Object-Oriented Programming with Java Lecture Notes

6 March 2008 • Testing and Debugging

Outline

• Logging

• Assertions

Logging Example

$ cat u.java
import java.util.logging.Logger;

class u {
  public static void main(String args[]) {
    final Logger printLog = 
      Logger.getLogger(Logger.GLOBAL_LOGGER_NAME);
    printLog.info("In main()");
    }
  }

$ javac -Xlint u.java

$ java u
Mar 5, 2008 5:10:25 PM u main
INFO: In main()

$ 

Logging Classes

• Logging involves two main classes:

  1. The Logger class and

  2. the Handler class.

  

• A Logger instance accepts messages, filters them, and adds other loggin information.

• A Handler instance accepts logging messages, formats them, and writes them to output.

Logging Levels

• There are seven logging levels defined:

  SEVERE, WARNING, INFO, CONFIG, 
  FINE, FINER, FINEST
  

  • Only messages at INFO or higher levels are logged by default.

• There are seven corresponding methods defined:

  severe(String msg)
warning(String msg)
  and so on

Level Filtering

• Loggers and handlers filter on message level.

  • Messages lower than a given level are thrown away.

  • Both the logger and handler filter by level.

    • Level changes need to be made in two places.

The Logger Manager

• How could several classes use the same log?

  • They would have to access the same Logger instance.

• This could be done by a class variable or by a constructor parameter.

Class-Variable Example

class LoggerHome {
  public static final Logger logger;
  static {
    // initialize logger appropriately.
    }
  }

class Blob {
  int moo() {
    LoggerHome.logger.info("In Blob.moo()");
    }
  }

Constructor-Parameter Example

public static void main(String arg[]) {
  Logger logger = ...;
  Blob b = new Blob(logger);
  Glob g = new Glob(logger);
  }

• More flexible and finer grain control than a class variable.

• May be more intrusive too.

Logger Manager

• The LogManager singleton instance provides a third way to manage Logger instances.

  • LogManager.getLogManager() returns a reference to the instance.

• The log manager provides two global services:

  1. A name space for referencing logger instances.

  2. A configuration dictionary (key-value pairs).

The Logger Name Space

• The log manager's global name space is a labeled tree similar to the package structure.

  

• Each node is a logger instance named by the path to the root.

  body.neurological.brain.cerebrum
  

Name-Space Access

• LogManager.getLogger() retrives the named logger instance.

  final Logger log = 
    LogManager.getLogger(
      "body.digestive.esophagus.throat");
  
  log.info("gulp");
  

• The first access creates the logger instance.

Assertions

• The assert statement

  assert bool-expr
  

  throws an AssertionError if the boolean expression evaluates to false.

  • It does nothing if it evaluates to true.

• It's an assertion Error; that is, a catastrophic event.

  • An unchecked exception that ends the show.

Enabling Assertions

• Assertions are disabled by default.

• Use the -ea or -enableassertions java command-line arguments.

• Assertions in selected packages or classes can also be enabled.

  -ea:package-name
  -ea:class-name
  

  • Repeat as needed.

Example

$ cat u.java
class u {
  public static void
  main(String args[]) {
    assert false;
    }
  }

$ javac u.java

$ java u

$ java -ea u
Exception in thread "main" java.lang.AssertionError
 at u.main(u.java:4)

$ 

Assertion Messages

• An assertion can take an optional argument

  assert bool-expr : expr
  

  used as a message for the assertion exception.

  • Calling the toString() if needed.

Example

$ cat u.java
class u {
  public static void
  main(String args[]) {
    assert false : "boom!";
    }
  }

$ javac u.java

$ java -ea u
Exception in thread "main" 
        java.lang.AssertionError: boom!
        at u.main(u.java:4)

$ 

Using Assertions

• Use assertions in programs to

  • document assumptions about class or instance state and,

  • check that the assumptions hold during execution.

• Example:

  int i = path.lastIndexOf('/')
  assert i > -1 : "Missing / in path"
  assert i < path.length() : "Missing name"
  

Unreliable Assertions

• Remember that assertions can be disabled.

  • Don't do useful work in assert statements.

  • Don't check for legal argument values, for instance.

• Example:

  int moo(Object o)
    assert o != null : "null argument";
    if (o == null)
      throw new IllegalArgumentException();