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File

File

依赖

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<dependency>
  <groupId>commons-io</groupId>
  <artifactId>commons-io</artifactId>
  <version>2.5</version>
</dependency>

InputStream 转为字符串

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File file = new File("a.txt");
IOUtils.toString(file.getInputStream(), Charset.forName("UTF-8"));

InputStream 转为文件

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File file = new File("a.txt");
FileUtils.copyInputStreamToFile(response.getInputStream(), file);

Bytes 转为文件

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File file = new File("targetFile");
FileUtils.writeByteArrayToFile(file, "ABC".getBytes());

拷贝文件

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public static void copyFileUsingJava7Files( File from, File to ) throws IOException {
    Files.copy( from.toPath(), to.toPath() );
}

使用 Channel:

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private static void copyFileUsingChannel(File source, File dest) throws IOException {
    FileChannel sourceChannel = null;
    FileChannel destChannel = null;
    try {
        sourceChannel = new FileInputStream(source).getChannel();
        destChannel = new FileOutputStream(dest).getChannel();
        destChannel.transferFrom(sourceChannel, 0, sourceChannel.size());
    } finally {
        sourceChannel.close();
        destChannel.close();
    }
}

使用 Stream:

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private static void copyFileUsingStream(File source, File dest) throws IOException {
    InputStream is = null;
    OutputStream os = null;
    try {
        is = new FileInputStream(source);
        os = new FileOutputStream(dest);
        byte[] buffer = new byte[1024];
        int length;
        while ((length = is.read(buffer)) > 0) {
            os.write(buffer, 0, length);
        }
    } finally {
        is.close();
        os.close();
    }
}

Paths

注意这两个的区别:

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new File(srcPath).toPath()

第二种:

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Paths.get(srcPath)

这完全是两码事，在 Windows 平台上使用第二种会抛出异常:

Java 路径

不要天真的认为分隔符总是 /，在 Windows 操作系统上问题变得异常突出，应该使用:

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File.separator

Java 删除目录

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static void deleteDir(File file) {
    File[] contents = file.listFiles();
    if (contents != null) {
        for (File f : contents) {
            deleteDir(f);
        }
    }
    file.delete();
}

文件锁

在 RocketMQ 中，用来防止启动多个 MessageStore 服务:

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File file = new File(rootDir + File.separator + "lock");
RandomAccessFile lockFile = new RandomAccessFile(file, "rw");
lock = lockFile.getChannel().tryLock(0, 1, false);
if (lock == null || lock.isShared() || !lock.isValid()) {
    throw new RuntimeException("Lock failed,MQ already started");
}

RandomAccessFile

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public class RandomAccessFile {

    public final void writeUTF(String str) throws IOException {
        DataOutputStream.writeUTF(str, this);
    }
    
}

public class DataOutputStream {

    static int writeUTF(String str, DataOutput out) throws IOException {
        // ...
        
        if (utflen > 65535)
            throw new UTFDataFormatException(
                "encoded string too long: " + utflen + " bytes");

        // ...
    }
    
}

所以 writeUTF 方法写入的字符串长度不能太长。推荐办法是使用 RandomAccessFile 的 write(byte b[]) 方法来写入:

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public void write(byte b[]) throws IOException {
    writeBytes(b, 0, b.length);
}

deleteOnExit()

File 类本身的 deleteOnExit() 方法存在一定的误导性，它不是指这个文件如果存在就会被删除。而是指当虚拟机结束运行的时候自动将这一文件删除掉。

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