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## Contents

- Contents
- Using JCL Diagnostics
  - When To Use Diagnostic Logging
  - How To Use Diagnostic logging
  - OIDs
  - Diagnostic Message Prefix
  - ClassLoader Hierarchy Tree
  - LogFactory Class Bootstrap
  - Construction Of LogFactoryImpl Instances
  - Log Discovery Diagnostics
- Containers With Custom LogFactory Implementations
  - The Incompatible LogFactory Issue
    - Symptoms
    - Explanation
    - Fixes
- Containers With Custom ClassLoading Behaviour for Logging
  - Apache Tomcat
  - JBoss Application Server
  - Other Containers

## Using JCL Diagnostics

Diagnostics is a feature introduced in JCL 1.1 as an aid to debugging problems with JCL configurations. When diagnostics are switched on, messages are logged to a stream (specified by the user) by the two main classes involved in discovery in JCL (`LogFactory` and `LogFactoryImpl`). Diagnostics are intended to be used in conjunction with the source. The source contains numerous and lengthy comments. Often these are intended to help explain the meaning of the messages.

### When To Use Diagnostic Logging

Diagnostic logging is intended only to be used when debugging a problematic configuration. It should be switched off for production.

### How To Use Diagnostic logging

Diagnostic logging is controlled through the system property `org.apache.commons.logging.diagnostics.dest`. Setting the property value to the special strings `STDOUT` or `STDERR` (case-sensitive) will output messages to `System.out` and `System.err` respectively. Setting the property value to a valid file name will result in the messages being logged to that file.

### OIDs

Diagnostics uses the concept of an Object ID (OID). This allows the identity of objects to be tracked without relying on useful `toString` implementations. These are of the form: `

    classname@system identity hash code

` The system identity hash code is found by calling `System.identityHashCode()` which should uniquely identify a particular instance. The classname is usually the fully qualified class name though in a few cases, `org.apache.commons.logging.impl.LogFactoryImpl` may be shortened to `LogFactoryImpl` to increase ease of reading. For example: `

    sun.misc.Launcher$AppClassLoader@20120943
    LogFactoryImpl@1671711

` OIDs are intended to be used to cross-reference. They allow particular instances of classloaders and JCL classes to be tracked in different contexts. This plays a vital role in building up the understanding of the classloader environment required to diagnose JCL problems.

### Diagnostic Message Prefix

Each diagnostic message is prefixed with details of the relevant class in a standard format. This takes the form: `

    [class-identifier from ClassLoader OID]

` ClassLoader OID is the OID of a classloader which loaded the class issuing the message. class-identifier identifies the object issuing the message. In the case of `LogFactory`, this is just `LogFactory`. For example (line split): `

    [LogFactory
       from sun.misc.Launcher$AppClassLoader@20120943] BOOTSTRAP COMPLETED

` In the case of `LogFactoryImpl`, the prefix is the instance OID. This can be cross referenced to discover the details of the TCCL used to manage this instance. For example (line split): `

    [LogFactoryImpl@1671711
       from sun.misc.Launcher$AppClassLoader@20120943] Instance created.

`

### ClassLoader Hierarchy Tree

Understanding the relationships between classloaders is vital when debugging JCL. At various points, JCL will print to the diagnostic log the hierarchy for important classloaders. This is obtained by walking the tree using `getParent`. Each classloader is represented (visually) by an OID (to allow cross referencing) and the relationship indicated in `child --> parent` fashion. For example (line split for easy reading): `

    ClassLoader tree:java.net.URLClassLoader@3526198
          --> sun.misc.Launcher$AppClassLoader@20120943 (SYSTEM)
          --> sun.misc.Launcher$ExtClassLoader@11126876
          --> BOOT

` Represents a hierarchy with four elements ending in the boot classloader.

### LogFactory Class Bootstrap

Whenever the `LogFactory` class is initialized, diagnostic messages about the classloader environment are logged. The content of each of these messages is prefixed by `[ENV]` to help distinguish them. The extension directories, application classpath, details of the classloader (including the OID and `toString` value) used to load `LogFactory` and the classloader tree for that classloader are logged. Many Sun classloaders have confusing `toString` values. For example, the OID may be `

    sun.misc.Launcher$AppClassLoader@20120943

` with a `toString` value of `

    sun.misc.Launcher$AppClassLoader@133056f

` Other classloader implementations may give very useful information (such as the local classpath). Finally, once initialization is complete a `BOOTSTRAP COMPLETED` message is issued.

### Construction Of LogFactoryImpl Instances

`LogFactoryImpl` is the standard and default `LogFactory` implementation. This section obviously only applies to configurations using this implementation.

Before assigning a `Log` instance, `LogFactory` loads a `LogFactory` implementation. The content is prefixed by `[LOOKUP]` for each diagnostic message logged by this process.

The implementation used can vary per Thread context classloader (TCCL). If this is the first time that a Log has been requested for a particular TCCL a new instance will be created.

Information of particular interest is logged at this stage. Details of the TCCL are logged allowing the OID later to be cross-referenced to the `toString` value and the classloader tree. For example, the following log snippet details the TCCL (lines split):

`

    [LogFactory from sun.misc.Launcher$AppClassLoader@20120943]
        [LOOKUP] LogFactory implementation requested for the first time for context
            classloader java.net.URLClassLoader@3526198
    [LogFactory from sun.misc.Launcher$AppClassLoader@20120943]
        [LOOKUP] java.net.URLClassLoader@3526198 == 'java.net.URLClassLoader@35ce36'
    [LogFactory from sun.misc.Launcher$AppClassLoader@20120943]
        [LOOKUP] ClassLoader tree:java.net.URLClassLoader@3526198
            --> sun.misc.Launcher$AppClassLoader@20120943 (SYSTEM)
              --> sun.misc.Launcher$ExtClassLoader@11126876
                --> BOOT

`

### Log Discovery Diagnostics

The standard `LogFactoryImpl` issues many diagnostic messages when discovering the `Log` implementation to be used.

During discovery, environment variables are loaded and values set. This content is prefixed by `[ENV]` to make it easier to distinguish this material.

The possible messages issued during discovery are numerous. To understand them, the source should be consulted. Attention should be paid to the classloader hierarchy trees for the classloader used to load `LogFactory` and to the TCCL.

## Containers With Custom LogFactory Implementations

Some containers use a custom `LogFactory` implementation to adapt JCL to their particular logging system. This has some important consequences for the deployment of applications using JCL within these containers.

Containers known to use this mechanism:

- WebSphere Application Server from IBM (versions 5 and 6).

Containers suspected to use this mechanism:

- WebSphere Application Server (other versions).

The Apache Commons team would be grateful if reports were posted to the development list of other containers using a custom implementation.

### The Incompatible LogFactory Issue

### Symptoms

An exception is thrown by JCL with a message similar to:

`

      The chosen LogFactory implementation does not extend LogFactory. Please check your configuration.
      (Caused by java.lang.ClassCastException: The application has specified that a custom LogFactory
      implementation should be used but Class 'com.ibm.ws.commons.logging.TrLogFactory' cannot be converted
      to 'org.apache.commons.logging.LogFactory'. The conflict is caused by the presence of multiple
      LogFactory classes in incompatible classloaders. Background can be found in
      http://commons.apache.org/logging/tech.html. If you have not explicitly specified a custom
      LogFactory then it is likely that the container has set one without your knowledge.
      In this case, consider using the commons-logging-adapters.jar file or specifying the standard
      LogFactory from the command line. Help can be found @http://commons.apache.org/logging.

`

This is a WebSphere example so the name of the custom LogFactory is `com.ibm.ws.commons.logging.TrLogFactory`. For other containers, this class name will differ.

### Explanation

A custom `LogFactory` implementation can only be used if the implementation class loaded dynamically at runtime can be cast to the `LogFactory` class that loaded it. There are several ways in which this cast can fail. The most obvious is that the source code may not actually extend `LogFactory`. The source may be compatible but if the `LogFactory` class against which the source is compiled is not binary compatible then the cast will also fail.

There is also another more unusual way in which this cast can fail: even when the binary is compatible, the implementation class loaded at runtime may be linked to a different instance of the `LogFactory` class. For more information, see the tech guide.

This situation may be encountered in containers which use a custom `LogFactory` implementation. The implementation will typically be provided in a shared, high level classloader together with JCL. When an application classloader contains `LogFactory`, the implementation will be loaded from that higher level classloader. The implementation class will be linked to the `LogFactory` class loaded by the higher level classloader. Even if the `LogFactory` implementations are binary compatible, since they are loaded by different classloaders the two `LogFactory` Class instances are not equal and so the cast must fail.

The policy adopted by JCL in this situation is to re-throw this exception. Additional information is included in the message to help diagnosis. The reasoning behind this choice is that a particular `LogFactory` implementation has been actively specified and this choice should not be ignored. This policy has unfortunate consequences when running in containers which have custom implementations: the above runtime exception may be thrown under certain classloading policies without the user knowingly specifying a custom implementation.

### Fixes

There are various ways to fix this problem. Which fix is right depends on the circumstances.

If you are happy using another classloading policy for the application, select a classloading policy which ensures that `LogFactory` will be loaded from the shared classloader containing the custom implementation.

If you want to bypass the container adaption mechanism then set the appropriate system property to the default value when the container is started:

`

     -Dorg.apache.commons.logging.LogFactory=org.apache.commons.logging.impl.LogFactoryImpl

`

If you want to continue to use the default container mechanism then:

- Find and replace the commons-logging implementation used by the container with the most modern release
- Replace the commons-logging jar in the application with the commons-logging-adapters jar. This will ensure that application classloader will delegate to it's parent when loading `LogFactory`.

If you encounter difficulties when applying the fixes recommended, please turn on diagnostics and consult the logs.

## Containers With Custom ClassLoading Behaviour for Logging

Because commons-logging is such a fundamental library, some containers modify the way in which classloading behaves for commons-logging classes.

### Apache Tomcat

At the current date, Tomcat 5.5.16 is the current release. All releases from version 4.1.x through 5.5.16 have a startup process that places jarfile ${tomcat.home}/bin/commons-logging-api.jar in the system classpath and then prevents any webapp from overriding the classes in that jarfile. Effectively, all webapps behave as if "parent-first" classloading were enabled for those classes.

This has some benefits; in particular it means that there are no problems in these Tomcat versions with having multiple copies of the commons-logging Log interface in the classpath (which avoids the "Log does not implement Log" problem described elsewhere).

However it also means that no webapp can override the core commons-logging classes by including an updated commons-logging jarfile in WEB-INF/lib; any class already loaded via the container takes priority. In particular, as Tomcat bundles logging 1.0.4 only, the new diagnostics and memory-leak-prevention features of the 1.1 release will not be available unless the container's library version is updated.

Because the commons-logging-api.jar in the container does not contain any log-library-adapter classes, updated behaviour for these will be seen when logging 1.1 is bundled in WEB-INF/lib. In particular, the support for log4j's TRACE level will take effect without having to update the container.

If you do wish to update Tomcat's version of commons-logging, then you must use the commons-logging-1.1-api jar only, not the full jar. Classes in the webapp cannot override classes loaded from the system classpath set up during Tomcat's startup process, and logging adapters can only see their matching concrete logging library if that library is available in the same classpath. Bundling the full commons-logging jarfile (with adapters) into the system classpath therefore means that logging libraries (eg log4j) within WEB-INF/lib are not accessable.

Note that the behaviour described here only applies if the standard Tomcat startup process is run. When Tomcat is embedded in a larger framework (eg run embedded within an IDE) this may not apply.

### JBoss Application Server

The JBoss Application Server can be configured to prevent deployed code from overriding classes higher in the hierarchy, effectively forcing "parent-first" behaviour for selected classes. By default, commons-logging is in this list (at least for some JBoss versions starting with 4.0.2), and therefore including an updated version of commons-logging in WEB-INF/lib or similar will have no effect. See the JBoss classloading documentation for more details.

### Other Containers

As more information becomes available on this topic, it may be added to the commons-logging wiki site.