Java - illyfrancis/scribble GitHub Wiki

Explanation of heap

http://www.avricot.com/blog/?post/2010/05/03/Get-started-with-java-JVM-memory-(heap%2C-stack%2C-xss-xms-xmx-xmn...)

And the memory allocation in 32bit OS etc. Because JVM requires a contiguous memory, in 32bit OS the max VM heap allocation is usually 1.2G to 1.6G according to the blog. Although that's that the limitation of the VM. The alternative is to use 64bit OS.

Swicth

-Xms : Sets the initial heap size for when the JVM starts.
-Xmx : Sets the maximum heap size.
-Xmn : Sets the size of the Young Generation.
-XX:PermSize : Sets the starting size of the Permanent Generation.
-XX:MaxPermSize : Sets the maximum size of the Permanent Generation

The Xmx and permgen space are separate. [http://rimuhosting.com/knowledgebase/linux/java/-Xmx-settings]

If you get java.lang.OutOfMemoryError: PermGen, increase pergen using XX:MaxPermSize like -XX:MaxPermSize=128m, the default is 64M.

Out of memory errors

http://www.kdgregory.com/index.php?page=java.outOfMemory

Caused by:

Permgen (as above), which is separate from the 'main' heap
Thread allocation
- Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread
and more refer to the blog

Garbage collection

Visual Tutorial from Oracle
http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html#RequiredSoftware
Presentation: Visualizing Java Garbage Collection
- according to presentation the 'mandatory' flags are:
  - -verbose:gc
  - -Xloggc:<pathtofile> path to the log output, make sure you've got disk space
  - -XX:+PrintGCDetails minimum information for tools to help, place -verbose:gc with this
  - -XX:+PrintTenuringDistribution premature promotion information
- it's no longer need to set -Xms and -Xmx to the same value, it was a workaround 3 - 4 years ago but no longer need to. (around 0:42)

Visual VM

Worth checking out

ThreadLocal

http://javapapers.com/core-java/threadlocal/

public class ThreadLocalExample {
    // SimpleDateFormat is not thread-safe, so give one to each thread
    private static final ThreadLocal<SimpleDateFormat> formatter = new ThreadLocal<SimpleDateFormat>() {
        @Override
        protected SimpleDateFormat initialValue()
        {
            return new SimpleDateFormat("yyyyMMdd HHmm");
        }
    };

    public String formatIt(Date date)
    {
        return formatter.get().format(date);
    }
}

ThreadLocalRandom

http://javapapers.com/core-java/threadlocal/

Since Java 7.

private static final ThreadLocal localRandom =
    new ThreadLocal() {
        protected ThreadLocalRandom initialValue() {
            return new ThreadLocalRandom();
        }
};