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Java Examples - How do I perform Logging in Java?

Updated on June 11, 2018
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I am a software engineer. I have been working with C++, MFC, and .net technologies for 15 years. I like playing video games & reading books.

1. Introduction to Logging

The application running in the end user system may malfunction at specific conditions. This is a programming flaw. When this situation rises, application debugging as we do it the development machine is not possible. Because the application installer will not deploy the debugging information in the user machine.

The application developer uses Logging to capture the application activities from the deployed system. Logging avoids deploying the Debugging Symbols in the production machine. Have a look at the below depiction:

Debugging Symbols are additional information packed with the binary output of a program. These additional information will help in debugging the errors as we can easily map it to the source code.

Capturing Log at Production System
Capturing Log at Production System | Source

The logging goes as a binary component in the application itself. In most cases, the configuration file takes care of turning ON or turning OFF the logging. When misbehavior of the application or crash happens, the Logging System will be turned ON to produce the “Log File”. The Log file captured in the production will be shipped to the Application Development or Support for further research to know what happened on the production system.

Log Files are mostly an AscII format readable files which has application activities captured in it.

In the above model, in place of deploying the debug information along with binary, the logging system is deployed as a component to capture the application information. OK. Let us see how we can produce Log information using "java.util.logging" package. Note that in this example we will not create any log file. But, we will display the Log Information in the console window.

2. The LogManager

An Application can comprise only one "LogManager" instance. This single LogManager object maintains multiple Loggers, Handlers, etc. That means, once we have LogManager instance, we can claim other objects needed for the logging from it.

The LogManager Class has a static method called "getLogManager()". This method constructs the LogManager object and returns that to the caller. It is not possible for the application to change LogManager object after its retrieval.

3. The Logger

The "getLogger" method of the LogManager class returns the Logger object to its caller. While retrieving the Logger, we can give a name to it by sending a string as a parameter to the getLogger(). The naming follows a hierarchical notation which allows the parent-child relationship between Loggers when multiple Logger exists.

Application running in the production uses the Logger to capture the run-time conditions. Logging output can be delivered to a special receiver like Network Socket, Files or Console Window etc. Java uses "Handler" object to accomplish this.

Each logging call to the Logger goes with a “Logging Level” which supports message filtering. For Example, let us mention the Log Level is 1,2,3,4 where 1 is a high level (Critical) message and 4 is a low-level message (developer debug information). Let us further say, the LogLevel in the Logger is 1. Now, the logger will pump only the Critical Message to the Log Receiver (File, Console etc) and it rejects all other log request with a log level higher than 1.

In this example, we will see how we log a message. We will deal with the Logging Level in a distinct Example.

4. Simple Logging Example

To perform logging, we require Logger from the “java.util.logging” package. To use Logger we need to have the "LogManager". The below import statements imports the required classes to our example.

Listing 1: Package inclusion for Logging

//Snippet 01: Package inclusion
import java.util.logging.LogManager;
import java.util.logging.Logger;
import java.util.logging.Level;

First, we get the application wide LogManager instance. We store that in the reference called "lgMan". We call the "getLogManager() static method" on the LogManager class. This retrieve the LogManager Class instance. Below is the code:

Listing 2: Get LogManager Instance

//Snippet 02: Get the Log Manager Instance
LogManager lgMan = LogManager.getLogManager();

After getting the LogManager, we are setting a name for our Logger using the GLOBAL_LOGGER_NAME constant from the Logger class. We are passing this global constant to the getLogger() method of the LogManager instance and the method returns a Logger instance. Note, it is also possible to pass our own logger name to the getLogger method. For this basic example, we used the name assigned by the system.

Now, the Logger is ready for logging the application specific information. Before we go for actual logging, we are setting the log level to the logger by calling the “setLevel()” method. The “Level.ALL” constant tells the logger not to filter any message. Below is the code

Listing 3: Get the Logger

//Snippet 03: Get Logger from Log Manager
String LoggerName = Logger.GLOBAL_LOGGER_NAME;
Logger Logr = lgMan.getLogger(LoggerName);

The “log()” method of the Logger class performs the logging. In the below code fragment, we are logging two informational messages. While logging, we say that both the messages are information messages by sending a Constant “Level.INFO” as the first parameter. The second parameter is a string containing the actual information message we want to log.

Since we set Logger with the "Level.ALL" Logging Level, the logger will log any message sent to it. This is a Console Application, and the logged messages go to Console Output Window once log() method is called. Below is the code:

Listing 4: Perform Logging

//Snippet 04: Perform the Logging
Logr.log(Level.INFO, "First Log Entry");
Logr.log(Level.INFO, "Second Log Entry");

To send the logging information to a File, we need to set a suitable handler to our logger and we will see that in a separate example. Below are the Complete example and its output.

Listing 5: Complete Example

//Snippet 01: Package inclusion
import java.util.logging.LogManager;
import java.util.logging.Logger;
import java.util.logging.Level;

public class Main
    public static void main(String[] args)
        //Snippet 02: Get the Log Manager Instance
        LogManager lgMan = LogManager.getLogManager();

        //Snippet 03: Get Logger from Log Manager
        String LoggerName = Logger.GLOBAL_LOGGER_NAME;
        Logger Logr = lgMan.getLogger(LoggerName);

        //Snippet 04: Perform the Logging
        Logr.log(Level.INFO, "First Log Entry");
        Logr.log(Level.INFO, "Second Log Entry");
Output of Logger Example
Output of Logger Example | Source

About the Program Output

Note that in the output we also get the Date and Time in which the information is logged. The purpose of Logging is to diagnose the behaviour of the application at a given time. This is the reason why the logger is appending the Date & Time for each and every logged message. It is marked as yellow in the output and the actual log messages are marked with a blue line. Light Blue?

What is your choice to perform Logging in Java Application?

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