private Log discoverLogImplementation()

in src/main/java/org/apache/commons/logging/impl/LogFactoryImpl.java [496:578]
42 lines of code
9 McCabe index (conditional complexity)

    private Log discoverLogImplementation(final String logCategory)
        throws LogConfigurationException {
        if (isDiagnosticsEnabled()) {
            logDiagnostic("Discovering a Log implementation...");
        }

        initConfiguration();

        Log result = null;

        // See if the user specified the Log implementation to use
        final String specifiedLogClassName = findUserSpecifiedLogClassName();

        if (specifiedLogClassName != null) {
            if (isDiagnosticsEnabled()) {
                logDiagnostic("Attempting to load user-specified log class '" +
                    specifiedLogClassName + "'...");
            }

            result = createLogFromClass(specifiedLogClassName,
                                        logCategory,
                                        true);
            if (result == null) {
                final StringBuilder messageBuffer =  new StringBuilder("User-specified log class '");
                messageBuffer.append(specifiedLogClassName);
                messageBuffer.append("' cannot be found or is not useable.");

                // Mistyping or misspelling names is a common fault.
                // Construct a good error message, if we can
                informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_LOG4J_LOGGER);
                informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_JDK14_LOGGER);
                informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_LUMBERJACK_LOGGER);
                informUponSimilarName(messageBuffer, specifiedLogClassName, LOGGING_IMPL_SIMPLE_LOGGER);
                throw new LogConfigurationException(messageBuffer.toString());
            }

            return result;
        }

        // No user specified log; try to discover what's on the classpath
        //
        // Note that we deliberately loop here over classesToDiscover and
        // expect method createLogFromClass to loop over the possible source
        // class loaders. The effect is:
        //   for each discoverable log adapter
        //      for each possible class loader
        //          see if it works
        //
        // It appears reasonable at first glance to do the opposite:
        //   for each possible class loader
        //     for each discoverable log adapter
        //        see if it works
        //
        // The latter certainly has advantages for user-installable logging
        // libraries such as Log4j; in a webapp for example this code should
        // first check whether the user has provided any of the possible
        // logging libraries before looking in the parent class loader.
        // Unfortunately, however, Jdk14Logger will always work in jvm>=1.4,
        // and SimpleLog will always work in any JVM. So the loop would never
        // ever look for logging libraries in the parent classpath. Yet many
        // users would expect that putting Log4j there would cause it to be
        // detected (and this is the historical JCL behavior). So we go with
        // the first approach. A user that has bundled a specific logging lib
        // in a webapp should use a commons-logging.properties file or a
        // service file in META-INF to force use of that logging lib anyway,
        // rather than relying on discovery.

        if (isDiagnosticsEnabled()) {
            logDiagnostic(
                "No user-specified Log implementation; performing discovery" +
                " using the standard supported logging implementations...");
        }
        for(int i=0; i<classesToDiscover.length && result == null; ++i) {
            result = createLogFromClass(classesToDiscover[i], logCategory, true);
        }

        if (result == null) {
            throw new LogConfigurationException
                        ("No suitable Log implementation");
        }

        return result;
    }