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The structure of logging information can vary in terms of the message content, the type of content, and the way the information is presented. But in abstract terms, all logging information can be organized into a header, a body, and a footer. The header may present an introduction to the message; the body contains the logging information; and the footer section can contain some hint about the content of the message. You can see how this resembles the physical documents that people deal with everyday. Bearing this idea in mind, log4j provides an abstract class called Layout that offers a skeleton implementation of all the common operations. All other individual Layout objects are basically subtypes of this abstract superclass. Individual Layout objects differ in the styles they use to format the logging information passed to them. These individual Layout objects inherit all the common operations from their abstract superclass and implement their own formatting methods. They may override and provide their own implementation of any method in the base class. Figure 4-1 depicts the class diagram of different Layout objects within the log4j API.

As its name implies, the Save function saves exception log information to the database using the SqlConnection passed in as a parameter The function begins by defining a parameterized SQL insert statement, and then uses that statement and the SqlConnection to create a new SqlCommand object Because the command object contains a parameterized SQL statement, it needs parameter values for all those parameters You can see directly under the SqlCommand definition that the function creates a parameter for each field in the statement and passes in the value of the corresponding class property It uses the CheckEmpty function to ensure string values are set to an empty string "" and not to Nothing If a parameter value is Nothing, then the SqlCommand object throws an execution when the command executes because it thinks you never supplied a value.

Here 11.330is where all the work should take place. We start with the implementation of the actual library lib.c (see Listing 2-3). This file lives in the native folder within the project folder.

The ExceptionID field is not included in this list because the database auto-generates its value during the insert After populating the command object with parameters and values, the Save function executes the command using the ExecuteNonQuery method, which returns the number of records affected by the query If no records were affected, then the Save failed and the function returns false If at least one record was affected, then the Save succeeded and the function goes on to acquire the new ExceptionID value Because SQL Server automatically generates the ExceptionID during the insert, you have to select it out of the database The @@IDENTITY variable stores the value of the last autogenerated number for the current connection.

Inside the affected record check, you can see that the Save function updates the SqlCommand object s CommandText to acquire the autogenerated number and assigns it to the ExceptionID property using the ExecuteScalar method ExecuteScalar returns a single value from the database without the need for a data reader Finally, the function returns true indicating that the Save was successful..

Figure 4-1. The Layout object hierarchy All Layout objects receive a LoggingEvent object from the Appender objects. The Layout objects then retrieve the message argument from the LoggingEvent and apply the appropriate ObjectRenderer to obtain the String representation of the message, as discussed in the previous chapter.

This function checks an incoming string to determine whether it is set to Nothing. If so, it returns an empty string "". If not, the function returns the actual string s value.

Note Native libraries in Linux (also known as shared objects) are the equivalents of dynamic link libraries (DLLs) in Windows. By convention, shared objects are named as lib<NAME><VERSION>.so.

LoadByID accepts an ExceptionID and a SqlConnection as parameters and loads the requested exception information from the database into the ExceptionLog object. It begins by creating a Boolean variable named ReturnVal and setting it to false. This variable keeps track of whether or not the ExceptionID requested is actually found and loaded. The command-building process for LoadByID is similar to that of the Save function. It starts by defining a parameterized query, builds a SqlCommand object that uses the query and the SqlConnection, and then creates the ExceptionID parameter value. LoadByID then runs the ExecuteReader method on the SqlCommand object and stores the resulting data reader in the dr variable. After getting a reference to the data reader, the function checks to see if the data reader is pointing to any information by calling the Read method. If Read returns True, the

function knows that the exception information was successfully located. It then copies the data from the data reader to the object using the MapData function and sets the ReturnVal to True indicating that the information was successfully located. Finally, the function closes the data reader and returns ReturnVal as the result of the function. If the exception information was not located, then ReturnVal will still be False.