java - 我是否必须关闭由 PrintStream 包装的 FileOutputStream？

Question

我将 FileOutputStream 与 PrintStream 一起使用，如下所示:

class PrintStreamDemo {  
    public static void main(String args[]) { 
        FileOutputStream out; 
        PrintStream ps; // declare a print stream object
        try {
            // Create a new file output stream
            out = new FileOutputStream("myfile.txt");

            // Connect print stream to the output stream
            ps = new PrintStream(out);

            ps.println ("This data is written to a file:");
            System.err.println ("Write successfully");
            ps.close();
        }
        catch (Exception e) {
            System.err.println ("Error in writing to file");
        }
    }
}

我只关闭了 PrintStream。我还需要关闭 FileOutputStream (out.close();) 吗？

Answer 1

不需要，只需要关闭最外层的流即可。它将一直委托(delegate)给包装流。

但是，您的代码包含一个概念上的错误，关闭应该发生在 finally 中，否则当代码在打开和关闭之间抛出异常时它永远不会关闭。

例如

public static void main(String args[]) throws IOException { 
    PrintStream ps = null;

    try {
        ps = new PrintStream(new FileOutputStream("myfile.txt"));
        ps.println("This data is written to a file:");
        System.out.println("Write successfully");
    } catch (IOException e) {
        System.err.println("Error in writing to file");
        throw e;
    } finally {
        if (ps != null) ps.close();
    }
}

(请注意，我将代码更改为抛出异常，以便您了解问题的原因，异常即包含有关问题原因的详细信息)

或者，当您已经在使用 Java 7 时，您还可以使用 ARM(自动资源管理；也称为 try-with-resources)，这样您就不需要自己关闭任何东西:

public static void main(String args[]) throws IOException { 
    try (PrintStream ps = new PrintStream(new FileOutputStream("myfile.txt"))) {
        ps.println("This data is written to a file:");
        System.out.println("Write successfully");
    } catch (IOException e) {
        System.err.println("Error in writing to file");
        throw e;
    }
}