You can easily customize a Spring Boot application to your liking, either in the initial configuration or later in the development cycle. For example, if you prefer Maven, then you can easily make change s in your POM file to replace the Spring Boot default value. Starters are a big part of the magic of Spring Boot, used to limit the amount of manual dependency configuration that you have to do. A starter is essentially a set of dependencies such as a Maven POM that are specific to the type of application the starter represents.
All starters use the naming convention: spring-boot-starter-XYZ , where XYZ is the type of application you want to build. Here are some popular Spring Boot starters:. The Spring Boot starter reference page lists lots more starters. Check it out to see the POM and dependencies for each starter. If you let it, Spring Boot will use its EnableAutoConfiguration annotation to automatically configure your application.
Take a look : Fire up your Spring Boot application with the --debug option and an auto-configuration report will be generated to the console. To run your application, you start Java like this:. A shaded JAR simply contains the. But as the complexity of your application grows and dependencies increase, shaded JARs can suffer from two issues:.
Examples in this section are based on a simple application called HelloSpringBoot. I encourage you to work through the application development example with me, but if you want to jump right in you can download the application code from Github. Select maven-archetype-quickstart , as shown in Figure 2. Now open App. Arrays; import org. Logger; import org. LoggerFactory; import org.
CommandLineRunner; import org. SpringApplication; import org. SpringBootApplication; import org. ApplicationContext; import org. Then create a new class called HelloRestController in the same package as App that looks like this:. RequestMapping; import org.
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All your base are belong to us. Listing 1. This tells Spring Boot that the application is a web application. Spring Boot will form its opinions accordingly. Lines 25 through 30 tell Maven to use the spring-boot-maven-plugin plugin to generate the Spring Boot application. Notice there is no XML. Based on this starter, Spring Boot has formed the following opinions about the application:. Talk about opinionated! But remember how I said Spring Boot is customizable?
Will Spring Boot still work? Listing 2. Make sure to add a repackage goal execution to the spring-boot-maven-plugin as explained here. You have two options for building the executable JAR using Maven: run the Maven build in Eclipse or run it from the command line. In the Goals text field, enter clean and package , then click the Run button.
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To run the Maven build from the command line, open a Mac terminal window or a Windows command prompt, navigate to the HelloSpringBoot project directory, and execute the command:. You should see a message in the terminal window or command prompt indicating a successful build. If you have configured it differently, please make the appropriate substitution in the command above. App : Starting App v1. App : Running with Spring Boot v1.
App : Started App in After forming an opinion about the type of application you intend to build, Spring Boot delivers a set of Maven dependencies. Note that Tomcat is the default embedded web server container. Listing 3. Notice below that the Maven dependencies for Tomcat are gone thanks to lines 5 through 10 in Listing 3 , replaced with dependencies for Jetty lines 12 through Actually, there are more Jetty dependencies than would fit in a single screenshot, but they are all there, and the Tomcat dependencies are gone.
It enables lightweight messaging within Spring-based applications and supports integration with external systems through declarative adapters. It includes some programming tips to help you make the most of Spring Integration. In its broadest sense, this means that the framework handles responsibilities on behalf of the components that are managed within its context.
The components themselves are simplified, because they are relieved of those responsibilities. For example, dependency injection relieves the components of the responsibility of locating or creating their dependencies. Likewise, aspect-oriented programming relieves business components of generic cross-cutting concerns by modularizing them into reusable aspects.
In each case, the end result is a system that is easier to test, understand, maintain, and extend. Furthermore, the Spring framework and portfolio provide a comprehensive programming model for building enterprise applications. Developers benefit from the consistency of this model and especially from the fact that it is based upon well established best practices, such as programming to interfaces and favoring composition over inheritance.
Spring Integration is motivated by these same goals and principles. It supports message-driven architectures where inversion of control applies to runtime concerns, such as when certain business logic should run and where the response should be sent. It supports routing and transformation of messages so that different transports and different data formats can be integrated without impacting testability. In other words, the messaging and integration concerns are handled by the framework.
Business components are further isolated from the infrastructure, and developers are relieved of complex integration responsibilities. That API is based upon well defined strategy interfaces and non-invasive, delegating adapters. Developers who have read that book should be immediately comfortable with the Spring Integration concepts and terminology. The framework should enforce separation of concerns between business logic and integration logic.
Extension points should be abstract in nature but within well-defined boundaries to promote reuse and portability. From a vertical perspective, a layered architecture facilitates separation of concerns, and interface-based contracts between layers promote loose coupling. Spring-based applications are typically designed this way, and the Spring framework and portfolio provide a strong foundation for following this best practice for the full stack of an enterprise application.
Message-driven architectures add a horizontal perspective, yet these same goals are still relevant. It is important to note that these two high-level paradigms are not mutually exclusive. In Spring Integration, a message is a generic wrapper for any Java object combined with metadata used by the framework while handling that object. It consists of a payload and headers. The payload can be of any type, and the headers hold commonly required information such as ID, timestamp, correlation ID, and return address. Headers are also used for passing values to and from connected transports.
For example, when creating a message from a received file, the file name may be stored in a header to be accessed by downstream components. Developers can also store any arbitrary key-value pairs in the headers. Producers send messages to a channel, and consumers receive messages from a channel. The message channel therefore decouples the messaging components and also provides a convenient point for interception and monitoring of messages.
A message channel may follow either point-to-point or publish-subscribe semantics.
With a point-to-point channel, no more than one consumer can receive each message sent to the channel. Publish-subscribe channels, on the other hand, attempt to broadcast each message to all subscribers on the channel. Spring Integration supports both of these models. In Spring Integration, pollable channels are capable of buffering Messages within a queue.
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The advantage of buffering is that it allows for throttling the inbound messages and thereby prevents overloading a consumer. However, as the name suggests, this also adds some complexity, since a consumer can only receive the messages from such a channel if a poller is configured. On the other hand, a consumer connected to a subscribable channel is simply message-driven. Message Channel Implementations has a detailed discussion of the variety of channel implementations available in Spring Integration. One of the primary goals of Spring Integration is to simplify the development of enterprise integration solutions through inversion of control.
This means that you should not have to implement consumers and producers directly, and you should not even have to build messages and invoke send or receive operations on a message channel. Instead, you should be able to focus on your specific domain model with an implementation based on plain objects. The components responsible for these connections are message endpoints. This does not mean that you should necessarily connect your existing application code directly.
Any real-world enterprise integration solution requires some amount of code focused upon integration concerns such as routing and transformation. The important thing is to achieve separation of concerns between the integration logic and the business logic. In other words, as with the Model-View-Controller MVC paradigm for web applications, the goal should be to provide a thin but dedicated layer that translates inbound requests into service layer invocations and then translates service layer return values into outbound replies.
In other words, the application code should ideally have no awareness of the message objects or the message channels. This is similar to the role of a controller in the MVC paradigm. Just as a controller handles HTTP requests, the message endpoint handles messages. Just as controllers are mapped to URL patterns, message endpoints are mapped to message channels. The goal is the same in both cases: isolate application code from the infrastructure. These concepts and all of the patterns that follow are discussed at length in the Enterprise Integration Patterns book.
Here, we provide only a high-level description of the main endpoint types supported by Spring Integration and the roles associated with those types. The chapters that follow elaborate and provide sample code as well as configuration examples. Probably the most common type of transformer is one that converts the payload of the message from one format to another such as from XML to java.
A message filter determines whether a message should be passed to an output channel at all. This simply requires a boolean test method that may check for a particular payload content type, a property value, the presence of a header, or other conditions. If the message is accepted, it is sent to the output channel. If not, it is dropped or, for a more severe implementation, an Exception could be thrown. Message filters are often used in conjunction with a publish-subscribe channel, where multiple consumers may receive the same message and use the criteria of the filter to narrow down the set of messages to be processed.
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A message router is responsible for deciding what channel or channels if any should receive the message next. A message router is often used as a dynamic alternative to a statically configured output channel on a service activator or other endpoint capable of sending reply messages. Likewise, a message router provides a proactive alternative to the reactive message filters used by multiple subscribers, as described earlier.
A splitter is another type of message endpoint whose responsibility is to accept a message from its input channel, split that message into multiple messages, and send each of those to its output channel. Basically a mirror-image of the splitter, the aggregator is a type of message endpoint that receives multiple messages and combines them into a single message.
In fact, aggregators are often downstream consumers in a pipeline that includes a splitter. Technically, the aggregator is more complex than a splitter, because it is required to maintain state the messages to be aggregated , to decide when the complete group of messages is available, and to timeout if necessary. Furthermore, in case of a timeout, the aggregator needs to know whether to send the partial results, discard them, or send them to a separate channel.
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Spring Integration provides a CorrelationStrategy , a ReleaseStrategy , and configurable settings for timeout, whether to send partial results upon timeout, and a discard channel. A Service Activator is a generic endpoint for connecting a service instance to the messaging system. The input message channel must be configured, and, if the service method to be invoked is capable of returning a value, an output message Channel may also be provided.
That reply message is sent to the output channel. A channel adapter is an endpoint that connects a message channel to some other system or transport. Channel adapters may be either inbound or outbound. Typically, the channel adapter does some mapping between the message and whatever object or resource is received from or sent to the other system file, HTTP Request, JMS message, and others. Depending on the transport, the channel adapter may also populate or extract message header values.
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Spring Integration provides a number of channel adapters, which are described in upcoming chapters. Consuming endpoints anything with an inputChannel consist of two beans, the consumer and the message handler. The consumer has a reference to the message handler and invokes it as messages arrive. Consider the following example of an annotated POJO:. Starting with version 5. Consider the following example of an annotated bean:.
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The EndpointId annotation creates names as created by the id attribute with XML configuration, as long as you use the convention of appending. This wrapper supports request handler advice handling and emits the normal 'produced no reply' debug log messages. Its bean name is the handler bean name plus. Handler: someAdapter. Throughout this document, you can see references to XML namespace support for declaring elements in a Spring Integration flow. This support is provided by a series of namespace parsers that generate appropriate bean definitions to implement a particular component.
For example, many endpoints consist of a MessageHandler bean and a ConsumerEndpointFactoryBean into which the handler and an input channel name are injected. The first time a Spring Integration namespace element is encountered, the framework automatically declares a number of beans a task scheduler, an implicit channel creator, and others that are used to support the runtime environment. The EnableIntegration annotation is also useful when you have a parent context with no Spring Integration components and two or more child contexts that use Spring Integration.
It lets these common components be declared once only, in the parent context. The EnableIntegration annotation registers many infrastructure components with the application context. In particular, it:. Adds several BeanFactoryPostProcessor instances to enhance the BeanFactory for global and default integration environment. Adds several BeanPostProcessor instances to enhance or convert and wrap particular beans for integration purposes. Adds annotation processors to parse messaging annotations and registers components for them with the application context.
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The IntegrationComponentScan annotation also permits classpath scanning. This annotation plays a similar role as the standard Spring Framework ComponentScan annotation, but it is restricted to components and annotations that are specific to Spring Integration, which the standard Spring Framework component scan mechanism cannot reach.
For an example, see MessagingGateway Annotation. The EnablePublisher annotation registers a PublisherAnnotationBeanPostProcessor bean and configures the default-publisher-channel for those Publisher annotations that are provided without a channel attribute. If more than one EnablePublisher annotation is found, they must all have the same value for the default channel. See Annotation-driven Configuration with the Publisher Annotation for more information.
The GlobalChannelInterceptor annotation has been introduced to mark ChannelInterceptor beans for global channel interception. GlobalChannelInterceptor annotations can be placed at the class level with a Component stereotype annotation or on Bean methods within Configuration classes. In either case, the bean must implement ChannelInterceptor. Previously, interceptors were not applied when beans were created after the application context was refreshed. You can place IntegrationConverter annotations at the class level with a Component stereotype annotation or on Bean methods within Configuration classes.
See Annotation Support for more information about messaging annotations. You should use plain old java objects POJOs whenever possible and only expose the framework in your code when absolutely necessary. If you do expose the framework to your classes, there are some considerations that need to be taken into account, especially during application startup:. If your component is ApplicationContextAware , you should generally not use the ApplicationContext in the setApplicationContext method.
Instead, store a reference and defer such uses until later in the context lifecycle.