Friday, March 22, 2013

Java: Best Practices for Using Exceptions

The next set of best practices show how the client code should deal with an API that throws checked exceptions.

1. Always clean up after yourself


If you are using resources like database connections or network connections, make sure you clean them up. If the API you are invoking uses only unchecked exceptions, you should still clean up resources after use, with try - finally blocks.

public void dataAccessCode(){
    Connection conn = null;
    try{
        conn = getConnection();
        ..some code that throws SQLException
    }catch(SQLException ex){
        ex.printStacktrace();
    } finally{
        DBUtil.closeConnection(conn);
    }
}

class DBUtil{
    public static void closeConnection
        (Connection conn){
        try{
            conn.close();
        } catch(SQLException ex){
            logger.error("Cannot close connection");
            throw new RuntimeException(ex);
        }
    }
}

DBUtil is a utility class that closes the Connection. The important point is the use of finally block, which executes whether or not an exception is caught. In this example, the finally closes the connection and throws a RuntimeException if there is problem with closing the connection.

2. Never use exceptions for flow control


Generating stack traces is expensive and the value of a stack trace is in debugging. In a flow-control situation, the stack trace would be ignored, since the client just wants to know how to proceed.
In the code below, a custom exception, MaximumCountReachedException, is used to control the flow.
public void useExceptionsForFlowControl() {
    try {
        while (true) {
            increaseCount();
        }
    } catch (MaximumCountReachedException ex) {
    }
    //Continue execution
}

public void increaseCount()
    throws MaximumCountReachedException {
    if (count >= 5000)
        throw new MaximumCountReachedException();
}

The useExceptionsForFlowControl() uses an infinite loop to increase the count until the exception is thrown. This not only makes the code difficult to read, but also makes it slower. Use exception handling only in exceptional situations.

3. Do not suppress or ignore exceptions


When a method from an API throws a checked exception, it is trying to tell you that you should take some counter action. If the checked exception does not make sense to you, do not hesitate to convert it into an unchecked exception and throw it again, but do not ignore it by catching it with {} and then continue as if nothing had happened.

4. Do not catch top-level exceptions


Unchecked exceptions inherit from the RuntimeException class, which in turn inherits from Exception. By catching the Exception class, you are also catching RuntimeException as in the following code:

try{
..
}catch(Exception ex){
}
The code above ignores unchecked exceptions, as well.

5. Log exceptions just once


Logging the same exception stack trace more than once can confuse the programmer examining the stack trace about the original source of exception. So just log it once.