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Does depedent project in java take log4j config of parent project?

I am having project A and project B, A has jar dependency of project B. I have defined log4j.xml in project A but I am not able to see logs of sub-project(B.jar) in file appender as well as tomcat server console. Does project B will take log4j.xml form parent project A or not then which config does it use?

There is one log4j config for your entire JVM (unless you're working in a containerized environment using class loaders and.... that's not what's described).
Missing log messages implies that the configuration from log4j either (a) isn't what you think it is (i.e. a different log4j.xml is being used) or (b) doesn't have the right settings for the missing log lines.
Adding the following to the JVM at startup may help:
-Dlog4j.debug
It may also be possible to browse the log4j settings via MBeans in jconsole.

If you want all apps (WAR files) in a Tomcat instance to have the same logging configs, the simple solution is to arrange that all WAR files have a copy of the same config file.
If you want the apps to share a common logging framework (with a single configuration), then you should consider using Context Selectors, as described in the Log4j 2 documentation.
Using Context Selectors
There are a few patterns for achieving the desired state of logging separation using ContextSelectors:
Place the logging jars in the container's classpath and set the system property log4j2.contextSelector to org.apache.logging.log4j.core.selector.BasicContextSelector. This will create a single LoggerContext using a single configuration that will be shared across all applications.
Place the logging jars in the container's classpath and use the default ClassLoaderContextSelector. Follow the instructions to initialize Log4j 2 in a web application. Each application can be configured to share the same configuration used at the container or can be individually configured. If status logging is set to debug in the configuration there will be output from when logging is initialized in the container and then again in each web application.
Follow the instructions to initialize Log4j 2 in a web application and set the system property or servlet context parameter log4j2.contextSelector to org.apache.logging.log4j.core.selector.JndiContextSelector. This will cause the container to use JNDI to locate each web application's LoggerContext. Be sure to set the isLog4jContextSelectorNamed context parameter to true and also set the log4jContextName and log4jConfiguration context parameters.
The exact method for setting system properties depends on the container. For Tomcat, edit $CATALINA_HOME/conf/catalina.properties. Consult the documentation for other web containers.
I don't think there is a direct equivalent in Log4j 1.x.

Initializing Log4j within Web Context(tomcat)?

I checked log4j setup in my tomcat environment. Although there is no log4j-web.jar file in my webapp or in "common libs" folder, log4j is initialized properly. I only added log4j.properties in my classpath. And that's it, log4j is working.
I am using spring mvc, sl4j-api.jar and sl4j-jcl.jar are in my classpath.
My question is : How does log4j work properly and pickup the configuration?

You say that you used log4.properties so I will be assuming you are using log4j 1.2.x (log4 2 use log4j2.properties)
Log4 perform default initialization when log4j classes are loaded into memory within the static initializer of the LogManager class.
See the section: Default Initialization Procedure in the manual
As soon as you dropped log4j.properties into your classpath it trigger log4j logging.
This procedure will be executed in any environment (tomcat container or other).
if you want to skip this procedure please note item 1 in the procedure:
Setting the log4j.defaultInitOverride system property to any other
value then "false" will cause log4j to skip the default initialization
procedure (this procedure).
Log4Web is an extension for depending on log4j 2 so It's irrelevant in your case.
sl4j-api.jar and sl4j-jcl.jar requires log4j-over-slf4j if you what to migrate existing code to use SLF4J without changing the code itself.
If you are not interest in such migration you may ignore them.

How to set different log levels in Eclipse Scout framework?

I'm having some trouble configuring proper logging in Eclipse Scout framework. My claims aren't that high as I only want to be able to set different log levels for different parts of my program in a configuration/properties/XML file.
The logging configuration in the config.ini of my Scout server plugin currently looks like this:
eclipse.consoleLog=true
org.eclipse.scout.log=eclipse
org.eclipse.scout.log.level=INFO
So as you can see this is the default logging configuration using Eclipse logging. It works fine for logging at a global level. The only thing I would like to do is to write something like this to set the different log levels:
packagename.ClassName=LOGLEVEL
As this is a very basic logging use case I think there must be some easy way to do this in Scout. Otherwise I would appreciate some help how to configure log4j, JUL or others for the use with Scout. The Eclipse Scout Wiki hasn't helped me so far. I created the example logger fragment to the host plugin 'org.eclipse.scout.commons' and removed the logging configuration lines from my config.ini but nothing happens. I'm also not sure where to put the log4j.poperties or how this is done otherwise.
I'm a bit ashamed for being unable to figure out such a basic problem, but would be very happy about some quick help.

I can tell you how to configure the logging if you choose the java logger (config.ini: org.eclipse.scout.log=java).
For the eclipse logger, I barely found any information at all.
Now, to configure the java (JUL) logging: You can do this in a file called logging.properties.
You can configure the logging by specifying the configuration file in your product:
Create your configuration file - say logging.properties inside the folder where your product file (for server or client respectively) is located. Typically this is in a folder named 'products'.
Open your product file and go to the "Launching" tab and specify your logging configuration file in the "VM Arguments" tab. Use the "java.util.logging.config.file" system property to do so:
-Djava.util.logging.config.file="${resource_loc:/com.yourapp.server/products/logging.properties}"
Now, you should be able to specify the log levels in your new logging.properties file:
### Root level of your application, all below are ignored
.level=INFO
### Handlers
handlers=java.util.logging.ConsoleHandler
### Handler properties
java.util.logging.ConsoleHandler.level=FINEST
### Override the logging level for certain classes
com.yourapp.server.SomeService.level=FINE
Alternatively, you can also use a class to initialize the logging with the java.util.logging.config.class option. See this wiki page for a detailed example.
Also, when building a WAR file, you might be interested in this answer.

Set log4j log level

I'm currently working on a project that uses log4j.
I'm running a testcase (junit) and would like to set the log level to trace so that I can see if all the values are correct. Classes that use logging in the project contain a line like the following:
private static final Log LOG = LogFactory.getLog(MatchTaskTest.class);
and use a like like this to do the actual debugging
LOG.trace("value");
I have never used log4j before, does anybody know how I can change the log level just for the testcase, preferably simply by defining a parameter in eclipse's run configuration dialog.

Using another configuration file
Perhaps you could point to another configuration file.
java -Dlog4j.configuration=config file yourApp
Where:
config, you file of configuration, e.g. log4j.properties or log4j.xml.
file, the log file, e.g. myApp.log
yourApp, you app, e.g. MyAppGUI
Or you can use a class
java -Dlog4j.configurationClass=config class yourApp
Where:
config, you file of configuration, e.g. log4j.properties or log4j.xml.
class, any customized initialization class, like LogManager, should implement
the org.apache.log4j.spi.Configurator
yourApp, you app, e.g. MyAppGUI
You can see more in Apache log4j 1.2 - Short introduction to log4j on Default Initialization Procedure section.
Modifying the level programmatically
Moreover, you can also use the methods that offers the Logger class, like public void setLevel(Level level), e.g.:
Logger.getRootLogger().setLevel(Level.TRACE);
Since you want only for testing purposes, you could use them. But it is recommended not to use in client code because they overwrite the default configuration parameters in hard coded. The best way is to use an external configuration file.

In your junit class put:
Logger.getRootLogger().setLevel(Level.TRACE);
somewhere before the execution of the tested method. It will set the threshold level of the root logger to TRACE.

If you're using Maven, you can have two log4j configuration files:
one in src/main/resources, containing your production logging config
one in src/test/resources, containing your test-time logging config
Maven will automatically use the latter at test time, and bundle the former into your artifact (JAR, WAR, etc) so that it's used in production. You don't have to mess around with command line switches or anything.

I don't think this is possible.
The config file is going to let you configure what log messages actually surface in the log, not what level each message is going be logged at. This makes sense - the config should not affect the level of the message.
The javadoc has a method for each log level and a generic log method, which takes in a priority, so I'm not sure there's even a default to be set.
You can set a config file explictly on the command line via -Dlog4j.configuration=<FILE_PATH>, so you could set up a specific config for that test case.

I have no idea why some of the above didn't work for me. (I don't want to write config file). following works for me
Logger log1 = Deencapsulation.getField(Some.class,"logger");
log1.setLevel(Level.DEBUG);

NB that the log4j2.properties file may include the line
filter.threshold.level = debug
You can waste an entire afternoon trying to figure out why your LOG.trace() statements aren't outputting anything!

I actually put it in the #Before method. But I believe it could (and should) be placed in the #BeforeClass.
If you set it in the class body you'll get compiler errors.

What is the default classpath for EJBs?

Please forgive my pitiful knowledge of Java EJBs but, when an EJB is deployed to an application server as a .jar file, where do things like Hibernate and log4j first look for their configuration files (hibernate.cfg.xml and log4j.properties) in the .jar file?

(...) when an EJB is deployed to an application server as a .jar file, where do things like Hibernate and log4j first look for their configuration files (hibernate.cfg.xml and log4j.properties) in the .jar file?
This depends on the implementation of the tool and is unrelated to the fact that you are using EJBs. For Hibernate, the documentation writes:
3.7. XML configuration file
An alternative approach to
configuration is to specify a full
configuration in a file named
hibernate.cfg.xml. This file can be
used as a replacement for the
hibernate.properties file or, if both
are present, to override properties.
The XML configuration file is by
default expected to be in the root of
your CLASSPATH.
Regarding Log4J, the procedure is described below:
Default Initialization Procedure
The log4j library does not make any
assumptions about its environment. In
particular, there are no default log4j
appenders. Under certain well-defined
circumstances however, the static
inializer of the Logger class will
attempt to automatically configure
log4j. The Java language guarantees
that the static initializer of a class
is called once and only once during
the loading of a class into memory. It
is important to remember that
different classloaders may load
distinct copies of the same class.
These copies of the same class are
considered as totally unrelated by the
JVM.
The default initialization is very
useful in environments where the exact
entry point to the application depends
on the runtime environment. For
example, the same application can be
used as a stand-alone application, as
an applet, or as a servlet under the
control of a web-server.
The exact default initialization
algorithm is defined as follows:
Setting the log4j.defaultInitOverride system property to any other value then
"false" will cause log4j to skip the
default initialization procedure (this
procedure).
Set the resource string variable to the value of the
log4j.configuration system property. The preferred way to
specify the default initialization
file is through the
log4j.configuration system property. In case the system property
log4j.configuration is not defined, then set the string variable
resource to its default value
"log4j.properties".
Attempt to convert the resource variable to a URL.
If the resource variable cannot be converted to a URL, for example due to
a MalformedURLException, then search
for the resource from the classpath by
calling
org.apache.log4j.helpers.Loader.getResource(resource,
Logger.class) which returns a URL.
Note that the string
"log4j.properties" constitutes a
malformed URL. See
Loader.getResource(java.lang.String)
for the list of searched locations.
If no URL could not be found, abort default initialization. Otherwise,
configure log4j from the URL. The
PropertyConfigurator will be used to
parse the URL to configure log4j
unless the URL ends with the ".xml"
extension, in which case the
DOMConfigurator will be used. You
can optionaly specify a custom
configurator. The value of the
log4j.configuratorClass system property is taken as the fully
qualified class name of your custom
configurator. The custom configurator
you specify must implement the
Configurator interface.
To summarize, if you put both files at the root of your EJB-JAR, they should be found.
Regarding the title of your question, I suggest to read Packaging EJB 3 Applications that I'm quoting below:
Dependencies between Java EE modules
Unfortunately, no Java EE
specification provides a standard for
class loading, and each application
server implements class loaders in
whatever way seems best to the vendor.
However, Java EE defines the
visibility and sharing of classes
between different modules, and we can
depict the dependency between
different modules as shown in figure
4.
As illustrated in figure 4, the EAR
class loader loads all JARs in the lib
directory that is shared between
multiple modules. Typically a single
EJB class loader loads all EJB
packaged in all EJB-JAR modules. The
EJB class loader is often the child of
the application class loader, and
loads all EJB classes. Because the EJB
is a child to the EAR class loader,
all classes loaded at the> EAR level
will be visible to the EJBs.
(source: developer.com)
Figure 4: Illustration of class
visibility of an EAR file containing
multiple web modules, EJBs, and shared
library modules. The EAR class loader
loads the classes in the JARs packaged
as library modules, and all classes
loaded by the EAR class loader are
visible to the EJBs. The classes
loaded by EJB class loader are
typically visible to the web module in
most containers because the WAR class
loader is a child of the EJB class
loader.

I think Log4j would look in more than one place for log4j.properties file. Anyway, all configuration files in an ejb-jar go inside the META-INF directory.