Storing error message in string after error has occured java?

Question

I need to store an error message in a string after the error has occurred. I am unable to use the try and catch blocks as the error will appear after the initial code is run.

Before I was using the code below to store the error message in a file when it appeared:

PrintStream printS = new PrintStream("file.txt");  
System.setErr(pst);

However I want to store it and a string and send this string else where after the error occured. I have tried already using a byte array output stream:

ByteArrayOutputStream byteArray = new ByteArrayOutputStream();
PrintStream printS = new PrintStream(byteArray);  
System.setErr(pst);
String tester = byteArray.toString(StandardCharsets.UTF_8);

But when I try to print the string it is empty and does not contain the error. Does anyone know how I can run code after a error has occurred, so I can send this error message elsewhere?

You are creating byteArray, and the trying the byteArray.toString. You are no using it. — pringi, Mar 22 '22 at 17:11

score 0 · Answer 1 · answered Mar 22 '22 at 17:33

Java just runs code on the spot, you're not defining triggers for the future. You are creating a new empty byte array output stream, setting it up as 'target' for any System.err calls later, and then immediately turning it into a string. It's still blank at this point, of course.

What you want is quite complicated - you'd have to write a custom PrintStream that saves anything sent to it, until it 'sees' a newline and then acts. Something like:

OutputStream myCustomReportingStream = new OutputStream() {
  ByteArrayOutputStream buffer = new ByteArrayOutputStream();

  @Override public void write(int b) {
    if (b == '\r') continue;
    if (b == '\n') {
      process(new String(buffer.getBytes(), StandardCharsets.UTF_8));
      buffer.reset();
    } else {
      buffer.write(b);
    }
  }

  private void process(String message) {
    // YOUR CODE HERE
  }
};
System.setErr(new PrintStream(myCustomreportingStream, true, StandardCharsets.UTF_8));

A bunch of things are happening here:

We're defining a custom OutputStream which buffers everything sent to it, until it sees \n (which is "newline" in ASCII, UTF-8, and many other encodings), at which point it turns the buffer into a string, sends that string to the process method, and clears out the buffer. It also ignores \r to deal with \r\n which is windows-style newlines.
We then use that OutputStream as basis for a new PrintStream, and then set up syserr to use this printstream.
This code does not deal with anything that sends data to System.err but doesn't send a newline symbol.
Anytime anybody anywhere runs System.err.println, your process() method will be run.

thankyou, but the code above has one error, that is "The method getBytes() is undefined for the type ByteArrayOutputStream". — Phillip Smith, Mar 22 '22 at 19:31
I wrote it from memory - just check the API docs. It's .toByteArray() or some such. — rzwitserloot, Mar 22 '22 at 21:28

Storing error message in string after error has occured java?

1 Answers1