Java基础之写文件——使用多个视图缓冲区（PrimesToFile2）

控制台程序。本例将对应于每个素数的数据以三个连续数据项的形式写入：

1、以二进制值表示的字符串长度值（最好是整型，但本例使用double类型）；

2、素数值的字符串表示”Prime=nnn“，其中数字的位数明显是变化的；

3、将素数以long类型的二进制表示。

基本策略是，创建一个字节缓冲区，之后创建一系列将三种不同类型数据映射到其中的视图缓冲区。一种简单的方法是每次写入一个素数的数据。

 import static java.lang.Math.ceil;
 import static java.lang.Math.sqrt;
 import static java.lang.Math.min;
 import static java.nio.file.StandardOpenOption.*;
 import java.nio.file.*;
 import java.nio.channels.*;
 import java.nio.*;
 import java.util.*;
 import java.io.IOException;

 public class PrimesToFile2 {
   public static void main(String[] args) {
     int primesRequired = 100;                                          // Default count
     if (args.length > 0) {
       try {
         primesRequired = Integer.valueOf(args[0]).intValue();
       } catch (NumberFormatException e) {
         System.out.println("Prime count value invalid. Using default of " + primesRequired);
       }
     }

       long[] primes = new long[primesRequired];                        // Array to store primes

       getPrimes(primes);
       Path file = createFilePath("Beginning Java Struff","primes.txt");
       writePrimesFile(primes,file);
     }
       //Calculate enough primes to fill the array
       private static long[] getPrimes(long[] primes) {
           primes[0] = 2L;                                                  // Seed the first prime
           primes[1] = 3L;                                                  // and the second
           // Count of primes found ?up to now, which is also the array index
           int count = 2;
           // Next integer to be tested
           long number = 5L;

           outer:
           for (; count < primes.length; number += 2) {

             // The maximum divisor we need to try is square root of number
             long limit = (long)ceil(sqrt((double)number));

             // Divide by all the primes we have up to limit
             for (int i = 1 ; i < count && primes[i] <= limit ; ++i)
               if (number % primes[i] == 0L)                                // Is it an exact divisor?
                 continue outer;                                            // yes, try the next number

             primes[count++] = number;                                      // We got one!
           }
           return primes;
         }
     //Create the path for the named file in the specified directory
     //in the user home directory
     private static Path createFilePath(String directory, String fileName) {
         Path file = Paths.get(System.getProperty("user.home")).resolve(directory).resolve(fileName);
         try {
           Files.createDirectories(file.getParent());                       // Make sure we have the directory
         } catch (IOException e) {
           e.printStackTrace();
           System.exit(1);
         }
         System.out.println("New file is: " + file);
         return file;
     }

     //Write the array contents to file
     private static void writePrimesFile(long[] primes, Path file) {
         final int BUFFERSIZE = 100;                                        // Byte buffer size
         try (WritableByteChannel channel = Files.newByteChannel(file, EnumSet.of(WRITE, CREATE))){
           ByteBuffer buf = ByteBuffer.allocate(BUFFERSIZE);
           DoubleBuffer doubleBuf = buf.asDoubleBuffer();
           buf.position(8);
           CharBuffer charBuf = buf.asCharBuffer();
           LongBuffer longBuf = null;
           String primeStr = null;

         for (long prime : primes) {
             primeStr = "prime = " + prime;                                 // Create the string
             doubleBuf.put(0,(double)primeStr.length());                    // Store the string length
             charBuf.put(primeStr);                                         // Store the string
             buf.position(2*charBuf.position() + 8);                        // Position for 3rd buffer
             longBuf = buf.asLongBuffer();                                  // Create the buffer
             longBuf.put(prime);                                            // Store the binary long value
             buf.position(buf.position() + 8);                              // Set position after last value
             buf.flip();                                                    // and flip
             channel.write(buf);                                            // Write the buffer as before

             buf.clear();
             doubleBuf.clear();
             charBuf.clear();
         }
           System.out.println("File written is " + ((FileChannel)channel).size() + " bytes.");
         } catch (IOException e) {
           e.printStackTrace();
         }
     }
 }