Why does PrintStream.close() end up getting called twice?

Question

Somewhat to my surprise, the following code prints out "Close" twice. Running through the debugger, it seems MyPrintStream.close() calls super.close(), which ends up calling MyPrintStream.close() again.

    
import java.io.*;

public class PrintTest
{
    static class MyPrintStream extends PrintStream
    {
        MyPrintStream(OutputStream os)
        {
            super(os);
        }

        @Override
        public void close()
        {
            System.out.println("Close");
            super.close();
        }
    }

    public static void main(String[] args) throws IOException
    {
        PrintStream ps = new MyPrintStream(new FileOutputStream(File.createTempFile("temp", "file")));
        ps.println("Hello");
        ps.close();
    }
}

Why is this happening? Should I not be extending PrintStream?

Answer 1

If you look at your code in a debugger and set a breakpoint in the close() method, it'll reveal the stacktraces of who is calling your close() method:

1. your main method
2. sun.nio.cs.StreamEncoder$CharsetSE.implClose() line 431

the complete stacktrace for the latter looks like:

PrintTest$MyPrintStream.close() line: 20    
sun.nio.cs.StreamEncoder$CharsetSE.implClose() line: 431 [local variables unavailable]  
sun.nio.cs.StreamEncoder$CharsetSE(sun.nio.cs.StreamEncoder).close() line: 160 [local variables unavailable]    
java.io.OutputStreamWriter.close() line: 222 [local variables unavailable]  
java.io.BufferedWriter.close() line: 250 [local variables unavailable]  
PrintTest$MyPrintStream(java.io.PrintStream).close() line: 307  
PrintTest$MyPrintStream.close() line: 20    
PrintTest.main(java.lang.String[]) line: 27 

Sadly though I can't tell why StreamEncoder would call back into your PrintStream though, as my IDE doesn't have a source attachment for sun.nio.cs.StreamEncoder :( This is JDK 6 btw, if that matters.

By the way, if you are asking this question because you noticed that custom code in your close() method is running twice, you should really be checking if this.closing. PrintStream.close() sets this to true (and the class's comments state /* To avoid recursive closing */ ).