General debugging

• What is your go-to debugging method?
• Basic printed statements
• Debugger

General debugging

• Printed statements are the easiest
• Don't even require much knowledge of the language
• Often effective for simple problems

static void transferMoney(User user1, User user2, int amount) {
    
    SomethingChecker.checkSomething(user1, user2);
    user1.deduct(amount);
    user1.doSomethingElse();
    
    System.out.println("Did we get here?");
    
    user2.add(amount);
    user2.doSomeCheck();
  }

General debugging

• Debugger is the most powerful
• Not always trivial to set up
• Generally want to avoid having to use it

Errors & Exceptions

• Expected runtime exceptions (e.g. input validation, file not found, incorrect access rights...)
• Unexpected runtime exceptions (e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException)
• Unexpected unrecoverable errors (e.g. NoClassDefFoundError, OutOfMemoryError, IllegalAccessError)

Errors & Exceptions

• Error and Exception are subclasses of Throwable
• Indication that an unusual situation happened
• Errors not to be handled, thrown by JVM
• Exceptions handleable - Java has predefined Exceptions (SQLException, FileNotFoundException...)

Errors & Exceptions

• You can make your own by extending the Exception class:

public class IncorrectFileNameException extends Exception {
  public IncorrectFileNameException(String errorMessage) {
    super(errorMessage);
  }
}

• Then throw it in a fitting situation:

try {
    
    // Attempt to do something where incorrect file name can be inputted.
        
} catch (FileNotFoundException e) {
    
    if (!isCorrectFileName(fileName)) {
        throw new IncorrectFileNameException("Incorrect filename : " + fileName);
    }
        
    // Continue handling the situation.
        
} finally {
    
    // Do this in any case.
}

try {
    
    // Attempt to do something where incorrect file name can be inputted.
        
} catch (FileNotFoundException e) {
    
    if (!isCorrectFileName(fileName)) {
        throw new IncorrectFileNameException("Incorrect filename : " + fileName, e);
    }
        
    // Continue handling the situation.
        
} finally {
    
    // Do this in any case.
}

Errors & Exceptions

• Handling exceptions should not serve to simply prevent the application from crashing.
• Errors and exceptions should thrown in appropriate situations.
• The message should be informative.
• The goal is to make debugging as easy as possible.

Logging

• A more sophisticated approach to tracking what is happening with the code and data.
• Can be outputted to the console, persisted in a file or re-directed to any other I/O stream.
• Again, the point is to make debugging easier.

Logging

• Events should be labeled with an appropriate level:
  • TRACE - the most granular information; used to "trace" the path through the code (line level)
  • DEBUG - variable value and other information of interest to the dev, not the admin (method level)
  • INFO - general statements about what the application is doing (feature level)

Logging

• WARN - statement about a potentially risky action (e.g. config file not found, using a backup server...)
• ERROR - something actually went wrong
• FATAL - something went so wrong the application needs to shut down
• Logs can be filtered based on that.
• Watch out for size of the log file!

Logging

• Java has a custom java.util.logging package.
• Allows for creation, filtering, memory handling and basic formatting of logs

source: Oracle.com

public class Bank {
    
  // Obtain a logger object.
  private static Logger logger = Logger.getLogger("cz.muni.fi.bank");
  
  public static void main(String args[]) {
    
      // Log a TRACE level message
    logger.fine("Describing what I'm doing in detail.");
    
    try {
      user.withdraw(amount);
    } catch (Exception ex) {
      // Log the exception
      logger.log(Level.WARNING, "Cannot withdraw the inputted amount.", ex);
    }
    logger.fine("done");
  }
}

Logging

• Can be configured to an extent.
• Handlers can be set up for writing of logs to different streams.
• FileHandler, StreamHandler, ConsoleHandler, SocketHandler...

public static void main(String[] args) {
    
  Handler fileHandler = new FileHandler("%t/bank.log");
  Logger.getLogger("").addHandler(fileHandler);
  Logger.getLogger("cz.muni.fi").setLevel(Level.FINEST);
  ...
}

Logging

• The default Java logger is okay for basic use cases.
• Several logging frameworks generally used for finer control and extra features (e.g. automatic reloading of config files, graceful recovery from IO failures, better performance...)
• E.g.: Log4J 2, Logback, SLF4J...

Logging

• As external packages logging frameworks need to be included in your POM.xml.

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-api</artifactId>
    <version>${log-4j-api-version}</version>
</dependency>
<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>${log-4j-core-version}</version>
</dependency>

Logging

• Log4J is configured using the log4j2.xml.
• The end-user needs to define an appender, which contains information about the output stream, formatting...

<Appenders>
  <File name="logger" fileName="output-file.log" append="true">
    <PatternLayout>
      <Pattern>%d{yyyy-MM-dd HH:mm:ss} %p %m%n</Pattern>
    </PatternLayout>
  </File>
</Appenders>

• %d - date pattern, %p - log level, %m - message, %n - new line

Logging

• And then enable it by including it among Loggers:

<Loggers>
  <Root level="error">
    <AppenderRef ref="logger"/>
  </Root>
</Loggers>

Logging

• Usage is similar to the default Java logger:

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;

public class Something {

    private static Logger logger = LogManager.getLogger(Something.class);

    public static void main(String[] args) {
        logger.debug("Debug log message");
        logger.info("Info log message");
        logger.error("Error log message");
    }
}

Transactions

• Transactions are a way of preserving consistency in data.
• They ensure that when we are making a change in data, that we will either do it correctly or not at all.

Transactions

• ACID principles:
  • Atomicity - a transaction cannot be broken into smaller parts; either the whole action is completed or it is not done at all
  • Consistency - a transaction cannot being in or end in an invalid state
  • Isolation - a transaction is always done one at a time
  • Durability - once the transaction is finished, it stays finished forever (e.g. saved to disk)

Transactions

• The operation takes Runnable as an argument.
• Runnable object is an executable block of code, similar to Main.
• All calls in the block are executed or none of them are.

@Override
public void importData(String filePath) {
  transactionExecutor.executeInTransaction(() -> 
        
        // delete data       
        // validate import data
        // import all data       
}