我最喜欢的读取小文件的方法是使用BufferedReader和StringBuilder。它非常简单和切中要害(虽然不是特别有效，但对大多数情况来说已经足够好了):

BufferedReader br = new BufferedReader(new FileReader("file.txt"));
try {
    StringBuilder sb = new StringBuilder();
    String line = br.readLine();

    while (line != null) {
        sb.append(line);
        sb.append(System.lineSeparator());
        line = br.readLine();
    }
    String everything = sb.toString();
} finally {
    br.close();
}

有人指出，在Java 7之后，你应该使用try-with-resources(即自动关闭)功能:

try(BufferedReader br = new BufferedReader(new FileReader("file.txt"))) {
    StringBuilder sb = new StringBuilder();
    String line = br.readLine();

    while (line != null) {
        sb.append(line);
        sb.append(System.lineSeparator());
        line = br.readLine();
    }
    String everything = sb.toString();
}

当我读取这样的字符串时，我通常希望每行都做一些字符串处理，所以我选择了这个实现。

虽然如果我只是想将文件读入字符串，我总是使用Apache Commons IO和类IOUtils.toString()方法。你可以在这里查看源代码:

http://www.docjar.com/html/api/org/apache/commons/io/IOUtils.java.html

FileInputStream inputStream = new FileInputStream("foo.txt");
try {
    String everything = IOUtils.toString(inputStream);
} finally {
    inputStream.close();
}

在Java 7中甚至更简单:

try(FileInputStream inputStream = new FileInputStream("foo.txt")) {     
    String everything = IOUtils.toString(inputStream);
    // do something with everything string
}

以下是三种工作和测试的方法:

使用BufferedReader

package io;
import java.io.*;
public class ReadFromFile2 {
    public static void main(String[] args)throws Exception {
        File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
        BufferedReader br = new BufferedReader(new FileReader(file));
        String st;
        while((st=br.readLine()) != null){
            System.out.println(st);
        }
    }
}

使用扫描仪

package io;

import java.io.File;
import java.util.Scanner;

public class ReadFromFileUsingScanner {
    public static void main(String[] args) throws Exception {
        File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
        Scanner sc = new Scanner(file);
        while(sc.hasNextLine()){
            System.out.println(sc.nextLine());
        }
    }
}

使用FileReader

package io;
import java.io.*;
public class ReadingFromFile {

    public static void main(String[] args) throws Exception {
        FileReader fr = new FileReader("C:\\Users\\pankaj\\Desktop\\test.java");
        int i;
        while ((i=fr.read()) != -1){
            System.out.print((char) i);
        }
    }
}

使用Scanner类读取整个文件，而不使用循环

package io;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ReadingEntireFileWithoutLoop {

    public static void main(String[] args) throws FileNotFoundException {
        File file = new File("C:\\Users\\pankaj\\Desktop\\test.java");
        Scanner sc = new Scanner(file);
        sc.useDelimiter("\\Z");
        System.out.println(sc.next());
    }
}

这里有另一种不使用外部库的方法:

import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public String readFile(String filename)
{
    String content = null;
    File file = new File(filename); // For example, foo.txt
    FileReader reader = null;
    try {
        reader = new FileReader(file);
        char[] chars = new char[(int) file.length()];
        reader.read(chars);
        content = new String(chars);
        reader.close();
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        if(reader != null){
            reader.close();
        }
    }
    return content;
}

import java.util.stream.Stream;
import java.nio.file.*;
import java.io.*;

class ReadFile {

 public static void main(String[] args) {

    String filename = "Test.txt";

    try(Stream<String> stream = Files.lines(Paths.get(filename))) {

          stream.forEach(System.out:: println);

    } catch (IOException e) {

        e.printStackTrace();
    }

 }

 }

只需使用java 8流。

try (Stream<String> stream = Files.lines(Paths.get(String.valueOf(new File("yourFile.txt"))))) {
    stream.forEach(System.out::println);
} catch (IOException e) {
    e.printStackTrace();
}

新文件(< path_name >)

通过将给定的路径名字符串转换为抽象路径名来创建一个新的File实例。如果给定的字符串是空字符串，那么结果就是空的抽象路径名。 参数: pathname -路径名字符串 抛出: NullPointerException -如果路径名参数为空

文件。lines返回String流

Stream<String> Stream = Files.lines(路径。get(字符串。返回对象的值(新文件(“yourFile.txt”)))) 可以抛出nullpointerexction, FileNotFoundException，所以，保持它在尝试将照顾异常在运行时

stream.forEach(System.out::println);

这用于在控制台中迭代流和打印 如果您有不同的用例，您可以提供您的自定义函数来操作行流

Java 11 Files.readString中引入了最直观的方法

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

public class App {
    public static void main(String args[]) throws IOException {
        String content = Files.readString(Paths.get("D:\\sandbox\\mvn\\my-app\\my-app.iml"));
        System.out.print(content);
    }
}

PHP几十年来一直享有这种特权!☺