Java IO流详解

引言

IO流是Java处理输入输出的核心API。本文系统讲解Java IO流的使用方法。

一、IO流分类

字节流
├── InputStream（输入）
│   ├── FileInputStream
│   ├── BufferedInputStream
│   └── ObjectInputStream
└── OutputStream（输出）
    ├── FileOutputStream
    ├── BufferedOutputStream
    └── ObjectOutputStream

字符流
├── Reader（输入）
│   ├── FileReader
│   ├── BufferedReader
│   └── InputStreamReader
└── Writer（输出）
    ├── FileWriter
    ├── BufferedWriter
    └── OutputStreamWriter

二、字节流

2.1 FileInputStream和FileOutputStream

// 读取文件
try (FileInputStream fis = new FileInputStream("input.txt")) {
    int data;
    while ((data = fis.read()) != -1) {
        System.out.print((char) data);
    }
} catch (IOException e) {
    e.printStackTrace();
}

// 写入文件
try (FileOutputStream fos = new FileOutputStream("output.txt")) {
    String content = "Hello, World!";
    fos.write(content.getBytes());
} catch (IOException e) {
    e.printStackTrace();
}

2.2 BufferedInputStream和BufferedOutputStream

// 带缓冲的读取（效率更高）
try (BufferedInputStream bis = new BufferedInputStream(
        new FileInputStream("input.txt"))) {
    byte[] buffer = new byte[1024];
    int len;
    while ((len = bis.read(buffer)) != -1) {
        System.out.println(new String(buffer, 0, len));
    }
}

// 带缓冲的写入
try (BufferedOutputStream bos = new BufferedOutputStream(
        new FileOutputStream("output.txt"))) {
    bos.write("Hello".getBytes());
    bos.flush();  // 刷新缓冲区
}

三、字符流

3.1 FileReader和FileWriter

// 读取文本文件
try (FileReader reader = new FileReader("input.txt")) {
    int data;
    while ((data = reader.read()) != -1) {
        System.out.print((char) data);
    }
}

// 写入文本文件
try (FileWriter writer = new FileWriter("output.txt")) {
    writer.write("Hello, World!");
}

3.2 BufferedReader和BufferedWriter

// 按行读取
try (BufferedReader br = new BufferedReader(
        new FileReader("input.txt"))) {
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
}

// 按行写入
try (BufferedWriter bw = new BufferedWriter(
        new FileWriter("output.txt"))) {
    bw.write("Line 1");
    bw.newLine();
    bw.write("Line 2");
}

四、转换流

// InputStreamReader：字节流转字符流
try (InputStreamReader isr = new InputStreamReader(
        new FileInputStream("input.txt"), "UTF-8")) {
    int data;
    while ((data = isr.read()) != -1) {
        System.out.print((char) data);
    }
}

// OutputStreamWriter：字符流转字节流
try (OutputStreamWriter osw = new OutputStreamWriter(
        new FileOutputStream("output.txt"), "UTF-8")) {
    osw.write("Hello, 世界!");
}

五、对象流

// 对象序列化
class Person implements Serializable {
    private static final long serialVersionUID = 1L;
    private String name;
    private int age;
    
    // 构造器、getter、setter
}

// 写入对象
try (ObjectOutputStream oos = new ObjectOutputStream(
        new FileOutputStream("person.dat"))) {
    Person person = new Person("Alice", 25);
    oos.writeObject(person);
}

// 读取对象
try (ObjectInputStream ois = new ObjectInputStream(
        new FileInputStream("person.dat"))) {
    Person person = (Person) ois.readObject();
    System.out.println(person);
}

六、文件操作

import java.io.File;

// 创建文件
File file = new File("test.txt");
if (!file.exists()) {
    file.createNewFile();
}

// 创建目录
File dir = new File("mydir");
dir.mkdir();      // 创建单级目录
dir.mkdirs();     // 创建多级目录

// 删除文件
file.delete();

// 文件信息
System.out.println("文件名: " + file.getName());
System.out.println("绝对路径: " + file.getAbsolutePath());
System.out.println("文件大小: " + file.length());
System.out.println("是否是目录: " + file.isDirectory());
System.out.println("是否是文件: " + file.isFile());

// 列出目录内容
File[] files = dir.listFiles();
for (File f : files) {
    System.out.println(f.getName());
}

七、NIO

7.1 Channel和Buffer

// 使用NIO读取文件
try (FileChannel channel = FileChannel.open(
        Paths.get("input.txt"), StandardOpenOption.READ)) {
    ByteBuffer buffer = ByteBuffer.allocate(1024);
    
    while (channel.read(buffer) != -1) {
        buffer.flip();  // 切换到读模式
        while (buffer.hasRemaining()) {
            System.out.print((char) buffer.get());
        }
        buffer.clear();  // 清空缓冲区
    }
}

7.2 Files工具类

// 读取所有行
List<String> lines = Files.readAllLines(Paths.get("input.txt"));

// 写入所有行
Files.write(Paths.get("output.txt"), lines);

// 复制文件
Files.copy(Paths.get("source.txt"), Paths.get("target.txt"));

// 删除文件
Files.delete(Paths.get("file.txt"));

// 遍历目录
Files.walk(Paths.get("mydir"))
     .forEach(System.out::println);

八、实战示例

8.1 文件复制

public void copyFile(String source, String target) {
    try (FileInputStream fis = new FileInputStream(source);
         FileOutputStream fos = new FileOutputStream(target)) {
        
        byte[] buffer = new byte[1024];
        int len;
        while ((len = fis.read(buffer)) != -1) {
            fos.write(buffer, 0, len);
        }
    } catch (IOException e) {
        e.printStackTrace();
    }
}

8.2 文本文件统计

public void analyzeFile(String filename) throws IOException {
    try (BufferedReader br = new BufferedReader(
            new FileReader(filename))) {
        int lines = 0;
        int words = 0;
        int chars = 0;
        
        String line;
        while ((line = br.readLine()) != null) {
            lines++;
            chars += line.length();
            words += line.split("\\s+").length;
        }
        
        System.out.println("行数: " + lines);
        System.out.println("单词数: " + words);
        System.out.println("字符数: " + chars);
    }
}