RabbitMQ Outbound gateway using java configuration

any example reference of RabbitMQ Outbound Gateway to post messages to rabbitmq cluster, I am looking for Java configuration with Spring Boot

See the reference manual. There are xml, Java and Java DSL examples there.

@Bean
@ServiceActivator(inputChannel = "amqpOutboundChannel")
public AmqpOutboundEndpoint amqpOutbound(AmqpTemplate amqpTemplate) {
    AmqpOutboundEndpoint outbound = new AmqpOutboundEndpoint(amqpTemplate);
    outbound.setExpectReply(true);
    outbound.setRoutingKey("foo"); // default exchange - route to queue 'foo'
    return outbound;
}

or

@Bean
public IntegrationFlow amqpOutbound(AmqpTemplate amqpTemplate) {
    return IntegrationFlows.from(amqpOutboundChannel())
            .handle(Amqp.outboundGateway(amqpTemplate)
                    .routingKey("foo")) // default exchange - route to queue 'foo'
            .get();
}

Gateways are for request/reply processing not just "post" ing; you need a channel adapter if you are just sending.

@Bean
@ServiceActivator(inputChannel = "amqpOutboundChannel")
public AmqpOutboundEndpoint amqpOutbound(AmqpTemplate amqpTemplate) {
    AmqpOutboundEndpoint outbound = new AmqpOutboundEndpoint(amqpTemplate);
    outbound.setRoutingKey("foo"); // default exchange - route to queue 'foo'
    return outbound;
}

or

@Bean
public IntegrationFlow amqpOutbound(AmqpTemplate amqpTemplate) {
    return IntegrationFlows.from(amqpOutboundChannel())
            .handle(Amqp.outboundAdapter(amqpTemplate)
                        .routingKey("foo")) // default exchange - route to queue 'foo'
            .get();
}