类别中的java log4j异常

Question

In my test application I'm getting the below exception

java.lang.NullPointerException
at org.apache.log4j.CategoryKey.<init>(CategoryKey.java:20)
at org.apache.log4j.Hierarchy.getLogger(Hierarchy.java:252)
at org.apache.log4j.Hierarchy.getLogger(Hierarchy.java:233)
at org.apache.log4j.LogManager.getLogger(LogManager.java:179)
at org.apache.log4j.Category.getInstance(Category.java:514)

Even though I am using the line

static final Logger logger1 = Logger.getLogger(Test.class);

Can someone help me in resolving this....

Thanks

Answer 1

I believe that I may have the answer to this. I stumbled upon this very same stack trace recently when I recompiled an old application. I was inadvertently using Java 1.6 to compile my application that was originally built on Java 1.4. I then deployed it along with log4j-1.2.8 inside of weblogic 8 container running a Java 1.4 virtual machine. So my web app, compiled under javac from 1.6 was dropped into weblogic and I hit the deploy button. Bang! Got the same stack trace as shown here.

It finally occurred to me that it might be a cross-version issue. As best I can tell, the Java 1.4 VM loaded my newer class and tried to initialize it. When my newer class dynamically linked to the older log4j library, something in the resolution of class.getName failed to return a valid string and a null was passed down to the Logger class instead of a valid string.

I fixed my issue by forcing my build script to use the java 1.4 compiler.

Answer 2

I got same exception when my logger name was null:

Logger.getLogger(null);

When I changed looger name to some string or class the problem was resolved:

Logger.getLogger(MyClass.class);

or

Logger.getLogger("MyLoggerName");

Answer 3

Odd, I did some searching around and that NPE is caused by a null Category name, but when you specify a Logger.getLogger(Test.class) you should not have that problem. I recommend getting the log4j source and debugging it.