原文地址：（英文版）
https://howtodoinjava.com/core-java/io/java-read-file-to-string-examples/

Learn to read file to string in java. Examples use Files.readAllBytes, Files.lines and FileReader & BufferedReader to read file content.

学习读取文件,并赋值到String中。 示例使用

• Files.readAllBytes
• Files.lines
• FileReader＆BufferedReader
  读取文件内容。

Source code of all 3 examples for reading files into string.
三种方法读文件方法的源代码

//Example 1
 
//Read file content into string with - Files.lines(Path path, Charset cs)
 
private static String readLineByLineJava8(String filePath)
{
    StringBuilder contentBuilder = new StringBuilder();
    try (Stream<String> stream = Files.lines( 
                                     Paths.get(filePath), StandardCharsets.UTF_8))
    {
        stream.forEach(s -> contentBuilder.append(s).append("\n"));
    }
    catch (IOException e)
    {
        e.printStackTrace();
    }
    return contentBuilder.toString();
}
 
//Example 2
 
//Read file content into string with - Files.readAllBytes(Path path)
 
private static String readAllBytesJava7(String filePath)
{
    String content = "";
    try
    {
        content = new String ( Files.readAllBytes( Paths.get(filePath) ) );
    }
    catch (IOException e)
    {
        e.printStackTrace();
    }
    return content;
}
 
//Example 3
 
//Read file content into string with - using BufferedReader and FileReader
//You can use this if you are still not using Java 8
 
private static String usingBufferedReader(String filePath)
{
    StringBuilder contentBuilder = new StringBuilder();
    try (BufferedReader br = new BufferedReader(new FileReader(filePath)))
    {
 
        String sCurrentLine;
        while ((sCurrentLine = br.readLine()) != null)
        {
            contentBuilder.append(sCurrentLine).append("\n");
        }
    }
    catch (IOException e)
    {
        e.printStackTrace();
    }
    return contentBuilder.toString();
}

I have placed a file data.txt at location c:/temp. I will read this file in all 3 examples.
准备一个文件，文件名为data.txt，放到c:/temp目录下面，然后会用三种方法把文件内容读出来

File content in c:/temp/data.txt
文件内容如下

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Read File to String using Files.lines() [≥ Java 8]

使用Files.lines()读取文件，需要JDK版本为Java 8 以上

lines() method read all lines from a file to stream and populates lazily as the stream is consumed. Bytes from the file are decoded into characters using the specified charset.

lines（）方法从文件中读取所有行，通过流（stream）的方式读到内存。 使用指定的字符集将文件中的二进制字节解码转换为字符。

import java.io.IOException;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;
 
public class ReadFileToString
{
    public static void main(String[] args)
    {
        String filePath = "c:/temp/data.txt";
 
        System.out.println( readLineByLineJava8( filePath ) );
    }
 
 
    //Read file content into string with - Files.lines(Path path, Charset cs)
 
    private static String readLineByLineJava8(String filePath)
    {
        StringBuilder contentBuilder = new StringBuilder();
 
        try (Stream<String> stream = Files.lines( 
                                        Paths.get(filePath), StandardCharsets.UTF_8))
        {
            stream.forEach(s -> contentBuilder.append(s).append("\n"));
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
 
        return contentBuilder.toString();
    }
}

打开你的IDE，COPY下面代码，然后运行，就可以在控制台看到输出

使用Files.readAllBytes()读取文件，需要JDK版本为Java 7 以上

Read File to String using Files.readAllBytes() [≥ Java 7]

readAllBytes() method 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.

After reading all bytes, we pass those bytes toStringclass constructor to create a string.

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
 
public class ReadFileToString
{
    public static void main(String[] args)
    {
        String filePath = "c:/temp/data.txt";
 
        System.out.println( readAllBytesJava7( filePath ) );
    }
 
    //Read file content into string with - Files.readAllBytes(Path path)
 
    private static String readAllBytesJava7(String filePath)
    {
        String content = "";
 
        try
        {
            content = new String ( Files.readAllBytes( Paths.get(filePath) ) );
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
 
        return content;
    }
}

Read File to String using BufferedReader [< Java 7]

使用BufferedReader 读取文件，最老的方式

If you are still not using java 7 or later, then use BufferedReader class. It’sreadLine()method reads the file one line at a time and return the content.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;
 
public class ReadFileToString
{
    public static void main(String[] args)
    {
        String filePath = "c:/temp/data.txt";
 
        System.out.println( usingBufferedReader( filePath ) );
    }
 
    //Read file content into string with - using BufferedReader and FileReader
    //You can use this if you are still not using Java 8
 
    private static String usingBufferedReader(String filePath)
    {
        StringBuilder contentBuilder = new StringBuilder();
        try (BufferedReader br = new BufferedReader(new FileReader(filePath)))
        {
 
            String sCurrentLine;
            while ((sCurrentLine = br.readLine()) != null)
            {
                contentBuilder.append(sCurrentLine).append("\n");
            }
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
        return contentBuilder.toString();
    }
}