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https://issues.apache.org/jira/browse/LOG4J2-2596?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16827020#comment-16827020
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Gary Gregory commented on LOG4J2-2596:
--------------------------------------
Hi [~Hecoz],
Log4j 1 has reached EOL, please update to Log4j 2.
Gary
> AppenderSkeleton may introduce NullPointerException
> ---------------------------------------------------
>
> Key: LOG4J2-2596
> URL: https://issues.apache.org/jira/browse/LOG4J2-2596
> Project: Log4j 2
> Issue Type: Bug
> Components: Appenders
> Reporter: Hiram
> Priority: Critical
> Labels: easyfix
>
> Hello, I am a software tester. I found this problem in all versions of log4j.
> AppenderSkeleton may cause a NullPointerException error.
>
> //Error code fragment
> public boolean isAsSevereAsThreshold(Priority priority){
> return this.threshold == null ||
> priority.isGreaterOrEqual(this.threshold);
> }
> public void setThreshold(Priority threshold){
> this.threshold = threshold;
> }
>
> In the AppenderSkeleton class, if the isAsSevereAsThreshold() method and
> setThreshold are executed simultaneously by two threads,
> isAsSevereAsThreshold method checks that this.threshold is not null, another
> thread calls the setThreshold method and sets the threshold to null. When the
> thread switches back to the isAsSevereAsThreshold method to continue
> execution, it will trigger a NullPointerException.
>
> There are many classes that extends AppenderSkeleton, which can cause this
> error if they are interleaved by different threads.
> This is an atomic violation.
> This error can be triggered by the following test:
>
> public static void main(String[] args) throws Exception {
> AppenderSkeleton app = new NullAppender();
> Thread t1 = new Thread(new Runnable() {
> @Override
> public void run() {
> for (int i = 0; i < 90000;i++){
> app.setThreshold(Priority.DEBUG);
> app.isAsSevereAsThreshold(Priority.DEBUG);
> }
> }
> });
> Thread t2 = new Thread(new Runnable() {
> @Override
> public void run() {
> for (int i = 0; i < 90000;i++){
> app.setThreshold(null);
> }
> }
> });
> t1.start();
> t2.start();
> t1.join();
> t2.join();
> }
>
> Error message：
> Exception in thread "Thread-0" java.lang.NullPointerException
> at org.apache.log4j.Priority.isGreaterOrEqual(Priority.java:123)
> at
> org.apache.log4j.AppenderSkeleton.isAsSevereAsThreshold(AppenderSkeleton.java:219)
> at test.TestSequence$1.run(TestSequence.java:19)
> at java.lang.Thread.run(Thread.java:748)
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