Java's birth challenges the traditional computer models, computer software development and software industries have had a profound impact:
(1) software 4A objectives and requirements of the software can reach any person in any place at any time for any electronic device can be applied. This software platform can meet each other in operation, with the scalability and reusability of the distributed computing model can be plug and play needs.
(2) Construction of the development approach based on the rise, leads to the CORBA architecture and the international standard software framework for multi-application system. Based on this platform and formed Java.2. NET platform, the two major factions, and promote the entire IT industry.
(3) of the software industry and industrial enterprises have had a profound impact on software development from the center shifted to a service center. Intermediate providers, component providers, server software and consulting service providers appear. Businesses must reinvent themselves, B2B e-commerce will drive the new economy, markets, giving enterprises new value, new growth, new business opportunities, new management.
(4) software development has brought a new revolution, emphasis on integration with third-party components, using the platform of infrastructure services, achieving various stages of development of important technologies, attention to the development team, organizational and cultural ideas, collaboration, creativity, responsibility, Credibility is the basic quality of human resources.
In short, the current Java to see the importance of the information age, the future will continue to develop, Java applications will be in a more promising future. 1.1 Java language development 1.1.1 Java language generation
The early 90s of last century, Sun companies, in a smart device for future programming language, the language should have some new features, in order to avoid some deficiencies of C + +.
The language originally named Oak, Gosling from the Office of the language of the oak tree outside the window (Oak). Later in the conflict when it came up, so to think of hot coffee from the hands of a rich coffee to the islands of Indonesia, the Chinese name of Java, Java language name here.
With the rapid development of Internet, Web application is becoming more widespread, Java language has also been developing rapidly. In 1994, Gosling Java developers with a real-time high, reliable, safe, interactive features of the new Web browser, it does not depend on any hardware platform and software platform. The browser name is HotJava, and in 1995 together with the Java language, officially released to the industry, caused a great sensation, Java attendant position was reaffirmed. After moving very quickly.
Java programming language and syntax similar to C + + syntax, semantics and Small Talk TM is the semantic similarity. Java programming language can be any conventional programming language used to create applications that can be created. Java programming language, designed the main objectives are:
? provide an easy programming language, thereby eliminating the other languages such as pointer arithmetic and memory management of the robustness of the defects.
? use of object-oriented concepts to make the program truly become a fully object-oriented programming.
? To make the code clear and reasonable as possible, provide a concise and smooth way.
? For the following two points provide an interpretation of environmental benefits: Improve the development speed to eliminate compile ─ ─ ─ Links - load - test cycle; ─ ─ portability code so that the operating system runtime environment for system-level calls to do.
? to run more than one active thread of the program provides a way.
? module by allowing the download code to run when the program can also change the dynamic support programs.
? To ensure the safety of those who load the code module which provides a screening method.
Well-developed Java core technology for these goals to provide a guarantee, including the following main technologies:
? Java Virtual Machine
? Automatic garbage collection ? Security Code
1.1.2 Java language development
Java language development goals is not just a programming language, but also to build a development environment, an application environment, a deployment environment. As the most basic support for Java language, Sun released in 1996, Java Development Kit JDK 1.0 (JDK is the Java Develop Kit for short), including the need for Java developers of various utility programs (compilation, execution, documentation generators, etc.), basic class library (equivalent to C language library and C + + class library), instances and so on. In 1998, Sun has released an updated JDK 1.2, because the thinking in technology, many improvements with the previous, so the next Java technology is generally known as Java 2. Subsequently, the characteristics of different areas, Java technology is divided into three different platforms (the latest title has removed the ambiguous meaning of 2, such as JavaSE):
? J2SE - Java Platform Standard
J2SE is the standard version of Java language, refers to the JDK (1.2 and later versions), including the Java Foundation Class Library and grammar. It is used to develop rich GUI (graphical user interface), complex logic and high-performance desktop applications. ? J2EE - Enterprise Java platform
Built on top of J2EE J2SE for the development and implementation of enterprise applications. It is a standard multi-tier architecture, enterprise applications can be divided into client layer, presentation layer, business layer and data layer, mainly for developing and deploying distributed, component-based, secure, reliable, scalable and easy management of enterprise-class applications.
? J2ME - Embedded Java technology platform
J2ME is built on top of J2SE, mainly for the development of a limited connection, memory, and user interface capabilities of the device application. Such as mobile phones (mobile phones), PDA (e), to access the set-top box or cable service terminal and other consumer
electronic products.
Any language to establish the types of applications are more or less running the application related to the environment, and Java language can create the following two general procedures:
? Applications
Applications is a stand-alone program, it is a typical general procedure, which can run on any Java runtime environment with the device.
? Applets
Applets are stored in a WWW server with the Java programming language program, it is usually downloaded by the browser to the client system, and run through a browser. Applets are usually small to reduce download time, which consists of the Hypertext Markup Language (HTML) for Web pages to call.
Java runtime environment has some special, or a lot of people to establish a special runtime environment, Java programming is often the establishment of various components, they can be run in a specific environment, such as Servlet, JavaBean, JSP and so on.
In the Java technology system, there are many free or free third-party Java components, they often provide a solution to a particular aspect can be used in many projects in the development process.
Learning needs to know the Java language Java technology system, starting from the local details of the study, carried out from the overall direction of awareness and choice, from the application to consolidate to improve. 1.2 Java Language Features
Java language for Internet environment, is a widely used web programming language, it has the following features:
? Simple
Java language and C + + syntax rules are similar, but canceled the Java language and multiple inheritance pointer, unified by reference to indicate the object (C + + in two forms, is actually two ways to generate objects, and Java, there is only one) by automatic garbage collection eliminates the need for programmers working for the release of the memory block.
? object-oriented (nearly complete)
Java language in order to improve efficiency, defined a few basic data types to achieve a non-class manner, and the remaining class of all data types in the form of packaging, the program structure of the system unit is also a class. Therefore considered to be almost completely object-oriented.
? platform independent (portable, cross-platform)
Java Virtual Machine (JVM) is true in a variety of machine architectures implemented using software to simulate an imaginary machine, when necessary, can be implemented by hardware.
Of course, the internal implementation of these virtual machines vary, but its function is the same - the implementation of a unified Java virtual machine instructions.
Java compiler Java application's source code files (. Java) translated into Java byte code file (. Class), which is composed of Java virtual machine instructions. Because it is a
virtual machine, and thus the implementation of Java Virtual Machine Java programs commonly referred to as the process of interpretation.
Relies on virtual machine technology, Java language has nothing to do with the characteristics of the machine architecture, that Java programs, once after being written, can be transplanted without modification to any one on a different machine architecture. From the operating system point of view, the process of implementation of a Java program is the process of implementation of a Java virtual machine process.
? oriented network programming
Java language for generating the beginning of the network of support in the JDK includes TCP / IP, HTTP and FTP protocols such as class libraries.
? Multi-threading support
Multithreading is to simultaneously perform multiple tasks of a function. Multi-threading mechanism that enables applications to perform multiple tasks in parallel, the various thread synchronization mechanisms to ensure proper operation of the shared data. ? Good security code
Runtime (Runtime) term perspective emphasizes the dynamic process of the dynamic changes when the program is running, but also with the expression of the runtime environment similar to the meaning of the word.
Java technology is a lot of work done at run time, such as strengthening security code verification operations.
In general, Java technology runtime environment performs the following three major tasks: ? loading code - the implementation by the class loader
Class loader for the implementation of the procedures required to load all classes (as far as possible and not necessarily the same).
? validation code - the implementation of the byte code verifier
Java code before the actual operation after several tests. Byte code verifier for four times on the validation code, which can ensure code compliance and does not destroy the JVM specification system integrity. If - Check forged pointers, object access rights violation or attempt to change the object type of illegal code.
? code execution - from the implementation of the runtime interpreter ? Automatic garbage collection
Many programming languages allow programs running in the memory block dynamically allocated, the process of allocating memory blocks because of the language syntax varies, but always point to the store to return to the starting position of the pointer. In C, C + + and other languages, the programmer is responsible for deallocation of memory blocks. Sometimes this is a very difficult task. Because programmers do not always know in advance when a block of memory should be released. When not in the system can be allocated memory block, it can cause the program to a standstill, this procedure is called a memory leak.
When the allocated memory block is no longer needed, the program should be canceled or the operating environment to allocate memory block.
? garbage collection is no longer needed recovery of allocated memory block.
? in other languages in general, is the programmer's responsibility to cancel the allocation.
? Java programming language provides a system-level threads to keep track of memory allocation, to complete the garbage collection:
? no longer need to check and release the memory block ? the above work can be completed automatically
? in the JVM implementation cycle, resulting in unexpected changes ? good robustness of the code
Java program to check at compile and run-time errors. Type of examination can help detect many errors early in development. Meanwhile, many integrated development tools (IDE) to compile and run the emergence of Java programs easier and a lot of integrated development tools (eg Eclipse) are free.
1.3 Object-oriented and process-oriented differences
The production of a programming language, programming is not only a technical improvement, but also includes the expression of ideas and cognitive progress.
The C language as the representative of the early part of the language, known as process-oriented language, not only because of their expression of program design is based on the basic elements of the process, in essence, is now more understanding of the computerized system the dominant ideology or control flow or data flow, constitute elements of the system is the module - processing logic.
The generation of object-oriented languages, because the understanding of the system or the abstract to a more advanced level. Cognitive thinking at this time not only closer to the real world, and its high degree of abstraction. Thus, both easy to understand on the one hand, there are also difficult to understand the other hand, to see the realm of understanding.
1.3.1 Review process-oriented thinking
Process-oriented programming language, thought and expression, can be simply summarized as the following few:
Understanding of the system structure analysis of the process - top-down, stepwise refinement to the functional decomposition;
Each function is a data processing module, which fully describe the IPO described the internal and external features of the module;
Each module is control flow, and therefore have a typical structure of the three procedures - sequence, branch, loop;
The relationship between the module by calling the preserve, which has become a function module, which is the most important process-oriented way of code reuse under way;
Although the function call parameters and return values, but the data and functions are always separate, not only with the famous assertion - program = algorithm + data, but also makes the absolute equality of the disorder have trouble - all the functions and data relationship is no limit;
If you want to use the data flow, then the data can only be produced in an appropriate time after the call function.
The main drawback of this period are: understanding the system more difficult, the systematic development of low efficiency, code reusability is poor.