Java ConcurrentHashMap.keySet() hangs

Question

I am observing a strange behavior on a Linux box. The code works fine on Win 7. The offending code hangs on xmlToJavaMap.keySet(). Neither of the two log statements are logged!!! No deadlock found in heap dump.

    ConcurrentHashMap<String,String> xmlToJavaMap = ApplicationContext.getBean("map");
    logger.info("before for loop");
    for (String key : xmlToJavaMap.keySet()) {
        logger.info("key: " + key);
        ...
    }   
    logger.info("map processed.");  

Platform: java version "1.7.0_11" Java(TM) SE Runtime Environment (build 1.7.0_11-b21) Java HotSpot(TM) 64-Bit Server VM (build 23.6-b04, mixed mode) Red Hat 4.4.7

Answer 1

Use jps -v to watch your java process. Then use jstack to watch your stack of threads. That could help you find the solution.

Answer 2

Here is what was actually happening :) xmlToJavaMap.keySet() was actually failing with NoSuchMethod and the thread was terminating. The error stack trace was getting logged in a different log file which was causing the confusion. Once the error was addressed , everything back to normal.

Answer 3

I found the same case, hang at keySet() method. actually the thread exit by NoSuchMethodError error. solution is that just declare

ConcurrentHashMap<String,String> xmlToJavaMap = ApplicationContext.getBean("map");

to

Map<String,String> xmlToJavaMap = ApplicationContext.getBean("map");

or

make sure code and dependency jar is compiled by same jdk version.

the root reason is that "Undefined reference: .. ConcurrentHashMap.keySet()" when building in Java 8 , cause by： in version 7 and 8, keySet() method is not the same return , this error maybe is not in your project's log or console, but in heap. you can dump heap to find java.lang.NoSuchMethodError related to keySet()