Question

我遇到了PrintWriter的一些问题。我写了一些代码，它们只是从文件中获取一些输入，然后将其输出到另一个。

尽管已创建文件，但该文件仍为空。所需的输入可以轻松地在控制台中打印出来，这意味着FileInputStream正常工作。

为什么PrintWriter不打印任何内容？

public static void writeInFile(File in, File out) throws FileNotFoundException {
    PrintWriter outputStream = null
    Scanner scanner = new Scanner(new FileInputStream(in));
    outputStream = new PrintWriter(new FileOutputStream(out));
    outputStream.print("test");
    while(scanner.hasNext()) {
    outputStream.print(scanner.nextLine() + "\n");
    }
    scanner.close();
}

Answer 1

确保始终关闭OutputStreams：

        while(scanner.hasNext()) {
            String s = scanner.nextLine();
            outputStream.print(s+"\n");
            System.out.println("Test "+s); //d.h. das Problem liegt am outputstream
        }
        outputStream.close();
        scanner.close();

编辑：当您close outputStream时，它会自动调用flush，这会将缓冲区写入文件。如果不关闭它，缓冲区可能永远不会被清空/写入文件，就像这里一样。

另请参阅this answer。

Answer 2

在处理需要清理的IO时，我更喜欢使用自动资源清理。这是最基本的需求：

public static void writeInToOut(InputStream in, OutputStream out) {
    try(PrintWriter outputStream = new PrintWriter(out);
        Scanner scanner = new Scanner(in)) {
        while(scanner.hasNext()) {
            outputStream.print(scanner.nextLine()+"\n");
        }
    }
}

您现在可以通过几种方式重载此功能：

public static void writeInToOut(File file, OutputStream out) {
    try (InputStream in = new FileInputStream(file)) {
        writeInToOut(in, out);
    } catch (IOException e) {
        Logger.getAnonymousLogger().log(Level.WARNING, "IOError", e);
    }
}

public static void writeInToOut(File inFile, File outFile) {
    try (InputStream in = new FileInputStream(inFile);
         OutputStream out = new FileOutputStream(outFile)) {
        writeInToOut(in, out);
    } catch (IOException e) {
        Logger.getAnonymousLogger().log(Level.WARNING, "IOError", e);
    }
}

public static void writeStdInToFile(File file) {
    try (OutputStream out = new FileOutputStream(file)) {
        writeInToOut(System.in, out);
    } catch (IOException e) {
        Logger.getAnonymousLogger().log(Level.WARNING, "IOError", e);
    }
}

Java：PrintWriter无法写入文件

2 个答案: