JDK1.7 之java.nio.file.Files 读取文件仅需一行代码实现

JDK1.7中引入了新的文件操作类java.nio.file这个包，其中有个Files类它包含了很多有用的方法来操作文件，比如检查文件是否为隐藏文件，或者是检查文件是否为只读文件。开发者还可以使用Files.readAllBytes(Path)方法把整个文件读入内存，此方法返回一个字节数组，还可以把结果传递给String的构造器，以便创建字符串输出。此方法确保了当读入文件的所有字节内容时，无论是否出现IO异常或其它的未检查异常，资源都会关闭。这意味着在读文件到最后的块内容后，无需关闭文件。要注意，此方法不适合读取很大的文件，因为可能存在内存空间不足的问题。开发者还应该明确规定文件的字符编码，以避免任异常或解析错误。

readAllBytes(Path)方法的源码：

<span style="font-size:32px;"> </span><span style="font-size:18px;">/**
 * Reads all the bytes from a file. The method ensures that the file is
 * closed when all bytes have been read or an I/O error, or other runtime
 * exception, is thrown.
 * 注意该方法只适用于简单的情况，这种简单的情况能够很方便地将所有的字节读进一个字节数组，但并不适合用来读取大文件
 * <p> Note that this method is intended for simple cases where it is
 * convenient to read all bytes into a byte array. It is not intended for
 * reading in large files.
 *
 * @param  path
 *     the path to the file
 *
 * @return a byte array containing the bytes read from the file
 *
 * @throws IOException
 *     if an I/O error occurs reading from the stream
 *     如果大于文件2G，将抛出内存溢出异常
 * @throws OutOfMemoryError
 *     if an array of the required size cannot be allocated, for
 *     example the file is larger that {@code 2GB}
 * @throws SecurityException
 *     In the case of the default provider, and a security manager is
 *     installed, the {@link SecurityManager#checkRead(String) checkRead}
 *     method is invoked to check read access to the file.
 */</span><span style="font-size:18px;">
  public static byte[] readAllBytes(Path path) throws IOException {
    try (SeekableByteChannel sbc = Files.newByteChannel(path);
       InputStream in = Channels.newInputStream(sbc)) {//JDK1.7 try-with-resource
      long size = sbc.size();
      if (size > (long)MAX_BUFFER_SIZE)
        throw new OutOfMemoryError("Required array size too large"); 

      return read(in, (int)size);
    }
  }</span> 

读取文件只要一行

package entryNIO; 

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths; 

public class BufferAndChannel {
  public static void main(String[] args) {
    try {
        System.out.println(
         new String(Files.readAllBytes(Paths.get("C:\\FileChannelImpl.java")))
        ); 

    } catch (IOException e) {
      e.printStackTrace();
    }
  }
} 

readAllLines方法的源码

public static List<String> readAllLines(Path path, Charset cs) throws IOException {
    try (BufferedReader reader = newBufferedReader(path, cs)) {
      List<String> result = new ArrayList<>();
      for (;;) {
        String line = reader.readLine();
        if (line == null)
          break;
        result.add(line);
      }
      return result;
    }
  } 

package entryNIO; 

import java.util.List;
import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths; 

public class BufferAndChannel {
  public static void main(String[] args) {
    //如果是文本文件也可以这么读 调用readAllLines 方法
    try {<span style="white-space:pre">               </span>//JDK1.8以后可以省略第二个参数，默认是UTF-8编码
      List<String> lines = Files.readAllLines(Paths.get("C:\\FileChannelImpl.java"), StandardCharsets.UTF_8);
      StringBuilder sb = new StringBuilder();
      for (String line : lines) {
        sb.append(line+"\n");// \r\n 换行符
      }
      String fromFile = sb.toString();
      System.out.println(fromFile); 

    } catch (IOException e) {
      e.printStackTrace();
    }
  }
} 

使用Java8 流的方式：

先看源码实现

public static Stream<String> lines(Path path) throws IOException {
    return lines(path, StandardCharsets.UTF_8);
  } 

package entryNIO; 

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths; 

public class BufferAndChannel {
  public static void main(String[] args) {
    //Java8 新增lines方法
    try {
       // Java8用流的方式读文件，更加高效
      Files.lines(Paths.get(<span style="font-family: Arial, Helvetica, sans-serif;">"C:\\FileChannelImpl.java"</span>)).forEach(System.out::println);
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

读文件一行写文件也只需要一行

package entryNIO; 

import java.util.Arrays;
import java.util.List;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;
public class BufferAndChannel {
  public static void main(String[] args){
    //Java8 新增lines方法
    String filePath="C:\\FileChannelImpl.java";
    try {
       // Java8用流的方式读文件，更加高效
      /*Files.lines(Paths.get(filePath)).forEach((line)->{
          try {
            Files.write(Paths.get("\\1.java"), line.getBytes(), StandardOpenOption.APPEND);
            //Files.copy(in, target, options);
          } catch (IOException e) {
            e.printStackTrace();
          } 

      }); */ 

      /* Files.readAllLines(Path path)方法返回值为List<String>类型，就是为Files.write()而设计的
       * 因为Files.write()需要传入一个Iterable<? extends CharSequence>类型的参数
       *
       * Files.write(Path path, Iterable<? extends CharSequence> lines, OpenOption... options)
       */
      List<String> stringStream=Files.readAllLines(Paths.get(filePath));
      //因为Files.lines(Path path)返回的是Stream<String>,所以可以通过下面这种方法变成List<String>
      //List<String> stringStream2=Arrays.asList((String[])Files.lines(Paths.get(filePath)).toArray()); 

      //StandardOpenOption为枚举类 ,如果当前所Paths.get()的文件不存在，第三个参数可选择StandardOpenOption.CREATE_NEW
      //文件存在则抛java.nio.file.FileAlreadyExistsException异常
      Files.write(Paths.get("C:\\2.java"), stringStream, StandardOpenOption.CREATE_NEW); 

    } catch (IOException e) {
      e.printStackTrace();
    }
  }
} 

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