Java IO 系统的介绍

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JAVA I/O 系统
对编程语言的设计者来说,创建一套好的输入输出(I/O)系统,是一项难度极高的任务。
这一点可以从解决方案的数量之多上看出端倪。这个问题难就难在它要面对的可能性太多了。不仅是因为有那么多I/O的源和目地(文件,控制台,网络连接等等),而且还有很多方法(顺序的『sequential』,随机的『random-SQLServer2000』,缓存的『buffered』,二进制的『binary』,字符方式的『character』,行的『by lines』,字的『by words』,等等)。
Java类库的设计者们用"创建很多类"的办法来解决这个问题。坦率地说Java I/O系统的类实在是太多了,以至于初看起来会把人吓着(但是,具有讽刺意味的是,这种设计实际上是限制了类的爆炸性增长)。此外,Java在1.0版之后又对其I/O类库作了重大的修改,原先是面向byte的,现在又补充了面向Unicode字符的类库。为了提高性能,完善功能,JDK 1.4又加了一个nio(意思是"new I/O"。这个名字会用上很多年)。这么以来,如果你想对JAVA的I/O类库有个全面了解,并且做到运用自如,你就得先学习大量的类。此外,了解I/O类库的演化的历史也是相当重要的。可能你的第一反应是"别拿什么历史来烦我了,告诉我怎么用就可以了!"但问题是,如果你对这段历史一无所知,很快就会被一些有用或是没用的类给搞糊涂了。
本章会介绍JAVA标准类库中的各种I/O类,及其使用方法。
File 类
在介绍直接从流里读写数据的类之前,我们先介绍一下处理文件和目录的类。
File类有一个极具欺骗性的名字;或许你会认为这是一个关于文件的类,但它不是。你可以用它来表示某个文件的名字,也可以用它来表示目录里一组文件的名字。如果它表示的是一组文件,那么你还可以用list( )方法来进行查询,让它会返回String数组。由于元素数量是固定的,因此数组会比容器更好一些。如果你想要获取另一个目录的清单,再建一个File对象就是了。实际上,叫它 "FilePath"可能会更好一些。下面我们举例说明怎样使用这个类及其相关的FilenameFilter接口。
目录列表器
假设你想看看这个目录。有两个办法。一是不带参数调用list( )。它返回的是File对象所含内容的完整清单。但是,如果你要的是一个"限制性列表(restricted list)"的话 —— 比方说,你想看看所有扩展名为.JAVA的文件 —— 那么你就得使用"目录过滤器"了。这是一个专门负责挑选显示File对象的内容的类。
DirFilter实现了FilenameFilter接口。我们来看看FilenameFilter究竟有多简单:
public interface FilenameFilter {
           boolean accept(File dir, String name);
}
也就是说,这类对象的任务就是提供一个accept( )的方法。之所以要创建这个类,就是要给list( )提供一个accept( )方法,这样当list( )判断该返回哪些文件名的时候,能够"回过头来调用"accept( )方法。因此,这种结构通常被称为回调(callback)。更准确地说,由于list( )实现了基本功能,而FilenameFilter提供了"对外服务所需的算法",因此这是一种"策略模式(Strategy Pattern)"。由于list( )拿FilenameFilter对象当参数,因此你可以将任何实现FilenameFilter接口的对象传给它,并以此(甚至是在运行时)控制list( )的工作方式。回调能提高程序的灵活性。
DirFilter还告诉我们,interface只是包含了一些方法,它没说你只能写这些方法。(但是,你至少要定义接口里有的方法。) 这里我们还定义了DirFilter的构造函数。
accept( )方法需要两个参数,一个是File对象,表示这个文件是在哪个目录里面的;另一个是String,表示文件名。虽然你可以忽略它们中的一个,甚至两个都不管,但是你大概总得用一下文件名吧。记住,list( )会对目录里的每个文件调用accept( ),并以此判断是不是把它包括到返回值里;这个判断依据就是accept( )的返回值。
切记,文件名里不能有路径信息。为此你只要用一个String对象来创建File对象,然后再调用这个File对象的getName( )就可以了。它会帮你剥离路径信息(以一种平台无关的方式)。然后再在accept( )里面用正则表达式(regular expression)的matcher对象判断,regex是否与文件名相匹配。兜完这个圈子,list( )方法返回了一个数组。

The JAVA I/O System
Creating a good input/output (I/O) system is one of the more difficult tasks for the language designer.
This is evidenced by the number of different approaches. The challenge seems to be in covering all eventualities. Not only are there different sources and sinks of I/O that you want to communicate with (files, the console, network connections, etc.), but you need to talk to them in a wide variety of ways (sequential, random-SQLServer2000, buffered, binary, character, by lines, by words, etc.).
The Java library designers attacked this problem by creating lots of classes. In fact, there are so many classes for Java’s I/O system that it can be intimidating at first (ironically, the Java I/O design actually prevents an explosion of classes). There was also a significant change in the I/O library after Java 1.0, when the original byte-oriented library was supplemented with char-oriented, Unicode-based I/O classes. In JDK 1.4, the nio classes (for “new I/O,” a name we’ll still be using years from now) were added for improved performance and functionality. As a result, there are a fair number of classes to learn before you understand enough of JAVA’s I/O picture that you can use it properly. In addition, it’s rather important to understand the evolution history of the I/O library, even if your first reaction is “don’t bother me with history, just show me how to use it!” The problem is that without the historical perspective, you will rapidly become confused with some of the classes and when you should and shouldn’t use them.
This chapter will give you an introduction to the variety of I/O classes in the standard JAVA library and how to use them.
The File class
Before getting into the classes that actually read and write data to streams, we’ll look at a utility provided with the library to assist you in handling file directory issues.
The File class has a deceiving name; you might think it refers to a file, but it doesn’t. It can represent either the name of a particular file or the names of a set of files in a directory. If it’s a set of files, you can ask for that set using the list( ) method, which returns an array of String. It makes sense to return an array rather than one of the flexible container classes, because the number of elements is fixed, and if you want a different directory listing, you just create a different File object. In fact, “FilePath” would have been a better name for the class. This section shows an example of the use of this class, including the associated FilenameFilter interface.
A directory lister
Suppose you’d like to see a directory listing. The File object can be listed in two ways. If you call list( ) with no arguments, you’ll get the full list that the File object contains. However, if you want a restricted list—for example, if you want all of the files with an extension of .JAVA—then you use a “directory filter,” which is a class that tells how to select the File objects for display.
The DirFilter class “implements” the interface FilenameFilter. It’s useful to see how simple the FilenameFilter interface is:
public interface FilenameFilter {
  boolean accept(File dir, String name);
}