Given a config snippet like this, where we explicitely use the DefaultTimeBasedFileNamingAndTriggeringPolicy to cause the problem:

<appender name="sandbox" class="ch.qos.logback.core.rolling.RollingFileAppender">
<file>${catalina.base}/log/sandbox.log</file>
<append>true</append>
<rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy">
<fileNamePattern>${catalina.base}/log/sandbox.log.%d

{yyyyMMdd}

-000000</fileNamePattern>
<timeBasedFileNamingAndTriggeringPolicy
class="ch.qos.logback.core.rolling.DefaultTimeBasedFileNamingAndTriggeringPolicy"/>
</rollingPolicy>
<encoder>
<pattern>%d

{ISO8601}

%9relative [%thread] %-5level %logger - %msg%n</pattern>
</encoder>
</appender>

we get this stacktrace:
-INFO in ch.qos.logback.classic.joran.action.JMXConfiguratorAction - begin
-INFO in ch.qos.logback.core.joran.action.AppenderAction - About to instantiate appender of type [ch.qos.logback.core.rolling.RollingFileAppender]
-INFO in ch.qos.logback.core.joran.action.AppenderAction - Naming appender as [sandbox]
-ERROR in ch.qos.logback.core.joran.spi.Interpreter@22:100 - RuntimeException in Action for tag [timeBasedFileNamingAndTriggeringPolicy] java.lang.NullPointerException
at ch.qos.logback.core.rolling.TimeBasedFileNamingAndTriggeringPolicyBase.start(TimeBasedFileNamingAndTriggeringPolicyBase.java:44)
at ch.qos.logback.core.rolling.DefaultTimeBasedFileNamingAndTriggeringPolicy.start(DefaultTimeBasedFileNamingAndTriggeringPolicy.java:32)
at ch.qos.logback.core.joran.action.NestedComplexPropertyIA.end(NestedComplexPropertyIA.java:167)

Why is that? The code in TimeBasedRollingPolicy that sets the default implementation if none is given
does so AFTER the "tbrp" member is set. Also, at the same location the setContext(context) and
setTimeBasedRollingPolicy(this) calls are made, then start() is called on the TBFNATP (in a fully
pre-initialized state).

But before that and with an explicit policy configuration, Joran calls the start() method when the tag is
closed on the newly created TBFNATP (see above stack trace), and that call must be ignored. That fact
is suprising at first and should be documented, and the default TBFNATP should be augmented to be
able to be used explicitely as above.

My own TBFNATP, derived from the default, thus has this code:

@Override
public void start() {
if (tbrp == null)

{ // Containing policy did not initialize us yet! // This happens when Joran calls our start() when the config tag is closed. addInfo("Premature start ignored."); return; }

super.start();
...

which prevents the problem. Maybe there could also be a more global solution that tells Joran
that this is a nested, yet incomplete element and to not call start() in the first place, but let the
containing element do it.