A question about exception catching in Java

Question

I have an instance method, and within it, I did a simple webpage parsing:

public void doOperation(final AuthorAccess authorAccess, ArgumentsMap arguments) throws  IllegalArgumentException,AuthorOperationException
{

    final String server = "chiexist1.worldbook.com";
    final String port = "8080";
    try {

             docBuilder = docFactory.newDocumentBuilder();
             doc = docBuilder.parse("http://" + server + ":" + port + "/exist/webdav/db/portfolio/config/products.xml");
        ...}
       catch{}
  }

Because currently I am hard-coding the server address in the string, there might be situations where the server name is not right, so in that case, I want it to automatically to change the server URL string to "localhost".

I think an if-else statement probably would work, but I am not very sure how to determine the boolean variable for detecting whether this parsing is failed or not. I also think of putting this in the catch statement, but what about other statements are also throwing exceptions?

I've also checked the API for DocumentBuilder, the parse() method always return a Document Type but not boolean. So I would be grateful if anyone could give me some suggestions here on how to detect the wronged URL and then change to parsing localhost instead, thanks in advance.

I would guess it returns an IOException if the URL is invalid, but there are probably many other reasons why an IOException would be thrown. By the way, can you be more precise about what you mean when you say "the server name is not right"? — President James K. Polk, Dec 28 '10 at 17:40
http://stackoverflow.com/questions/4177864/checking-a-url-exist-or-not — jmj, Dec 28 '10 at 18:01
@Gregs that basically means I am accessing some other companies server which I don't have access to, so I am blocked out. However, the localhost will always work. — Kevin, Dec 28 '10 at 18:03
thanks for the info. By the way, I see that I wrote "returns an IOException" instead of "throws an IOException". I will report to the flogging chamber shortly. — President James K. Polk, Dec 28 '10 at 19:30
@Gregs: I thought it should be IOException earlier, but actually it turned out, in my code, that only SAXException could correctly catch the exception. — Kevin, Dec 29 '10 at 15:03

score 3 · Answer 1 · edited May 23 '17 at 12:04

3

I think that here you can find an answer: Validating URL in Java

edited May 23 '17 at 12:04

Community

1
1

answered Dec 28 '10 at 17:39

yegor256

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or even [this](http://stackoverflow.com/questions/4177864/checking-a-url-exist-or-not) – jmj Dec 28 '10 at 18:00

jjnguy · Accepted Answer · 2010-12-28T18:07:05.723

2

The following code should do what you like. Basically, in the catch just build another doc from localhost.

final String server = "chiexist1.worldbook.com";
final String port = "8080";
docBuilder = docFactory.newDocumentBuilder();
doc = null;
try {
    doc = docBuilder.parse("http://" + server + ":" + port + "/exist/webdav/db/portfolio/config/products.xml");
} catch{
    try{ 
        doc = docBuilder.parse("http://" + "localhost" + ":" + port + "/exist/webdav/db/portfolio/config/products.xml");
    } catch {
        // now we have an error we can't recover from
    }
}
...  // I meant to do this before.

edited Dec 28 '10 at 18:07

answered Dec 28 '10 at 17:40

jjnguy

136,852
53
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It might be a better idea to wrap only the "doc = docBuilder.parse(...)" statement in the try. That way it avoids catching exceptions from further statements. If the type of exception could be narrowed down, it would be even better. – R. Martinho Fernandes Dec 28 '10 at 17:46
@Marti, yeah, that was my fault. I forgot to move the `...` – jjnguy Dec 28 '10 at 17:47
@marc, oops, yeah. I didn't mean to do that. – jjnguy Dec 28 '10 at 17:48
@jjnguy, the question is, I need to keep the outermost try block. Could I nest them? – Kevin Dec 28 '10 at 18:05

score 1 · Answer 3 · answered Dec 28 '10 at 17:39

A general tip: don't just write "catch". Always specify which types of exceptions you're catching (preferably your own custom exception types), so that you know the code does exactly what you mean it to do. Otherwise, as you said, the catch statement may catch some other type of exception you didn't mean to occur, that should actually be thrown upwards.

moinudin · Answer 4 · 2010-12-28T17:48:33.453

Keep track of the number of attempts in a variable and loop until success or max attempts (I chose 5) is passed:

String server = "chiexist1.worldbook.com";
final String port = "8080";
int attempts = 0;
final int MAX_ATTEMPTS = 5;
boolean success = false;
docBuilder = docFactory.newDocumentBuilder();
while (!success && attempts < MAX_ATTEMPTS) {
    try {
        doc = docBuilder.parse("http://" + server + ":" + port + "/exist/webdav/db/portfolio/config/products.xml");
        success = true;
    } catch (IOException e) {
        if (++attempts < MAX_ATTEMPTS) {
            System.err.println("Attempt to connect to " + server + ":" + port + " failed. Retrying" + (attempts == 1 ? ", but connecting to localhost:" + port + " this time" : "") + ".");
            server = "localhost";
        } else {
            System.err.println("Attempt to connect to " + server + ":" + port + " failed. Giving up after " + attempts + " failed attempts.");
        }
    } 
}
if (!success) {
    // Tidy up and exit
}
// Do stuff

score 0 · Answer 5 · answered Dec 28 '10 at 17:44

You should catch the respective exceptions separately and perform different error handling logic depending on which exception you catch. That is considered the best approach since it separates error condiions from normal logic and u can have different errors handled differently.

It is considered better than if else statements within your normal flow of logic.

score 0 · Answer 6 · edited May 23 '17 at 12:26

Try something like this:

static String hostname = "hostname";
static{
   if(doesHostExist(hostName)==false){
     hostname="localhost";
   }
}

private static boolean doesHostExist(String host){
  try {
    URL url = new URL("http://" + host);
    URLConnection conn = url.openConnection();
    conn.connect();
    return true;
  } catch (MalformedURLException e) {
    return false;
  } catch (IOException e) {
    return false;
  }

}

*edit additional code lifted from a previous answer: Validating URL in Java

A question about exception catching in Java

6 Answers6