JDK7文件處理

實用的工具類，Path，Paths，Files，FileSystem 

有一些很靈活的處理方法： 

//得到一個Path對象
Path path = Paths.get("/test/a.txt");
//Path轉換File
File file = path.toFile();

Files.readAllBytes(path);
Files.deleteIfExists(path);
Files.size(path);

正確拼接路徑不要手動拼接路徑

不好的代碼： 

String game = "foo";
File file = new File("~/test/" + game + ".txt");

即使是要手動拼接路徑，請使用下面兩個平臺無關的變量： 

System.out.println(File.pathSeparator);
System.out.println(File.separator);

正確簡潔的方法是使用Paths類： 

Path path = Paths.get("~/test/", "foo", "bar", "a.txt");
System.out.println(path);
//  ~/test/foo/bar/a.txt

讀取文件的所有內容，文件的所有行

讀取文件所有內容前，先判斷文件大小，防止OOM。 

public static byte[] readAllBytes(String fileName, long maxSize) throws IOException {
    Path path = Paths.get(fileName);
    long size = Files.size(path);
    if (size > maxSize) {
        throw new IOException("file: " + path + ", size:" + size + "> " + maxSize);
    }
    return Files.readAllBytes(path);
}

public static List<String> readAlllines(String fileName, Charset charset, long maxSize) throws IOException {
    Path path = Paths.get(fileName);
    long size = Files.size(path);
    if (size > maxSize) {
        throw new IOException("file: " + path + ", size:" + size + "> " + maxSize);
    }
    return Files.readAllLines(path, charset);
}

利用JDK7的特性，auto close，遠離一堆的catch, close

Path path = Paths.get("~/test/", "foo", "bar", "a.txt");
try (InputStream in = Files.newInputStream(path)) {
    // process
    //in.read();
}

歷遍目錄

DK7新特性，FileVisitor 

public class MyFileVisitor extends SimpleFileVisitor<Path>{
    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
        System.out.println(file);
        return FileVisitResult.CONTINUE;
    }

    public static void main(String[] args) throws IOException {
        Path path = Paths.get("/home/user/test");
        Files.walkFileTree(path, new MyFileVisitor());
    }
}

判斷文件是否在父路徑下

網上流傳一種遞歸判斷parent的方式，http://stackoverflow.com/questions/18227634/check-if-file-is-in-subdirectory

但是查閱jdk代碼後，發現getParent()函數是通過處理文件名得到的。所以直接比較文件前綴即可。 

請務必注意，file.getCanonicalPath()函數 。

public static boolean isSubFile(File parent, File child) throws IOException {
    return child.getCanonicalPath().startsWith(parent.getCanonicalPath());
}


public static boolean isSubFile(String parent, String child) throws IOException {
    return isSubFile(new File(parent), new File(child));
}