https://issues.apache.org/bugzilla/show_bug.cgi?id=51783
Bug #: 51783
Summary: WriterAppender NullPointerException when calling close
and append concurrently
Product: Log4j
Version: 1.2
Platform: PC
OS/Version: Linux
Status: NEW
Severity: major
Priority: P2
Component: Appender
AssignedTo: log4j-dev@logging.apache.org
ReportedBy: nist...@illinois.edu
Classification: Unclassified
Created attachment 27471
--> https://issues.apache.org/bugzilla/attachment.cgi?id=27471
test
Hi,
I am encountering a NullPointerException in WriterAppender. I am using
Log4j version 1.2.16.
The NullPointerException manifests when calling close() and append()
concurrently. I have attached a test that exposes this problem. For
this test, the expected output is:
Expected output:
0
1
2
3
4
.....
499998
499999
DONE
But when the bug manifests (almost always for this test), the output
is:
Bug output:
0
1
2
3
4
...
16
Exception in thread "main" java.lang.NullPointerException
at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:317)
at org.apache.log4j.WriterAppender.append(WriterAppender.java:162)
at org.apache.log4j.Test.test(Test.java:37)
at org.apache.log4j.Test.main(Test.java:16)
or
0
1
2
3
4
Exception in thread "main" java.lang.NullPointerException
at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:318)
at org.apache.log4j.WriterAppender.append(WriterAppender.java:162)
at org.apache.log4j.Test.test(Test.java:37)
at org.apache.log4j.Test.main(Test.java:16)
The test iteration where the NullPointerException is thrown varies
from run to run, depending on how the threads interleave.
The reason for NullPointerException is that append() is not
synchronized.
Here is the thread scenario that triggers the bug:
(1) append() starts to execute. It checks checkEntryConditions(). This
check succeeds because close was not yet executed(). append()
continue to subAppend(event); but does not yet enter subAppend()
(e.g., if there is a delay due to high load)
(2) the other thread executes close(). close() calls reset() which
sets this.qw = null;
(3) append() continue to execute subAppend(); subAppend() calls
several methods on this.qw, which by now is null. These methods
are:
this.qw.write(this.layout.format(event));
this.qw.write(s[i]);
this.qw.write(Layout.LINE_SEP);
Calling these methods throws NullPointerException.
The solution is to synchronize append(). close() is already
synchronized.
Is this a bug? Does synchronizing append() fully solve the problem?
Are there other scenarios that I am missing and are not solved by
synchronizing append()?
Thanks!
Adrian
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