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Is it possible to stop the garbage collector OR to force it to run at specific time? [duplicate]

Is it possible to force garbage collection in Java, even if it is tricky to do? I know about System.gc(); and Runtime.gc(); but they only suggest to do GC. How can I force GC?

Your best option is to call System.gc() which simply is a hint to the garbage collector that you want it to do a collection. There is no way to force and immediate collection though as the garbage collector is non-deterministic.

The jlibs library has a good utility class for garbage collection. You can force garbage collection using a nifty little trick with WeakReference objects.
RuntimeUtil.gc() from the jlibs:
/**
* This method guarantees that garbage collection is
* done unlike <code>{#link System#gc()}</code>
*/
public static void gc() {
Object obj = new Object();
WeakReference ref = new WeakReference<Object>(obj);
obj = null;
while(ref.get() != null) {
System.gc();
}
}

The best (if not only) way to force a GC would be to write a custom JVM. I believe the Garbage collectors are pluggable so you could probably just pick one of the available implementations and tweak it.
Note: This is NOT an easy answer.

Using the Java™ Virtual Machine Tool Interface (JVM TI), the function
jvmtiError ForceGarbageCollection(jvmtiEnv* env)
will "Force the VM to perform a garbage collection." The JVM TI is part of the JavaTM Platform Debugger Architecture (JPDA).

YES it is almost possible to forced you have to call to methods in the same order and at the same time this ones are:
System.gc ();
System.runFinalization ();
even if is just one object to clean the use of this two methods at the same time force the garbage collector to use the finalise() method of unreachable object freeing the memory assigned and doing what the finalize() method states.
HOWEVER it is a terrible practice to use the garbage collector because the use of it could introduce an over load to the software that may be even worst than on the memory, the garbage collector has his own thread which is not possible to control plus depending on the algorithm used by the gc could take more time and is consider very inefficient, you should check your software if it worst with the help of the gc because it is definitely broke, a good solution must not depend on the gc.
NOTE: just to keep on mind this will works only if in the finalize method is not a reassignment of the object, if this happens the object will keep alive an it will have a resurrection which is technically possible.

Under the documentation for OutOfMemoryError it declares that it will not be thrown unless the VM has failed to reclaim memory following a full garbage collection. So if you keep allocating memory until you get the error, you will have already forced a full garbage collection.
Presumably the question you really wanted to ask was "how can I reclaim the memory I think I should be reclaiming by garbage collection?"

You can trigger a GC from the command line. This is useful for batch/crontab:
jdk1.7.0/bin/jcmd <pid> GC.run
See :
https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/tooldescr006.html

To manually Request GC (not from System.gc()) :
Go To : bin folder in JDK eg.-C:\Program Files\Java\jdk1.6.0_31\bin
Open jconsole.exe
Connect to the desired local Process.
Go To memory tab and click perform GC.

How to Force Java GC
Okay, here are a few different ways to force Java GC.
Click JConsole's Perform GC button
Use JMap's jmap -histo:live 7544 command where 7544 is the pid
Call the Java Diagnostic Console's jcmd 7544 GC.run command
Call System.gc(); in your code
Call Runtime.getRuntime().gc(); in your code
None of these work
Here's the dirty little secret. None of these are guaranteed to work. You really can't force Java GC.
The Java garbage collection algos are non-deterministic, and while all of these methods can motivate the JVM to do GC, you can't actually force it. If the JVM has too much going on and a stop-the-world operation is not possible, these commands will either error out, or they will run but GC won't actually happen.
if (input.equalsIgnoreCase("gc")) {
System.gc();
result = "Just some GC.";
}
if (input.equalsIgnoreCase("runtime")) {
Runtime.getRuntime().gc();
result = "Just some more GC.";
}
Fix the darn problem
If you've got a memory leak or object allocation problem, then fix it. Sitting around with your finger on Java Mission Control's Force Java GC button only kicks the can down the road. Profile your app with Java Flight Recorder, view the results in VisualVM or JMC, and fix the problem. Trying to force Java GC is a fools game.

.gc is a candidate for elimination in future releases - a Sun Engineer once commented that maybe fewer than twenty people in the world actually know how to use .gc() - I did some work last night for a few hours on a central / critical data-structure using SecureRandom generated data, at somewhere just past 40,000 objects the vm would slow down as though it had run out of pointers. Clearly it was choking down on 16-bit pointer tables and exhibited classic "failing machinery" behavior.
I tried -Xms and so on, kept bit twiddling until it would run to about 57,xxx something. Then it would run gc going from say 57,127 to 57,128 after a gc() - at about the pace of code-bloat at camp Easy Money.
Your design needs fundamental re-work, probably a sliding window approach.

JVM specification doesn't say anything specific about garbage collection. Due to this, vendors are free to implement GC in their way.
So this vagueness causes uncertainty in garbage collection behavior. You should check your JVM details to know about the garbage collection approaches/algorithms. Also there are options to customize behavior as well.

If you need to force garbage collection, perhaps you should consider how you're managing resources. Are you creating large objects that persist in memory? Are you creating large objects (e.g., graphics classes) that have a Disposable interface and not calling dispose() when done with it? Are you declaring something at a class level that you only need within a single method?

It would be better if you would describe the reason why you need garbage collection. If you are using SWT, you can dispose resources such as Image and Font to free memory. For instance:
Image img = new Image(Display.getDefault(), 16, 16);
img.dispose();
There are also tools to determine undisposed resources.

Another options is to not create new objects.
Object pooling is away to reduce the need GC in Java.
Object pooling is generally not going to be faster than Object creation (esp for lightweight objects) but it is faster than Garbage Collection. If you created 10,000 objects and each object was 16 bytes. That's 160,000 bytes GC has to reclaim. On the other hand, if you don't need all 10,000 at the same time, you can create a pool to recycle/reuse the objects which eliminates the need to construct new objects and eliminates the need to GC old objects.
Something like this (untested).
And if you want it to be thread safe you can swap out the LinkedList for a ConcurrentLinkedQueue.
public abstract class Pool<T> {
private int mApproximateSize;
private LinkedList<T> mPool = new LinkedList<>();
public Pool(int approximateSize) {
mApproximateSize = approximateSize;
}
public T attain() {
T item = mPool.poll();
if (item == null) {
item = newInstance();
}
return item;
}
public void release(T item) {
int approxSize = mPool.size(); // not guaranteed accurate
if (approxSize < mApproximateSize) {
recycle(item);
mPool.add(item);
} else if (approxSize > mApproximateSize) {
decommission(mPool.poll());
}
}
public abstract T newInstance();
public abstract void recycle(T item);
public void decommission(T item) { }
}

You can try using Runtime.getRuntime().gc() or use utility method System.gc() Note: These methods do not ensure GC. And their scope should be limited to JVM rather than programmatically handling it in your application.

We can trigger jmap -histo:live <pid> using the java runtime. This will force a full GC on heap to mark all the live objects.
public static void triggerFullGC() throws IOException, InterruptedException {
String pid = ManagementFactory.getRuntimeMXBean().getName().split("#")[0];
Process process = Runtime.getRuntime().exec(
String.format("jmap -histo:live %s", pid)
);
System.out.println("Process completed with exit code :" + process.waitFor());
}

I did some experimentation (see https://github.com/mikenakis/ForcingTheJvmToGarbageCollect) trying about a dozen different ways of performing a garbage collection, including ways described in this answer, and more, and I found that there is absolutely no frigging way to deterministically force the JVM to do a complete garbage collection. Even the best answers to this question are only partially successful in that the best they achieve is some garbage collection, but never a guaranteed full garbage collection.
My experimentation has showed that the following code snippet yields the best (least bad) results:
public static void ForceGarbageCollection()
{
long freeMemory = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed();
for( ; ; )
{
Runtime.getRuntime().gc();
Runtime.getRuntime().runFinalization();
long newFreeMemory = ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed();
if( newFreeMemory == freeMemory )
break;
freeMemory = newFreeMemory;
sleep( 10 );
}
}
Where the sleep() function is as follows:
private static void sleep( int milliseconds )
{
try
{
Thread.sleep( milliseconds );
}
catch( InterruptedException e )
{
throw new RuntimeException( e );
}
}
Unfortunately, that number 10 in that sleep( 10 ) is magic; it assumes that you are doing a moderate number of memory allocations per second, which incur a moderate amount of finalization. If you are going through objects faster, then 10 might be inadequate and you may need to wait longer. You could set it to 100 to be sure, but no matter what you set it to, there will always be a chance that it will not be enough.
That having been said, in a controlled environment where that 10 is enough, this approach has been observed to consistently eliminate all unreachable objects from memory, while no other approach mentioned in this Q&A does. The experiment code I linked to on github proves so.
In my opinion, the fact that the Java Virtual Machine provides no means of performing a forced-on-demand, unconditional, deterministic, absolutely thorough, stop-the-world garbage collection makes it BROKEN.
To put it in a different way, the creators of the JVM are so full of hubris as to think that they know better than us whether we want to do that or whether we should want to do that. Don't be so arrogant. If something works as if by magic, then some means of bypassing the magic must be provided.

I wanted to force gc, because my code was frozen for a long time when it happened. The aim is to smooth the charge, by regularly cause gc.
The solutions listed doesnt forced anything in my environment.
So:
I request the memory for temporary variable,
simply, by increments,
and monitor the memory and stop the operation as soon as gc is triggered.
It works easily but you have to tune.
Runtime rt = Runtime.getRuntime();
double usedMB = (rt.totalMemory() - rt.freeMemory()) / 1024 / 1024;
if (usedMB > 1000) // only when necessary
{
byte[][] for_nothing = new byte[10][];
for (int k = 0; k < 10 ; k ++)
for_nothing[k] = new byte[100_000_000];
}
System.gc();
Runtime.getRuntime().gc();
Runtime.getRuntime().runFinalization();

If you are running out of memory and getting an OutOfMemoryException you can try increasing the amount of heap space available to java by starting you program with java -Xms128m -Xmx512m instead of just java. This will give you an initial heap size of 128Mb and a maximum of 512Mb, which is far more than the standard 32Mb/128Mb.

Really, I don't get you. But to be
clear about "Infinite Object Creation"
I meant that there is some piece of
code at my big system do creation of
objects whom handles and alive in
memory, I could not get this piece of
code actually, just gesture!!
This is correct, only gesture. You have pretty much the standard answers already given by several posters. Let's take this one by one:
I could not get this piece of code
actually
Correct, there is no actual jvm - such is only a specification, a bunch of computer science describing a desired behaviour ... I recently dug into initializing Java objects from native code. To get what you want, the only way is to do what is called aggressive nulling. The mistakes if done wrong are so bad doing that we have to limit ourselves to the original scope of the question:
some piece of code at my big system
do creation of objects
Most of the posters here will assume you are saying you are working to an interface, if such we would have to see if you are being handed the entire object or one item at a time.
If you no longer need an object, you can assign null to the object but if you get it wrong there is a null pointer exception generated. I bet you can achieve better work if you use NIO
Any time you or I or anyone else gets: "Please I need that horribly." it is almost universal precursor to near total destruction of what you are trying to work on .... write us a small sample code, sanitizing from it any actual code used and show us your question.
Do not get frustrated. Often what this resolves to is your dba is using a package bought somewhere and the original design is not tweaked for massive data structures.
That is very common.

FYI
The method call System.runFinalizersOnExit(true) guarantees that finalizer methods
are called before Java shuts down. However, this method is inherently unsafe
and has been deprecated. An alternative is to add “shutdown hooks” with the method
Runtime.addShutdownHook.
Masarrat Siddiqui

There is some indirect way for forcing garbage collector. You just need to fill heap with temporary objects until the point when garbage collector will execute. I've made class which forces garbage collector in this way:
class GarbageCollectorManager {
private static boolean collectionWasForced;
private static int refCounter = 0;
public GarbageCollectorManager() {
refCounter++;
}
#Override
protected void finalize() {
try {
collectionWasForced = true;
refCounter--;
super.finalize();
} catch (Throwable ex) {
Logger.getLogger(GarbageCollectorManager.class.getName()).log(Level.SEVERE, null, ex);
}
}
public int forceGarbageCollection() {
final int TEMPORARY_ARRAY_SIZE_FOR_GC = 200_000;
int iterationsUntilCollected = 0;
collectionWasForced = false;
if (refCounter < 2)
new GarbageCollectorManager();
while (!collectionWasForced) {
iterationsUntilCollected++;
int[] arr = new int[TEMPORARY_ARRAY_SIZE_FOR_GC];
arr = null;
}
return iterationsUntilCollected;
}
}
Usage:
GarbageCollectorManager manager = new GarbageCollectorManager();
int iterationsUntilGcExecuted = manager.forceGarbageCollection();
I don't know how much this method is useful, because it fills heap constantly, but if you have mission critical application which MUST force GC - when this may be the Java portable way to force GC.

I would like to add some thing here. Please not that Java runs on Virtual Machine and not actual Machine. The virtual machine has its own way of communication with the machine. It may varry from system to system. Now When we call the GC we ask the Virtual Machine of Java to call the Garbage Collector.
Since the Garbage Collector is with Virtual Machine , we can not force it to do a cleanup there and then. Rather that we queue our request with the Garbage Collector. It depends on the Virtual Machine, after particular time (this may change from system to system, generally when the threshold memory allocated to the JVM is full) the actual machine will free up the space. :D

On OracleJDK 10 with G1 GC, a single call to System.gc() will cause GC to clean up the Old Collection. I am not sure if GC runs immediately. However, GC will not clean up the Young Collection even if System.gc() is called many times in a loop. To get GC to clean up the Young Collection, you must allocate in a loop (e.g. new byte[1024]) without calling System.gc(). Calling System.gc() for some reason prevents GC from cleaning up the Young Collection.

If you are using JUnit and Spring, try adding this in every test class:
#DirtiesContext(classMode = DirtiesContext.ClassMode.AFTER_CLASS)

Verify object deletion in Java

In my code there are many objects created and deleted during runtime which can have long reference chains (i.e. be reachable in terms of garbage collection from many places).
In my unit test I want to verify that when I "delete" an object, e.g. remove it from the list that primarily contains it, all references to that object are deleted. To be clear: I want to verify that no reference to this object exists anymore in my application.
How can I achieve this?
So far I came only up with this (testing that a PhantomReference is enqueued):
#Test
public void test_objectX_can_be_garbage_collected() throws Exception {
ReferenceQueue<MyClass> refsToBeDeleted = new ReferenceQueue<>();
MyClass obj = new MyClass();
PhantomReference<MyClass> phantomRef = new PhantomReference<MyClass>(obj, refsToBeDeleted);
obj = null; // delete last strong reference to obj
System.out.println("Reference is enqueued:" + phantomRef.isEnqueued()); // "false"
System.gc(); // invoke garbage collection
Thread.sleep(1000L); // required - see text
System.gc(); // invoke garbage collection again
System.out.println("Reference is enqueued:" + phantomRef.isEnqueued()); // true
}
with
public static class MyClass {
}
The doc says:
public static void gc()
Runs the garbage collector.
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.
(emphasis mine)
This is quite vague ("best effort") and seems quite non-deterministic, e.g. I have to either wait 1 second and call gc() again, or call it e.g. four times in a row for the reference to be enqueued.
Is there any way to do this properly? I want to test that objects don't persist and will eventually flood my memory after months of uptime.
Remarks:
1.) I know you should not call gc() because it's bad code. True, but I am calling it from test code to verify production code.
2.) I know that gc() is actually not deterministic from a local point of view. I'm aware of this and asking for a better alternative.
Edit:
I realized that this can be a kind of "special" unit test: If it succeeds once for a given code base, I know that this reference is garbage collected; if the unit test fails it might be because gc() didn't want to delete it, or it could not be deleted. So this test can verify that the memory is released, but it might need several attempts for that verification. However, this is "special" (read: "bad") because I would have to accept a randomly failing unit test.

AFAIK there is no way to make sure that an object has been garbage collected. Calling System.gc() will usually result in garbage collection, but not always.
I also have a infinitely running Java process - I call System.gc() at the beginning of each iteration and record the resulting amount of used memory. If this amount does not significantly change over time, I can be reasonably sure not to "leak" memory.

I've observed that when a heap dump is triggered, it usually does a full GC. If that is a policy of JVM to force GC during a Heap collection, then you can trigger a heap collection by
kill -3 pid
and see if it works.
You might need to add jvm arguments for getting a heap dump.

I do not see a problem using reference and System.gc() for testing (though I prefer weak references instead phantom ones). I do not remember any stability problems for such approach.
Reference based approach work only if you can get reference to object which may be leaked.
I have forked version of head dump analysis library from NetBeans, which I use to automate certain types of memory usage reports.
It is possible, for test code, to take heap dump of itself and inspect this dump assert specific number of objects of given type to be present.
Here you can find example of such approach.

java finalize method use

My system suffer from memory leak, i try to optimize it by releasing memory as soon as possible.
Is this a good use of the finalize method?
Where to i catch the "Throwable" object?
public class OrderSendBulkHandler extends PandaSoapHandler {
// <login,Data>
private HashMap<String,OrderSendBulkData> followersData = new HashMap<String,OrderSendBulkData>();
// <login,error_code>
private HashMap<String,BulkDataResponse> positionResponses = new HashMap<String,BulkDataResponse>();
private Position position;
private Float guruBalance;
private float partialRatio = -1;
private boolean ignorePosition = false;
#Override
protected void finalize() throws Throwable {
try {
followersData.clear();
followersData = null;
positionResponses.clear();
positionResponses = null;
position = null;
guruBalance = null;
}
catch (Exception e) {
e.printStackTrace();
}
super.finalize();
}
// more code of the class here ...
// .....
}

Finalize is not the place to do anything related to code optimization for memory leak or so.
In fact thumb rule is that "Never use finalize for anything on which your program logic/success depends".

Using finalize() method actually slows the release of memory. When you use finalize, the object has to be added to a queue and thus is not cleaned up until it is called in another thread which means delaying its actual clean up for at least one GC cycle.
If you don't use finalize() it can be cleaned up in one cycle, this included anything the object references as well.
The best way to reduce memory consumption is to use a memory profiler. I would look at reducing the load on the GC by creating less garbage and by reducing the amount of memory retained by looking at the breakdown of the heap esp looking at where objects are allocated.
If you don't have a commercial profiler I suggest getting Java Mission Control supported in Java 7 update 40.

No, it's not, JVM is good at traversing object graph and removing unused objects. Implementing your own finalize method only slows down performance.
you don't have to
You should use the following strategy for tracking the memory leak:
add the following property to your application -XX:+HeapDumpOnOutOfMemoryError (assuming you are using HotSpot JVM)
load your application with tests of some kind so you eventually get OutOfMemoryError - after that you can find an .hprof file in app directory
download http://www.eclipse.org/mat/
open .hprof file in MAT tool, open Dominators tree and try to find the root cause of the issue

Is this a good use of the finalize method?
No.Since here you are making the objects eligible for garbage collection in finalize().
But According to java docs
finalize() method Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
So you can use finally block instead to make the objects eligible for garbage collection.
These are the SO questions worth reading
1.In Java, what purpose do the keywords `final`, `finally` and `finalize` fulfil?
2.Clean up code in finalize() or finally()?

No. Your finalize() method does exactly nothing relevant that wouldn't already happen by default.

Memory leaks ram java program with opencv and processing

i have this code in java and memory in ram almost explodes in just a few seconds. I release the IplImage "imagensMedia ". Why??
EDIT: This function runs many times per second
int largura=1280;
int altura=800;
IplImage[] imagens = new IplImage[5];
IplImage imagensSoma=cvCreateImage(cvSize(largura,altura), 32, 3);
int indiceImagem=0;
for(x=0; x<imagens.length;x++)
imagens[x]=cvCreateImage(cvSize(largura,altura), 8, 3);
public void imageArrayBuilder() //Constroi Array de Imagens e a Sua Media
{
int z;
IplImage imagensMedia =cvCreateImage(cvSize(largura,altura), 8, 3);
cam.read();
opencv.copy(cam.get());
if(imagemTotalFlag)
{
cvSub(imagensSoma,imagens[indiceImagem],imagensSoma, null);
cvAcc(opencv.Buffer, imagensSoma, null);
cvConvertScale(imagensSoma, imagensMedia, 1.0/imagens.length,0);
cvCopy(opencv.Buffer,imagens[indiceImagem],null);
indiceImagem++;
if(indiceImagem==imagens.length)
indiceImagem=0;
opencv.copy(imagensMedia);
}
else
{
if(indiceImagem<imagens.length)
{
cvCopy(opencv.Buffer,imagens[indiceImagem],null);
indiceImagem++;
if(indiceImagem==imagens.length)
{
imagemTotalFlag=true;
for(z = 0; z < imagens.length; z++)
cvAcc(imagens[z], imagensSoma, null);
cvConvertScale(imagensSoma, imagensMedia, 1.0/imagens.length,0);
indiceImagem=0;
opencv.copy(imagensMedia);
}
}
}
cvReleaseImage(imagensMedia);
}
Someone help me please...

I don't think the concept of a "memory leak" really applies in java, as it is a garbage collected language (references are automatically deleted when no other objects refer to them). The comments have given a few possibilities about where the memory is being used; I would recommend looking into how garbage collection works (see this link for starters). If all else fails, calling System.gc() will run the garbage collector, or more specifically:
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.
Beyond that, check for infinite loops or infinite recursion.
Hope this helps.

From what I understand, the Java wrapper for OpenCV's Mat has a tiny footprint. The underlying native objects may be using gigabytes of memory (it was in my case) and the GC knows nothing about it.
The only 2 options are to:
Manually deallocate native memory using Mat.release().
Call System.gc() every so often to force garbage collection of unreferenced Mat wrapper objects. Note, if you are using something like VideoCapture to read in frames, you will absolutely want to NOT call System.gc() every frame read as it will destroy performance. I call it about every 20 seconds in my application.

Understanding Java Memory Management

Java programmers know that JVM runs a Garbage Collector, and System.gc() would just be a suggestion to JVM to run a Garbage Collector. It is not necessarily that if we use System.gc(), it would immediately run the GC.Please correct me if I misunderstand Java's Garbage Collector.
Is/are there any other way/s doing memory management other than relying on Java's Garbage Collector?If you intend to answer the question by some sort of programming practice that would help managing the memory, please do so.

The most important thing to remember about Java memory management is "nullify" your reference.
Only objects that are not referenced are to be garbage collected.
For example, objects in the following code is never get collected and your memory will be full just to do nothing.
List objs = new ArrayList();
for (int i = 0; i < Integer.MAX_VALUE; i++) objs.add(new Object());
But if you don't reference those object ... you can loop as much as you like without memory problem.
List objs = new ArrayList();
for (int i = 0; i < Integer.MAX_VALUE; i++) new Object();
So what ever you do, make sure you remove reference to object to no longer used (set reference to null or clear collection).
When the garbage collector will run is best left to JVM to decide. Well unless your program is about to start doing things that use a lot of memory and is speed critical so you may suggest JVM to run GC before going in as you may likely get the garbaged collected and extra memory to go on. Other wise, I personally see no reason to run System.gc().
Hope this helps.

Below is little summary I wrote back in the days (I stole it from some blog, but I can't remember where from - so no reference, sorry)
There is no manual way of doing garbage collection in Java.
Java Heap is divided into three generation for the sake of garbage collection. These are the young generation, tenured or old generation, and Perm area.
New objects are created in the young generation and subsequently moved to the old generation.
String pool is created in Perm area of Heap, Garbage collection can occur in perm space but depends on upon JVM to JVM.
Minor garbage collection is used to move an object from Eden space to Survivor 1 and Survivor 2 space, and Major collection is used to move an object from young to tenured generation.
Whenever Major garbage collection occurs application, threads stops during that period which will reduce application’s performance and throughput.
There are few performance improvements has been applied in garbage collection in Java 6 and we usually use JRE 1.6.20 for running our application.
JVM command line options -Xms and -Xmx is used to setup starting and max size for Java Heap. The ideal ratio of this parameter is either 1:1 or 1:1.5 based on my experience, for example, you can have either both –Xmx and –Xms as 1GB or –Xms 1.2 GB and 1.8 GB.
Command line options: -Xms:<min size> -Xmx:<max size>

Just to add to the discussion: Garbage Collection is not the only form of Memory Management in Java.
In the past, there have been efforts to avoid the GC in Java when implementing the memory management (see Real-time Specification for Java (RTSJ)). These efforts were mainly dedicated to real-time and embedded programming in Java for which GC was not suitable - due to performance overhead or GC-introduced latency.
The RTSJ characteristics
Immortal and Scoped Memory Management - see below for examples.
GC and Immortal/Scoped Memory can coexist withing one application
RTSJ requires a specially modified JVM.
RTSJ advantages:
low latency, no GC pauses
delivers predictable performance that is able to meet real-time system requirements
Why RTSJ failed/Did not make a big impact:
Scoped Memory concept is hard to program with, error-prone and difficult to learn.
Advance in Real-time GC algoritms reduced the GC pause-time in such way that Real-time GCs replaced the RTSJ in most of the real-time apps. However, Scoped Memories are still used in places where no latencies are tolerated.
Scoped Memory Code Example (take from An Example of Scoped Memory Usage):
import javax.realtime.*;
public class ScopedMemoryExample{
private LTMemory myMem;
public ScopedMemoryExample(int Size) {
// initialize memory
myMem = new LTMemory(1000, 5000);
}
public void periodicTask() {
while (true)) {
myMem.enter(new Runnable() {
public void run() {
// do some work in the SCOPED MEMORY
new Object();
...
// end of the enter() method, the scoped Memory is emptied.
}
});
}
}
}
Here, a ScopedMemory implementation called LTMemory is preallocated. Then a thread enters the scoped memory, allocates the temporary data that are needed only during the time of the computation. After the end of the computation, the thread leaves the scoped memory which immediately makes the whole content of the specific ScopedMemory to be emptied. No latency introduced, done in constant time e.g. predictable time, no GC is triggered.

From my experience, in java you should rely on the memory management that is provided by JVM itself.
The point I'd focus on in this topic is to configure it in a way acceptable for your use case. Maybe checking/understanding JVM tuning options would be useful: http://docs.oracle.com/cd/E15523_01/web.1111/e13814/jvm_tuning.htm

You cannot avoid garbage collection if you use Java. Maybe there are some obscure JVM implementations that do, but I don't know of any.
A properly tuned JVM shouldn't require any System.gc() hints to operate smoothly. The exact tuning you would need depends heavily on what your application does, but in my experience, I always turn on the concurrent-mark-and-sweep option with the following flag: -XX:+UseConcMarkSweepGC. This flag allows the JVM to take advantage of the extra cores in your CPU to clean up dead memory on a background thread. It helps to drastically reduce the amount of time your program is forcefully paused when doing garbage collections.

Well, the GC is always there -- you can't create objects that are outside its grasp (unless you use native calls or allocate a direct byte buffer, but in the latter case you don't really have an object, just a bunch of bytes). That said, it's definitely possible to circumvent the GC by reusing objects. For instance, if you need a bunch of ArrayList objects, you could just create each one as you need it and let the GC handle memory management; or you could call list.clear() on each one after you finish with it, and put it onto some queue where somebody else can use it.
Standard best practices are to not do that sort of reuse unless you have good reason to (ie, you've profiled and seen that the allocations + GC are a problem, and that reusing objects fixes that problem). It leads to more complicated code, and if you get it wrong it can actually make the GC's job harder (because of how the GC tracks objects).

Basically the idea in Java is that you should not deal with memory except using "new" to allocate new objects and ensure that there is no references left to objects when you are done with them.
All the rest is deliberately left to the Java Runtime and is - also deliberately - defined as vaguely as possible to allow the JVM designers the most freedom in doing so efficiently.
To use an analogy: Your operating system manages named areas of harddisk space (called "files") for you. Including deleting and reusing areas you do not want to use any more. You do not circumvent that mechanism but leave it to the operating system
You should focus on writing clear, simple code and ensure that your objects are properly done with. This will give the JVM the best possible working conditions.

You are correct in saying that System.gc() is a request to the compiler and not a command. But using below program you can make sure it happens.
import java.lang.ref.WeakReference;
public class GCRun {
public static void main(String[] args) {
String str = new String("TEMP");
WeakReference<String> wr = new WeakReference<String>(str);
str = null;
String temp = wr.get();
System.out.println("temp -- " + temp);
while(wr.get() != null) {
System.gc();
}
}
}

I would suggest to take a look at the following tutorials and its contents
This is a four part tutorial series to know about the basics of garbage collection in Java:
Java Garbage Collection Introduction
How Java Garbage Collection Works?
Types of Java Garbage Collectors
Monitoring and Analyzing Java Garbage Collection
I found This tutorial very helpful.

"Nullify"ing the reference when not required is the best way to make an object eligible for Garbage collection.
There are 4 ways in which an object can be Garbage collected.
Point the reference to null, once it is no longer required.
String s = new String("Java");
Once this String is not required, you can point it to null.
s = null;
Hence, s will be eligible for Garbage collection.
Point one object to another, so that both reference points to same object and one of the object is eligible for GC.
String s1 = new String("Java");
String s2 = new String("C++");
In future if s2 also needs to pointed to s1 then;
s1 = s2;
Then the object having "Java" will be eligible for GC.
All the objects created within a method are eligible for GC once the method is completed. Hence, once the method is destroyed from the stack of the thread then the corresponding objects in that method will be destroyed.
Island of Isolation is another concept where the objects with internal links and no extrinsic link to reference is eligible for Garbage collection.
"Island of isolation" of Garbage Collection
Examples:
Below is a method of Camera class in android. See how the developer has pointed mCameraSource to null once it is not required. This is expert level code.
public void release() {
if (mCameraSource != null) {
mCameraSource.release();
mCameraSource = null;
}
}
How Garbage Collector works?
Garbage collection is performed by the daemon thread called Garbage Collector. When there is sufficient memory available that time this demon thread has low priority and it runs in background. But when JVM finds that the heap is full and JVM wants to reclaim some memory then it increases the priority of Garbage collector thread and calls Runtime.getRuntime.gc() method which searches for all the objects which are not having reference or null reference and destroys those objects.