Java Binding User Guide

Exonum Java Binding is a framework for building blockchain applications in Java, powered by Exonum.

With Exonum Java Binding you can define stateful Exonum Services in Java, configure a network of nodes and run your services in it.

Exonum Java Binding provides:

  • SDK for service development as a number of Java libraries.
  • Exonum Java Application — an Exonum node with built-in support for running Java services.


To run a node with your Java service you need to use Exonum Java application.

There are several installation options:

Windows is not supported by Java Binding at the moment, consider using WSL or Docker.

Manual Installation

You can download an archive containing the application and all the necessary dependencies on the Releases page on GitHub. We suggest using debug version during development and release version for deployment.

  1. Download and unpack the archive from the Releases page into some known location. To install the latest release to ~/bin:

    mkdir -p ~/bin
    cd ~/bin
    unzip /path/to/downloaded/
  2. Install Libsodium as the necessary runtime dependency.


    Exonum Java is built with Libsodium 23, which means it will not work on some older Linux distributions, like Ubuntu 16.04. Libsodium 23 is available in Ubuntu 18.04 or can be installed from a custom PPA.

    sudo apt-get update && sudo apt-get install libsodium-dev
    brew install libsodium
  3. Follow the steps in the After Install section below

Homebrew Package

For Mac users, we provide a Homebrew repository, which gives the easiest way of installing Exonum Java App:

brew tap exonum/exonum
brew install exonum-java

This will install exonum-java binary with all the necessary dependencies. However, you still need to follow the steps mentioned in After Install section below.

Build from Source

It is also possible to build Exonum Java application from sources. To do so, follow the instructions in Contribution Guide.

After Install

  1. Create an environment variable EXONUM_HOME pointing at installation location.

    # The path is provided in after-install message in case of Homebrew
    export EXONUM_HOME=~/bin/exonum-java-0.10.0-release
    # Setting PATH variable is not needed in case of Homebrew
    export PATH="$PATH:$EXONUM_HOME/bin"
  2. Install JDK 11.

  3. Install Maven 3 which is essential for developing and building Java service.

Creating Project

The easiest way to create a Java service project is to use a template project generator. After installing Maven 3, run the command:

mvn archetype:generate \
    -DinteractiveMode=false \
    -DarchetypeGroupId=com.exonum.binding \
    -DarchetypeArtifactId=exonum-java-binding-service-archetype \
    -DarchetypeVersion=0.10.0 \
    -DgroupId=com.example.myservice \
    -DartifactId=my-service \

You can also use the interactive mode:

mvn archetype:generate \
    -DarchetypeGroupId=com.exonum.binding \
    -DarchetypeArtifactId=exonum-java-binding-service-archetype \

The build definition files for other build systems (e.g., Gradle) can be created similarly to the template. For more information see an example.

Service Development

The service abstraction serves to extend the framework and implement the business logic of an application. The service defines the schema of the stored data that constitute the service state; transaction processing rules that can make changes to the stored data; handles events occurring in the ledger; and defines an API for external clients that allows interacting with the service from outside of the system. See more information on the software model of services in the corresponding section.

In Java, the abstraction of a service is represented by Service interface. Implementations can use the abstract class AbstractService.

Schema Description

Exonum provides several collection types to persist service data. The main types are sets, lists and maps. Data organization inside the collections is arranged in two ways – ordinary collections and Merkelized collections; the latter allow providing cryptographic evidence of the authenticity of data to the clients of the system (for example, that an element is stored in the collection under a certain key). The blockchain state is influenced only by the Merkelized collections.

For the detailed description of all Exonum collection types see the corresponding documentation section. In Java, implementations of collections are located in a separate package. Said package documentation describes their use.

Collections are instantiated using a database view. The database view works as an index factory. The views may be based on a Snapshot — a read-only view corresponding to the database state as of the latest committed block; or Fork, which is mutable and allows performing modifying operations. The database view is provided by the framework: Snapshot can be requested at any time, while Fork – only when the transaction is executed. The lifetime of these objects is limited by the scope of the method to which they are passed to.

Exonum stores elements in collections as byte arrays. Therefore, serializers for values stored in collections must be provided. See Serialization for details.

void putEntry(Prefixed access, String key, String value) {
  // Access the 'entries' ProofMap
  var indexAddress = IndexAddress.valueOf("entries");
  var stringSerializer = StandardSerializers.string();
  var entries = access.getProofMap(indexAddress, stringSerializer,
  // Put the key-value pair in the ProofMap index
  entries.put(key, value);

A set of named collections constitute a service schema. A service schema is usually created using a Prefixed database access object, which provides isolation of all service indexes from other instances.

For convenient access to service collections you can implement a factory of service collections.

The state of the service in the blockchain is determined by the index hashes of its Merkelized collections. Exonum performs the state aggregation of all non-grouped Merkelized collections automatically. See the “State Aggregation” section of MerkleDB documentation for details.

final class FooSchema implements Schema {

  private final Prefixed view;

   * Creates a schema given a prefixed database access object.
   * The database state is determined by the access object.
  FooSchema(Prefixed access) {
    this.view = access;

   * Provides access to a test ProofMap.
   * <p>Such factory methods may be used in transactions and read requests
   * to access a collection of a certain type and name. Here,
   * a ProofMap with String keys and values is created with a full name
   * {@code "<service name>.test-map"}.
  ProofMapIndexProxy<String, String> testMap() {
    var address = IndexAddress.valueOf("test-map");
    var stringSerializer = StandardSerializers.string();
    return view.getProofMap(address, stringSerializer, stringSerializer);

Blockchain Data

BlockchainData is the object providing access to blockchain data of a particular service instance.

The service instance data is accessible via a Prefixed access which isolates the service data from all the other instances.

On top of that, this class provides read-only access to persistent data of:

  • Exonum Core, containing information on blocks, transactions, execution results, consensus configuration, etc.
  • Dispatcher, containing information on deployed service artifacts and active service instances.
  • Other services.


As Exonum storage accepts data in the form of byte arrays, storing user data requires serialization. Java Binding provides a set of built-in serializers that you can find in the StandardSerializers utility class. The list of serializers covers the most often-used entities and includes:

  • Standard types: boolean, float, double, byte[] and String. Integers with various encoding types, see StandardSerializers Java documentation and the table below.
  • Exonum types: PrivateKey, PublicKey and HashCode.
  • Any Protobuf messages using StandardSerializers#protobuf.

Besides the available built-in serializers, users can still implement their own serializers for storing their data in a custom format instead of using the built-in ones.

Integer Encoding Types Comparison Table
Type Description The most efficient range
fixed32 Always four bytes If values are often greater than 2^28
uint32 Unsigned int that uses variable-length encoding If values are in range [0; 2^21-1]
sint32 Signed int that uses variable-length encoding If values are in range [-2^20; 2^20-1]
fixed64 Always eight bytes If values are often greater than 2^56
uint64 Unsigned int that uses variable-length encoding If values are in range [0; 2^49-1]
sint64 Signed int that uses variable-length encoding If values are in range [-2^48; 2^48-1]

Transactions Description

Exonum transactions allow you to perform modifying atomic operations with the storage. Transactions are executed sequentially, in the order determined by the consensus of the nodes in the network.

Transactions are transmitted by external service clients to the framework as Exonum messages. The transaction message includes the transaction arguments that can be serialized using an arbitrary algorithm supported by both the service client and the service itself.

For more details about transactions in Exonum – their properties and processing rules – see the corresponding section of our documentation.

Transaction Definition

A transaction is a Service method that is annotated with the @Transaction annotation and defines the transaction business logic. The transaction method describes the operations that are applied to the current storage state when the transaction is executed.

A transaction method must accept two parameters:

  1. arguments either as a protobuf message or as a byte[]. Protobuf messages are deserialized using a #parseFrom(byte[]) method of the actual parameter type.
  2. execution context as ExecutionContext.

The execution context provides:

  • A mutable access to the blockchain data, which allows performing modifying operations;
  • The service instance name and numeric ID;
  • Some information about the corresponding transaction message: its SHA-256 hash that uniquely identifies it, and the author’s public key.

A service schema object can be used to access data collections of this service.

/** A numeric identifier of the "put" transaction. */
private static final int PUT_TX_ID = 0;

 * Puts an entry (a key-value pair) into the test proof map.
 * <p>{@code Transactions.PutTransactionArgs} is a Protoc-generated
 * class with the transaction method arguments.
public void putEntry(Transactions.PutEntryArgs arguments,
    ExecutionContext context) {
  var schema = new FooSchema(context.getServiceData());
  String key = arguments.getKey();
  String value = arguments.getValue();
      .put(key, value);
syntax = "proto3";


option java_package = "";

message PutEntryArgs {
  string key = 1;
  string value = 2;

An implementation of the Transaction method must be a pure function, i.e. it must produce the same observable result on all the nodes of the system for the given transaction. An observable result is the one that affects the blockchain state hash: a modification of a collection that affects the service state hash, or an execution exception.


Transaction methods may throw ExecutionException to notify Exonum about an error in a transaction execution.

The ExecutionException contains an integer error code and a message with error description. The error code allows the clients of this method to distinguish the different types of errors that may occur in it. For example, a transaction transferring tokens from one account to another might have the following error conditions that the client needs to distinguish:

  • Unknown receiver ID
  • Unknown sender ID
  • Insufficient balance
  • Same sender and receiver.

An exception of any other type will be recorded with no error code as an “unexpected” execution error.

If transaction execution fails, the changes made by the transaction are rolled back, while the error data is stored in the database for further user reference. Light clients also provide access to information on the transaction execution result (which may be either success or failure) to their users.

Submitting Transactions to Self

If the service itself needs to create a transaction on a particular node, it can use the Node#submitTransaction method. This method will create and sign a transaction message using the service key of that particular node (meaning that the node will be the author of the transaction), and submit it to the network. Invoking this method on each node unconditionally will produce N transactions that have the same payloads, but different authors’ public keys and signatures, where N is the number of nodes in the network.

Before and After Transactions Handlers

Services can receive notifications:

  • before any transactions in a block has been processed
  • after all transactions in a block has been processed, but before the block has been committed.

Such notification allow inspecting the changes made by the transactions and modifying the service state.

Exonum delivers these notification to implementations of Service#beforeTransactions and Service#afterTransactions methods. The beforeTransactions method sees the state of the database as of the last committed block, possibly, modified by the previously invoked beforeTransactions handlers of other services. The afterTransactions handlers see the blockchain state after all transactions in the block have been applied.

Both methods may make any changes to the service state, which will be included in the block. As transactions, the implementations must produce the same observable result on all nodes of the system. If exceptions occur in this handler, Exonum will roll back any changes made to the persistent storage in it.

The execution result of before- / afterTransactions is also saved in the database.

Blockchain Events

A service can also handle a block commit event that occurs each time the framework commits a new block. The framework delivers this event to implementations of Service#afterCommit(BlockCommittedEvent) callback in each deployed service. Each node in the network processes that event independently from other nodes. The event includes a BlockchainData snapshot, allowing a read-only access to the database state exactly after the commit of the corresponding block.

As services can read the database state in the handler, they may detect any changes in it, e.g., that a certain transaction is executed; or some condition is met. Services may also create and submit new transactions using Node#submitTransaction. Using this callback to notify other systems is another common use case, but the implementations must pay attention to not perform any blocking operations such as synchronous I/O in this handler, as it is invoked synchronously in the same thread that handles transactions. Blocking that thread will delay transaction processing on the node.

External Service API

The external service API is used for the interaction between a service and external systems. A set of operations defined by a service usually includes read requests for the blockchain data with the provision of the corresponding cryptographic proof. Exonum provides an embedded web framework for implementing the REST-interface of the service.

Service#createPublicApiHandlers method is used to set the handlers for HTTP requests. These handlers are available at the common path corresponding to the service name. Thus, the /balance/:walletId handler for balance requests in the “cryptocurrency” service will be available at /api/services/cryptocurrency/balance/:walletId.

See documentation on the possibilities of Vert.x used as a web framework.


Java services use a separate server from Rust services. The TCP ports of Java and Rust servers are specified when a node is started, see “Running the node” section for details.


Initialization and Initial Configuration

Service can initialize its persistent state on its start using Service#initialize.

Exonum invokes this method when a new service instance is added to the blockchain. Administrators, initiating this action, may pass some initialization parameters. A service is responsible for validation of these parameters. If they are incorrect, it shall throw an ExecutionException, which will abort the start of this service instance.

A service shall also update its persistent state based on these parameters. It shall not derive the Service object state from them, as Exonum may instantiate multiple Service objects for a single service instance (e.g., when a node is stopped and then restarted).

For example, a service may set some initial values in its collections, or save all or some configuration parameters as is for later retrieval in transactions and/or read requests.


Exonum supervisor service provides a mechanism to reconfigure the started service instances. The re-configuration protocol for services is similar to the one for consensus configuration. This protocol includes the following steps:

  1. a proposal of a new configuration;
  2. verification of its correctness;
  3. approval of the proposal; and
  4. application of the new approved configuration.

The protocol of the proposal and approval steps is determined by the installed supervisor service. The service can implement verification and application of the parameters by implementing Configurable interface.

Exonum includes a standard implementation of the supervisor service. See its documentation to learn how to invoke the re-configuration process of a started service instance. Also, consider using the standard client application — exonum-launcher.

A service that does not need the same protocol for its reconfiguration as for consensus reconfiguration may implement it itself as a set of transactions.

Dependencies Management

Exonum uses Guice to describe the dependencies of the service components (both system-specific ones, for example, Exonum time service, and external ones). Each service should define a Guice module describing implementations of the Service and its dependencies, if any.

A service module shall:

  1. extend AbstractServiceModule
  2. be annotated with @org.pf4j.Extension.
  3. be public.
public final class ServiceModule extends AbstractServiceModule {

  protected void configure() {
    // Define the Service implementation

The fully-qualified name of the module class is recorded in the service artifact metadata and is used by the framework to instantiate services.

For more information on using Guice, see the project wiki.


Exonum Java Binding provides a powerful testing toolkit — exonum-testkit library. TestKit allows testing transaction execution in the synchronous environment by offering simple network emulation (that is, without consensus algorithm and network operation involved).

Project Configuration

New projects

exonum-testkit is already included in projects generated with exonum-java-binding-service-archetype and you can skip the following instructions.

For existing projects include the following dependency into your pom.xml:


As the TestKit uses a library with the implementation of native methods, pass java.library.path system property to JVM:


EXONUM_HOME environment variable should point at the installation location, as specified in “After Install” section.

Packaged artifact should be available for integration tests that use TestKit, so Maven Failsafe Plugin should be configured as follows:


Creating Test Network

To perform testing, we first need to create a network emulation – the instance of TestKit. TestKit allows recreating behavior of a single full node (a validator or an auditor) in an emulated Exonum blockchain network.

To instantiate the TestKit, use TestKit.Builder. It allows configuration of:

  • Type of the emulated node (either Validator or Auditor)
  • Service artifacts - artifact ID and filename
  • Directory that stores service artifacts
  • Service instances with which the TestKit is instantiated - their artifact ID, service instance name, service instance ID and an optional service configuration
  • TimeProvider if usage of Time Oracle is needed (for details see Time Oracle Testing)
  • Number of validators in the emulated network


Note that regardless of the configured number of validators, only a single node will be emulated. This node will create the service instances, execute operations of those instances (e.g., afterCommit(BlockCommittedEvent) method logic), and provide access to their state.

Default TestKit can be instantiated with a single validator as an emulated node, a single service with no configuration and without Time Oracle. To instantiate the TestKit do the following:

ServiceArtifactId artifactId =
    ServiceArtifactId.newJavaId("com.exonum.binding:test-service", "1.0.0");
String artifactFilename = "test-service.jar";
String serviceName = "test-service";
int serviceId = 46;
Path artifactsDirectory = Paths.get("target");
try (TestKit testKit = TestKit.forService(artifactId, artifactFilename,
    serviceName, serviceId, artifactsDirectory)) {
  // Test logic

The TestKit can be also instantiated using a builder, if different configuration is needed:

// This TestKit will be instantiated with two instances of the
// same service ARTIFACT_ID.
try (TestKit testKit = TestKit.builder()
    .withDeployedArtifact(ARTIFACT_ID, ARTIFACT_FILENAME)
    // The first service will be instantiated with some custom configuration
    // The second — with no (empty) configuration
    .build()) {
  // Test logic

In TestKit code examples, ARTIFACT_ID, ARTIFACT_FILENAME, and ARTIFACTS_DIRECTORY are constants defining the corresponding properties of the artifact and the environment. As the values of these properties are usually defined in the build configuration, it is recommended to pass them to the test from the build configuration (e.g., via system properties set in maven-failsafe-plugin).

Transactions Testing

The TestKit allows testing transaction execution by submitting blocks with the given transaction messages. Here is an example of a test that verifies the execution result of a valid transaction and the changes it made in service schema:

try (TestKit testKit = TestKit.forService(ARTIFACT_ID, ARTIFACT_FILENAME,
  // Construct a valid transaction
  TransactionMessage validTx = createPutTransaction(SERVICE_ID, "key1", "v1");

  // Commit block with this transaction

  // Retrieve a snapshot of the current database state
  BlockchainData blockchainData = testKit.getBlockchainData(SERVICE_NAME);
  // It can be used to access the core schema, for example to check the
  // transaction execution result:
  Blockchain blockchain = blockchainData.getBlockchain();
  Optional<ExecutionStatus> txResult = blockchain.getTxResult(validTx.hash());
  // And also to verify the changes the transaction made to the service state:
  FakeSchema schema = new FakeSchema(blockchainData.getExecutingServiceData());
  // Perform assertions on the data in the service schema
  MapIndex<String, String> testMap = schema.testMap();

And a test that verifies that a transaction that throws an exception during its execution will fail:

try (TestKit testKit = TestKit.forService(ARTIFACT_ID, ARTIFACT_FILENAME,
  // Construct a transaction that will throw an ExecutionException
  // with the given error code during its execution
  byte errorCode = 100;
  TransactionMessage validTx = createErrorTransaction(SERVICE_ID, errorCode);

  // Commit block with this transaction

  // Retrieve a snapshot of the current database state
  BlockchainData blockchainData = testKit.getBlockchainData(SERVICE_NAME);
  // Check that the transaction failed
  Blockchain blockchain = blockchainData.getBlockchain();
  Optional<ExecutionStatus> txResult = blockchain.getTxResult(validTx.hash());
  ExecutionStatus executionStatus = txResult.get();
  ExecutionError executionError = executionStatus.getError();
      .contains("Diagnostic message");
  // ExecutionError also includes some properties of the error
  // that are set by the framework: error kind, call site, etc.
  // (not shown here, see ExecutionError API)

The TestKit also allows creating blocks that contain all current in-pool transactions. In the example below, a service that submits a transaction in its afterCommit method is instantiated. Such transactions are placed into the transaction pool and committed with createBlock() method:

try (TestKit testKit = TestKit.forService(ARTIFACT_ID, ARTIFACT_FILENAME,
  // Create a block so that the `afterCommit` method is invoked

  // This block will contain the in-pool transactions - namely, the transaction
  // submitted in the `afterCommit` method
  Block block = testKit.createBlock();

  // Check the resulting block or blockchain state

The TestKit provides getTransactionPool() and findTransactionsInPool(Predicate<TransactionMessage> predicate) methods to inspect the transaction pool. These methods are useful when there is a need to verify transactions that the service instance submitted itself (e.g., in afterCommit method) into the transaction pool.


Note that blocks that are created with TestKit.createBlockWithTransactions(Iterable<TransactionMessage> transactionMessages) will ignore in-pool transactions. As of 0.10.0, there is no way to create a block that would contain both given and in-pool transactions with a single method.

Checking the Blockchain State

In order to test service read operations and verify changes in the blockchain state, the TestKit provides a snapshot of the current database state (i.e., the one that corresponds to the latest committed block). There are several ways to access it:

  • Snapshot getSnapshot() Returns a snapshot of the current database state
  • void withSnapshot(Consumer<Snapshot> snapshotFunction) Performs the given function with a snapshot of the current database state
  • <ResultT> ResultT applySnapshot(Function<Snapshot, ResultT> snapshotFunction) Performs the given function with a snapshot of the current database state and returns a result of its execution

Read operations can also be tested through service web API.


Note that withSnapshot and applySnapshot methods destroy the snapshot once the passed closure completes. When using getSnapshot, created snapshots are only disposed when the TestKit is closed. That might cause excessive memory usage if many snapshots are created. Therefore, it is recommended to use the first two methods if a large number (e.g. more than a hundred) of snapshots needs to be created.

Time Oracle Testing

The TestKit allows to use Time Oracle in integration tests if your service depends on it. To do that, the TestKit should be created with TimeProvider. Its implementation, FakeTimeProvider, mocks the source of the external data (current time) and, therefore, allows to manually manipulate time that is returned by the time service. Note that the time must be set in UTC time zone. When tests do not need to control the current time, system time provider might be used.

void timeOracleTest() {
  String timeServiceName = "time-service";
  int timeServiceId = 10;
  ZonedDateTime initialTime =;
  FakeTimeProvider timeProvider = FakeTimeProvider.create(initialTime);
  try (TestKit testKit = TestKit.builder()
      .withDeployedArtifact(ARTIFACT_ID, ARTIFACT_FILENAME)
      .withTimeService(timeServiceName, timeServiceId, timeProvider)
      .build()) {
    // Create an empty block
    // The time service submitted its first transaction in `afterCommit`
    // method, but it has not been executed yet
    Optional<ZonedDateTime> consolidatedTime1 =
        getConsolidatedTime(testKit, timeServiceName);
    // No time is available till the time service transaction is processed

    // Increase the time value
    ZonedDateTime time1 = initialTime.plusSeconds(1);
    // The time service submitted its second transaction. The first must
    // have been executed, with consolidated time now available and equal to
    // initialTime
    Optional<ZonedDateTime> consolidatedTime2 =
        getConsolidatedTime(testKit, timeServiceName);

    // Increase the time value
    ZonedDateTime time2 = initialTime.plusSeconds(1);
    // The time service submitted its third transaction, and processed the
    // second one. The consolidated time must be equal to time1
    Optional<ZonedDateTime> consolidatedTime3 =
        getConsolidatedTime(testKit, timeServiceName);

private static Optional<ZonedDateTime> getConsolidatedTime(TestKit testKit,
    String timeServiceName) {
  BlockchainData blockchainData = testKit.getBlockchainData(timeServiceName);
  TimeSchema timeSchema = TimeSchema.newInstance(blockchainData, timeServiceName);
  return timeSchema.getTime().toOptional();

TestKit JUnit 5 Extension

The TestKit JUnit 5 extension simplifies writing tests that use TestKit. It allows to inject TestKit objects into test cases as a parameter and delete them afterwards. To enable it, define a TestKitExtension object annotated with @RegisterExtension and provided with a builder. The builder would be used to construct the injected TestKit objects:

TestKitExtension testKitExtension = new TestKitExtension(
        .withDeployedArtifact(ARTIFACT_ID, ARTIFACT_FILENAME)

void test(TestKit testKit) {
  // Test logic

It is possible to configure the injected TestKit instance with the following annotations:

  • @Validator sets an emulated TestKit node type to validator
  • @Auditor sets an emulated TestKit node type to auditor
  • @ValidatorCount sets a number of the validator nodes in the TestKit network

These annotations should be applied on the TestKit parameter:

private static final String SERVICE_NAME = "test-service";

TestKitExtension testKitExtension = new TestKitExtension(
        .withDeployedArtifact(ARTIFACT_ID, ARTIFACT_FILENAME)
        .withService(ARTIFACT_ID, SERVICE_NAME, 17)

void validatorTest(TestKit testKit) {
  // Injected TestKit has a default configuration, specified in the builder
  // above

void auditorTest(@Auditor @ValidatorCount(8) TestKit testKit) {
  // Injected TestKit has an altered configuration — "auditor" as an emulated
  // node and 8 validator nodes
  BlockchainData blockchainData = testKit.getBlockchainData(SERVICE_NAME);
  Blockchain blockchain = blockchainData.getBlockchain();
  Config consensusConfig = blockchain.getConsensusConfiguration();


Note that after the TestKit is instantiated in the given test context, it is not possible to reconfigure it again. For example, if the TestKit is injected in @BeforeEach method, it can't be reconfigured in @Test or @AfterEach methods. Also note that the TestKit cannot be injected in @BeforeAll and @AfterAll methods.


There are two ways to test API. First, API can be tested with unit tests by means of service mocks. TestKit is not required for these tests. Second, API can be tested with integration tests that use TestKit. For these tests TestKit provides the getPort() method that retrieves a TCP port to interact with the service REST API.

With either approach, you can use any HTTP client to send requests.

To unit test API implemented with Vertx tools, use the tools described in the project documentation.

An example of API service tests can be found in ApiControllerTest.

Using Libraries

An Exonum service can use any third-party library as its dependency. At the same time, Exonum comes with its own dependencies. Classes of these dependencies are used in Exonum public APIs:

Said dependencies are provided by the framework and must be used as provided. They will not be changed in an incompatible way in a compatible Exonum release. An up-to-date list is also available in the Exonum bill of materials (BOM).

On top of that, Guava can be and is recommended to be used as a provided library.


These dependencies do not have to be declared explicitly because any service depends on "exonum-java-binding-core" which has them as transitive dependencies.

These libraries must not be packaged into the service artifact. To achieve that in Maven, use the provided Maven dependency scope in the dependency declarations if you would like to specify them explicitly.

How to Build a Service Artifact

Exonum Java services are packaged as JAR archives with some extra metadata, required to identify the service and instantiate it.

If you used the service archetype to generate the project template, the build definition already contains all the required configuration. Hence you can invoke mvn verify and use the produced service artifact.

In case the service build definition needs to be configured, ensure that the following required metadata is present in the service artifact JAR:

  • Entries in the JAR manifest:
    • "Plugin-Id": must be set to "1:groupId/artifactId:version", e.g., 1:com.exonum.example.timestamping/timestamping-demo:1.0.2.
    • "Plugin-Version": must be set to the project version, e.g., 1.0.2.
  • A fully-qualified name of the service module class in "META-INF/extensions.idx" file. This file is automatically generated by the annotation processor of @Extension.

Built-In Services

Currently Java Binding includes the following built-in services:

  • Supervisor Service. The Supervisor service is the main service of Exonum. It is capable of deploying and starting new services.

  • Anchoring Service. The anchoring service writes the hash of the current Exonum blockchain state to the Bitcoin blockchain with a certain time interval. The anchored data is authenticated by a supermajority of validators using digital signature tools available in Bitcoin.

  • Time Oracle. Time oracle allows user services to access the calendar time supplied by validator nodes to the blockchain.

These services come pre-deployed in the genesis block. See Deploy and Start the Service for instructions on their instantiation.

Node Configuration

Exonum offers a three-step way of the node configuration process. It allows setting up a network of multiple nodes with multiple administrators without any risk of private keys leakage.

Exonum Java App offers the same configuration process as standard Exonum Rust services. In this guide we will describe how to configure a network of a single node.

Exonum Java App includes help for each of the available commands and its options. Use a -h flag for the short version of the CLI documentation, a --help flag for the detailed one. exonum-java <command-name> --help will print a detailed description of a specific command.

All paths in Exonum Java App CLI arguments are either absolute, or relative to the current working directory.

Step 1. Generate Template Config

First, we generate a common (also known as "template") part of the node configuration. Run this command only once per network, and distribute the resulting file among every node for the next step. However, the resulting file is not machine-specific and depends on the passed arguments only. Therefore, each administrator can run this command locally without distributing the file.

Provide a path to the resulting configuration file and a number of validator nodes in the network.

Optional supervisor-mode parameter denotes the mode of the Supervisor service. Possible values are "simple" or "decentralized". If no parameter is supplied, "simple" is used.

exonum-java generate-template \
    testnet/common.toml \
    --supervisor-mode "simple" \

Step 2. Generate Node Private and Public Configs

Note: the following examples omit setting up a password for private node keys protection and pass a --no-password flag. It is strictly recommended not to use this flag in production.

Second, we generate both private and public parts of the node configuration. Public part of the configuration must be distributed among every other administrator for the next step. Private parts must not be exposed to the outer world and are node-specific.

Provide a path to the common configuration file, a path to the directory for the generated configuration files and an external socket address of the node to use them for communication between the nodes.

exonum-java generate-config \
    testnet/common.toml \
    testnet \
    --no-password \

Step 3. Finalize Configuration

Third, combine private part of the node configuration and the public parts of the configuration of each node in the network.

Provide a path to the private part of the node configuration, a path to the resulting node configuration file and a list of paths to the public configuration files of every node.

exonum-java finalize \
    testnet/sec.toml \
    testnet/node.toml \
    --public-configs testnet/pub.toml

After completing each of the steps, the testnet/node.toml file contains the final node configuration. Use this file to run the node with the specified parameters.

Running the Node

Unlike configuration process, run command of Exonum Java App is different from the similar Exonum Rust command.

There are several required parameters here:

  • --db-path for a path to the database directory.
  • --node-config for a path to the final node configuration file.
  • --artifacts-path for a path to the directory with the compiled service artifacts.
  • --ejb-port for a port that Java services will use for communication. Java Binding does not use the public API address directly. Only Java services are accessible via this port.
  • --public-api-address and --private-api-address for the socket addresses for the node user API. The public API address is used by users to send transactions and requests to the blockchain, the private one is used by node administrators to perform different maintenance actions. Setting these addresses is not strictly necessary for every node, but you will need available API ports for deploying and starting a service using exonum-launcher.

There are also optional parameters useful for debugging purposes, logging configuration and JVM fine tuning:

  • --jvm-args-prepend and --jvm-args-append are additional parameters for the JVM that prepend and append the rest of arguments. Must not have a leading dash. For example, Xmx2G.
  • --jvm-debug allows remotely debugging the JVM over the JDWP protocol. Takes a socket address as a parameter in the following form — HOSTNAME:PORT. For example, localhost:8000.
  • --ejb-log-config-path for a path to Log4j two configuration files. The default config log4j-fallback.xml provided with Exonum Java app prints to STDOUT. See Logging Configuration for more information.
exonum-java run \
    --db-path testnet/db \
    --node-config testnet/node.toml \
    --artifacts-path artifacts \
    --ejb-port 7000 \
    --ejb-log-config-path "log4j.xml" \
    --master-key-pass pass \
    --public-api-address \

Changing the Used JVM

By default, Exonum Java App automatically finds the system JVM and uses it. To change this, set the JVM installation directory as the value for the JAVA_HOME environment variable.

Debugging the JVM

To enable remote debugging of the Java code on a running Exonum node, pass the --jvm-debug option with a socket address. This address will be used to connect a debugger:

exonum-java run -d testnet/db -c testnet/node.toml \
    --public-api-address \
    --private-api-address \
    --ejb-port 7000 \
    --jvm-debug localhost:8000

Now you can debug the service using any JDWP client, such as command line JDB or a debugger built into your IDE:

jdb -attach localhost:8000 -sourcepath /path/to/source

Deploy and Start the Service

Exonum Java Binding provides a way to dynamically deploy and start multiple services without stopping the nodes in the network. This can be done by sending particular transactions to the built-in Supervisor service. To simplify this process for the users, it is recommended to use exonum-launcher.

Place the compiled service JAR files into the artifacts directory (configured with the artifacts-path argument). The service files are needed for the whole time of their execution and cannot be moved or modified once deployed.

exonum-launcher is a Python application which is capable of forming and sending deploy transactions to the node, following the provided deploy configuration in the YAML file. exonum-launcher also has additional plugins for support of different runtimes and services.

Exonum Java Binding provides two plugins:

  • Exonum Java Runtime Plugin for Java services support.
  • Exonum Instance Configuration Plugin for support of services with custom initial configuration arguments encoded with Protobuf. Such arguments are sent to the service on its start and are typically used to customize the behavior of a particular service instance.

Launcher Installation

Follow the instructions in the plugins Readme

To deploy and start a specific list of services, use the following command with the prepared config.yml file:

python3 -m exonum_launcher -i config.yml

See the following section for instructions on creating the config.yml file for a specific service.

Writing the Configuration File

Start with specifying IP addresses of the blockchain nodes:

  - host: ""
    ssl: false
    public-api-port: 8080
    private-api-port: 8081

Specify every node for which you have access to its private API port. If you do not have access to every node in the network, the administrators of other nodes must run exonum-launcher with the same configuration file. The list of available nodes must be adjusted by every administrator accordingly.

The deadline height describes the maximum blockchain height for the deployment process. Make sure to specify the value larger than the current blockchain height.

deadline_height: 20000

Enable the Java runtime by specifying its identifier (1). The Rust runtime is enabled by default:

  java: 1

Add artifacts you want to deploy. For each artifact specify its name alias (as a YAML key) and its runtime (using the runtime field). Name aliases are used in other parts of the configuration for readability and easier refactoring. Java artifacts also need the name of the JAR file in the spec: artifact_filename field of the artifacts directory. The present example shows how to add the Java cryptocurrency-demo service, and two Rust services — the timestamping and time oracle services. Please note, that Rust services are pre-deployed in the genesis block, so we do not specify action: deploy for them.

    runtime: java
    name: "com.exonum.examples:cryptocurrency-demo:0.10.0"
      artifact_filename: "cryptocurrency-demo-0.10.0-artifact.jar"
    action: deploy
    runtime: rust
    name: "exonum-time:1.0.0"
    runtime: rust
    name: "exonum-timestamping:1.0.0"


Exonum Java comes with some built-in artifacts, which are already deployed. To see an up-to-date information on the coordinates (name and version) of the services built-in into the node, you may launch the node and request the list of services from the supervisor:

# Replace <public_port> with the node public port.
# You may also pipe the response into `jq` to pretty-print it,
# if you have jq installed:
curl -s<public_port>/api/services/supervisor/services # | jq

Add a plugins section to enable both Java Runtime plugin and Instance Configuration plugin. The runtime plugin is enabled for a specific runtime (java in the present example). The Instance Configuration plugin is enabled for a specific artifact name alias (timestamping in the present example).

    java: "exonum_java_runtime_plugin.JavaDeploySpecLoader"
    timestamping: "exonum_instance_configuration_plugin.InstanceSpecLoader"

The present example uses the Instance Configuration plugin to serialize initial configuration parameters of the timestamping service in Protobuf. Take a service.proto file with the message description from the service sources and place it inside some known directory.

syntax = "proto3";

package exonum.examples.timestamping;

message Config {
    string time_service_name = 1;

Finally, add an instances section that describes the list of service instances you want to start in the blockchain. For each instance specify its artifact name alias in the artifact field. Instance names are the keys in the YAML dictionary.

    artifact: cryptocurrency
    action: start
    artifact: time
    action: start

To instantiate a service which requires configuration parameters, config dictionary must be supplied.

The Instance Configuration plugin supports several configuration formats:

Text Configuration Formats

Requires the following parameters:

  • format. Describes the format of the configuration string. Possible values are: text, json and properties (see java.util.Properties).
  • value. A configuration string, may be used instead of from_file. If both value and from_file are present, value takes higher priority.
  • from_file. A path to a file containing configuration string. May be absolute or relative to the working directory.
    artifact: timestamping
    action: start
      format: "properties"
      from_file: "configs/"

Custom Configuration Message

Requires the following parameters:

  • sources. Points to a directory with the Protobuf sources of the service configuration message. The proto_sources directory is used.
  • config_message_source. A file name where the message_name message is located. In the present example the service.proto file is used.
  • message_name. A name of the Protobuf message used to represent the service configuration. Optional, defaults to Config.
  • data. Your actual configuration in the format corresponding to the message_name message.
    artifact: timestamping
    action: start
      sources: "proto_sources"
      config_message_source: "service.proto"
      message_name: "Config"
        time_service_name: "time"

See sample-config.yml for the final state of the configuration file.

Logging Configuration

Java Binding uses two different methods for logging — Log4J in Java modules and env_logger in Rust modules.

Rust Logging

Rust logs are produced by Exonum Core and can be used to monitor the status of the blockchain node, including information about the block commitment and the consensus status.

Rust logs are disabled by default and controlled by the RUST_LOG environment variable. It is recommended to set info logging level for Exonum modules and warn level for all other modules:

export RUST_LOG=warn,exonum=info,exonum-java=info

Log entries go to stderr by default.

See env_logger documentation for more information on possible configuration options.

Java Logging

Logs produced by Java code (the framework and its dependencies, and the deployed services) are handled by Log4J framework. The services can use either Log4J or SLF4J logging APIs.

Java logging configuration is controlled by the configuration file specified by the optional ejb-log-config-path parameter. If no file is provided, the default log4j-fallback.xml configuration file from the installation directory is used. This file allows printing INFO-level messages to STDOUT.

See Log4J documentation for more information on possible configuration options.

Common Library

Java Binding includes a library module that can be useful for Java client applications that interact with an Exonum service. The module does not have the dependency on Java Binding Core, but it contains Java classes obligatory for the core that can as well be easily used in clients, if necessary. The library provides the ability to create transaction messages, check proofs, serialize/deserialize data and perform cryptographic operations. For using the library just include the dependency in your pom.xml:


Known Limitations

  • Custom Rust services can be added to the application only by modifying and rebuilding thereof.
  • Exonum Java application does not support Windows yet.

See Also