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How do I get an exception message in a string variable in Java?

I need to handle an exception message when any exception is caught in Java.

I am working on a database connection class. When I give incorrect data like username, password, hostname, sid, etc., Control goes to the catch block and throws an error. I would like to receive this error message on the JSP side and redirect to the same page with this error message. However, when I get an error in Java, it always evaluates to null.

My sample code is here.

String errorMessage = null;
try{
// CODE Where Exception occure
}catch(SQLException se){
    errorMessage = se.getMessage();
}catch(Exception e){
    System. out.println("In Exception block.");
    errorMessage = e.getMessage();
}finally{
    System.out.println(errorMessage);
}

It will go into the Exception block, but the errorMessage will be null.

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5 answers


First, @ Artem Moskalev's answer should be correct in many ways. In your case, you would say:

It goes into the exception block, but errorMessage is null.

So let's try two cases to debug the behavior:

Firstly:

class Test1
{
    public static void main (String[] args) throws java.lang.Exception
    {
        String errorMessage = null;
            try{
                throw(new Exception("Let throw some exception message here"));
            }catch(Exception e){
                System.out.println("In Exception block.");
                errorMessage = e.getMessage();
            }finally{
                System.out.println(errorMessage);
            }
    }
}

Output:

In Exception block.
Let throw some exception message here


Seems to work as you'd expect.

Secondly:

class Test2
{
    public static void main (String[] args) throws java.lang.Exception
    {
        // String errorMessage = null;
        // To make the difference between non-initialized value 
        // and assigned null value clearer in this case,
        // we will set the errorMessage to some standard string on initialization
        String errorMessage = "Some standard error message";
            try{
                throw(new Exception());
            }catch(Exception e){
                System.out.println("In Exception block.");
                errorMessage = e.getMessage();
            }finally{
                System.out.println(errorMessage);
            }
    }
}

Output:

In Exception block.
null

Why is this? Because you access e.getMessage()

, but if emtpy message, e.getMessage()

will return null

. That way, null

not from initialization, but from the return value e.getMessage()

when e

it has no error message (for example if thrown NullPointerException

).

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This block is always executed:

...finally{
    System.out.println(errorMessage);
}


If yours errorMessage

was not previously assigned any other value (i.e. the exception thrown in your proposal try

) - System.out.println

will print you the value errorMessage

that is null

.

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your block catch(Exception e)

will catch all exceptions (except SQLException

which you specifically catch above). Some exceptions, for example. NullPointerException

can have a message with a null message, i.e. e.getMessage()

can reset zero. Therefore, it is better to print the details of the exception, such as the type of the exception - use e.ToString()

or e.printStacktrace()

for more details.

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Use is getMessage()

useless, you want either e.printStackTrace()

(for really simple programs) or for any correct error handling use a registration framework that allows you to write code like log.error("Something went wrong", e);

.

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    try {
        throw(new Exception());
    } catch (Exception e) {
        StringWriter stringWriter = new StringWriter();
        PrintWriter printWriter = new PrintWriter(stringWriter);
        e.printStackTrace(printWriter);
        stringWriter.toString();
    }
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