How to get the reference class logger message as a string from my class

Question

I have created a class named "A" and it has its reference class "B". I want to get all the logger message as String including reference class "B" using console appender. Note - B class as third party class which cannot update it.

For eg:

Class A {

    public static void main(String[] args) {
        final java.util.logging.Logger logger = java.util.logging.Logger.getLogger(A.getClassName());
        logger.setLevel(level);
        ConsoleHandler handler = new ConsoleHandler();
        handler.setLevel(Level.ALL);
        handler.setFormatter(new SimpleFormatter() {
            @Override public String format(LogRecord record) {
                return "Test---" + record.getMessage();
            }
        });
        logger.addHandler(handler);
        launch();
    }

    public static void launch() {

        B s = new B();
        s.getData();
        logger.info("A log");
    }
}

Class B {
    final java.util.logging.Logger logger = java.util.logging.Logger.getLogger(A.getClassName());

    getData() {
        logger.info("B log");
    }

}

Current Output:

B log
Test---A log

Expected Output:

Test---B log
Test---A log

Kawser Habib · Accepted Answer · 2021-02-21 15:51:37Z

1

Using reflection you can modify its behavior:

import java.lang.reflect.Field;
import java.util.logging.ConsoleHandler;
import java.util.logging.Level;
import java.util.logging.LogRecord;
import java.util.logging.Logger;
import java.util.logging.SimpleFormatter;

public class A {

    final static java.util.logging.Logger logger = Logger.getLogger(A.class.getName());

    public static void logFormater() {
        ConsoleHandler handler = new ConsoleHandler();
        handler.setLevel(Level.ALL);
        handler.setFormatter(new SimpleFormatter() {
            @Override
            public String format(LogRecord record) {
                return "Test---" + record.getMessage();
            }
        });
        logger.addHandler(handler);
    }

    public static void main(String[] args) {
        logFormater();
        launch();
    }

    public static void launch() {
        try {
            B bClass= new B();
            //using reflection get logger field of B class
            Field aField= bClass.getClass().getDeclaredField("logger");
            aField.setAccessible(true);
            //inject new behavior here
            aField.set(bClass, logger);
            
            bClass.getData();
            logger.info("A log");
        } catch (Exception ex) {
            //error msg
        }
    }
}

class B {
    final java.util.logging.Logger logger = Logger.getLogger(B.class.getName());

    void getData() {
        logger.info("B log");
    }
}

Console output:

Test---B logFeb 21, 2021 9:40:54 PM com.kawser.stackoverflow.problem.solution.B getData
INFO: B log
Test---A logFeb 21, 2021 9:40:54 PM com.kawser.stackoverflow.problem.solution.A launch
INFO: A log

edited Feb 21, 2021 at 15:51

answered Feb 21, 2021 at 15:45

Kawser Habib

1,4222 gold badges10 silver badges24 bronze badges

Sign up to request clarification or add additional context in comments.

7 Comments

user1698109 Over a year ago

Thanks for the details. I am not sure what is the field they might used in the B class since its third party class and i'am getting the below error Caused by: java.lang.NoSuchFieldException: logger. Even when i tried to get all the fields using reflection, it returns empty array

Kawser Habib Over a year ago

If you don't mind, can share your library/ 3rd party class. I will try to figure out this issue.

user1698109 Over a year ago

<dependency> <groupId>org.mule.services</groupId> <artifactId>mule-service-weave</artifactId> <version>2.3.0</version> <classifier>mule-service</classifier> </dependency>. Logger Class is "Feb 21, 2021 11:16:13 AM org.mule.weave.v2.model.service.DefaultLoggingService$ logInfo"

Kawser Habib Over a year ago

Thanks, let me check.

BogdanBiv Over a year ago

B.logger was made final exactly because the library author did not want to allow users to change it. Overriding this via reflection breaks encapsulation and the Java guarantees. B author could change B.logger at any point and it will break your program. Make sure you factor in this risk.

|

jmehrens · Accepted Answer · 2021-02-24 17:05:56Z

1

Remove all handlers from the root logger and attach your consolehander to the root logger. By default all log message travel up to the root logger handlers.

Class A {
        private static final java.util.logging.Logger root = java.util.logging.Logger.getLogger("");

        private static final java.util.logging.Logger logger = java.util.logging.Logger.getLogger(A.class.getName());

    public static void main(String[] args) {
        LogManager.getManager().reset();

        logger.setLevel(level);

        ConsoleHandler handler = new ConsoleHandler();
        handler.setLevel(Level.ALL);
        handler.setFormatter(new SimpleFormatter() {
            @Override public String format(LogRecord record) {
                return "Test---" + formatMessage(record);
            }
        });
        root.addHandler(handler);
        launch();
    }

    public static void launch() {

        B s = new B();
        s.getData();
        logger.info("A log");
    }
}

Class B {
    final java.util.logging.Logger logger = java.util.logging.Logger.getLogger(B.class.getName());

    getData() {
        logger.info("B log");
    }

}

answered Feb 24, 2021 at 17:05

jmehrens

11.1k1 gold badge41 silver badges49 bronze badges

1 Comment

user1698109 Over a year ago

Thanks It worked. I am able to get the logger as String

Collectives™ on Stack Overflow

How to get the reference class logger message as a string from my class

2 Answers 2

7 Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

7 Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related