How do I close a file after catching an IOExceptio

问题:

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,

回答1:

 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.

回答2:

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();

回答3:

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;
}

回答4:

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. 
    }
}

回答5:

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();

回答6:

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.
}

回答7:

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.