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());
    }
}

如果您有一个大文件，您可以使用Apache Commons IO迭代处理该文件，而不会耗尽可用内存。

try (LineIterator it = FileUtils.lineIterator(theFile, "UTF-8")) {
    while (it.hasNext()) {
        String line = it.nextLine();
        // do something with line
    }
}

我记录了15种用Java读取文件的方法，然后测试了它们在不同文件大小下的速度——从1kb到1gb，下面是最常用的三种方法:

java.nio.file.Files.readAllBytes() Tested to work in Java 7, 8, and 9. import java.io.File; import java.io.IOException; import java.nio.file.Files; public class ReadFile_Files_ReadAllBytes { public static void main(String [] pArgs) throws IOException { String fileName = "c:\\temp\\sample-10KB.txt"; File file = new File(fileName); byte [] fileBytes = Files.readAllBytes(file.toPath()); char singleChar; for(byte b : fileBytes) { singleChar = (char) b; System.out.print(singleChar); } } } java.io.BufferedReader.readLine() Tested to work in Java 7, 8, 9. import java.io.BufferedReader; import java.io.FileReader; import java.io.IOException; public class ReadFile_BufferedReader_ReadLine { public static void main(String [] args) throws IOException { String fileName = "c:\\temp\\sample-10KB.txt"; FileReader fileReader = new FileReader(fileName); try (BufferedReader bufferedReader = new BufferedReader(fileReader)) { String line; while((line = bufferedReader.readLine()) != null) { System.out.println(line); } } } } java.nio.file.Files.lines() This was tested to work in Java 8 and 9 but won't work in Java 7 because of the lambda expression requirement. import java.io.File; import java.io.IOException; import java.nio.file.Files; import java.util.stream.Stream; public class ReadFile_Files_Lines { public static void main(String[] pArgs) throws IOException { String fileName = "c:\\temp\\sample-10KB.txt"; File file = new File(fileName); try (Stream linesStream = Files.lines(file.toPath())) { linesStream.forEach(line -> { System.out.println(line); }); } } }

如果这是关于结构的简单性，请使用Java kiss:

import static kiss.API.*;

class App {
  void run() {
    String line;
    try (Close in = inOpen("file.dat")) {
      while ((line = readLine()) != null) {
        println(line);
      }
    }
  }
}

你可以使用readAllLines和join方法在一行中获取整个文件内容:

String str = String.join("\n",Files.readAllLines(Paths.get("e:\\text.txt")));

它默认使用UTF-8编码，可以正确读取ASCII数据。

你也可以使用readAllBytes:

String str = new String(Files.readAllBytes(Paths.get("e:\\text.txt")), StandardCharsets.UTF_8);

我认为readAllBytes更快更精确，因为它不会用\n替换新行，也不会用\r\n替换新行。哪一种合适取决于你的需要。

这里有一个简单的解决方案:

String content = new String(Files.readAllBytes(Paths.get("sample.txt")));

或读作列表:

List<String> content = Files.readAllLines(Paths.get("sample.txt"))