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copied!<p>Ok, this was a fun problem. Dosen't seem to be an elegant way of solving it for all <code>PrintStream</code> methods at once. (Unfortunately there is no <code>FilterPrintStream</code>.)</p> <p>I did write up an ugly reflection-based workaround though (not to be used in production code I suppose :)</p> <pre><code>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; } } </code></pre> <p>...that can be used like this:</p> <pre><code>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: -----\n" + lpsOut.buf); System.out.println("----- Log for System.err: -----\n" + lpsErr.buf); } } </code></pre> <p>Resulting output:</p> <pre><code>hello 5 Some error ----- Log for System.out: ----- hello 5 ----- Log for System.err: ----- Some error </code></pre> <p>(Note though, that the <code>out</code> field in <code>FilterOutputStream</code> is protected and documented, so it is part of the API :-)</p>

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