Emissary (formerly Deezpatch)

A simple-to-use, no dependency, yet 🗲BLAZING FAST🗲 messaging library for decoupling messages (requests and events) and message handlers 🚀

Emissary aims to take advantage of the simplicity of using the annotations for handlers (e.g. @RequestHandler/@EventHandler) without the drawbacks of reflection (slow).

Emissary aims to make it easy to build applications that apply the Command Query Responsibility Segregation (CQRS) pattern, but it is not in any way limited to that pattern only.

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Get Emissary

Important

Up until v1.1.0, the core library is published under the old deezpatch-core name. This has been renamed to emissary-core starting from v2.0.0 onwards.

Gradle

implementation "io.github.joel-jeremy.emissary:emissary-core:${version}"

Maven

<dependency>
<groupId>io.github.joel-jeremy.emissary</groupId>
<artifactId>emissary-core</artifactId>
<version>${version}</version>
</dependency>

Java 9 Module Names

Important

Up until v1.1.0, the core library has the module name io.github.joeljeremy.deezpatch.core. This has been renamed to io.github.joeljeremy.emissary.core starting from v2.0.0 onwards.

Emissary jars are published with Automatic-Module-Name manifest attribute:

• Core - io.github.joeljeremy.emissary.core

Module authors can use above module names in their module-info.java:

module foo.bar {
requires io.github.joeljeremy.emissary.core;
}

Performance

What differentiates Emissary from other messaging/dispatch libraries? It takes advantage of java.lang.invoke.LambdaMetafactory to avoid the cost of invoking methods reflectively. This results in performance close to directly invoking the request handler and event handler methods!

~ 1000% more throughput compared to other similar libraries (Spring’s ApplicationEventPublisher, Pipelinr, EventBus)

~ 90% faster compared to other similar libraries (Spring’s ApplicationEventPublisher, Pipelinr, EventBus)

Java 25 Benchmarks

Java 21 Benchmarks

Java 17 Benchmarks

Java 11 Benchmarks

Requests

Requests are messages that either:

1. Initiate a state change/mutation

• Commands in CQRS

1. Retrieve/query data

• Queries in CQRS

public class CreateFooCommand implements Request<Void> {
private final String name;

public CreateFooCommand(String name) {
this.name = name;
}

public String name() {
return name;
}
}

public class GetFooByNameQuery implements Request<Foo> {
private final String name;

public GetFooByNameQuery(String name) {
this.name = name;
}

public String name() {
return name;
}
}

Request Handlers

Requests are handled by request handlers. Request handlers can be registered through the use of the @RequestHandler annotation.

A request must only have a single request handler.

(@RequestHandlers fully support methods with void return types! No need to set method return type to Void and return null for no reason.)

public class CreateFooCommandHandler {
@RequestHandler
public void handle(CreateFooCommand command) {
insertFooToDatabase(command.name());
}
}

public class GetFooQueryHandler {
@RequestHandler
public Foo handle(GetFooByNameQuery query) {
return getFooFromDatabase(query.name());
}
}

Request Dispatcher

Requests are dispatched to a single request handler and this can be done through a dispatcher.

public static void main(String[] args) {
// Use Spring's application context as InstanceProvider in this example
// but any other DI framework can be used e.g. Guice, Dagger, etc.
ApplicationContext applicationContext = springApplicationContext();

// Emissary implements the Dispatcher interface.
Dispatcher dispatcher = Emissary.builder()
.instanceProvider(applicationContext::getBean)
.requests(config -> config.handlers(CreateFooCommandHandler.class, GetFooQueryHandler.class))
.build();

// Send command!
dispatcher.send(new CreateFooCommand("Emissary"));

// Send query!
Optional<Pong> pong = dispatcher.send(new GetFooByNameQuery("Emissary"));
}

Events

Events are messages that indicate that something has occurred in the system.

public class FooCreatedEvent implements Event {
private final String name;

public FooCreatedEvent(String name) {
this.name = name;
}

public String name() {
return name;
}
}

Event Handlers

Events are handled by event handlers. Event handlers can be registered through the use of the @EventHandler annotation.

An event can have zero or more event handlers.

public class FooEventHandler {
@EventHandler
public void notifyA(FooCreatedEvent event) {
helloFoo(event.name());
}

@EventHandler
public void notifyB(FooCreatedEvent event) {
kumustaFoo(event.name());
}
}

Event Publisher

Events are dispatched to zero or more event handlers and this can be done through a publisher.

public static void main(String[] args) {
// Use Spring's application context as InstanceProvider in this example
// but any other DI framework can be used e.g. Guice, Dagger, etc.
ApplicationContext applicationContext = springApplicationContext();

// Emissary implements the Publisher interface.
Publisher publisher = Emissary.builder()
.instanceProvider(applicationContext::getBean)
.events(config -> config.handlers(FooEventHandler.class))
.build();

// Publish event!
publisher.publish(new FooCreatedEvent("Emissary"));
}

Easy Integration with Dependency Injection (DI) Frameworks

The library provides an InstanceProvider interface as an extension point to let users customize how request/event handler instances should be instantiated. This can be as simple as new-ing up request/event handlers or getting instances from a DI framework such as Spring’s ApplicationContext, Guice’s Injector, etc.

Example with No DI framework

// Application.java

public static void main(String[] args) {
Emissary emissary = Emissary.builder()
.instanceProvider(Application::getInstance)
.requests(...)
.events(...)
.build();
}

private static Object getInstance(Class<?> handlerType) {
if (MyRequestHandler.class.equals(handlerType)) {
return new MyRequestHandler();
} else if (MyEventHandler.class.equals(handlerType)) {
return new MyEventHandler();
}

throw new IllegalStateException("Failed to get instance for " + handlerType.getName() + ".");
}

Example with Spring’s ApplicationContext

public static void main(String[] args) {
ApplicationContext applicationContext = springApplicationContext();
Emissary emissary = Emissary.builder()
.instanceProvider(applicationContext::getBean)
.requests(...)
.events(...)
.build();
}

Example with Guice’s Injector

public static void main(String[] args) {
Injector injector = guiceInjector();
Emissary emissary = Emissary.builder()
.instanceProvider(injector::getInstance)
.requests(...)
.events(...)
.build();
}

Custom Request/Event Handler Annotations

In cases where a project is built in such a way that bringing in external dependencies is considered a bad practice (e.g. domain layer/package in a Hexagonal (Ports and Adapters) architecture), Emissary provides a way to use custom request/event handler annotations (in addition to the built-in RequestHandler and EventHandler annotations) to annotate request/event handlers.

This way, Emissary can still be used without adding the core Emissary library as a dependency of a project’s domain layer/package. Instead, it may be used in the outer layers/packages to wire things up.

// Let's say below classes are declared in a project's core/domain package:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface AwesomeRequestHandler {}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface AwesomeEventHandler {}

public class MyRequestHandler {
@AwesomeRequestHandler
public void handle(TestRequest request) {
// Handle.
}
}

public class MyEventHandler {
@AwesomeEventHandler
public void handle(TestEvent event) {
// Handle.
}
}

// To wire things up:

public static void main(String[] args) {
// Use Spring's application context as InstanceProvider in this example
// but any other DI framework can be used e.g. Guice, Dagger, etc.
ApplicationContext applicationContext = springApplicationContext();

// Register handlers and custom annotations.
Emissary emissary = Emissary.builder()
.instanceProvider(applicationContext::getBean)
.requests(config -> config.handlerAnnotations(AwesomeRequestHandler.class).handlers(MyRequestHandler.class))
.events(config -> config.handlerAnnotations(AwesomeEventHandler.class).handlers(MyEventHandler.class))
.build();
}

Custom Invocation Strategies

The library provides Emissary.RequestHandlerInvocationStrategy and Emissary.EventHandlerInvocationStrategy interfaces as extension points to let users customize how request/event handler methods are invoked by the Dispatcher and Publisher.

Built-in implementations are:

• SyncRequestHandlerInvocationStrategy (Default)
• SyncEventHandlerInvocationStrategy (Default)
• AsyncEventHandlerInvocationStrategy

Users can create a new implementation and override the defaults by:

// Register custom invocation strategy.
Emissary emissary = Emissary.builder()
.requests(config -> config.invocationStrategy(new CustomRetryOnErrorInvocationStrategy()))
.events(config -> config.invocationStrategy(new CustomOrderGuaranteedInvocationStrategy()))
.build();

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