Log4J and Logback both capture a reference to System.out and/or System.err
and then use that reference thereafter. This means that any manipulation of
System.out or System.err -- using System.setOut(PrintStream) or
System.setErr(PrintStream) -- probably isn't going to play nicely with
logging in general.
Here's what that means for Geode tests...
If you're writing a Unit Test, then that test will be executed in a
long-lived JVM in which other Unit Tests are also going to be executed. The
SystemOutRule from system-rules will work with code that uses System.out
directly but will not work repeatedly with loggers.
If you're writing an Integration Test or Distributed Test, then that test
will be executed in a fresh JVM (fork every one is the config), so as long
as you can get the SystemOutRule before to invoke before logging, the test
will behave. In general that means you'll want to annotate it
with @BeforeClass.
Here are a couple examples in which log4j2 captures System.out to
illustrate why SystemOutRule can't intercept the output after invoking
System.setOut(PrintStream):
log4j-core/src/main/java/org/apache/logging/log4j/core/config/status/StatusConfiguration.java:43:
private static final PrintStream DEFAULT_STREAM = System.out;
log4j-api/src/main/java/org/apache/logging/log4j/simple/SimpleLoggerContext.java:87:
ps = System.out;
If SystemOutRule executes its before -- System.setOut(PrintStream) -- after
the above code, logging will be printing to a different PrintStream than
the PrintStream that SystemOutRule inserted into System.
