How do I close a file after catching an IOException in java?

Question

All,

I am trying to ensure that a file I have open with BufferedReader is closed when I catch an IOException, but it appears as if my BufferedReader object is out of scope in the catch block.

public static ArrayList readFiletoArrayList(String fileName, ArrayList fileArrayList)
{
    fileArrayList.removeAll(fileArrayList);

    try {
        //open the file for reading
        BufferedReader fileIn = new BufferedReader(new FileReader(fileName));

        // add line by line to array list, until end of file is reached
        // when buffered reader returns null (todo). 
        while(true){
                fileArrayList.add(fileIn.readLine());
            }
    }catch(IOException e){
        fileArrayList.removeAll(fileArrayList);
        fileIn.close(); 
        return fileArrayList; //returned empty. Dealt with in calling code. 
    }
}

Netbeans complains that it "cannot find symbol fileIn" in the catch block, but I want to ensure that in the case of an IOException that the Reader gets closed. How can I do that without the ugliness of a second try/catch construct around the first?

Any tips or pointers as to best practise in this situation is appreciated,

Yishai · Accepted Answer · 2010-04-15 22:09:24Z

25

 BufferedReader fileIn = null;
 try {
       fileIn = new BufferedReader(new FileReader(filename));
       //etc.
 } catch(IOException e) {
      fileArrayList.removeall(fileArrayList);
 } finally {
     try {
       if (fileIn != null) fileIn.close();
     } catch (IOException io) {
        //log exception here
     }
 }
 return fileArrayList;

A few things about the above code:

close should be in a finally, otherwise it won't get closed when the code completes normally, or if some other exception is thrown besides IOException.
Typically you have a static utility method to close a resource like that so that it checks for null and catches any exceptions (which you never want to do anything about other than log in this context).
The return belongs after the try so that both the main-line code and the exception catching have a return method without redundancy.
If you put the return inside the finally, it would generate a compiler warning.

answered Apr 15, 2010 at 22:09

Yishai

92.4k31 gold badges195 silver badges266 bronze badges

Sign up to request clarification or add additional context in comments.

11 Comments

BalusC Over a year ago

+1 for the finally. Surprisingly a lot of answers didn't mention about it. Here are some Sun links of interest: Lesson: Basic IO and Lesson: Exceptions

Dave Jarvis Over a year ago

You can remove the check for null and catch Exception, unless you plan on telling the user that you couldn't close the file versus couldn't open it. But, generally, users don't care.

Bert F Over a year ago

You closed the BufferdedRead, but not the FileReader?

DaveJohnston Over a year ago

Closing the BufferedReader will automatically close the FileReader.

BalusC Over a year ago

@Bert: Indeed, the Java IO classes follows the decorator pattern. So will under each close() delegate upwards to the decorated instance.

|

purecharger · Accepted Answer · 2010-04-15 22:08:29Z

1

Once you hit the catch block, any variables declared in the try are not scoped anymore. Declare BufferedReader fileIn = null; above the try block, then assign it inside. In your catch block, do if(fileIn != null) fileIn.close();

answered Apr 15, 2010 at 22:08

purecharger

1,2532 gold badges15 silver badges34 bronze badges

Comments

Yuliy · Accepted Answer · 2010-04-15 22:08:41Z

It's complaining about the symbol not being there because it's not. It's in the try block. If you want to refer to fileIn, you'll need to declare it outside the try.

However, it really sounds like you'd want to place the close in a finally block instead: you should close the file regardless of success or failure before returning.

public static ArrayList readFiletoArrayList(String fileName, ArrayList fileArrayList)
{
    fileArrayList.removeAll(fileArrayList);

    BufferedReader fileIn = null;
    try {
        //open the file for reading
        fileIn = new BufferedReader(new FileReader(fileName));

        // add line by line to array list, until end of file is reached
        // when buffered reader returns null (todo). 
        while(true){
                fileArrayList.add(fileIn.readLine());
            }
    }catch(IOException e){
        fileArrayList.removeAll(fileArrayList); 
    }finally{
        if(fileIn != null) fileIn.close();
    }
    return fileArrayList;
}

DaveJohnston · Accepted Answer · 2010-04-15 22:16:42Z

1

My preferred way of performing clean-up after an exception (when the clean-up can potentially also throw an exception) is to put the code in the try block inside another try/finally block, as follows:

public static ArrayList readFiletoArrayList(String fileName, ArrayList fileArrayList) {
    fileArrayList.removeAll(fileArrayList);

    try {
        //open the file for reading
        BufferedReader fileIn = null;

        try {
            fileIn = new BufferedReader(new FileReader(fileName));
            // add line by line to array list, until end of file is reached
            // when buffered reader returns null (todo). 
            while(true){
                fileArrayList.add(fileIn.readLine());
            }
        } finally {
            if (fileIn != null) {
                fileIn.close();
            }
        }
    }catch(IOException e){
        fileArrayList.removeAll(fileArrayList);
        return fileArrayList; //returned empty. Dealt with in calling code. 
    }
}

answered Apr 15, 2010 at 22:16

DaveJohnston

10.2k10 gold badges58 silver badges84 bronze badges

1 Comment

Florian F Over a year ago

This has the disadvantage of swallowing the wrong exception. If the readLine fails for some reason it throws an IOException. The file is then closed inside the finally. If the file close also fails, a new IOException is thrown and the first exception, with a possibly valuable error message, is lost.

Kaleb Pederson · Accepted Answer · 2010-04-15 22:07:41Z

0

Move the declaration out of the try block:

public static ArrayList readFiletoArrayList(String fileName, ArrayList fileArrayList)
{
    fileArrayList.removeAll(fileArrayList);

    BufferedReader fileIn = null;
    try {
        //open the file for reading
        fileIn = new BufferedReader(new FileReader(fileName));

        // add line by line to array list, until end of file is reached
        // when buffered reader returns null (todo). 
        while(true){
                fileArrayList.add(fileIn.readLine());
            }
    }catch(IOException e){
        fileArrayList.removeAll(fileArrayList);
        fileIn.close(); 
        return fileArrayList; //returned empty. Dealt with in calling code. 
    }
}

But, you'll still need to be careful that fileIn was actually initialized before trying to close it:

if (fileIn != null)
    fileIn.close();

answered Apr 15, 2010 at 22:07

Kaleb Pederson

46.7k21 gold badges105 silver badges148 bronze badges

2 Comments

T.J. Crowder Over a year ago

If there's an exception in the reader constructors (file not found, for instance), that'll blow up with a NullPointerException. Have to check the null in the catch.

SMBiggs Over a year ago

In your catch, you have fileIn.close(); which also throws an IOException. The compiler won't allow it.

John ClearZ · Accepted Answer · 2010-04-15 22:15:16Z

Declare the BufferedReader outside the try block and set it to null then use a finally block to close it if its not null. Also fileArrayList is passed by reference so any changes made to it will happen to the object you passed in so there is no need to also return it.

    public static ArrayList readFiletoArrayList(String fileName, ArrayList fileArrayList)
{
    fileArrayList.removeAll(fileArrayList);
    BufferedReader fileIn = null;
    try {
        //open the file for reading
        fileIn = new BufferedReader(new FileReader(fileName));

        // add line by line to array list, until end of file is reached
        // when buffered reader returns null (todo). 
        while(true){
                fileArrayList.add(fileIn.readLine());
            }
    }catch(IOException e){
        fileArrayList.removeAll(fileArrayList);  
    }finally
    {
       try
       {
           if(fillIn != null)
               fileIn.close();
       }
       catch(IOException e){}
    }
    return fileArrayList; //returned empty. Dealt with in calling code.
}

Community · Accepted Answer · 2017-05-23 12:17:45Z

0

It's better not to deal with null - the general idiom for resource acquisition and release in Java is :

final Resource resource = acquire();
try { use(resource); }
finally { resource.release(); }

so:

public static List<String> readFiletoArrayList(String fileName,
        List<String> fileArrayList, String charsetName) {
    fileArrayList.clear(); // why fileArrayList.removeAll(fileArrayList) ?
    try {
        InputStream file = new FileInputStream(fileName);
        try {
            InputStreamReader reader = new InputStreamReader(file, charsetName);
            BufferedReader buffer = new BufferedReader(reader);
            for (String line = buffer.readLine(); line != null; line = buffer
                    .readLine()) {
                fileArrayList.add(line);
            }
        } finally {
            try {
                file.close();
            } catch (IOException e) {
                e.printStackTrace(); // you do not want this to hide an
                // exception thrown earlier so swallow it
            }
        }
    } catch (IOException e) {
        fileArrayList.clear(); // returned empty. Dealt with in client
    }
    return fileArrayList;
}

See my points here

If you are using a reader you must specify the encoding as I am doing here. If you want to read bytes forget about the reader. Also if you use readLine() you must forget about the end of line characters - if this is a concern consider omitting the BufferedReader entirely.

edited May 23, 2017 at 12:17

CommunityBot

11 silver badge

answered Jul 18, 2013 at 21:21

Mr_and_Mrs_D

34.5k45 gold badges194 silver badges373 bronze badges

4 Comments

supercat Over a year ago

It would seem to me that if the file was open for writing rather than reading, safe operation would require that any IOException which occurs while closing the file not be swallowed unless there was already an IOException percolating up the call stack. It may be reasonable to swallow an IOException thrown by a file-close operation which is itself within a Catch(IOException e), but if the finally is reached via normal code execution, swallowing an IOException on close could cause the caller to erroneously believe a file was written when it actually wasn't.

Mr_and_Mrs_D Over a year ago

@supercat: you are right - but this is file reading. Writing must flush the decorator (and not rely on close) - if there is a decorator. I go through some peculiarities here - unjustly closed

supercat Over a year ago

The pattern of stifling cleanup exceptions is often applied without regard for what the cleanup entails. In some cases it's okay, but as a habit it seems dangerous. As for your linked (closed) question, I'd suggest that you start with a bit less "backstory". What you really want to know (from what I can tell) is how to deal with an exception thrown from a constructor which is supposed to acquire ownership if a passed-in stream. My suggestion would be that such constructors should close the passed-in stream if they throw an exception, or else be wrapped in a factory method that does so.

SMBiggs Over a year ago

While necessary, that is some ugly code. What should be a few lines has turned into a mess of scopes with a tangled spaghetti flow. And to think all this complexity was added for the sake of simplicity! Makes me yearn for the days of assembly.

Collectives™ on Stack Overflow

How do I close a file after catching an IOException in java?

7 Answers 7

11 Comments

Comments

Comments

1 Comment

2 Comments

Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

7 Answers 7

11 Comments

Comments

Comments

1 Comment

2 Comments

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related