change auto-configured client’s address for web3j.

    name: transaction-service
  port: ${PORT:8090}

5. Building applications

If we included web3j starter to the project dependencies all you need is to autowire Web3j bean. Web3j is responsible for sending transaction to Geth client node. It receives response with transaction hash if it has been accepted by the node or error object if it has been rejected. While creating transaction object it is important to set gas limit to minimum 21000. If you sent lower value, you will probably receive error Error: intrinsic gas too low.

public class BlockchainService {

    private static final Logger LOGGER = LoggerFactory.getLogger(BlockchainService.class);

    Web3j web3j;

    public BlockchainTransaction process(BlockchainTransaction trx) throws IOException {
        EthAccounts accounts = web3j.ethAccounts().send();
        EthGetTransactionCount transactionCount = web3j.ethGetTransactionCount(accounts.getAccounts().get(trx.getFromId()), DefaultBlockParameterName.LATEST).send();
        Transaction transaction = Transaction.createEtherTransaction(accounts.getAccounts().get(trx.getFromId()), transactionCount.getTransactionCount(), BigInteger.valueOf(trx.getValue()), BigInteger.valueOf(21_000), accounts.getAccounts().get(trx.getToId()),BigInteger.valueOf(trx.getValue()));
        EthSendTransaction response = web3j.ethSendTransaction(transaction).send();
        if (response.getError() != null) {
            return trx;
        String txHash = response.getTransactionHash();"Tx hash: {}", txHash);
        EthGetTransactionReceipt receipt = web3j.ethGetTransactionReceipt(txHash).send();
        if (receipt.getTransactionReceipt().isPresent()) {
  "Tx receipt: {}", receipt.getTransactionReceipt().get().getCumulativeGasUsed().intValue());
        return trx;


The @Service bean visible above is invoked by the controller. The implementation of POST method takes BlockchainTransaction object as parameter. You can send there sender id, receiver id, and transaction amount. Sender and receiver ids are equivalent to index in query eth.account[index].

public class BlockchainController {

    BlockchainService service;

    public BlockchainTransaction execute(@RequestBody BlockchainTransaction transaction) throws NoSuchAlgorithmException, NoSuchProviderException, InvalidAlgorithmParameterException, CipherException, IOException {
        return service.process(transaction);


You can send a test transaction by calling POST method using the following command.

$ curl --header "Content-Type: application/json" --request POST --data '{"fromId":2,"toId":1,"value":3}' http://localhost:8090/transaction

Before sending any transactions you should also unlock sender account.

Application bonus-service listens for transactions processed by Ethereum node. It subscribes for notifications from Web3j library by calling web3j.transactionObservable().subscribe(...) method. It returns the amount of received transaction to the sender’s account once per 10 transactions sent from that address. Here’s the implementation of observable method inside application bonus-service.

Web3j web3j;

public void listen() {
	Subscription subscription = web3j.transactionObservable().subscribe(tx -> {"New tx: id={}, block={}, from={}, to={}, value={}", tx.getHash(), tx.getBlockHash(), tx.getFrom(), tx.getTo(), tx.getValue().intValue());
		try {
			EthCoinbase coinbase = web3j.ethCoinbase().send();
			EthGetTransactionCount transactionCount = web3j.ethGetTransactionCount(tx.getFrom(), DefaultBlockParameterName.LATEST).send();"Tx count: {}", transactionCount.getTransactionCount().intValue());
			if (transactionCount.getTransactionCount().intValue() % 10 == 0) {
				EthGetTransactionCount tc = web3j.ethGetTransactionCount(coinbase.getAddress(), DefaultBlockParameterName.LATEST).send();
				Transaction transaction = Transaction.createEtherTransaction(coinbase.getAddress(), tc.getTransactionCount(), tx.getValue(), BigInteger.valueOf(21_000), tx.getFrom(), tx.getValue());
		} catch (IOException e) {
			LOGGER.error("Error getting transactions", e);


Blockchain and cryptocurrencies are not the easy topics to start. Ethereum simplifies development of applications that use blockchain, by providing a complete, scripting language. Using web3j library together with Spring Boot and Docker image of Ethereum Geth client allows to quickly start local development of solution implementing blockchain technology. IF you would like to try it locally just clone my repository available on GitHub


Generating large PDF files using JasperReports

During the last ‘Code Europe’ conference in Warsaw appeared many topics related to microservices architecture. Several times I heard the conclusion that the best candidate for separation from monolith is service that generates PDF reports. It’s usually quite independent from the other parts of application. I can see a similar approach in my organization, where first microservice running in production mode was the one that generates PDF reports. To my surprise, the vendor which developed that microservice had to increase maximum heap size to 1GB on each of its instances. This has forced me to take a closer look at the topic of PDF reports generation process.
The most popular Java library for creating PDF files is JasperReports. During generation process, this library by default stores all objects in RAM memory. If such reports are large, this could be a problem my vendor encountered. Their solution, as I have mentioned before, was to increase the maximum size of Java heap 🙂

This time, unlike usual, I’m going to start with the test implementation. Here’s simple JUnit test with 20 requests per second sending to service endpoint.

public class JasperApplicationTest {

	protected Logger logger = Logger.getLogger(JasperApplicationTest.class.getName());
	TestRestTemplate template = new TestRestTemplate();

	public void testGetReport() throws InterruptedException {
		List<HttpStatus> responses = new ArrayList<>();
		Random r = new Random();
		int i = 0;
		for (; i < 20; i++) {
			new Thread(new Runnable() {
				public void run() {
					int age = r.nextInt(99);
					long start = System.currentTimeMillis();
					ResponseEntity<InputStreamResource> res = template.getForEntity("http://localhost:2222/pdf/{age}", InputStreamResource.class, age);"Response (" +  (System.currentTimeMillis()-start) + "): " + res.getStatusCode());
					try {
					} catch (InterruptedException e) {

		while (responses.size() != i) {
		}"Test finished");

In my test scenario I inserted about 1M records into the person table. Everything works fine during running test. Generated files had about 500kb size and 200 pages. All requests were succeeded and each of them had been processed about 8 seconds. In comparison with single request which had been processed 4 seconds it seems to be a good result. The situation with RAM is worse as you can see in the figure below. After generating 20 PDF reports allocated heap size increases to more than 1GB and used heap size was about 550MB. Also CPU usage during report generation increased to 100% usage. I could easily image generating files bigger than 500kb in the production mode…


In our situation we have two options. We can always add more RAM memory or … look for another choice 🙂 Jasper library comes with solution – Virtualizers. The virtualizer cuts the jasper report print into different files and save them on the hard drive and/or compress it. There are three types of virtualizers:
JRFileVirtualizer, JRSwapFileVirtualizer and JRGzipVirtualizer. You can read more about them here. Now, look at the figure below. Here’s illustration of memory and CPU usage for the test with JRFileVirtualizer. It looks a little better than the previous figure, but it does not knock us down 🙂 However, requests with the same overload as for the previous test take much longer – about 30 seconds. It’s not a good message, but at least the heap size allocation is not increases as fast as for previous sample.


Same test has been performed for JRSwapFileVirtualizer. The requests was average processed around 10 seconds. The graph illustrating CPU and memory usage is rather more similar to in memory test than JRFileVirtualizer test.


To see the difference between those three scenarios we have to run our application with maximum heap size set. For my tests I set -Xmx128m -Xms128m. For test with file virtualizers we receive HTTP responses with PDF reports, but for in memory tests the exception is thrown by the sample application: java.lang.OutOfMemoryError: GC overhead limit exceeded.

For testing purposes I created Spring Boot application. Sample source code is available as usual on GitHub. Here’s full list of Maven dependencies for that project.


Here’s application main class. There are @Bean declarations of file virtualizers and JasperReport which is responsible for template compilation from .jrxml file. To run application for testing purposes type java -jar -Xms64m -Xmx128m -Ddirectory=C:\Users\minkowp\pdf sample-jasperreport-boot.jar.

public class JasperApplication {

	private String directory;

	public static void main(String[] args) {, args);

	JasperReport report() throws JRException {
		JasperReport jr = null;
		File f = new File("personReport.jasper");
		if (f.exists()) {
			jr = (JasperReport) JRLoader.loadObject(f);
		} else {
			jr = JasperCompileManager.compileReport("src/main/resources/report.jrxml");
			JRSaver.saveObject(jr, "personReport.jasper");
		return jr;

	JRFileVirtualizer fileVirtualizer() {
		return new JRFileVirtualizer(100, directory);

	JRSwapFileVirtualizer swapFileVirtualizer() {
		JRSwapFile sf = new JRSwapFile(directory, 1024, 100);
		return new JRSwapFileVirtualizer(20, sf, true);


There are three endpoints exposed for the tests:
/pdf/{age} – in memory PDF generation
/pdf/fv/{age} – PDF generation with JRFileVirtualizer
/pdf/sfv/{age} – PDF generation with JRSwapFileVirtualizer

Here’s method generating PDF report. Report is generated in fillReport static method from JasperFillManager. It takes three parameters as input: JasperReport which encapsulates compiled .jrxml template file, JDBC connection object and map of parameters. Then report is ganerated and saved on disk as a PDF file. File is returned as an attachement in the response.

	private ResponseEntity<InputStreamResource> generateReport(String name, Map<String, Object> params) {
		FileInputStream st = null;
		Connection cc = null;
		try {
			cc = datasource.getConnection();
			JasperPrint p = JasperFillManager.fillReport(jasperReport, params, cc);
			JRPdfExporter exporter = new JRPdfExporter();
			SimpleOutputStreamExporterOutput c = new SimpleOutputStreamExporterOutput(name);
			exporter.setExporterInput(new SimpleExporterInput(p));

			st = new FileInputStream(name);
			HttpHeaders responseHeaders = new HttpHeaders();
			responseHeaders.setContentDispositionFormData("attachment", name);
		    return new ResponseEntity<InputStreamResource>(new InputStreamResource(st), responseHeaders, HttpStatus.OK);
		} catch (Exception e) {
		} finally {
			if (cc != null)
				try {
				} catch (SQLException e) {
		return null;

To enable virtualizer during report generation we only have to pass one parameter to the map of parameters – instance of virtualizer object.

	JRFileVirtualizer fv;
	JRSwapFileVirtualizer sfv;
	DataSource datasource;
	JasperReport jasperReport;

	@RequestMapping(value = "/pdf/fv/{age}")
	public ResponseEntity<InputStreamResource> getReportFv(@PathVariable("age") int age) {"getReportFv(" + age + ")");
		Map<String, Object> m = new HashMap<>();
		m.put(JRParameter.REPORT_VIRTUALIZER, fv);
		m.put("age", age);
		String name = ++count + "personReport.pdf";
		return generateReport(name, m);

Template file report.jrxml is available under /src/main/resources directory. Inside queryString tag there is SQL query which takes age parameter in WHERE statement. There are also five columns declared all taken from SQL query result.

<?xml version = "1.0" encoding = "UTF-8"?>
<!DOCTYPE jasperReport PUBLIC "//JasperReports//DTD Report Design//EN"    "">

<jasperReport xmlns=""               xmlns:xsi=""               xsi:schemaLocation=""               name="report2" pageWidth="595" pageHeight="842"                columnWidth="555" leftMargin="20" rightMargin="20"               topMargin="20" bottomMargin="20">
    <parameter name="age" class="java.lang.Integer"/>
        <![CDATA[SELECT * FROM person WHERE age = $P{age}]]>
    <field name="id" class="java.lang.Integer" />
    <field name="first_name" class="java.lang.String" />
    <field name="last_name" class="java.lang.String" />
    <field name="age" class="java.lang.Integer" />
    <field name="pesel" class="java.lang.String" />

        <band height="15">

                <reportElement x="0" y="0" width="50" height="15" />

                <textElement textAlignment="Right" verticalAlignment="Middle"/>

                <textFieldExpression class="java.lang.Integer">

                <reportElement x="100" y="0" width="80" height="15" />

                <textElement textAlignment="Left" verticalAlignment="Middle"/>

                <textFieldExpression class="java.lang.String">

                <reportElement x="200" y="0" width="80" height="15" />

                <textElement textAlignment="Left" verticalAlignment="Middle"/>

                <textFieldExpression class="java.lang.String">

                <reportElement x="300" y="0" width="50" height="15"/>
                <textElement textAlignment="Right" verticalAlignment="Middle"/>

                <textFieldExpression class="java.lang.Integer">

                <reportElement x="380" y="0" width="80" height="15" />

                <textElement textAlignment="Left" verticalAlignment="Middle"/>

                <textFieldExpression class="java.lang.String">



And the last thing we have to do is to properly set database connection pool settings. A natural choice for Spring Boot application is Tomcat JDBC pool.

    name: jasper-service
    url: jdbc:mysql://
    username: datagrid
    password: datagrid
      initial-size: 20
      max-active: 30

Final words

In this article I showed you how to avoid out of memory exception while generating large PDF reports with JasperReports. I compared three solutions: in memory generation and two methods based on cutting the jasper print into different files and save them on the hard drive. For me, the most interesting was the solution based on single swapped file with JRSwapFileVirtualizer. It is slower a little than in memory generation but works faster than similar tests for JRFileVirtualizer and in contrast to in memory generation didn’t avoid out of memory exception for files larger than 500kb with 20 requests per second.