Log4j

import org.apache.log4j.Logger;

Logger logger = Logger.getLogger(Main.class);
logger.info("Application started");

import org.apache.log4j.Logger;
import org.apache.log4j.BasicConfigurator;

public class Main {

  private static final Logger logger = Logger.getLogger(Main.class);

  public static void main(String[] args) {

    // Basic configuration
    BasicConfigurator.configure();

    logger.info("Application Started");
    logger.debug("Debug message");
    logger.warn("Warning message");
    logger.error("Error message");

    try {
      int result = 10 / 0;
    } catch (Exception e) {
      logger.error("Exception occurred", e);
    }

  }
}

// Output (example logs):
// INFO  Application Started
// DEBUG Debug message
// WARN  Warning message
// ERROR Error message

• 1 Apache logging framework for Java.
• 2 Used to record runtime events.
• 3 Highly configurable.
• 4 Supports multiple appenders.

• 1 DEBUG – detailed debugging info.
• 2 INFO – general information.
• 3 WARN – warning messages.
• 4 ERROR – error messages.
• 5 FATAL – critical errors.

• 1 Logger – logs messages.
• 2 Appender – defines output destination.
• 3 Layout – formats log messages.
• 4 Configuration – controls logging behavior.

• 1 Better debugging support.
• 2 Production monitoring.
• 3 Flexible configuration.
• 4 Improves application maintainability.

• 1 Used in enterprise Java applications.
• 2 Used in legacy systems for logging.
• 3 Used in backend server applications.
• 4 Used in debugging production issues.
• 5 SaaS products use Log4j in Java in services, dashboards, background jobs, and API workflows.
• 6 ERP and banking systems apply Log4j in Java with validation, logging, review, and rollback plans.
• 7 E-commerce and healthcare platforms use Log4j in Java carefully because reliability and data correctness matter.

• 1 A Java program first evaluates the surrounding context, then applies the Log4j in Java rules to the current data.
• 2 The important mental model is input, transformation, result, and failure path.
• 3 In production, the same flow usually sits inside a larger layer such as a controller, service, repository, job, or UI component.

• 1 Choose the simplest implementation first, then measure real workloads.
• 2 Watch for repeated work inside loops, unnecessary allocations, and slow I/O in hot paths.
• 3 Prefer clear data structures and stable APIs before micro-optimizing syntax.

• 1 Treat external input as untrusted until it is validated.
• 2 Avoid hardcoded secrets and never print sensitive values in examples or logs.
• 3 Use established libraries for authentication, encryption, parsing, and database access.

• 1 Not configuring log4j properly.
• 2 Using outdated Log4j versions.
• 3 Logging sensitive information.
• 4 Not managing log files properly.
• 5 Skipping the small working example before adding framework code.
• 6 Ignoring null, empty, duplicate, and boundary inputs.
• 7 Mixing business logic, input handling, and output formatting in one place.
• 8 Using broad error handling that hides the real failure.
• 9 Forgetting to test the behavior after refactoring.
• 10 Adding clever code that future maintainers will struggle to read.

• 1 Use proper log levels (INFO, DEBUG, ERROR).
• 2 Configure log4j using properties or XML.
• 3 Avoid logging sensitive data.
• 4 Use updated Log4j2 instead of Log4j 1.x.
• 5 Start with clear requirements and one minimal working example.
• 6 Use meaningful names that explain business intent.
• 7 Keep examples small enough to debug line by line.
• 8 Validate input at every trust boundary.
• 9 Handle errors explicitly and preserve useful context.
• 10 Prefer simple control flow over deeply nested logic.
• 11 Separate domain logic from I/O and framework code.
• 12 Write tests for normal, boundary, and failure cases.
• 13 Review security assumptions before production use.
• 14 Measure performance before optimizing.
• 15 Document non-obvious decisions close to the code or in project notes.
• 16 Use official documentation when behavior is version-specific.
• 17 Keep dependencies current and remove unused code.
• 18 Avoid hardcoded secrets, credentials, and environment-specific paths.
• 19 Log operational events without exposing sensitive data.
• 20 Design examples so learners can safely modify and rerun them.

• 1 Beginner: rewrite the example with different names and values.
• 2 Intermediate: add validation and handle one expected failure case.
• 3 Advanced: place Log4j in Java inside a small service-style design with tests.

• 1 Build a small Java console feature that demonstrates Log4j in Java.
• 2 Accept input, process it with the concept, print a clear result, and handle invalid input.
• 3 Add a README note explaining the design choice and two edge cases you tested.

• 1 If the program does not compile, check spelling, imports, braces, and file/class names first.
• 2 If output is unexpected, print intermediate values and verify each branch of the logic.
• 3 If the design feels complex, reduce it to the smallest working example and add pieces back one at a time.

• 1 Practice Log4j in Java with a second example from a business domain such as inventory, payroll, banking, or e-commerce.
• 2 Review related Java topics that cover data flow, error handling, testing, and clean design.
• 3 Compare your solution with official documentation and simplify anything you cannot explain clearly.