I have a Java program that outputs some text into console. It uses print
, println
, and some other methods to do this.
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.
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
.)
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;
}
}
...that can be used like this:
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);
}
}
Resulting output:
hello 5
Some error
----- Log for System.out: -----
hello 5
----- Log for System.err: -----
Some error
(Note though, that the out
field in FilterOutputStream
is protected and documented, so it is part of the API :-)
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