java 基础知识之IO总结

java 基础知识之IO总结

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我计划在接下来的几篇文章中快速回顾一下Java，主要是一些基础的JDK相关的内容。

工作后，使用的技术随着项目的变化而变化，时而C#，时而Java，当然还有其他一些零碎的技术。总体而言，C#的使用时间要更长一些，其次是Java。我本身对语言没有什么倾向性，能干活的语言，就是好语言。而且从面向对象的角度来看，我觉得C#和Java对我来说，没什么区别。

这篇文章主要回顾Java中和I/O操作相关的内容，I/O也是编程语言的一个基础特性，Java中的I/O分为两种类型，一种是顺序读取，一种是随机读取。

我们先来看顺序读取，有两种方式可以进行顺序读取，一种是InputStream/OutputStream，它是针对字节进行操作的输入输出流；另外一种是Reader/Writer，它是针对字符进行操作的输入输出流。

下面我们画出InputStream的结构

java 基础知识之IO总结

FileInputStream：操作文件，经常和BufferedInputStream一起使用
PipedInputStream：可用于线程间通信
ObjectInputStream：可用于对象序列化
ByteArrayInputStream：用于处理字节数组的输入
LineNumberInputStream：可输出当前行数，并且可以在程序中进行修改

下面是OutputStream的结构

java 基础知识之IO总结

PrintStream：提供了类似print和println的接口去输出数据

下面我们来看如何使用Stream的方式来操作输入输出

使用InputStream读取文件

使用FileInputStream读取文件信息
public static byte[] readFileByFileInputStream(File file) throws IOException
{
  ByteArrayOutputStream output = new ByteArrayOutputStream();
  FileInputStream fis = null;
  try
  {
    fis = new FileInputStream(file);
    byte[] buffer = new byte[1024];
    int bytesRead = 0;
    while((bytesRead = fis.read(buffer, 0, buffer.length)) != -1)
    {
      output.write(buffer, 0, bytesRead);
    }
  }
  catch(Exception ex)
  {
    System.out.println("Error occurs during reading " + file.getAbsoluteFile());
  }
  finally
  {
    if (fis !=null) fis.close();
    if (output !=null) output.close();
  }
  return output.toByteArray();
}

使用BufferedInputStream读取文件
public static byte[] readFileByBufferedInputStream(File file) throws Exception
{
  FileInputStream fis = null;
  BufferedInputStream bis = null;
  ByteArrayOutputStream output = new ByteArrayOutputStream();
  try
  {
    fis = new FileInputStream(file);
    bis = new BufferedInputStream(fis);
    byte[] buffer = new byte[1024];
    int bytesRead = 0;
    while((bytesRead = bis.read(buffer, 0, buffer.length)) != -1)
    {
      output.write(buffer, 0, bytesRead);
    }
  }
  catch(Exception ex)
  {
    System.out.println("Error occurs during reading " + file.getAbsoluteFile());
  }
  finally
  {
    if (fis != null) fis.close();
    if (bis != null) bis.close();
    if (output != null) output.close();
  }
  return output.toByteArray();
}

使用OutputStream复制文件

使用FileOutputStream复制文件
public static void copyFileByFileOutputStream(File file) throws IOException
{
  FileInputStream fis = null;
  FileOutputStream fos = null;
  try
  {
    fis = new FileInputStream(file);
    fos = new FileOutputStream(file.getName() + ".bak");
    byte[] buffer = new byte[1024];
    int bytesRead = 0;
    while((bytesRead = fis.read(buffer,0,buffer.length)) != -1)
    {
      fos.write(buffer, 0, bytesRead);
    }
    fos.flush();
  }
  catch(Exception ex)
  {
    System.out.println("Error occurs during copying " + file.getAbsoluteFile());
  }
  finally
  {
    if (fis != null) fis.close();
    if (fos != null) fos.close();
  }
}

使用BufferedOutputStream复制文件
public static void copyFilebyBufferedOutputStream(File file)throws IOException
{
  FileInputStream fis = null;
  BufferedInputStream bis = null;
  FileOutputStream fos = null;
  BufferedOutputStream bos = null;
  try
  {
    fis = new FileInputStream(file);
    bis = new BufferedInputStream(fis);
    fos = new FileOutputStream(file.getName() + ".bak");
    bos = new BufferedOutputStream(fos);
    byte[] buffer = new byte[1024];
    int bytesRead = 0;
    while((bytesRead = bis.read(buffer, 0, buffer.length)) != -1)
    {
      bos.write(buffer, 0, bytesRead);
    }
    bos.flush();
  }
  catch(Exception ex)
  {
    System.out.println("Error occurs during copying " + file.getAbsoluteFile());
  }
  finally
  {
    if (fis != null) fis.close();
    if (bis != null) bis.close();
    if (fos != null) fos.close();
    if (bos != null) bos.close();
  }
}

这里的代码对异常的处理非常不完整，稍后我们会给出完整严谨的代码。

下面我们来看Reader的结构

java 基础知识之IO总结

这里的Reader基本上和InputStream能够对应上。

Writer的结构如下

java 基础知识之IO总结

下面我们来看一些使用Reader或者Writer的例子

使用Reader读取文件内容

使用BufferedReader读取文件内容
public static String readFile(String file)throws IOException
{
  BufferedReader br = null;
  StringBuffer sb = new StringBuffer();
  try
  {
    br = new BufferedReader(new FileReader(file));
    String line = null;
    
    while((line = br.readLine()) != null)
    {
      sb.append(line);
    }
  }
  catch(Exception ex)
  {
    System.out.println("Error occurs during reading " + file);
  }
  finally
  {
    if (br != null) br.close();
  }
  return sb.toString();
}

使用Writer复制文件

使用BufferedWriter复制文件
public static void copyFile(String file) throws IOException
{ 
  BufferedReader br = null;
  BufferedWriter bw = null;
  try
  {
    br = new BufferedReader(new FileReader(file));
    bw = new BufferedWriter(new FileWriter(file + ".bak"));
    String line = null;
    while((line = br.readLine())!= null)
    {
      bw.write(line);
    }
  }
  catch(Exception ex)
  {
    System.out.println("Error occurs during copying " + file);
  }
  finally
  {
    if (br != null) br.close();
    if (bw != null) bw.close();
  }
}

下面我们来看如何对文件进行随机访问，Java中主要使用RandomAccessFile来对文件进行随机操作。

创建一个大小固定的文件

创建大小固定的文件
public static void createFile(String file, int size) throws IOException
{
  File temp = new File(file);
  RandomAccessFile raf = new RandomAccessFile(temp, "rw");
  raf.setLength(size);
  raf.close();
}

向文件中随机写入数据

向文件中随机插入数据
public static void writeFile(String file, byte[] content, int startPos, int contentLength) throws IOException
{
  RandomAccessFile raf = new RandomAccessFile(new File(file), "rw");
  raf.seek(startPos);
  raf.write(content, 0, contentLength);
  raf.close();
}

接下里，我们来看一些其他的常用操作

移动文件

移动文件
public static boolean moveFile(String sourceFile, String destFile)
{
  File source = new File(sourceFile);
  if (!source.exists()) throw new RuntimeException("source file does not exist.");
  File dest = new File(destFile);
  if (!(new File(dest.getPath()).exists())) new File(dest.getParent()).mkdirs();
  return source.renameTo(dest);
}

复制文件

复制文件
public static void copyFile(String sourceFile, String destFile) throws IOException
{
  File source = new File(sourceFile);
  if (!source.exists()) throw new RuntimeException("File does not exist.");
  if (!source.isFile()) throw new RuntimeException("It is not file.");
  if (!source.canRead()) throw new RuntimeException("File cound not be read.");
  File dest = new File(destFile);
  if (dest.exists())
  {
    if (dest.isDirectory()) throw new RuntimeException("Destination is a folder.");
    else
    {
      dest.delete();
    }
  }
  else
  {
    File parentFolder = new File(dest.getParent());
    if (!parentFolder.exists()) parentFolder.mkdirs();
    if (!parentFolder.canWrite()) throw new RuntimeException("Destination can not be written.");
  }
  FileInputStream fis = null;
  FileOutputStream fos = null;
  try
  {
    fis = new FileInputStream(source);
    fos = new FileOutputStream(dest);
    byte[] buffer = new byte[1024];
    int bytesRead = 0;
    while((bytesRead = fis.read(buffer, 0, buffer.length)) != -1)
    {
      fos.write(buffer, 0, bytesRead);
    }
    fos.flush();
  }
  catch(IOException ex)
  {
    System.out.println("Error occurs during copying " + sourceFile);
  }
  finally
  {
    if (fis != null) fis.close();
    if (fos != null) fos.close();
  }
}

复制文件夹

复制文件夹
public static void copyDir(String sourceDir, String destDir) throws IOException
{
  
  File source = new File(sourceDir);
  if (!source.exists()) throw new RuntimeException("Source does not exist.");
  if (!source.canRead()) throw new RuntimeException("Source could not be read.");
  File dest = new File(destDir);
  if (!dest.exists()) dest.mkdirs();
  
  File[] arrFiles = source.listFiles();
  for(int i = 0; i < arrFiles.length; i++)
  {
    if (arrFiles[i].isFile())
    {
      BufferedReader reader = new BufferedReader(new FileReader(arrFiles[i]));
      BufferedWriter writer = new BufferedWriter(new FileWriter(destDir + "/" + arrFiles[i].getName()));
      String line = null;
      while((line = reader.readLine()) != null) writer.write(line);
      writer.flush();
      reader.close();
      writer.close();
    }
    else
    {
      copyDir(sourceDir + "/" + arrFiles[i].getName(), destDir + "/" + arrFiles[i].getName());
    }
  }
}

删除文件夹

删除文件夹
public static void del(String filePath)
{
  File file = new File(filePath);
  if (file == null || !file.exists()) return;
  if (file.isFile())
  {
    file.delete();
  }
  else
  {
    File[] arrFiles = file.listFiles();
    if (arrFiles.length > 0)
    {
      for(int i = 0; i < arrFiles.length; i++)
      {
        del(arrFiles[i].getAbsolutePath());
      }
    }
    file.delete();
  }
}

获取文件夹大小

获取文件夹大小
public static long getFolderSize(String dir)
{
  long size = 0;
  File file = new File(dir);
  if (!file.exists()) throw new RuntimeException("dir does not exist.");
  if (file.isFile()) return file.length();
  else
  {
    String[] arrFileName = file.list();
    for (int i = 0; i < arrFileName.length; i++)
    {
      size += getFolderSize(dir + "/" + arrFileName[i]);
    }
  }
  
  return size;
}

将大文件切分为多个小文件

将大文件切分成多个小文件
public static void splitFile(String filePath, long unit) throws IOException
{
  File file = new File(filePath);
  if (!file.exists()) throw new RuntimeException("file does not exist.");
  long size = file.length();
  if (unit >= size) return;
  int count = size % unit == 0 ? (int)(size/unit) : (int)(size/unit) + 1;
  String newFile = null;
  FileOutputStream fos = null;
  FileInputStream fis =null;
  byte[] buffer = new byte[(int)unit];
  fis = new FileInputStream(file);
  long startPos = 0;
  String countFile = filePath + "_Count";
  PrintWriter writer = new PrintWriter(new FileWriter( new File(countFile)));
  writer.println(filePath + "\t" + size);
  for (int i = 1; i <= count; i++)
  {
    newFile = filePath + "_" + i;
    startPos = (i - 1) * unit;
    System.out.println("Creating " + newFile);
    fos = new FileOutputStream(new File(newFile));
    int bytesRead = fis.read(buffer, 0, buffer.length);
    if (bytesRead != -1)
    {
      fos.write(buffer, 0, bytesRead);
      writer.println(newFile + "\t" + startPos + "\t" + bytesRead);
    }
    fos.flush();
    fos.close();
    System.out.println("StartPos:" + i*unit + "; EndPos:" + (i*unit + bytesRead));
  }
  writer.flush();
  writer.close();
  fis.close();
}

将多个小文件合并为一个大文件

将多个小文件合并成一个大文件
public static void linkFiles(String countFile) throws IOException
{
  File file = new File(countFile);
  if (!file.exists()) throw new RuntimeException("Count file does not exist.");
  BufferedReader reader = new BufferedReader(new FileReader(file));
  String line = reader.readLine();
  String newFile = line.split("\t")[0];
  long size = Long.parseLong(line.split("\t")[1]);
  RandomAccessFile raf = new RandomAccessFile(newFile, "rw");
  raf.setLength(size);
  FileInputStream fis = null;
  byte[] buffer = null;
  
  while((line = reader.readLine()) != null)
  {
    String[] arrInfo = line.split("\t");
    fis = new FileInputStream(new File(arrInfo[0]));
    buffer = new byte[Integer.parseInt(arrInfo[2])];
    long startPos = Long.parseLong(arrInfo[1]);
    fis.read(buffer, 0, Integer.parseInt(arrInfo[2]));
    raf.seek(startPos);
    raf.write(buffer, 0, Integer.parseInt(arrInfo[2]));
    fis.close();
  }
  raf.close();
}

执行外部命令

执行外部命令
public static void execExternalCommand(String command, String argument)
{
  Process process = null;
  try
  {
    process = Runtime.getRuntime().exec(command + " " + argument);
    InputStream is = process.getInputStream();
    BufferedReader br = new BufferedReader(new InputStreamReader(is));
    String line = null;
    while((line = br.readLine()) != null)
    {
      System.out.println(line);
    }
  }
  catch(Exception ex)
  {
    System.err.println(ex.getMessage());
  }
  finally
  {
    if (process != null) process.destroy();
  }
}

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