First, J2EE concept
At present, the Java 2 platform has 3 versions, which is a Java 2 platform for small devices and smart cards Micro version (Java 2 Platform Micro Edition, Java 2 Platform Standard (Java 2 Platform Standard Edition) , J2SE), suitable for Java 2 Platform Enterprise Edition (JAVA 2 Platform Enterprise Edition, J2EE), which creates server applications and services.
J2EE is an architecture using Java 2 platform to simplify the development, deployment, and management of business solutions. The basis of J2EE technology is the standard version of the core Java platform or Java 2 platform. J2EE not only consolidates many of the advantages in the standard version, such as "writing once, run everywhere", which is easy to access the JDBC API, CORBA technology, and capable of accessing the database. Safety mode for protecting data in the Internet application, etc., and also provides comprehensive support for EJB (Enterprise JavaBeans, Java Server Pages), and XML technology. The ultimate goal is to be an architecture that enables enterprise developers to significantly shorten the market time.
The J2 architecture provides an intermediate layer integration framework to meet the needs of applications that do not require too much cost and require high availability, high reliability, and scalability. By providing a unified development platform, J2EE reduces the cost and complexity of developing multi-layer applications, while providing strong support for existing applications, fully supporting Enterprise JavaBeans, with good guidance support packaging and deployment applications, add directory support , Enhance the safety mechanism and improve performance.
II. J2EE's advantage
J2EE provides a good mechanism for building a scalability, flexibility, and easy maintenance business system: 1. Retain existing IT assets: Since companies must adapt to new business needs, use investment in existing enterprise information systems Not to re-establish a full plan. Thus, a server-side platform mechanism established on an existing system in a progressive (rather than radical, all) is the company's needs. The J2EE architecture makes full use of the original investment, such as the BEA Tuxedo, IBM CICS, IBM Encina, Inprise Visibroker, and Netscape Application Server used by some companies. This has become possible because J2EE has a wide range of industry support and some important 'enterprise calculation' domain suppliers' participation. Every vendor provides an existing customer's upgrade route for existing investments and enters the portable J2EE field. Since products based on J2EE platforms can almost operate on any operating system and hardware configuration, existing operating systems and hardware can be reserved. 2. Efficient development: J2EE allows companies to hand over some general, cumbersome server tasks to middleware suppliers. This developers can concentrate on how to create business logic, reduce development time accordingly. Advanced Middleware Suppliers offer the following complex middleware services: O Status Management Services - Let developers write less code, do not care how to manage status, this can complete the program development faster. O Sustained Services - Let developers can write applications without coding data access logic, can generate more lightweight, unrelated to the database, which is easier to develop and maintain. O Distributed Shared Data Objects Cache Services - Let developers prepare high-performance systems, greatly improve overall deployment of scalability. 3. Support heterogeneous environments: J2EE can develop portable procedures deployed in heterogeneous environments. J2EE-based applications do not rely on any particular operating system, middleware, hardware. Therefore, a reasonable J2EE-based program is designed to be deployed to a variety of platforms. This is critical in a typical heterogeneous enterprise computing environment. The J2EE standard also allows customers to order off-the-shelf components compatible with J2EE, deploy them into the heterogeneous environment, saving the costs required by themselves. 4. Scalability: Enterprises must choose a server-side platform that meets excellent scalability to meet a large number of new customers who have commercial operations on their system. Applications based on J2EE platforms can be deployed on a variety of operating systems. For example, it can be deployed to high-end UNIX and mainframe systems, which can support 64 to 256 processors. (This is the supplier of the NT server) J2EE field provides a wider load balancing strategy. Can eliminate bottlenecks in the system to allow multiple servers integrated deployment. This deployment can reach thousands of processors, achieving highly scalable systems to meet future business applications. Stable availability: A server-side platform must be able to operate all-day to meet the needs of company customers, partners. Because Internet is global, there is everywhere, even if it is downtown at night, it may cause serious loss. If it is unexpected, there will be catastrophic consequences. J2EE deploys to a reliable operating environment that they support long-term availability. Some J2EE deployed in a Windows environment, customers can also choose a robust performance, such as Sun Solaris, IBM OS / 390. The most robust operating system can reach 99.999% availability or only 5 minutes a year. This is an ideal choice for real-time business systems.
Three. J2EE's four-layer model
J2EE uses multi-layered distributed application models, application logic is divided into components, and each application component is distributed in different machines based on their layers. In fact, the original intention of Sun Design J2EE is to solve the drawback of two layers (Client / Server), in traditional mode, the client acts as a large role and looks bloated, in this mode, the first deployment It is relatively easy, but it is difficult to upgrade or improve, stretchability is not ideal, and often based on some proprietary protocol - usually a database protocol. It makes it difficult to reuse business logic and interface logic. Now J2EE's multi-layer enterprise application model cuts different levels in the two-layer model into many layers. A multi-layer application can provide a separate layer for each of the services, the following is a Typical four-layer structure of J2EE: the client layer component running on the client machine is running on the J2EE server WEB component running in J2EE server The business logic layer component runs in the Enterprise Information System (Enterprise Information System) Software on the EIS server
The J2EE application component J2EE application is composed of components. J2ee components are software units with independent features, which are assembled into J2EE applications through related classes and files and interact with other components. The following J2EE components are defined in the J2EE manual:
Application client programs and applets are customer layer components. Java servlet and JavaServer Pages (JSP) are web layer components. Enterprise JavaBeans (EJB) is a business layer component.
The client layer component J2EE application can be a web-based, or may be based on a traditional manner.
The Web layer component J2EE Web layer component can be a JSP page or servlets. Follow the J2EE specification, static HTML pages and Applets are not a web layer component.
As the client layer shown below, the web layer may contain some JavaBean objects to handle user input, and send input to Enterprise Beans running on the business layer.
The logic of the business layer component business layer code is used to meet the needs of special business sectors such as banks, retail, and finance. The following image shows how an Enterprise bean receives data from the client program. For processing (if necessary), and send it to the EIS layer, this process can also be reverse.
There are three enterprise-class beans: Session Beans, entity (Entity) Beans, and messaging-driven) Beans. Session beans represent temporary interactions with client programs. When the client program is executed, the session bean And related data will disappear. In contrast, entity bean represents a permanent record in the table of the database. When the client program is aborted or the server is closed, there will be potential service to ensure that the entity bean's data is saved. Message driver bean combines The characteristics of the message listener of the session bean and JMS allow a business layer component to receive JMS messages asynchronously.
Enterprise Information Systems Enterprise Information System Layer Processing Enterprise Information System Software includes enterprise infrastructure systems such as enterprise resource programs (ERP), large machine transaction, database systems, and other legacy information systems. For example, J2EE application components may be for database connections Need to access the enterprise information system
Four. J2EE's structure
This component-free J2EE structure has a platform-independent J2EE structure that makes the J2EE program are very simple, as the service logic is encapsulated into a reuse component, and the J2EE server provides background service in the form of a container. Because you Don't develop this service yourself, so you can concentrate on solving your business problems.
The container and service container settings are customized for the J2EE server, including security, transaction management, Java Naming and Directory Interface Addressing, Remote Connections, etc. The most important services are listed below: J2EE Security ( The security) model allows you to configure web components or Enterprise Beans, so that only authorized users can access system resources. Each customer belongs to a special role, while each role only allows an activation of a particular method. You should declare roles and activation methods in the Arrangement description of Enterprise Beans. Due to this declarative approach, you don't have to write rules that strengthen security. J2EE Transaction Management Model allows you to specify a relationship between all methods in a transaction, such a transaction is used as a single unit. When the client activates the method in an Enterprise Bean, the container is involved in one management Transaction. Due to container management transactions, you don't have to encode the boundaries of your transaction in Enterprise Beans. The code that requires controlling distributed transactions will be very complicated. You only need to declare the transaction attribute of Enterprise Bean in the arrangement description file without writing and debugging complex code. The container will read this file and handle this Enterprise Bean's transaction. JNDI Addressing provides a unified interface to multiple names and directory services in the enterprise, so application components can access names and directory services. J2EE Remote Client Connectivity Model Management Client and Enterprise Bean Low-level interaction. When an Enterprise bean is created, a client can call it. It is like it and the client is on the same virtual machine. Life cycle management model manages Enterprise Bean creation and removal An Enterprise Bean will have several states in its survival cycle. The container creates Enterprise Beans and moves him in the available instance pool and the active state, and eventually removes it from the container. Even if you can call Enterprise Bean's Create and REMOVE methods, the container will also perform these tasks in the background. Database Connection Pooling model is a valuable resource. Getting a database connection is a time consuming, and the number of connections is very limited. The container is alleviated by managing the connection pool. Enterprise Beans can quickly get connected from the pool. The connection is used for other beans in the bean release. Container Type J2EE Applications can be installed to the following containers:
EJB container manages all of the execution of enterprise-class beans in the J2EE application. Enterprise beans and their containers are running on the J2EE server. Web container manages the execution of the JSP page and Servlet components in the J2EE application. Web components and their containers run On the J2EE server. Application client container manages execution of application client components in the J2EE application. The application client and their container are running on the J2EE server. Applet container is a web browser running on the client machine Combination of Java plug-ins..
5. J2EE's core API and components
The J2EE platform consists of a single service (SERVICES), the application interface (APIS), and protocol, which provides functional support for the development of web-based multi-layer applications, which is a simple description of 13 technical specifications in J2EE (limited to Here, only simple descriptions are possible): 1. JDBC API provides a unified way for accessing different databases, like ODBC, JDBC blocks some detail issues to developers, JDCB access to the database is also a platform-independent. 2. JNDI (Java Name and Directory Interface): JNDI API is used to perform names and directory services. It provides a consistent model to access and operate enterprise resources such as DNS and LDAP, local file systems, or objects in the application server. 3. EJB (Enterprise Javabean): J2EE technology Win one thing is one of the reasons why some of the body is EJB. They provide a framework to develop and implement distributed business logic, which significantly simplifies the development of enterprise applications with scalability and highly complex. The EJB specification defines how EJB components interact with their containers. The container is responsible for providing public services, such as directory services, transaction management, security, resource buffer pools, and fault tolerance. But here is worth noting that EJB is not the only way to implement J2EE. It is because of the openness of J2EE, so that some vendors can achieve the same purpose in a way and EJB. 4. RMI (Remote Method Invoke): As indicated by its name, the RMI protocol calls the method on the remote object. It uses a serialized manner to pass data on the client and server. RMI is a more underlying protocol used by EJB. 5. Java IDL / CORBA: With the support of Java IDL, developers can integrate Java and Corba. They can create Java objects and make it expanded in Corba ORB, or they can also create Java classes and customers who are expanded with other ORBs. The latter approach provides another way, through which Java can be used to integrate your new application and old system. 6. JSP (Java Server Pages): The JSP page is composed of HTML code and the Java code embedded in it. The server is processed after the page is requested by the client, and then returns the generated HTML page to the client's browser. 7. Java servlet: servlet is a small Java program that extends the functionality of the web server. As a server-side application, starting when the request is performed, which is similar to the CGI Perl script. Most of the functionality provided by servlet is similar to JSP, but the implementation is different. JSP is usually embedded in a small amount of Java code in most HTML code, while servlets are written by Java and generate HTML. 8. XML (Extensible Markup Language): XML is a language that can be used to define other tag language. It is used to share data in different business processes. The development of XML and Java are independent of each other, but it is the same objective as Java is platform independence. By combining Java and XML, you can get a perfect platform-independent solution. 9. JMS (Java Message Service): MS is an application interface (API) for communicating with a message-oriented middleware. It supports the domain of point-to-point, with domains that support publish / subscribe / subside types, and provide support for the following types: approved messaging, transaction messages, consistent messages, and subscriptions with persistence Support.
