JDK1.7 之java.nio.file.Files 读取文件只要一行

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：

/**  * 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.  */ 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); } } 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 { //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("C:\\FileChannelImpl.java")).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(); } } }