java 文件同步读写的几种方式

方案1：利用RandomAccessFile的文件操作选项s，s即表示同步锁方式写

RandomAccessFile file = new RandomAccessFile(file, "rws" );

方案2：利用FileChannel的文件锁

File file = new File( "test.txt" );
FileInputStream fis = new FileInputStream(file);
FileChannel channel = fis.getChannel();
FileLock fileLock = null ;
while ( true ) {
  fileLock = channel.tryLock( 0 , Long.MAX_VALUE, false ); // true表示是共享锁，false则是独享锁定
  if (fileLock!= null ) break ;
  else  // 有其他线程占据锁
   sleep( 1000 );
}

方案3：先将要写的内容写入一个临时文件，然后再将临时文件改名（Hack方案，利用了缓冲+原子操作的原理）

public class MyFile {
  private String fileName;
  public MyFile(String fileName) {
   this .fileName = fileName;
  }

  public synchronized void writeData(String data) throws IOException {
   String tmpFileName = UUID.randomUUID().toString()+ ".tmp" ;
   File tmpFile = new File(tmpFileName);
   FileWriter fw = new FileWriter(tmpFile);
   fw.write(data);
   fw.flush();
   fw.close();

   // now rename temp file to desired name, this operation is atomic operation under most os
   if (!tmpFile.renameTo(fileName) {
     // we may want to retry if move fails
     throw new IOException( "Move failed" );
   }
  }
}

方案4：根据文件路径封装文件，并且用synchronized控制写文件

public class MyFile {
  private String fileName;
  public MyFile(String fileName) {
   this .fileName = fileName;
  }
  public synchronized void writeData(String data) throws IOException {
   FileWriter fw = new FileWriter(fileName);
   fw.write(data);
   fw.flush();
   fw.close();
  }
}

方案5：通过切换设置读写权限控制，模拟设置一个可写标记量（蜕变成操作系统中经典的读写问题....）

public class MyFile {
  private volatile boolean canWrite = true ;
  private String fileName;
  public MyFile(String fileName) {
   this .fileName = fileName;
  }
  public void writeData(String data) {
   while (!canWrite) {
     try { Thread.sleep( 100 ); } catch (InteruptedException ie) { } // 可以设置一个超时写时间
   }
   canWrite = false ;

   // Now write file

   canWrite = true ;
  }
}