Java File相关源码分析：File、FileInputStream、InputStreamReader

该源码适用于windows环境下

最近做了个小系统，用到了File相关的类，如下：

<span style="font-size:14px;">InputStreamReader in = new InputStreamReader(new FileInputStream(new File(UrlUtil.getRootUrl() + "config/mango.properties")), "GBK");</span>

其中，getRootUrl定义如下：

<span style="font-size:14px;">        /*** @return 获取根目录 其实是classpath*/public static String getRootUrl() {URL rootUrl = UrlUtil.class.getClassLoader().getResource("");// 针对打包成jar包后获取rootUrl为null的问题if (null == rootUrl) {return "";}String rootUrlS = rootUrl.getPath();if (rootUrlS.contains("/bin/")) {return rootUrlS.substring(0, rootUrlS.length() - 4);}return rootUrlS;}</span>

文件结构分布如下：

从new File（UrlUtil.getRootUrl() + "config/mango.properties"）开始分析

一、File

An abstract representation of file and directory pathnames. 该类的对象可以代表一个具体的文件或文件夹

<span style="font-size:14px;">    /*** 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>*/public File(String pathname) {if (pathname == null) {throw new NullPointerException();}this.path = fs.normalize(pathname);               //将文件路径转换为正常路径this.prefixLength = fs.prefixLength(this.path);   //正常路径下的前缀长度}</span>

1.1其中fs为文件系统，在windows下获取的是Win32FileSystem

<span style="font-size:14px;">    /*** The FileSystem object representing the platform's local file system.*/static private FileSystem fs = FileSystem.getFileSystem();</span>

1.2 fs.normalize(pathname)

<span style="font-size:14px;">    /* Check that the given pathname is normal.  If not, invoke the realnormalizer on the part of the pathname that requires normalization.This way we iterate through the whole pathname string only once. */public String normalize(String path) {int n = path.length();char slash = this.slash;char altSlash = this.altSlash;char prev = 0;for (int i = 0; i < n; i++) {char c = path.charAt(i);if (c == altSlash)return normalize(path, n, (prev == slash) ? i - 1 : i);if ((c == slash) && (prev == slash) && (i > 1))return normalize(path, n, i - 1);if ((c == ':') && (i > 1))return normalize(path, n, 0);prev = c;}if (prev == slash) return normalize(path, n, n - 1);return path;}</span>

<span style="font-size:14px;">    private final char slash;private final char altSlash;private final char semicolon;public Win32FileSystem() {slash = AccessController.doPrivileged(new GetPropertyAction("file.separator")).charAt(0);semicolon = AccessController.doPrivileged(new GetPropertyAction("path.separator")).charAt(0);altSlash = (this.slash == '\\') ? '/' : '\\';}</span>

转换前/E:/MyEclipse/workspace/FreightSystem/config/mango.properties

转换后E:\MyEclipse\workspace\FreightSystem\config\mango.properties

1.3 fs.prefixLength(this.path)

<span style="font-size:14px;">    public int prefixLength(String path) {char slash = this.slash;int n = path.length();if (n == 0) return 0;char c0 = path.charAt(0);char c1 = (n > 1) ? path.charAt(1) : 0;if (c0 == slash) {if (c1 == slash) return 2;  /* Absolute UNC pathname "\\\\foo" */return 1;                   /* Drive-relative "\\foo" */}if (isLetter(c0) && (c1 == ':')) {if ((n > 2) && (path.charAt(2) == slash))return 3;               /* Absolute local pathname "z:\\foo" */return 2;                   /* Directory-relative "z:foo" */}return 0;                       /* Completely relative */}</span>

关于前缀的解释，看完后就能理解上述代码了

<span style="font-size:14px;"><li> For UNIX platforms, the prefix of an absolute pathname is always* <code>"/"</code>.  Relative pathnames have no prefix.  The abstract pathname* denoting the root directory has the prefix <code>"/"</code> and an empty* name sequence.** <li> For Microsoft Windows platforms, the prefix of a pathname that contains a drive* specifier consists of the drive letter followed by <code>":"</code> and* possibly followed by <code>"\\"</code> if the pathname is absolute.  The* prefix of a UNC pathname is <code>"\\\\"</code>; the hostname and the share* name are the first two names in the name sequence.  A relative pathname that* does not specify a drive has no prefix.</span>

对于E:\MyEclipse\workspace\FreightSystem\config\mango.properties获取到的前缀长度为3，也即是E:\都为前缀

二、FileInputStream

A <code>FileInputStream</code> obtains input bytes from a file in a file system. What files are available depends on the host environment. <code>FileInputStream</code> is meant for reading streams of raw bytes such as image data. For reading streams of characters, consider using<code>FileReader</code>.

通过FileInputStream可以从文件中获得字节流，如果要获得字符流需要适用FileReader，下面会介绍InputStreamReader，用来获取字符流。

<span style="font-size:14px;">    /*** Creates a <code>FileInputStream</code> by* opening a connection to an actual file,* the file named by the <code>File</code>* object <code>file</code> in the file system.* A new <code>FileDescriptor</code> object* is created to represent this file connection.* <p>* First, if there is a security manager,* its <code>checkRead</code> method  is called* with the path represented by the <code>file</code>* argument as its argument.* <p>* If the named file does not exist, is a directory rather than a regular* file, or for some other reason cannot be opened for reading then a* <code>FileNotFoundException</code> is thrown.** @param      file   the file to be opened for reading.* @exception  FileNotFoundException  if the file does not exist,*                   is a directory rather than a regular file,*                   or for some other reason cannot be opened for*                   reading.* @exception  SecurityException      if a security manager exists and its*               <code>checkRead</code> method denies read access to the file.* @see        java.io.File#getPath()* @see        java.lang.SecurityManager#checkRead(java.lang.String)*/public FileInputStream(File file) throws FileNotFoundException {String name = (file != null ? file.getPath() : null);SecurityManager security = System.getSecurityManager();if (security != null) {security.checkRead(name);}if (name == null) {throw new NullPointerException();}if (file.isInvalid()) {throw new FileNotFoundException("Invalid file path");}fd = new FileDescriptor();fd.incrementAndGetUseCount();this.path = name;open(name);    //打开文件，进行读取  这个是本地方法}</span>

其中的read方法，从其中的read方法也可看出只能读取字节

<span style="font-size:14px;">    /*** Reads a byte of data from this input stream. This method blocks* if no input is yet available.** @return     the next byte of data, or <code>-1</code> if the end of the*             file is reached.* @exception  IOException  if an I/O error occurs.*/public int read() throws IOException {Object traceContext = IoTrace.fileReadBegin(path);int b = 0;try {b = read0();} finally {IoTrace.fileReadEnd(traceContext, b == -1 ? 0 : 1);}return b;}private native int read0() throws IOException;</span></span>

三、InputStreamReader

An InputStreamReader is a bridge from byte streams to character streams: It reads bytes and decodes them into characters using a specified{@link java.nio.charset.Charset <code>charset</code>}. The charset that it uses may be specified by name or may be given explicitly, or the platform's default charset may be accepted.

InputStreamReader是从字节流到字符流转换的桥梁，它读出字节并按照提供的编码格式解码成字符。而其又委托给StreamDeCoder去做。

<span style="font-size:14px;">    /*** Creates an InputStreamReader that uses the default charset.** @param  in   An InputStream*/public InputStreamReader(InputStream in) {super(in);try {sd = StreamDecoder.forInputStreamReader(in, this, (String)null); // ## check lock object} catch (UnsupportedEncodingException e) {// The default encoding should always be availablethrow new Error(e);}}/*** Creates an InputStreamReader that uses the named charset.** @param  in*         An InputStream** @param  charsetName*         The name of a supported*         {@link java.nio.charset.Charset </code>charset<code>}** @exception  UnsupportedEncodingException*             If the named charset is not supported*/public InputStreamReader(InputStream in, String charsetName)throws UnsupportedEncodingException{super(in);if (charsetName == null)throw new NullPointerException("charsetName");sd = StreamDecoder.forInputStreamReader(in, this, charsetName);  //创建了一个StreamDecoder}</span>

<span style="font-size:14px;"><pre class="java" name="code">StreamDecoder位于sun.nio.cs包中</span>

<span style="font-size:14px;"> public static StreamDecoder forInputStreamReader(InputStream in,Object lock,String charsetName)throws UnsupportedEncodingException{String csn = charsetName;if (csn == null)csn = Charset.defaultCharset().name();   //默认编码格式为UTF-8try {if (Charset.isSupported(csn))    //检查jvm是否支持该编码格式,否则跑出异常return new StreamDecoder(in, lock, Charset.forName(csn));} catch (IllegalCharsetNameException x) { }throw new UnsupportedEncodingException (csn);}</span>

最终如下，初始化其中的cs，decoder，ch

<span style="font-size:14px;">StreamDecoder(InputStream in, Object lock, Charset cs) {this(in, lock,cs.newDecoder().onMalformedInput(CodingErrorAction.REPLACE).onUnmappableCharacter(CodingErrorAction.REPLACE));}StreamDecoder(InputStream in, Object lock, CharsetDecoder dec) {super(lock);this.cs = dec.charset();this.decoder = dec;// This path disabled until direct buffers are fasterif (false && in instanceof FileInputStream) {ch = getChannel((FileInputStream)in);if (ch != null)bb = ByteBuffer.allocateDirect(DEFAULT_BYTE_BUFFER_SIZE);}if (ch == null) {this.in = in;this.ch = null;bb = ByteBuffer.allocate(DEFAULT_BYTE_BUFFER_SIZE);}bb.flip();                      // So that bb is initially empty}</span>

解码最终交给了CharsetDecoder。

然后看其read方法

<span style="font-size:14px;">    /*** Reads a single character.** @return The character read, or -1 if the end of the stream has been*         reached** @exception  IOException  If an I/O error occurs*/public int read() throws IOException {return sd.read();}</span>

调用到StreamDecoder中的read方法

<span style="font-size:14px;">public int read() throws IOException {return read0();}private int read0() throws IOException {synchronized (lock) {// Return the leftover char, if there is oneif (haveLeftoverChar) {haveLeftoverChar = false;return leftoverChar;}// Convert more byteschar cb[] = new char[2];int n = read(cb, 0, 2);switch (n) {case -1:return -1;case 2:leftoverChar = cb[1];haveLeftoverChar = true;// FALL THROUGHcase 1:return cb[0];default:assert false : n;return -1;}}}</span>

继续read

<span style="font-size:14px;">public int read(char cbuf[], int offset, int length) throws IOException {int off = offset;int len = length;synchronized (lock) {ensureOpen();if ((off < 0) || (off > cbuf.length) || (len < 0) ||((off + len) > cbuf.length) || ((off + len) < 0)) {throw new IndexOutOfBoundsException();}if (len == 0)return 0;int n = 0;if (haveLeftoverChar) {// Copy the leftover char into the buffercbuf[off] = leftoverChar;off++; len--;haveLeftoverChar = false;n = 1;if ((len == 0) || !implReady())// Return now if this is all we can produce w/o blockingreturn n;}if (len == 1) {// Treat single-character array reads just like read()int c = read0();if (c == -1)return (n == 0) ? -1 : n;cbuf[off] = (char)c;return n + 1;}return n + implRead(cbuf, off, off + len);}}</span>

<span style="font-size:14px;">int implRead(char[] cbuf, int off, int end) throws IOException {// In order to handle surrogate pairs, this method requires that// the invoker attempt to read at least two characters.  Saving the// extra character, if any, at a higher level is easier than trying// to deal with it here.assert (end - off > 1);CharBuffer cb = CharBuffer.wrap(cbuf, off, end - off);if (cb.position() != 0)// Ensure that cb[0] == cbuf[off]cb = cb.slice();boolean eof = false;for (;;) {CoderResult cr = decoder.decode(bb, cb, eof);  //最终的解码if (cr.isUnderflow()) {if (eof)break;if (!cb.hasRemaining())break;if ((cb.position() > 0) && !inReady())break;          // Block at most onceint n = readBytes();if (n < 0) {eof = true;if ((cb.position() == 0) && (!bb.hasRemaining()))break;decoder.reset();}continue;}if (cr.isOverflow()) {assert cb.position() > 0;break;}cr.throwException();}if (eof) {// ## Need to flush decoderdecoder.reset();}if (cb.position() == 0) {if (eof)return -1;assert false;}return cb.position();}</span>

四、关于new FileInputStream(file)及new FileOutputStream(file)时java.io.FileNotFoundException问题

原因可能有三：

(1)文件名乱码。你工程的编码是 utf-8还是 gbk,如果是gbk那么小心文件名如果有中文，会乱码。

(2)没有创建文件夹，在没有文件夹情况下，创建文件将报错.解决方案为先创建文件夹，具体代码见下面。

(3)有文件夹，但文件所在位置无文件夹，即文件路径太深，超过二级。解决方案同2

要明确FileInputStream及FileOutputStream并不会帮你创建不存在的路径（从上述的源码中也可看出，检查完路径的合法性，直接open的），所以要先创建路径，再创建文件。

<span style="font-size:14px;">File cacheDir = new File("data/data/com.kavinapps.androidk.pcexam/databases/");//设置目录参数  cacheDir.mkdirs();//新建目录  Log.i("copySd2phone","新建data/data目录成功");  String filename;  //获得文件名的长度  filename = "carnum.db";   Log.i("createNewFile","filename= "+filename);  //文件名  cacheFile = new File(cacheDir,filename);//设置参数  cacheFile.createNewFile();//生成文件  Log.i("createNewFile","生成文件成功"+cacheFile.getName());  output = new FileOutputStream( "data/data/com.kavinapps.androidk.pcexam/databases/carnum.db" );
</span>

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