Liu Zhan (JERU@163.net) Bachelor of Information and Computational Science
This paper has been more comprehensive in J2EE from five aspects. From the concept of J2EE, to its advantage, to the J2EE typical four-layer model, and its framework structure, the final is a profile for J2EE thirteen core technology. This document has made a more detailed introduction to the service, components, hierarchies, containers and APIs in J2EE. I believe that readers will have a clearer understanding of J2EE.
I. Concept of J2EE
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 creating server applications and services, Java2 Platform Enterprise Edition, J2EE).
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 become a significant shorteness of enterprise developers
Architecture.
The J2EE architecture provides an intermediate-level integrated 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 commercial system with scalability, flexibility, and easy maintenance:
Retaining existing IT assets: Since companies must adapt to new business needs, using existing corporate information systems investments, rather than re-establishing a total plan is important. 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 are almost operated on any operating system and hardware configuration, existing operating systems and hardware are also
Can be reserved.
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 intermediate suppliers offer the following complex middleware services:
Status Management Services - Let developers write less code, do not care about how to manage status, this can complete the program development faster. 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.
Distributed Shared Data Objects Cache Services - Let developers prepare high-performance systems, greatly improve overall deployment
Portability.
Support heterogeneous environments: J2EE can develop portable procedures deployed in heterogeneous environments. J2EE-based application is not
Rely on any particular operating system, middleware, hardware. Therefore, the design-based J2EE-based procedure is only developed once.
You can deploy a variety of platforms. This is critical in a typical heterogeneous enterprise computing environment. J2EE standard is also allowed
Customer ordering the ready-made components of J2EE-compatible third-party, deploying them into the heterogeneous environment, saving by themselves
The cost required for the entire program is developed.
Scalability: Enterprises must choose a server-side platform that provides excellent scalability to meet a large number of new customers who make business 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 based on them.
The layers are located on different machines. In fact, the original intention of Sun Design J2EE is to solve two-story mode (CL
IENT / Server's drawbacks, in traditional mode, the client acts as too much role and looks bloated, in this model
In the first deployment, it is relatively easy, but it is difficult to upgrade or improve, and the stretchability is not ideal, and often
Based on some proprietary protocol - is usually a certain database protocol. It makes reuse business logic and interface logic very
difficult. Now J2EE's multi-layer enterprise application model cuts different levels in the two-layer model into many layers. One
Multi-layered applications provide a separate layer for each of the services, the following is a typical four-layer structure of J2EE:
Customer layer components running on the client machine
WEB layer components running on the J2EE server
Business logic layer components running on the J2EE server
Enterprise Information System (Enterprise Information System) Software running on the EIS server
J2EE application components
The J2EE application is composed of components. J2ee components are software units with independent features, they pass related classes.
And file assembled into J2EE applications 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.
Customer layer component
The 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 put
Enter the ENTERPRISE BEAN that is sent to the business layer to process.
Business layer components
The logic of business layer code is used to meet the needs of special business sectors such as banks, retail, and finance, by running on the business layer.
Enterprise bean is handled. The following figure shows how an Enterprise bean is from the client program.
Receive data, process (if necessary), and send it to the EIS layer, this process can also be reversed
Row.
There are three enterprise-class beans: session beans, entities (Entity) Beans, and Message Drivers (MES
Sage-Driven) Beans. Session bean means a temporary interaction with the client program. After the client program is executed
The session bean and related data will disappear. On the contrary, the entity bean represents a permanent record in the table of the database.
When the client program is aborted or when the server is turned off, there will be potential service to ensure that the data of the entity bean is saved.
. Message Driver BEAN combines the characteristics of the message listener of session bean and JMS, allowing a business layer component to be different
Step Receive JMS messages.
Enterprise information system layer
Enterprise Information System Layer Processing Enterprise Information System Software includes enterprise infrastructure systems such as enterprise resource programs (ERP)
, Large machine transaction processing, database system, and other legacy information systems. For example, J2EE application components may be
Database connection requires access to enterprise information systems
Four. J2EE's structure
This component-based, a platform-independent J2EE structure makes the J2EE program is very simple because
Logic is encapsulated into a reused component, and the J2EE server provides a background in the form of containers.
Service. Because you don't have to develop this service yourself, you can concentrate on solving your business problems.
Containers and services
The container settings customized the J2EE server, including security, transaction management, JNDI (Java Nami)
NG and Directory Interface Addressing, Remote Connection, etc. The following is the most important service:
J2EE Security (Security) model allows you to configure web components or Enterprise Beans, which is only authorized
Users can access system resources. Each customer belongs to a special role, and each role only allows activation of specific
Methods. You should declare roles and activation methods in the Arrangement description of Enterprise Beans. Due to this sound
Method, you don't have to write rules that strengthen security.
J2EE Transaction Management Model Let you specify all methods in a transaction
Relationship, all methods in such a transaction are regarded as a single unit. When the client activates an Enterprise
The method in the Se BEAN, the container involves a management transaction. Due to container management transactions, do not in Enterprise Bean
Must encode the boundary of the transaction. The code that requires controlling distributed transactions will be very complicated. You only need to describe
Declare the transaction attribute of Enterprise Bean without writing and debugging complex code. Container will read this article
Parts and handle this Enterprise Bean transaction.
JNDI Addressing Service provides a unified interface to multiple names and directory services in the enterprise.
The sample application program component can access the name and directory service.
J2EE Remote Connection (Remote Client Connectivity) model management client and Enterprise Bean
Low-level interaction. When an Enterprise Bean is created, a client can call it method like it and the customer.
Terminal is the same as the same virtual machine. Life cycle management model manages Enterprise Bean creation and removal, one
Enterprise Beans will have several states in its survival cycle. Container creates Enterprise Bean, and
You can move him in an instance pool with the active state, and finally remove 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. Get databases
The connection is a time consuming work, 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
The J2EE application component can be installed to the following containers:
EJB container manages the execution of enterprise-level beans in the J2EE application. Enterprise bean and their capacity
The operator runs on the J2EE server.
Web container manages all JSP pages and servlet components in the J2EE application. Web components and their capacity
The operator runs on the J2EE server.
Application Client Contract Management All J2EE applications are executed in the application client components. Application
The household and their containers are running on the J2EE server.
The AppleT container is a combination of web browsers and Java plugins running on the client machine.
5. J2EE's core API and components
The J2EE platform consists of a single service (SERVICES), the application interface (APIS), and protocol, which is based on the development base.
The multi-layer application of the Web provides functional support, and the following is a simple description of 13 technical specifications in J2EE (limited to
Convenient description here can only be described here):
JAVA DATABASE CONNECTIVITY:
JDBC API provides a unified way for accessing different databases, like ODBC, JDBC blocks developers
Some detail issues, in addition, JDCB access to the database has a platform-independent.
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.
EJB (Enterprise Javabean):
One of the reasons why J2EE technology wins some of the extensive attention is EJB. They provide a framework to develop and implement
Separate business logic, which significantly simplifies the development of enterprise-class 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
Yes, EJB is not the only way to implement J2EE. It is because of the opening of J2EE, making some manufacturers can
And the EJB parallel way to achieve the same purpose.
RMI (RemoteMETHOD Invoke):
As indicated by his name, the RMI protocol calls the remote object method. It uses a serialization mode in the client
And server-side delivery data. RMI is a more underlying protocol used by EJB.
Java IDL / CORBA:
With the support of Java IDL, developers can integrate Java and Corba. They can create java pairs
Icon and make it unfold in Corba ORB, or you can also create Java classes and expand as other orb
Corba objects of Corba objects. The latter approach provides another way, through which Java can be used to use your new
Application and old system integration.
JSP (Java Server Pages):
The JSP page is composed of HTML code and embedded Java code. The server is requested after the page is requested by the client.
These Java code processes, and then return the generated HTML page to the client's browser.
Java servlet:
Servlet is a small Java program that extends the functionality of the web server. As a server-side application
When the request is requested, this is similar to the CGI Perl script. Most of the features provided by servlets are similar to JSP.
However, it is different from the implementation. JSP is usually in most HTML code embedded in a small amount of Java code, while servlets
All is written by Java and generate HTML.
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
A combination of Java and XML, you can get a perfect platform-independent solution.
JMS (Java Message Service):
MS is an application interface (API) for communication with a message-oriented middleware. It supports the domain of point-to-point,
Support publish / subscribe (publish / subscribe) domain, and provide support for the following types: approved
Message delivery, transaction messaging, consistent messaging, and subscriber support for persistence. JMS also provides another
One way to integrate your application with the old background system.
JTA (Java Transaction Architecture):
JTA defines a standard API, which can be accessed by various transaction surveillance.
JTS (Java Transaction Service):
JTS is the basic implementation of CORBA OTS transaction monitoring. JTS specifies the implementation of the transaction manager. This transaction management
The device is in the high-level support Java Transaction API (JTA) specification, and implements omg OTS Specif at a lower layer.
The Java image of Ication. JTS Transaction Manager is an application server, resource manager, independent application, and communication
The resource manager provides a transaction service.
JavaMail:
JavaMail is an API for accessing a mail server, which provides an abstract class of a mail server. Not only support S
The MTP server also supports the IMAP server.
JTA (JavaBeans Activation Framework):
JavaMail uses JAF to handle MIME encoded mail attachments.
