在日常的java开发中少不了文件的读取和 写入，这就涉及到文件的I/O操作，今天就来总结下文件的IO操作，顺便文件的IO操作也需要File了的帮助，所以一起总结了。
以下图片为我根据其他博客所总结的内容进行了相应的总结和IO流的类结构图，类结构图中还少了几个类
简单描述下：
IO分为字节和字符流2中方式，字节流以byte为单位，字符流以字符为单位，1个字节8byte 0·255，字节流的抽象类为inputstream和outputstream，他们无法实例化，所以需要
其子类来实现。字符流通常按2个字节来表示则为16byte，字符流的抽象类为reader和writer。
字符流通常是需要将文件读取到内存才能进行操作的，而字节流则是直接操作文件的。
还是通过具体的例子来说明吧。
@Test
public void testFileReader() throws IOException {
File file = new File("D:/train/train.txt");
FileInputStream fs = new FileInputStream(file);
InputStreamReader reader = new InputStreamReader(fs,"utf8");
BufferedReader bufferedReader = new BufferedReader(reader);
System.out.println(bufferedReader.readLine());
// 关闭文件流
bufferedReader.flush(); bufferedReader.close();
reader.close();
fs.close();
} @Test
public void testFileOut() throws IOException {
File file = new File("D:/train/train.txt");
FileOutputStream out = new FileOutputStream(file);
String str = "hello world";
out.write(str.getBytes("utf8"));
out.close();
}
上面的代码包含了文件的读取和文件的写入，包含字符集设置，读取操作其实可以包含了IO中绝大多数的操作了。需要注意的是不管是字节流，还是字符流他们的read（）方法返回的结果都是int类型，需要将其转换成char类型。具体的源码可以根据类的结构树去查看。
一上为简单总结了IO流，下面是File类的总结：
File -> FileSystem -> WinNTFileSystem
操作文件其实通过操作系统的文件功能来实现的，不同的系统如windows和UNIX其实是不同的，新建的File通过getFileSystem（）方法获取系统操作文件的权限，每个新建的文件都包含2个重要参数path和prefixLength
/**
* This abstract pathname's normalized pathname string. A normalized
* pathname string uses the default name-separator character and does not
* contain any duplicate or redundant separators.
* 这个抽象路径的正常路径名称，"d:/program file/train"这种格式
* @serial
*/
private final String path; /**
* The length of this abstract pathname's prefix, or zero if it has no
* prefix. 抽象路径的前缀长度，不存在则为0
*/
private final transient int prefixLength;
文件系统的操作其实很多都是通过这个前缀来创建文件的。
通过看源码可以发现，File类其实有点像接口，而WinNTFileSystem类则像实现接口。
看下File的构造函数，其中通过uri的构造函数没有贴出，有需要可以自己去查阅
/**
* Creates a new <code>File</code> instance by converting the given
* pathname string into an abstract pathname. If the given string is
* the empty string, then the result is the empty abstract pathname.
*
* @param pathname A pathname string
* @throws NullPointerException
* If the <code>pathname</code> argument is <code>null</code> 最通用给构造函数，通过文件路径来创建文件
*/
public File(String pathname) {
if (pathname == null) {
throw new NullPointerException();
}
this.path = fs.normalize(pathname);
this.prefixLength = fs.prefixLength(this.path);
} /**
* Creates a new <code>File</code> instance from a parent pathname string
* and a child pathname string.
*
* <p> If <code>parent</code> is <code>null</code> then the new
* <code>File</code> instance is created as if by invoking the
* single-argument <code>File</code> constructor on the given
* <code>child</code> pathname string.
*
* <p> Otherwise the <code>parent</code> pathname string is taken to denote
* a directory, and the <code>child</code> pathname string is taken to
* denote either a directory or a file. If the <code>child</code> pathname
* string is absolute then it is converted into a relative pathname in a
* system-dependent way. If <code>parent</code> is the empty string then
* the new <code>File</code> instance is created by converting
* <code>child</code> into an abstract pathname and resolving the result
* against a system-dependent default directory. Otherwise each pathname
* string is converted into an abstract pathname and the child abstract
* pathname is resolved against the parent.
* 通过父路径和子路径来创建文件
* @param parent The parent pathname string
* @param child The child pathname string
* @throws NullPointerException
* If <code>child</code> is <code>null</code>
*/
public File(String parent, String child) {
if (child == null) {
throw new NullPointerException();
}
if (parent != null) {
if (parent.equals("")) {
this.path = fs.resolve(fs.getDefaultParent(), -->"/"
fs.normalize(child));
} else {
this.path = fs.resolve(fs.normalize(parent),
fs.normalize(child));
}
} else {
this.path = fs.normalize(child);
}
this.prefixLength = fs.prefixLength(this.path);
} /** 通过父文件和子路径来创建文件
* Creates a new <code>File</code> instance from a parent abstract
* pathname and a child pathname string.
*
* <p> If <code>parent</code> is <code>null</code> then the new
* <code>File</code> instance is created as if by invoking the
* single-argument <code>File</code> constructor on the given
* <code>child</code> pathname string.
*
* <p> Otherwise the <code>parent</code> abstract pathname is taken to
* denote a directory, and the <code>child</code> pathname string is taken
* to denote either a directory or a file. If the <code>child</code>
* pathname string is absolute then it is converted into a relative
* pathname in a system-dependent way. If <code>parent</code> is the empty
* abstract pathname then the new <code>File</code> instance is created by
* converting <code>child</code> into an abstract pathname and resolving
* the result against a system-dependent default directory. Otherwise each
* pathname string is converted into an abstract pathname and the child
* abstract pathname is resolved against the parent.
*
* @param parent The parent abstract pathname
* @param child The child pathname string
* @throws NullPointerException
* If <code>child</code> is <code>null</code>
*/
public File(File parent, String child) {
if (child == null) {
throw new NullPointerException();
}
if (parent != null) {
if (parent.path.equals("")) {
this.path = fs.resolve(fs.getDefaultParent(),
fs.normalize(child));
} else {
this.path = fs.resolve(parent.path, --> 将父文件转换成路径，然后和子路径拼接
fs.normalize(child));
}
} else {
this.path = fs.normalize(child);
}
this.prefixLength = fs.prefixLength(this.path);
}
其他的一些方法如创建新文件，creatNewFile(),创建文件夹mkdir(),mkdirs()等方法，相信看下源码就能很清楚了。