TR-Java-lecture01(JAVA语言介绍).ppt

JavaJava程序设计程序设计 Java Programming Spring, 2009 1成都信息工程学院成都信息工程学院 计算机系计算机系 Course Information nInstructor: lName: Tang, Rong(汤蓉) lTel:lE-mail: lMy Webpage: 教师主页 汤蓉 /Js/PFP/S7L/Default.htm 2 2 Department of Computer Science lLab：12 学时; lExperiment：6学时. 先修课程包括:数据结构、面向对象程序设计等 3 3 Department of Computer Science lFriday, Section 5 and do not leave your PC unattended while you are logged in any time it instead contains bytecodes the machine language of the Java Virtual Machine (JVM). lThe Java launcher tool then runs your application with an instance of the Java Virtual Machine. 2323 Department of Computer Science & TechnologyDepartment of Computer Science & Technology Java Virtual Machine (JVM) nan abstract computing machine nuses software to simulate a real computer nLike a real computing machine, it has an instruction set and manipulates various memory areas at run time ninterprets Java byte code into machine code Write Once, Run Anywhere! 2424 Department of Computer Science & TechnologyDepartment of Computer Science & Technology What is the JVM nA software simulation of a hypothetical computing machine that runs Java bytecodes (Java compiler output). nThe Java Virtual Machine allows Java code to be portable between various hardware and OS platforms. nJava虚拟机（JVM）- Java Virtual Machine，它是通过在实 际的计算机上用软件模拟各种计算机功能模块虚构出来的 一个计算机。Java虚拟机由5个部分组成：一组指令集、一 组寄存器、一个堆栈、一个无用垃圾内存单元收集器和一 个方法区域。 2525 Department of Computer Science & TechnologyDepartment of Computer Science & Technology The Java Virtual Machine 2626 Department of Computer Science & TechnologyDepartment of Computer Science & Technology JAVA 虚拟机 - JVM n为什么使用Java虚拟机？ Java语言最重要的特点就是可以在任何操作系统中运行 - 跨平台特性。 Java编译器生成与平台无关的字节码，作为虚拟机的输入，Java解释器得 到字节码后，对其进行解释，使之能在不同的平台下运行。 不同的操作系统有不同的虚拟机。 nJava虚拟机的基本原理 Java虚拟机屏蔽了与具体操作系统平台相关的信息，使得Java语言 编译程序只需生成在Java虚拟机上运行的目标代码（字节码），就 可以在多种平台上不加修改地运行。 Java虚拟机在执行字节码时，实际上最终还是把字节码解释成具体 平台上的机器指令执行。 2727 Department of Computer Science & TechnologyDepartment of Computer Science & Technology Translation of Java to machine code 2828 Department of Computer Science & TechnologyDepartment of Computer Science & Technology Translation of Java to machine code nA two step process Step 1: Compile source code to Java byte code. lByte code is a standard intermediate-level language. lInput: Java code in one or more files ending in .java lOutput: Byte code in one or more files ending in .class Step 2: Interpret Java byte code on a particular machine lThe interpreter converts each byte code statement to the specific type of machine code and then runs that machine code. 2929 Department of Computer Science & TechnologyDepartment of Computer Science & Technology Some Features of Java nPortable/platform independent nThe Java Virtual Machine nAutomatic Garbage Collection nNo pointers or pointer arithmetic nMultiple inheritance replaced by interfaces, eliminates complexities of multiple inheritance 3030 Department of Computer Science & TechnologyDepartment of Computer Science & Technology 可移植性-Portable/platform independent n程序的可移植性：指程序不经过修改而在不同的硬 件或软件平台上运行的特性。 n可移植性包括二个层次：源代码级和二进制级的可 移植性。 C、C+仅有一定的源代码级的可移植性。 Java编译器生成与平台无关的字节码，只要安装了Java 运行系统，其程序就可以在任意的处理器上运行 3131 Department of Computer Science & TechnologyDepartment of Computer Science & Technology Types of Java Programs nApplication Standalone Java program that can run independent of any Web browser nApplet Java program that runs within a Java-enabled Web browser nServlet Java software