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Crawford and Kaplan's J2EE Design Patterns approaches the subject in a unique , highly practical and pragmatic way. Rather than simply present another. O'Reilly books may be purchased for educational, business, or sales promotional use. . in the book Core J2EE Patterns and were separated into three main. O'Reilly books may be purchased for educational, business, or sales .. in the book Core J2EE Patterns and were separated into three main.

Oreilly J2ee Design Patterns Pdf

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J2EE Component. Design Patterns. Thomas Liou. Manager, Skills Development. Valtech Technologies, Inc. [email protected] O'Reilly. J2EE Design Patterns. Be watching for more books in the Head First series! Other related books from O'Reilly. Head First Java. Head First Servlets & JSP. PDF | In this paper we propose a model based on the Model-View-Controller design Keywords: Enterprise platforms, design patterns, J2EE, frameworks .. Prentice Hall. Crawford, W., Kaplan, J., J2EE design patterns. O'Reilly. Giang.

There are different API management solutions out there, and these come with all kinds of complexity ranging from simple frameworks and best practices to complete products, which have to be deployed as part of the product. It will help you to keep track of various aspects of your interfaces.

Most importantly, they allow you to dispatch based on service versions, and most of them offer load-balancing features. Besides monitoring, versioning, and load balancing, it is also important to keep track of the individual number of calls per service and version. This is the first step to actually acquiring a complete SLA overview and also tracking down issues with service usage and bottlenecks. Outside performance-relevant topics, API gateways and management solutions offer a broad range of additional features, including increased governance and security.

Use caches at the right layer Caching is the most important and performance-relevant part of microservices architectures. There are basically two different kinds of data in applications: the type that can be heavily cached, and the type that should never be cached.

The latter is represented by constantly refreshing data streams e. Everything else can be heavily cached on different levels. The UI aspects of a microservice can actually take advantage of the high-performance web technologies already available, such as edge caches, content delivery networks CDN , or simpler HTTP proxies.

All of these solutions rely on the cache expiry settings negotiated between the server and the client. A different layer of caching technology comes in at the backend. The easiest case is to use a second-level cache with a JPA provider or a dedicated in-memory datastore as a caching layer for your domain entities.

The biggest issue is maintaining consistency between cache replicas and between the cache and the backend data source. The best approach here is to use an existing implementation such as JBoss Infinispan. Independently Deployable and Fully Contained A microservices architecture will make it easier to scale development.

With this technology, there is no large team of developers responsible for a large set of features or individual layers of the monolith. However, with constantly shifting team setups and responsibilities for developers comes another requirement: services need to be independently deployable. Teams are fully responsible for everything from implementation to commissioning and this requires that they are in full control of the individual services they are touching. Another advantage is that this design pattern supports fault isolation.

If every service ideally comes with its own runtime, there is no chance a memory leak in one service can affect other services. Crosscutting Concerns Crosscutting concerns typically represent key areas of your software design that do not relate to a specific layer in your application.

This is where design concepts like dependency injection DI and aspect-oriented programming AOP can be used to complement object-oriented design principles to minimize tight coupling, enhance modularity, and better manage the crosscutting concerns. Figure Security layers for microservices First, is application-level security. Think about an authorized user who belongs to a role and has to access the entry point of the system.

The application will present a login service that a user has to access. The described mechanism is well known and works perfectly fine in monolithic applications. Keeping a credential accessible application-wide makes it easy to resolve application-level security authorizations when needed. This is completely different in a heavily distributed application that is composed of individual services, eventually running on different containers and implemented in different programming languages.

Although there is still a lack of suitable security standards to overcome this problem, there are different options to implement. Possible solutions range from custom tokens that get passed on with every request to developing industry standards like OAuth2 to infrastructure-based enterprise access management EAM solutions.

A distributed identity needs to be made available to all services. The next level is the user-level security. Both methods have pros and cons. Finding the correct approach is highly specific to the business requirement of the individual services and depends on a number of factors. These include the time needed to retrieve the user information, the additional payload size, the complete number of downstream chained service requests, and potentially more.

Last but not least, there is network-level security. Network-level security is typically the most important layer in enterprise scenarios.

Additionally, it might be feasible to also train an application security manager ASM solution with the allowed and wanted requests for your externally available services. Logging Although logging in typical enterprise environments only has to fulfill a few basic needs such as developer support, debugging in production, and business transaction logging , the nature of a distributed system requires a lot more.

Because one service request can be split out to many different subsequent requests and produce an error somewhere downstream, logging should be able to follow the complete request path down to the error.

And all the distributed logging sources need to be collected in a single application-wide log. Depending on the existing environment, this can be done with logging frameworks that support syslog or other existing centralized logging solutions but also built using the ELK Elasticsearch, Logstash, and Kibana stack.

Health Checks Health checks are an important part of DevOps. Every part needs to be controlled and monitored from the very beginning. However, there are different ways of approaching this requirement. A simple approach is to select an API management solution that not only deals with governance and load balancing but also handles the SLA and implicit health monitoring of every service. Although this is strongly recommended, there are plenty of other solutions starting from custom implementations up to more complex monitoring approaches.

Integration Testing While integration testing for Java EE applications has always been important but complex, it is even harder for microservices-based, distributed systems. As usual, testing begins with the so-called module or developer tests.

Typically running on a single developer machine, integration tests for distributed systems require the presence of all downstream services.

With everything completely automated, this also includes controlling the relevant containers with the dependent services. First and foremost, this requires a decent infrastructure, including complete test and integration systems for the various teams.

But working with microservices and being successful in enterprise settings will require having a PaaS offering, which can spin up needed instances easily and still be cost-effective. Depending on the delivery model that has been chosen, this might involve building container images and spinning up new instances as needed.

There are very few integration testing frameworks available today that can handle these kind of requirements. The most complete one is Arquillian together with the Cube extension. It can run a complete integration test including image build and spinning up containers as needed as developer local instances or even using remote PaaS offerings. The traditional test concepts and plans as executed by enterprise-grade developments have to step back a bit at this point in favor of more agile and DevOps-related approaches.

Java EE and Microservices Java EE began with less than 10 individual specifications, but it has grown over time through subsequent updates and releases to encompass Compared to microservices-based architectures, Java EE and its included specifications were originally designed for a different development and deployment model.

Only one monolithic server runtime or cluster hosted many different applications packaged according to standards. Such a model runs opposite to the goal of microservices.

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Of course, this has its limits and is not meant to be adjusted for quickly changing requirements or excessive load situations. But the latest versions of Java EE added a ton of developer productivity to the platform alongside a streamlined package. With the sloping modularity of Java and the JVM, an established platform, and skilled developers alongside individual implementations, Java EE is considered to be a reasonable solution for microservices development. If you are starting to build microservices architectures on top of Java EE, make sure to look at the asynchronous features and try to use the best available parts.

One important item to keep in mind: Java EE was never built to work with distributed applications or microservices. So every decision and line of code should be carefully inspected and validated to maximize asynchronicity. JAX-RS 2. Session interface.

This is an instance of the nested interface of the javax. Servlet 3. The parts that need to be implemented are thread pools using the ExecutorService , AsyncContext, the runnable instance of work, and a Filter to mark the complete processing chain as asynchronous.

Asynchronous annotation. This is placed on a Stateless, Stateful, or Singleton EJB to make all the contained methods asynchronous or placed at the method level itself.

A method with the Asynchronous annotation can either return void fire and forget or an instance of java. Future if the asynchronous method result needs to be tracked.

This is done by calling the Future. The Missing Pieces To build a complete and reliable microservices application, you need something more than what typical Java EE servers provide today. Those missing pieces are relevant, and you only have the chance to build them individually or put an infrastructure in place to deliver them.

Service Registry Multiple microservices are composed to create an application, and each microservice can scale independently. Service registration allows each microservice to register itself with a registry using a logical name. This name is bound to a physical URI and additional metainformation. By using the logical name, a consumer can locate and invoke the microservice after a simple registry query. If the microservice goes down, then the consumers are notified accordingly or alternative services get returned.

The registry should work closely together with the API gateway. Security In a traditional multitiered server architecture, a server-side web tier deals with authenticating the user by calling out to a relational database or a Lightweight Directory Access Protocol LDAP server. An HTTP session is then created containing the required authentication and user details.

Having a central component that authenticates a user and propagates a token containing the relevant information downstream is unavoidable. Enterprise access management EAM systems mostly provide the needed features in an enterprise environment.

In addition, some API management solutions also contain security features on top of their government engine. And last but not least, there are dedicated products, like JBoss Keycloak. Migration Approaches Putting the discussion in Designing Software for a Scalable Enterprise about greenfield versus brownfield development into practice, there are three different approaches to migrating existing applications to microservices.

After the initial assessment, you know exactly which parts of the existing application can take advantage of a microservices architecture. And while moving out individual services one at a time, the team has a fair chance to adapt to the new development methodology and make its first experience with the technology stack a positive one. A load balancer or proxy decides which requests need to reach the original application and which go to the new parts. There are some synchronization issues between the two stacks.

Parallel operations: strangler pattern Big Bang: Refactor an Existing System In very rare cases, complete refactoring of the original application might be the right way to go. This is the least recommended approach because it carries a comparably high risk of failure. Microservices Design Pattern Functional decomposition of an application with the help of DDD is a prerequisite for building a microservices architecture.

Only this approach allows you to effectively design for loose coupling and high cohesion. However, unlike with applications, which are tied together by the frontend, microservices can interact with each other and span a network of service calls.

To keep the variety of interactions comprehensible and maintainable, a first set of patterns have emerged that will help you to model the service interaction. These patterns were first published by Arun Gupta , but have been revised for this report. Common Principles Every microservice has some common basic principles that need to be taken into account.

One reason for this is that teams can be fully responsible for putting new versions into production. It also enables the team to use the needed downstream services at the correct revision by querying the repository.

Compare Independently Deployable and Fully Contained. Needing to replicate state across various services is a strong indicator of a bad design.

Index of /~jacques/cursos/j2ee/recursos

Services are fully contained and independent and should be able to work without any prepopulated state. Compare Designing Software for a Scalable Enterprise. The Data Access Layer Is Cached In order to keep service response times to a minimum, you should consider data caching in every service you build. And keep in mind Design for Performance. It is already well known from the Enterprise Integration pattern catalog and has proven to be useful outside microservices architecture.

The primary goal of this pattern is to act as a special filter that receives a stream of responses from service calls and identifies or recognizes the responses that are correlated. Once all the responses have been been collected, the aggregator correlates them and publishes a single response to the client for further processing. In its most basic form, aggregator is a simple, single-page application e.

Assuming all three services in this example are exposing a REST interface, the application simply consumes the data and exposes it to the user. The services in this example should be application services compare above and do not require any additional business logic in the frontend. If they represent domain services, they should be called by an application service first and brought into a representable state.

Software design pattern

It is totally valid to use different protocols. Because the aggregator is another business service heavily accessing asynchronous domain services, it uses a message-driven approach with the relevant protocols on top e.

The wrapper service can add additional functionality to the service of interest without changing its code. Proxy pattern The proxy may be a simple pass-through proxy, in which case it just delegates the request to one of the proxied services. It is usually called a smart proxy when additional logic is happening inside the proxy service.

The applicable logic varies in complexity and can range from simple logging to adding a transaction.

If used as a router, it can also proxy requests to different services by parameter or client request. Pipeline Pattern In more complex scenarios, a single request triggers a complete series of steps to be executed.

In this case, the number of services that have to be called for a single response is larger than one. A pipeline can be triggered synchronously or asynchronously, although the processing steps are most likely synchronous and rely on each other. But if the services are using synchronous requests, the client will have to wait for the last step in the pipeline to be finished. As a general rule of thumb, according to usability studies , one-tenth of a second is about the limit for having the user feel that the system is reacting instantaneously.

Normally, no special feedback is necessary during delays of more than 0. Shared Resources One of the critical design principles of microservices is autonomy.

Especially in migration scenarios see "Migration Approaches" on page 43 , it might be hard to correct design mistakes made a couple of years ago. And instead of reaching for the big bang, there might be a more reasonable way to handle those special cases. The key here is to keep the business domain closely related and not to treat this exception as a rule; it may be considered an antipattern but business needs might require it.

With that said, it is certainly an antipattern for greenfield applications. Most likely, they are implemented in a synchronous and therefore blocking manner. Even if this can be changed in Java EE, and the implementations support asynchronous calls, it might still be considered a second-class citizen in the enterprise systems you are trying to build.

Message-oriented middleware MOM is a more reasonable solution to integration and messaging problems in this field, especially when it comes to microservices that are exposed by host systems and connected via MOMs. Asynchronous messaging Conclusion The world of IT as we know it is changing dramatically. In the next point we describe the proposed model for the development of J2EE enterprise applications.

The Intercepting Filter is used in our model to implement the requests pre-processor; this system initially manages the entry requests from clients in the presentation layer. Therefore, when a request arrives to the application, it should pass through a set of verifications before reaching the main processing phase —called the Front Controller—: authentication, session validation, client IP address checking, request authorization, data codification, auditory or browser type used [3].

The Intercepting Filter pattern is a flexible and highly decoupled way to intercept a request, applying a set of filters, thus rejecting or allowing the request to arrive to the initial process [4]. In the proposed model see fig 1 , our controller is designed following the Service To Worker pattern see fig 2 , which combines a set of smaller patterns that provide a complete and flexible solution to fulfil the requirements for an MVC controller while allowing the separation between actions the model , the view and the controller [7].

The Front Controller pattern [4, 3, 7] describes a central point that manages the requests. In order to reach a greater flexibility and independence between the view and the model, the Front Controller assumes only the request analysis task, delegating in the Request Dispatcher the selection of the view and the action to perform.

The Command pattern represents each request by means of an object, therefore providing a very simple way to introduce new operations [4]. In our model, the Command pattern is responsible for encapsulating the request information, parameters and the current state into a command object that contains the business logic.

This command is then sent through the network to the model, where it is finally executed EJBCommand. By using this approach, we achieve a complete decoupling between the controller and the model [24]. On the other hand, we define another set of patterns in order to integrate our model with inherited systems or other business models.

This scheme provides the advantages of the Command pattern in an environment where the business logic is distributed, therefore allowing the execution of business rules without overloading the application by a massive usage of EJBs. The other two patterns —the Business Delegate and the Session Facade— jointly provide a good solution in order to decouple the model from the view and the controller. For the second case, we have defined two patterns to ease the integration between the business model and the inherited systems or other business models.

The Data Object Access pattern supplies a mechanism to abstract and encapsulate access to the data sources, therefore achieving warehouse independency [3]. The Service Activator pattern describes a way to access other business models and services in an asynchronous manner. When a message is received, the Service Activator locates and invokes the business methods necessary to resolve the request asynchronously [3].

The view is responsible for showing the data output by the MVC model. In this scheme, the Request Dispatcher returns the data initially output by the model, as an XML document. Since XSLT can return any format, it is a notably flexible technology, supporting multiple types of clients [12]. Besides, it provides a simple way to modify the view without changing the model, and facilitates parallel application development. In the next point we describe the implementation of the proposed model, based on SUN's recommendations [24] —use of existing frameworks in the market—.

We will detail the process of frameworks selection and the integration tasks carried out, finalizing with the advantages and the power offered by the obtained implementation. There are also several open source frameworks supported by a solid community and widely spread in the last years [24]. In order to select the framework that is most appropriate for our purposes, we carried out a survey which focused on open source frameworks, due to its inexpensive costs and the technological maturity reached by some of them.

In the next point, we describe each of these frameworks and how they fit into our model. The view is based on the plug-ins concept. For these reasons, Struts is the ideal choice in order to implement our model. This component extracts from the configuration file strut-config. After performing these tasks, it delegates the control in the RequestProcessor. For each operation or use case, the developer creates an action object that inherits from the Action component.

It is a mature technology, created and maintained by the Apache Software Foundation.

J2EE Design Patterns

When Cocoon receives a request, it is analyzed based on the configuration parameters stored in the site-map. For this purpose, Cocoon uses the Generator [21] interface, which allows specifying different source types to get the XML document. To achieve this, Cocoon uses the Transformer [21] interface, which is implemented depending on the desired transformation type.

Cocoon and Struts class diagram 4. To replace this framework we must substitute the associated plug-in for a new one. In [5], Cocoon Plug-in for Struts is described as a framework that allows the integration between Struts and Cocoon.

In order to use the mentioned plug-in, we must replace the Tiles plug-in with it this can be configured in the struts- config. Like the RequestProcessor, it creates and executes the corresponding action. This framework provides the architecture and design necessary to integrate Struts with a distributed model based on EJBs.

Its function consists in creating an instance of the StrutsEJB command EJBCommand associated to the request and to store in this command the parameters and session variables required to run its business logic. The ServiceLocator is the helper component —an implementation of the Service Locator pattern— used to look for services. Usually, a particular SessionFacade is created for each set of use cases.

In this way, SessionFacades provide a distributed environment in order to execute such commands.

The StatefulFacade is to be used in applications where it is necessary to maintain the state; otherwise, the StatelessFacade will be used. It is an abstract component that encapsulates the business logic for each use case or set of use cases. Therefore, it is necessary to extend this component for each operation or set of related operations , creating a new command. In this way, developers do not need to use EJBs in their applications although they can still use them voluntarily , but the business logic is kept inside the EJBs container.

EJBCommands are created from the Web tier by means of an action DefaultAction , which sends them to the model where they will be executed by a SessionFacade. This modification solved the problem of passing the XML document generated in the model to the CocoonRequestProcessor, as well as other internacionalization, multiple devices support and error managing problems.

In order to implement this pattern, we have not used any framework, but a J2EE Filter, which provides a mechanism similar to a Servlet. The Filter is controlled by a Web container and can be inserted, in a declarative way, into the request-reply HTTP [1] process.

For each Filter needed, the developer creates a new class that extends the Filter interface and incorporates it to the application adding the corresponding directives to the configuration file, web. Intercepting Filter model 4. One of the most important is the user access security authentication and authorization.Proxy pattern The proxy may be a simple pass-through proxy, in which case it just delegates the request to one of the proxied services.

You can use Vert. All the required Java EE dependencies are already available to the application with the application server base installation, and containers provide additional features like transactions and security.

It might be easier to verbalize a process service by putting its mission statement up front, such as the following: This service lists similar courses for a given course.

Then they go on to show how to apply the patterns when writing realworld software. And last but not least, there are dedicated products, like JBoss Keycloak. Service Registry Multiple microservices are composed to create an application, and each microservice can scale independently.

Pipeline Pattern In more complex scenarios, a single request triggers a complete series of steps to be executed. Event-driven design Another approach to transactions is the event-driven design of services.

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