In this lab we look at how servers host a website, and also go over debugging ideas and technique. We will look at how to make a program that handles requests in the form of URI paths, and how to make a website with that in mind. And for debuggin we will look at the core definition of bugs, symptoms, and how to categorize an issue.
In this part of the lab, we are tasked with creating a class called StringServer that will host a web server and handle requests. The first step is to build a handler that implements the java URLHandler interface. That is, we are to make a class that defines the methods outlined in the URLHandler. Once we have designed our implementation, and allowed it to handle our expected requests, in this case "add-message?s=ourString", we can create the StringServer class which calls this class in its main and creates the web server.
import java.io.IOException;
import java.net.URI;
class stringHandler implements URLHandler {
String theString = "";
public String handleRequest(URI url) {
if (url.getPath().equals("/")) {
return theString;
}
else if (url.getPath().contains("/add-message")) {
String[] parameters = url.getQuery().split("=");
if (parameters[0].equals("s")) {
theString = theString + parameters[1] + "\n";
return theString;
}
return "404 Not Found!";
} else {
System.out.println("Path: " + url.getPath());
return "404 Not Found!";
}
}
}
class StringServer {
public static void main(String[] args) throws IOException {
if(args.length == 0){
System.out.println("Missing port number! Try any number between 1024 to 49151");
return;
}
int port = Integer.parseInt(args[0]);
Server.start(port, new stringHandler());
}
}After calling Stringserver and giving it a command line argument for the port, we get this local web server:
Here the input path is add-message?s=The first message.
In this example the URI called url gets the whole url as a string. getPath returns the path portion of the url. and theString is currently empty. When the path is checked to include the add-message key word, then the = delimiter is used to separate the string from the command, and that string is added to the currently empty string. theString is then appended with a \n return character.
Here the input path is add-message?s=And this will be the second message.
In this case, the new url variable will hold the newer longer url. However, theString still has date from the previous request. Once the path is checked to have the add-message key, the string will have the new message appended as well as another endline character.
In this part of the lab, we look into how to debug in a formulaic way. We start by defining a bug, a symptom, and interpreting them based in expected versus actual output. In this case, a bug is something in the progream that causes the program to not work as expected, and a symptom is a specific case of the program not having an expected output.
Failure inducing input:
@Test
public void testMergeB(){
String[] theArray = {"a", "cheery"};
ArrayList<String> theList1 = new ArrayList<String>(Arrays.asList(theArray));
theArray = new String[] {"bright","day", "eh?"};
ArrayList<String> theList2 = new ArrayList<String>(Arrays.asList(theArray));
String[] outArray = {"a","bright", "cheery","day", "eh?"};
ArrayList<String> theMergedList = new ArrayList<String>(ListExamples.merge(theList1, theList2));
assertArrayEquals(theMergedList.toArray(),outArray);
}passing input:
public void testMergeA(){
String[] theArray = {"a", "cheery", "eh?"};
ArrayList<String> theList1 = new ArrayList<String>(Arrays.asList(theArray));
theArray = new String[] {"bright","day"};
ArrayList<String> theList2 = new ArrayList<String>(Arrays.asList(theArray));
String[] outArray = {"a","bright", "cheery","day", "eh?"};
ArrayList<String> theMergedList = new ArrayList<String>(ListExamples.merge(theList1, theList2));
assertArrayEquals(theMergedList.toArray(),outArray);
}Symptom: running out of heap
After about a minute the test failed.
before:
// Takes two sorted list of strings (so "a" appears before "b" and so on),
// and return a new list that has all the strings in both list in sorted order.
static List<String> merge(List<String> list1, List<String> list2) {
List<String> result = new ArrayList<>();
int index1 = 0, index2 = 0;
while(index1 < list1.size() && index2 < list2.size()) {
if(list1.get(index1).compareTo(list2.get(index2)) < 0) {
result.add(list1.get(index1));
index1 += 1;
}
else {
result.add(list2.get(index2));
index2 += 1;
}
}
while(index1 < list1.size()) {
result.add(list1.get(index1));
index1 += 1;
}
while(index2 < list2.size()) {
result.add(list2.get(index2));
index1 += 1;
}
return result;
}after:
static List<String> merge(List<String> list1, List<String> list2) {
List<String> result = new ArrayList<>();
int index1 = 0, index2 = 0;
while(index1 < list1.size() && index2 < list2.size()) {
if(list1.get(index1).compareTo(list2.get(index2)) < 0) {
result.add(list1.get(index1));
index1 += 1;
}
else {
result.add(list2.get(index2));
index2 += 1;
}
}
while(index1 < list1.size()) {
result.add(list1.get(index1));
index1 += 1;
}
while(index2 < list2.size()) {
result.add(list2.get(index2));
index2 += 1;
}
return result;
}The fix was actually quite simple. The issue was that once one list is entirely added to the new sorted list, all of the remaining elements in the other list must be added to the end of the merged list. This was accomplished with two while loops. However, in one of the while loops, the index was index1 when it needed to be index2. This caused an infinite loop. However, the infinite loop will only occur when the second array has the last element alphabetically.
One of the coolest things I learned in these labs was just how server hosting works. I had no idea about how local web servers were a thing either.
I also really appreciate having a formal and structured way of thinking about bugs and debugging.
And also I finally understand JUnit through the command line.
ChatGPT was used to provide a template for the markdown file. THe template provided is shown:
