Java finalize方法，SocksSocketImpl，Profiling等

Question

We have a batch job, that reads data from database and writes it to a file. The job is written on top of Spring Batch 2.8. We have noticed, it's taking a lot of memory and are trying to tune it. I have some questions while performing the exercise

1. I know implementing finalize() is not recommended. In numerous posts I have read - they say the object might be pushed to a ReferenceQueue and a Finalizer thread polls the queue. So is there a way, I can check the queue myself? (I know a memory dump will show this, but sometimes the file is too huge to transfer from a live system to a local machine and perform diagnostics).

2. What exactly is the difference between a trivial and non-trivial finalize() method? Couldn't find an article which explains this.

3. If finalize() method is not a good idea, why does AbstractPlainSocketImpl,FileInputStream and others implement it? And how is that, their finalize method helps them in GC and not to be part of unreachable or dead code?

4. While using jvisualvm or jmc - the committed heap size is shown as 600MB. But when I execute the below command on our Linux environment it shows me the memory as 800MB+.

    ps --cols 500 -C java -o user,ppid,pid,pcpu,rss,size,vsize,cmd | grep <uid> | awk '{ print $3,$4,$5/1024}' 

Environment Details

    Number of cores: 2
    OS: Red Hat Enterprise Linux Server release 6.5
    Total Physical Memory: 7.69GB
    JVM: Java HotSpot(TM) 64-Bit Server VM (24.45-b08) for linux-amd64 JRE (1.7.0_45-b18)
    JVM Command Line Arguments: -Xloggc:../logs/Job-gc.log -verbose:gc -XX:+PrintGCDetails -XX:+PrintGCDateStamps -XX:+PrintHeapAtGC -XX:-PrintTenuringDistribution -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=5 -XX:GCLogFileSize=5M -XX:-CITime -XX:-PrintClassHistogram -XX:-PrintConcurrentLocks -XX:-PrintAdaptiveSizePolicy -XX:-TraceClassLoading -XX:-TraceClassUnloading -XX:+UnlockCommercialFeatures -XX:+FlightRecorder -Dcom.sun.management.jmxremote=true -Dcom.sun.management.jmxremote.port=7091 -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.ssl=false -Djava.rmi.server.hostname=server.com

    Garbage Collector used (this info is from JMC): ParallelScavenge for Young and ParallelOld for Old.

Answer 1

I know a memory dump will show this, but sometimes the file is too huge to transfer from a live system to a local machine and perform diagnostics

Command tar -czf dump.tar.gz dump.hprof may help you to transfer that big file.

3GB -> 100MB.

The compress rate of dump file would be so great.