如何访问从另一个类引发的异常

Question

My requirement is to send automatic mail whenever the exception arises

public Connection dbConnect(String dbconnectionstring,String userid,String userpassword)
{
    Connection connection = null;
    System.out.println();
  try {
     Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver").newInstance();
     connection = DriverManager.getConnection(dbconnectionstring,userid,userpassword);
     System.out.println("connected to" + dbconnectionstring );

    } catch (InstantiationException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (IllegalAccessException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

  return connection;

now in the other class, if I want to access the content of Exception e (if any exception raises), what should be written in my code to know if any exception raised? and if the exception raised, the content of e should be mailed.

Answer 1

Re-throw the same exception, or wrap it inside an application-specific exception class (my preference):

    ...
} catch (InstantiationException e) {
    ...
    throw new MyAppException(e);
}

Answer 2

It depends what "other" is. If it is the caller of your doConnect() method just do not catch the exceptions but throw them and catch in caller method:

public void doConnect(....) throws InstantiationException, IllegalAccessException, .......{
}

public void theCaller() {
    try {
    doConnect(......)
    } catch(InstantiationException e) {
    }
    } catch(IllegalAccessException e) {
    }
/// etc
}

BTW in java 7 you can catch all exceptions in one block:

} catch(IllegalAccessException | InstantioationException | SqlException e)

Alternatively you can wrap specific exception using other exception that can be either checked or unchecked (runtime), eg

public Connection dbConnect(String dbconnectionstring,String userid,String userpassword)
{
    Connection connection = null;
    System.out.println();
  try {
     Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver").newInstance();
     connection = DriverManager.getConnection(dbconnectionstring,userid,userpassword);
     System.out.println("connected to" + dbconnectionstring );

    } catch (InstantiationException | IllegalAccessException  | ClassNotFoundException  | SQLException e) {
        throw new IllegalStateException("Connection failure", e);
    }

  return connection;
}

Now the caller may catch IllegalStateException and extract its cause.

If however you need some kind of asynchronous listener that manages all exceptions you can implement one class like ExceptionManager that is being updated by other classes when exception happens there. The ExceptionManager may be singleton, so the code will look like this:

public void doConnect() {
    try {
       doConnect(......)
    catch(IllegalAccessException | InstantiationException | SqlException e)
        ExceptionManager.getInstance().exceptionThrown(e);
    }
}

Answer 3

You can design it like a log manager: get the application-level instance of your report class, and in the exception handling, make it process the exception.

Or you can rethrow the exception and catch it at the highest level of the application, where it can handle any unexpected exception (used in Swing applications to avoid closing on a simple NPE).