How to Force Garbage Collector to Run

How to force garbage collector to run?

System.GC.Collect() forces garbage collector to run. This is not recommended but can be used if situations arise.

Forcing Java virtual machine to run garbage collector

No, You cant force garbage collection.

Even using

System.gc();  

You can just make a request for garbage collection but it depends on JVM to do it or not.

Also Garbage collector are smart enough to collect unused memory when required so instead of forcing garbage collection you should check if you are handling objects in a wrong way.

If you are handling objects in a wrong way (like keeping reference to unnecessary objects) there is hardly anything JVM can do to free the memory.

From Doc

Calling the gc method suggests that the Java Virtual Machine expend
effort toward recycling unused objects in order to make the memory
they currently occupy available for quick reuse. When control returns
from the method call, the Java Virtual Machine has made a best effort
to reclaim space from all discarded objects.

Open Bug regarding System.gc() documentation

The documentation for System.gc() is extremely misleading and fails to
make reference to the recommended practise of never calling
System.gc().

The choice of language leaves it unclear what the behaviour would be
when System.gc() is called and what external factors will influence
the behaviour.

Few useful link to visit when you think you should force JVM to free up some memory

1. How does garbage collection work

2. When does System.gc() do anything

3. Why is it bad practice to call System.gc()?

All says

1. You dont have control over GC in Java even System.gc() dont guarantee it.

2. Also its bad practise as forcing it may have adverse effect on performance.

3. Revisit your design and let JVM do his work :)

Can I Force Garbage Collection in Java?

Nope, System.gc() is as close as you can get. Java isn't C or C++, the JVM manages memory for you, so you don't have that kind of fine grained control. If you set objects you're no longer using to null, or loose all references, they will get cleaned up. And the GC is pretty smart, so it should take good care of you.

That said, if you are on a unix box, and force a thread dump (kill -3), it'll pretty much force garbage collection.

Best Practice for Forcing Garbage Collection in C#

The best practise is to not force a garbage collection.

According to MSDN:

"It is possible to force garbage
collection by calling Collect, but
most of the time, this should be
avoided because it may create
performance issues. "

However, if you can reliably test your code to confirm that calling Collect() won't have a negative impact then go ahead...

Just try to make sure objects are cleaned up when you no longer need them. If you have custom objects, look at using the "using statement" and the IDisposable interface.

This link has some good practical advice with regards to freeing up memory / garbage collection etc:

http://msdn.microsoft.com/en-us/library/66x5fx1b.aspx

How do you Force Garbage Collection from the Shell?

You can do this via the free jmxterm program.

Fire it up like so:

java -jar jmxterm-1.0-alpha-4-uber.jar

From there, you can connect to a host and trigger GC:

$>open host:jmxport
#Connection to host:jmxport is opened
$>bean java.lang:type=Memory
#bean is set to java.lang:type=Memory
$>run gc
#calling operation gc of mbean java.lang:type=Memory
#operation returns: 
null
$>quit
#bye

Look at the docs on the jmxterm web site for information about embedding this in bash/perl/ruby/other scripts. I've used popen2 in Python or open3 in Perl to do this.

UPDATE: here's a one-liner using jmxterm:

echo run -b java.lang:type=Memory gc | java -jar jmxterm-1.0-alpha-4-uber.jar -n -l host:port

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