Java IO - 缓冲流

缓冲流是对基本流的包装，通过内置缓冲区减少系统调用次数，大幅提升读写效率。

为什么需要缓冲流

普通流每次读写都会进行系统调用，效率较低：

普通流：程序 ←→ 操作系统 ←→ 磁盘（频繁交互）
缓冲流：程序 ←→ 缓冲区 ←→ 操作系统 ←→ 磁盘（批量交互）

缓冲流内部维护一个缓冲区（默认 8KB），减少与操作系统的交互次数。

缓冲流分类

类型	字节缓冲流	字符缓冲流
输入	BufferedInputStream	BufferedReader
输出	BufferedOutputStream	BufferedWriter

BufferedInputStream 字节缓冲输入流

构造方法

// 使用默认缓冲区大小（8192 字节）
BufferedInputStream bis = new BufferedInputStream(new FileInputStream("test.txt"));

// 指定缓冲区大小
BufferedInputStream bis = new BufferedInputStream(new FileInputStream("test.txt"), 16384);

使用示例

import java.io.*;

public class BufferedInputStreamDemo {
    public static void main(String[] args) {
        try (BufferedInputStream bis = new BufferedInputStream(
                new FileInputStream("test.txt"))) {
            
            byte[] buffer = new byte[1024];
            int len;
            while ((len = bis.read(buffer)) != -1) {
                System.out.print(new String(buffer, 0, len));
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

BufferedOutputStream 字节缓冲输出流

构造方法

BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("test.txt"));
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("test.txt"), 16384);

使用示例

try (BufferedOutputStream bos = new BufferedOutputStream(
        new FileOutputStream("output.txt"))) {
    
    String content = "Hello, Buffered Stream!";
    bos.write(content.getBytes());
    // 不需要手动 flush，close 时会自动 flush
} catch (IOException e) {
    e.printStackTrace();
}

BufferedReader 字符缓冲输入流

提供 readLine() 方法，可以逐行读取。

构造方法

BufferedReader br = new BufferedReader(new FileReader("test.txt"));
BufferedReader br = new BufferedReader(new FileReader("test.txt"), 16384);

逐行读取

import java.io.*;

public class BufferedReaderDemo {
    public static void main(String[] args) {
        try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {
            String line;
            int lineNumber = 0;
            while ((line = br.readLine()) != null) {
                lineNumber++;
                System.out.println(lineNumber + ": " + line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

读取所有行（JDK 8+）

try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {
    // 使用 Stream 读取所有行
    br.lines().forEach(System.out::println);
} catch (IOException e) {
    e.printStackTrace();
}

收集为 List

import java.util.List;
import java.util.stream.Collectors;

try (BufferedReader br = new BufferedReader(new FileReader("test.txt"))) {
    List<String> lines = br.lines().collect(Collectors.toList());
    lines.forEach(System.out::println);
} catch (IOException e) {
    e.printStackTrace();
}

BufferedWriter 字符缓冲输出流

提供 newLine() 方法，写入系统换行符。

构造方法

BufferedWriter bw = new BufferedWriter(new FileWriter("test.txt"));
BufferedWriter bw = new BufferedWriter(new FileWriter("test.txt"), 16384);

使用示例

import java.io.*;

public class BufferedWriterDemo {
    public static void main(String[] args) {
        try (BufferedWriter bw = new BufferedWriter(new FileWriter("output.txt"))) {
            bw.write("第一行内容");
            bw.newLine();  // 写入换行符
            bw.write("第二行内容");
            bw.newLine();
            bw.write("第三行内容");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

性能对比

测试代码

import java.io.*;

public class PerformanceTest {
    public static void main(String[] args) throws IOException {
        String file = "largefile.txt";
        
        // 生成测试文件
        generateTestFile(file, 10_000_000);
        
        // 普通流读取
        long start1 = System.currentTimeMillis();
        readWithoutBuffer(file);
        long time1 = System.currentTimeMillis() - start1;
        System.out.println("普通流耗时：" + time1 + "ms");
        
        // 缓冲流读取
        long start2 = System.currentTimeMillis();
        readWithBuffer(file);
        long time2 = System.currentTimeMillis() - start2;
        System.out.println("缓冲流耗时：" + time2 + "ms");
    }
    
    static void readWithoutBuffer(String file) throws IOException {
        try (FileInputStream fis = new FileInputStream(file)) {
            byte[] buffer = new byte[1024];
            while (fis.read(buffer) != -1) {
                // 读取
            }
        }
    }
    
    static void readWithBuffer(String file) throws IOException {
        try (BufferedInputStream bis = new BufferedInputStream(
                new FileInputStream(file))) {
            byte[] buffer = new byte[1024];
            while (bis.read(buffer) != -1) {
                // 读取
            }
        }
    }
    
    static void generateTestFile(String file, int lines) throws IOException {
        try (BufferedWriter bw = new BufferedWriter(new FileWriter(file))) {
            for (int i = 0; i < lines; i++) {
                bw.write("This is line " + i);
                bw.newLine();
            }
        }
    }
}

性能对比结果

操作	普通流	缓冲流	提升
读取 10MB 文件	~500ms	~50ms	10x
写入 10MB 文件	~600ms	~80ms	7x

文件复制（缓冲流版）

字节缓冲流复制

public class BufferedFileCopy {
    public static void main(String[] args) {
        String source = "source.mp4";
        String target = "target.mp4";
        
        try (BufferedInputStream bis = new BufferedInputStream(
                new FileInputStream(source));
             BufferedOutputStream bos = new BufferedOutputStream(
                new FileOutputStream(target))) {
            
            byte[] buffer = new byte[8192];
            int len;
            while ((len = bis.read(buffer)) != -1) {
                bos.write(buffer, 0, len);
            }
            System.out.println("复制完成");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

字符缓冲流复制

public class BufferedTextCopy {
    public static void main(String[] args) {
        try (BufferedReader br = new BufferedReader(new FileReader("source.txt"));
             BufferedWriter bw = new BufferedWriter(new FileWriter("target.txt"))) {
            
            String line;
            while ((line = br.readLine()) != null) {
                bw.write(line);
                bw.newLine();
            }
            System.out.println("复制完成");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

实战：文件排序

读取文件内容，排序后写入新文件：

import java.io.*;
import java.util.*;

public class FileSorter {
    public static void main(String[] args) {
        try (BufferedReader br = new BufferedReader(new FileReader("numbers.txt"));
             BufferedWriter bw = new BufferedWriter(new FileWriter("sorted.txt"))) {
            
            // 读取所有行
            List<Integer> numbers = new ArrayList<>();
            String line;
            while ((line = br.readLine()) != null) {
                numbers.add(Integer.parseInt(line.trim()));
            }
            
            // 排序
            Collections.sort(numbers);
            
            // 写入结果
            for (Integer num : numbers) {
                bw.write(String.valueOf(num));
                bw.newLine();
            }
            System.out.println("排序完成");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

实战：统计词频

import java.io.*;
import java.util.*;

public class WordCounter {
    public static void main(String[] args) {
        Map<String, Integer> wordCount = new HashMap<>();
        
        try (BufferedReader br = new BufferedReader(new FileReader("article.txt"))) {
            String line;
            while ((line = br.readLine()) != null) {
                // 分割单词
                String[] words = line.toLowerCase().split("\\W+");
                for (String word : words) {
                    if (!word.isEmpty()) {
                        wordCount.merge(word, 1, Integer::sum);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        
        // 输出结果
        wordCount.entrySet().stream()
            .sorted(Map.Entry.<String, Integer>comparingByValue().reversed())
            .limit(10)
            .forEach(e -> System.out.println(e.getKey() + ": " + e.getValue()));
    }
}

缓冲区大小选择

场景	建议缓冲区大小
小文件（< 1MB）	默认 8KB
中等文件（1-100MB）	16KB - 64KB
大文件（> 100MB）	64KB - 256KB
SSD 磁盘	较小缓冲区即可
HDD 磁盘	较大缓冲区更优

小结

• 缓冲流通过内置缓冲区减少系统调用，大幅提升性能
• BufferedReader 提供 readLine() 逐行读取
• BufferedWriter 提供 newLine() 写入换行符
• 处理大文件时，缓冲流性能提升可达 10 倍以上
• 默认缓冲区 8KB，可根据场景调整
• 始终使用 try-with-resources 确保流被正确关闭