java.lang.ClassCastException with org.apache.logging.log4j.Logger

Question

I have a cast issue whose generates a java.lang.ClassCastException exception:

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.core.LoggerContext;
import org.apache.logging.log4j.message.MessageFactory;

public class MyLogger extends org.apache.logging.log4j.core.Logger {

    MyLogger(LoggerContext context, String name, MessageFactory messageFactory) {
        super(context, name, messageFactory);
        // TODO Auto-generated constructor stub
    }

    public static MyLogger getLogger(String name) {
        org.apache.logging.log4j.core.Logger logger_ = (org.apache.logging.log4j.core.Logger) LogManager
            .getLogger(name);
        return (MyLogger) logger_;
    }
}

While this is the same principle as this one:

public class test {

    private static class A {

    }

    private static class B extends A {

    }

    public static void main(String [] args) {
        A a = new A();
        B b = new B();
        a = (A) b;
    }
}

Can someone give an explaination?

Are you sure LogManager.getLogger(name) returns an instance of MyLogger? Why would it? — sp00m
– sp00m, Commented Oct 30, 2014 at 14:12

ToYonos · Accepted Answer · 2014-10-30 14:32:53Z

1

LogManager.getlogger(name) returns a org.apache.logging.log4j object (the interface)

your are trying to store this object in a org.apache.logging.log4j.core.Logger object (the implementation), it won't work. Here is an example :

You can store a String in an Object :

String s = "foo";
Object o = s;

But you can't store an Object in a String :

Object o = new Object();
String s = o; // Won't compile

Here, the Object is org.apache.logging.log4j and the String is org.apache.logging.log4j.core.Logger

Here is a solution :

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.core.LoggerContext; 
import org.apache.logging.log4j.message.MessageFactory;

public class MyLogger extends org.apache.logging.log4j.core.Logger
{
    MyLogger(LoggerContext context, String name, MessageFactory messageFactory) {
        super(context, name, messageFactory);
    }

    public static Logger getLogger(String name) {
        Logger logger_ = LogManager.getLogger(name);
        return logger_;
    }
}

edited Oct 30, 2014 at 14:32

answered Oct 30, 2014 at 14:12

ToYonos

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1 Comment

Cyril Rezhol Over a year ago

thank you I have missed that this function returns the interface and not the object, it's clear now :)

Olivier Masseau · Accepted Answer · 2014-10-30 14:15:10Z

0

In english please! (En anglais svp!)

In your code:

MyLogger extends Logger

means that you can cast MyLogger to Logger, but you cannot surely cast Logger to MyLogger.

For example if Dog extends Animal -> you can say that a Dog is an Animal but you cannot say that an Animal is always a Dog !

answered Oct 30, 2014 at 14:15

Olivier Masseau

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