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();
		}
	}
}