从命令行-java.lang.ClassNotFoundException：org.slf4j.LoggerFactory

Question

OK. I know there are other questions like this one out there and this is not the first time that slf4j has kicked my butt. However, I have looked at my PATH in Environment Variables and below are the two slf4j jar files included in my PATH as well as my project dependencies.

C:\Users\pdl\.m2\repository\org\slf4j\slf4j-api\1.7.13\slf4j-api-1.7.13.jar
C:\Users\pdl\.m2\repository\org\slf4j\slf4j-simple\1.7.13\slf4j-simple-1.7.13.jar

This is what is in my pom file:

    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-ext</artifactId>
        <version>1.7.13</version>
    </dependency>

Other applications we are running contain slf4j jar dependencies in the IDE but they are not listed in the pom file. I am so confused about where to put what that I can't see straight.

This is what the application dependencies look like:

I can run the application from the IDE (Netbeans) but I get the following error when I try to run from command prompt.

C:\Users\pdl\Projects\WeatherTestDrive>java -cp WeatherApp.jar;WeatherOpenWeatherMap.jar;WeatherClient.jar com.a2i.weatherclient.Client
Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
        at com.a2i.weatherclient.Client.<clinit>(Client.java:22)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 1 more

Adding slf4j in my VM classpath and I still get the error.

C:\Users\pdl\Projects\WeatherTestDrive>java -cp WeatherApp.jar;WeatherOpenWeatherMap.jar;WeatherClient.jar;C:\Users\pdl\.m2\repository\org\slf4j\slf4j-api\1.7.13\slf4j-api-1.7.13.jar;C:\Users\pdl\.m2\repository\org\slf4j\slf4j-simple\1.7.13\slf4j-simple-1.7.13.jar com.a2i.weatherclient.Client
Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
        at com.a2i.weatherclient.Client.<clinit>(Client.java:22)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 1 more

Can somebody please help me figure out what I am doing wrong?

1. Should I be adding slf4j-api to my dependencies instead of slf4j-exe? Or even something else?
2. I guess whichever one I use, I should add it to my VM classpath.
3. Does it even need to be in my pom file?

------------------------------ EDIT ----------------------------------

I created a simple HelloWorld app to log my name. As soon as I added the Logger to my Hello class it was highlighted in red, so I added the slf4j-simple to my dependencies and slf4j-api came with it. But when I opened the pom file only the slf4j-simple was added:

    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-simple</artifactId>
        <version>1.7.13</version>
    </dependency>

When I run from the IDE, everything works well. But when I run from command line I still get the error:

C:\Users\pdl\Projects\HelloWorld\target>java -cp HelloWorld-1.0-SNAPSHOT.jar;C:\Users\pdl\.m2\repository\org\slf4j\slf4j-simple\1.7.13\slf4j-simple-1.7.13.jar com.a2i.helloworld.Hello
Exception in thread "main" java.lang.NoClassDefFoundError: org/slf4j/LoggerFactory
        at com.a2i.helloworld.Hello.<clinit>(Hello.java:17)
Caused by: java.lang.ClassNotFoundException: org.slf4j.LoggerFactory
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 1 more

Answer 1

Where's slf4j in your VM classpath? Try running as follows:

java -cp WeatherApp.jar;WeatherOpenWeatherMap.jar;WeatherClient.jar;C:\Users\pdl\.m2\repository\org\slf4j\slf4j-api\1.7.13\slf4j-api-1.7.13.jar;C:\Users\pdl\.m2\repository\org\slf4j\slf4j-simple\1.7.13\slf4j-simple-1.7.13.jar com.a2i.weatherclient.Client

You're missing slf4j from your runtime. Hope that helps.

Answer 2

Maven, and thus the pom-file, are intended to build the classpath for you. So yes, all depedencies you like to use should be in your pom file.

Regarding slf4j: slf4j-api is a dependency that only defines an api (or interface). To make it work you also have to add an implementation. See here for explanation. So you have to add at least one more dependency. For example:

    <dependency>
        <groupId>ch.qos.logback</groupId>
        <artifactId>logback-classic</artifactId>
        <version>1.1.3</version>
    </dependency>

Regarding slf4j-ext, i am quite sure you dont need it to simply log. Maybe you have more elaborate use cases.

To start your application from command line you have to run java with the -cp argument. The classpath is a list all used classes or archives containing classes. For applications with a lot of dependencies it can become quiet cumbersome to build it manually.

Use mvn dependency:build-classpath to let maven build this big string of jar-paths for you.

Answer 3

This concerns Java SE projects built in IntelliJ where slf4j - or any other library for that matter - is included (ie slf4j-api-1.7.26.jar => The API & slf4j-simple-1.7.26.jar => the API implementation)...

Step #1:

create folder \\libs in project root and place the two JARs in there click to see project structure

step #2:

Build code and make sure that the Logger is working properly inside the IDE.

step #3:

Launch cmd and navigate using cd command inside the .\\out directory up until the packages of the main (check 1st line of Main.java). ie

cd C:\\Users.....\\out...\\testcp

step #4

Do right click on sjf4j-api jar and copy path (path#1) &

Do right click on sjf4j-simple jar and copy path (path#2) &

Do right click on Main.class and copy path (path#3. SOS: EXCLUDE THE Main.class part).

Now, run java command with -cp flag as:

java -cp path#1;path#2;path#3; org.me.Main

That's it

Note : Eclipse users do not run into this problem since this is taken care of by using the build path utility.