Java I/O Annoyances

While making some code updates the other day, I discovered a couple of potentially confusing features of the java.io package that I wasn't previously aware of:

  1. The PrintStream and PrintWriter classes consume exceptions. From the PrintWriter documentation:

    Methods in this class never throw I/O exceptions, although some of its constructors may. The client may inquire as to whether any errors have occurred by invoking checkError().

    Because I had incorrectly assumed that a PrintWriter would propagate any exceptions thrown by the underlying stream, I hadn't been calling checkError(). The result was that my servlet class, which was using the print writer returned by ServletResponse#getWriter(), failed to detect when a connection had been terminated by the client. The servlet simply continued writing to the output stream. Once I started calling checkError(), the response was correctly terminated:

    if (writer.checkError()) {
        throw new IOException("Error writing to output stream.");
    }

    I couldn't find any explanation as to why these two classes were written this way, while all of the other classes in java.io appear to simply propagate exceptions.

  2. The read(byte[], int, int) method of the InputStream class also consumes exceptions. From the Javadoc:

    The read(b, off, len) method for class InputStream simply calls the method read() repeatedly. If the first such call results in an IOException, that exception is returned from the call to the read(b, off, len) method. If any subsequent call to read() results in a IOException, the exception is caught and treated as if it were end of file.

    This is extremely misleading, since it completely obscures the fact that an error occurred and makes it appear as though the stream terminated normally.

So, even though this behavior is not what I had expected, at least it is documented, and is something I'll now be aware of when using these classes in the future.

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