[Java]读取文件方法大全

1、按字节读取文件内容
2、按字符读取文件内容
3、按行读取文件内容

4、随机读取文件内容

public class ReadFromFile {

    //以字节为单位读取文件，常用于读二进制文件，如图片、声音、影像等文件。

    public static void readFileByBytes(String fileName) {

        File file = new File(fileName);

        InputStream in = null;

        try {

            System.out.println("以字节为单位读取文件内容，一次读一个字节：");

            // 一次读一个字节

            in = new FileInputStream(file);

            int tempbyte;

            while ((tempbyte = in.read()) != -1) {

                System.out.write(tempbyte);

            }

            in.close();

        } catch (IOException e) {

            e.printStackTrace();

            return;

        }

        try {

            System.out.println("以字节为单位读取文件内容，一次读多个字节：");

            // 一次读多个字节

            byte[] tempbytes = new byte[100];

            int byteread = 0;

            in = new FileInputStream(fileName);

            ReadFromFile.showAvailableBytes(in);

            // 读入多个字节到字节数组中，byteread为一次读入的字节数

            while ((byteread = in.read(tempbytes)) != -1) {

                System.out.write(tempbytes, 0, byteread);

            }

        } catch (Exception e1) {

            e1.printStackTrace();

        } finally {

            if (in != null) {

                try {

                    in.close();

                } catch (IOException e1) {

                }

            }

        }

    }

   //以字符为单位读取文件，常用于读文本，数字等类型的文件

    public static void readFileByChars(String fileName) {

        File file = new File(fileName);

        Reader reader = null;

        try {

            System.out.println("以字符为单位读取文件内容，一次读一个字节：");

            // 一次读一个字符

            reader = new InputStreamReader(new FileInputStream(file));

            int tempchar;

            while ((tempchar = reader.read()) != -1) {

                // 对于windows下，\r\n这两个字符在一起时，表示一个换行。

                // 但如果这两个字符分开显示时，会换两次行。

                // 因此，屏蔽掉\r，或者屏蔽\n。否则，将会多出很多空行。

                if (((char) tempchar) != '\r') {

                    System.out.print((char) tempchar);

                }

            }

            reader.close();

        } catch (Exception e) {

            e.printStackTrace();

        }

        try {

            System.out.println("以字符为单位读取文件内容，一次读多个字节：");

            // 一次读多个字符

            char[] tempchars = new char[30];

            int charread = 0;

            reader = new InputStreamReader(new FileInputStream(fileName));

            // 读入多个字符到字符数组中，charread为一次读取字符数

            while ((charread = reader.read(tempchars)) != -1) {

                // 同样屏蔽掉\r不显示

                if ((charread == tempchars.length)

                        && (tempchars[tempchars.length - 1] != '\r')) {

                    System.out.print(tempchars);

                } else {

                    for (int i = 0; i < charread; i++) {

                        if (tempchars[i] == '\r') {

                            continue;

                        } else {

                            System.out.print(tempchars[i]);

                        }

                    }

                }

            }

        } catch (Exception e1) {

            e1.printStackTrace();

        } finally {

            if (reader != null) {

                try {

                    reader.close();

                } catch (IOException e1) {

                }

            }

        }

    }

   // 以行为单位读取文件，常用于读面向行的格式化文件

    public static void readFileByLines(String fileName) {

        File file = new File(fileName);

        BufferedReader reader = null;

        try {

            System.out.println("以行为单位读取文件内容，一次读一整行：");

            reader = new BufferedReader(new FileReader(file));

            String tempString = null;

            int line = 1;

            // 一次读入一行，直到读入null为文件结束

            while ((tempString = reader.readLine()) != null) {

                // 显示行号

                System.out.println("line " + line + ": " + tempString);

                line++;

            }

            reader.close();

        } catch (IOException e) {

            e.printStackTrace();

        } finally {

            if (reader != null) {

                try {

                    reader.close();

                } catch (IOException e1) {

                }

            }

        }

    }

   //随机读取文件内容

    public static void readFileByRandomAccess(String fileName) {

        RandomAccessFile randomFile = null;

        try {

            System.out.println("随机读取一段文件内容：");

            // 打开一个随机访问文件流，按只读方式

            randomFile = new RandomAccessFile(fileName, "r");

            // 文件长度，字节数

            long fileLength = randomFile.length();

            // 读文件的起始位置

            int beginIndex = (fileLength > 4) ? 4 : 0;

            // 将读文件的开始位置移到beginIndex位置。

            randomFile.seek(beginIndex);

            byte[] bytes = new byte[10];

            int byteread = 0;

            // 一次读10个字节，如果文件内容不足10个字节，则读剩下的字节。

            // 将一次读取的字节数赋给byteread

            while ((byteread = randomFile.read(bytes)) != -1) {

                System.out.write(bytes, 0, byteread);

            }

        } catch (IOException e) {

            e.printStackTrace();

        } finally {

            if (randomFile != null) {

                try {

                    randomFile.close();

                } catch (IOException e1) {

                }

            }

        }

    }

   //显示输入流中还剩的字节数

    private static void showAvailableBytes(InputStream in) {

        try {

            System.out.println("当前字节输入流中的字节数为:" + in.available());

        } catch (IOException e) {

            e.printStackTrace();

        }

    }

    public static void main(String[] args) {

        String fileName = "C:/temp/newTemp.txt";

        ReadFromFile.readFileByBytes(fileName);

        ReadFromFile.readFileByChars(fileName);

        ReadFromFile.readFileByLines(fileName);

        ReadFromFile.readFileByRandomAccess(fileName);

    }

}

5、将内容追加到文件尾部

   

public class AppendToFile {

   //A方法追加文件：使用RandomAccessFile

    public static void appendMethodA(String fileName, String content) {

        try {

            // 打开一个随机访问文件流，按读写方式

            RandomAccessFile randomFile = new RandomAccessFile(fileName, "rw");

            // 文件长度，字节数

            long fileLength = randomFile.length();

            //将写文件指针移到文件尾。

            randomFile.seek(fileLength);

            randomFile.writeBytes(content);

            randomFile.close();

        } catch (IOException e) {

            e.printStackTrace();

        }

    }

   //B方法追加文件：使用FileWriter

    public static void appendMethodB(String fileName, String content) {

        try {

            //打开一个写文件器，构造函数中的第二个参数true表示以追加形式写文件

            FileWriter writer = new FileWriter(fileName, true);

            writer.write(content);

            writer.close();

        } catch (IOException e) {

            e.printStackTrace();

        }

    }

    public static void main(String[] args) {

        String fileName = "C:/temp/newTemp.txt";

        String content = "new append!";

        //按方法A追加文件

        AppendToFile.appendMethodA(fileName, content);

        AppendToFile.appendMethodA(fileName, "append end. \n");

        //显示文件内容

        ReadFromFile.readFileByLines(fileName);

        //按方法B追加文件

        AppendToFile.appendMethodB(fileName, content);

        AppendToFile.appendMethodB(fileName, "append end. \n");

        //显示文件内容

        ReadFromFile.readFileByLines(fileName);

    }

}