我如何将 Java 控制台输出读入字符串缓冲区

问题描述

我有一个将一些文本输出到控制台的 Java 程序.它使用 print、println 和其他一些方法来执行此操作.

I have a Java program that outputs some text into console. It uses print, println, and some other methods to do this.

在程序结束时，我想读取控制台中的所有文本并将其复制到字符串缓冲区中.我怎么能在 Java 中做到这一点?我需要分别阅读stdout和stderr.

At the end of the program , I want to read all the text in console and copy it into a String buffer. How could I do this in Java ? I need to read stdout and stderr separately.

推荐答案

好的，这是一个有趣的问题.似乎不是一次解决所有 PrintStream 方法的优雅方法.(遗憾的是没有FilterPrintStream.)

Ok, this was a fun problem. Dosen't seem to be an elegant way of solving it for all PrintStream methods at once. (Unfortunately there is no FilterPrintStream.)

我确实写了一个丑陋的基于反射的解决方法(我想不要在生产代码中使用:)

I did write up an ugly reflection-based workaround though (not to be used in production code I suppose :)

class LoggedPrintStream extends PrintStream {

    final StringBuilder buf;
    final PrintStream underlying;

    LoggedPrintStream(StringBuilder sb, OutputStream os, PrintStream ul) {
        super(os);
        this.buf = sb;
        this.underlying = ul;
    }

    public static LoggedPrintStream create(PrintStream toLog) {
        try {
            final StringBuilder sb = new StringBuilder();
            Field f = FilterOutputStream.class.getDeclaredField("out");
            f.setAccessible(true);
            OutputStream psout = (OutputStream) f.get(toLog);
            return new LoggedPrintStream(sb, new FilterOutputStream(psout) {
                public void write(int b) throws IOException {
                    super.write(b);
                    sb.append((char) b);
                }
            }, toLog);
        } catch (NoSuchFieldException shouldNotHappen) {
        } catch (IllegalArgumentException shouldNotHappen) {
        } catch (IllegalAccessException shouldNotHappen) {
        }
        return null;
    }
}

...可以这样使用:

public class Test {
    public static void main(String[] args) {

        // Create logged PrintStreams
        LoggedPrintStream lpsOut = LoggedPrintStream.create(System.out);
        LoggedPrintStream lpsErr = LoggedPrintStream.create(System.err);

        // Set them to stdout / stderr
        System.setOut(lpsOut);
        System.setErr(lpsErr);

        // Print some stuff
        System.out.print("hello ");
        System.out.println(5);
        System.out.flush();

        System.err.println("Some error");
        System.err.flush();

        // Restore System.out / System.err
        System.setOut(lpsOut.underlying);
        System.setErr(lpsErr.underlying);

        // Print the logged output
        System.out.println("----- Log for System.out: -----
" + lpsOut.buf);
        System.out.println("----- Log for System.err: -----
" + lpsErr.buf);
    }
}

结果输出:

hello 5
Some error
----- Log for System.out: -----
hello 5

----- Log for System.err: -----
Some error

(请注意，FilterOutputStream 中的 out 字段受到保护和记录，因此它是 API 的一部分 :-)

(Note though, that the out field in FilterOutputStream is protected and documented, so it is part of the API :-)

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