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Java - JDBC Memory Use After Closing Result/Statement/Connection [duplicate]

I was browsing some old books and found a copy of "Practical Java" by Peter Hagger. In the performance section, there is a recommendation to set object references to null when no longer needed.
In Java, does setting object references to null improve performance or garbage collection efficiency? If so, in what cases is this an issue? Container classes? Object composition? Anonymous inner classes?
I see this in code pretty often. Is this now obsolete programming advice or is it still useful?

It depends a bit on when you were thinking of nulling the reference.
If you have an object chain A->B->C, then once A is not reachable, A, B and C will all be eligible for garbage collection (assuming nothing else is referring to either B or C). There's no need, and never has been any need, to explicitly set references A->B or B->C to null, for example.
Apart from that, most of the time the issue doesn't really arise, because in reality you're dealing with objects in collections. You should generally always be thinking of removing objects from lists, maps etc by calling the appropiate remove() method.
The case where there used to be some advice to set references to null was specifically in a long scope where a memory-intensive object ceased to be used partway through the scope. For example:
{
BigObject obj = ...
doSomethingWith(obj);
obj = null; <-- explicitly set to null
doSomethingElse();
}
The rationale here was that because obj is still in scope, then without the explicit nulling of the reference, it does not become garbage collectable until after the doSomethingElse() method completes. And this is the advice that probably no longer holds on modern JVMs: it turns out that the JIT compiler can work out at what point a given local object reference is no longer used.

No, it's not obsolete advice. Dangling references are still a problem, especially if you're, say, implementing an expandable array container (ArrayList or the like) using a pre-allocated array. Elements beyond the "logical" size of the list should be nulled out, or else they won't be freed.
See Effective Java 2nd ed, Item 6: Eliminate Obsolete Object References.

Instance fields, array elements
If there is a reference to an object, it cannot be garbage collected. Especially if that object (and the whole graph behind it) is big, there is only one reference that is stopping garbage collection, and that reference is not really needed anymore, that is an unfortunate situation.
Pathological cases are the object that retains an unnessary instance to the whole XML DOM tree that was used to configure it, the MBean that was not unregistered, or the single reference to an object from an undeployed web application that prevents a whole classloader from being unloaded.
So unless you are sure that the object that holds the reference itself will be garbage collected anyway (or even then), you should null out everything that you no longer need.
Scoped variables:
If you are considering setting a local variable to null before the end of its scope , so that it can be reclaimed by the garbage collector and to mark it as "unusable from now on", you should consider putting it in a more limited scope instead.
{
BigObject obj = ...
doSomethingWith(obj);
obj = null; // <-- explicitly set to null
doSomethingElse();
}
becomes
{
{
BigObject obj = ...
doSomethingWith(obj);
} // <-- obj goes out of scope
doSomethingElse();
}
Long, flat scopes are generally bad for legibility of the code, too. Introducing private methods to break things up just for that purpose is not unheard of, too.

In memory restrictive environments (e.g. cellphones) this can be useful. By setting null, the objetc don't need to wait the variable to get out of scope to be gc'd.
For the everyday programming, however, this shouldn't be the rule, except in special cases like the one Chris Jester-Young cited.

Firstly, It does not mean anything that you are setting a object to null. I explain it below:
List list1 = new ArrayList();
List list2 = list1;
In above code segment we are creating the object reference variable name list1 of ArrayList object that is stored in the memory. So list1 is referring that object and it nothing more than a variable. And in the second line of code we are copying the reference of list1 to list2. So now going back to your question if I do:
list1 = null;
that means list1 is no longer referring any object that is stored in the memory so list2 will also having nothing to refer. So if you check the size of list2:
list2.size(); //it gives you 0
So here the concept of garbage collector arrives which says «you nothing to worry about freeing the memory that is hold by the object, I will do that when I find that it will no longer used in program and JVM will manage me.»
I hope it clear the concept.

One of the reasons to do so is to eliminate obsolete object references.
You can read the text here.

How to delete a java object from Heap?

You have one big object in java. it has got 4 or five references. you don't know all those references. At time on deletion you know only one reference and you want to delete that object completely. How to achieve that? and also if you want to know other references then to what is the best way to do that.

It is not in our hand.. You can just nullify it from your end..
Object a = new Object();
a = null; // after that, if there is no live thread which is accessing members,it will be deleted by garbage collector

You could try Finalize() or System.runFinalization() but frankly, if there are references still pointing to the object, then I think the GC will ignore your request.

It is not possible in Java.
If you have strong reference referring your object, you cannot force JVM to GC that object. It simply cannot guarantee the program will work.
If codes of all other references are in your control, consider changing them to use WeakReference or SoftReference

There are some things that are not in our hands and its better to leave it to the JRE to handle it. All we can do that we make sure that the we make them null explicitly after using them.
{
// Some block
HugeObject obj = HugeObject.getInstance();
// Use it
obj = null;
}
// end of block

Java memory handling is just built to prevent that. An object is guaranteed to live as long as a reference to this object exists. As far as I know there is no (official) way to get to know the other references to an object (and there should be no need for that).

In Java GC(Garbage collector) handles heap cleanup. If an Object has no live references to it then it will automatically be cleaned up. So you need to make sure there are no live references to the Object.
Making it null is one of the way. But it will not guarantee it's cleanup if there is some other Object pointing to the same reference. That is why writing good code involves closing all the resources after use which includes making it to null.
If you are running low on heap you can try increasing heap size or calling System.gc() but again calling gc manually does not guarantee gc will actually be performed. it depends on lot of parameters which are JVM dependent.

What kind of references are these to the object? Are these references created by you and at runtime you don't keep track of of those references. If this is the case, you can wrap your references to the object in soft/ weak reference and then explicitly run the GC request. Otherwise, on runtime, if any live thread has access to the object. GC shall not delete that object.

It is hard to answer no knowing your use case, but if there is one location that you want to be able to remove it from then you can store every other reference to it as a WeakReference. Java normally uses strong refrences when referencing objects and the GC will only clear something when it has no more strong references. However, if you use WeakRefrences and your strong refrence ever goes out of scope there is no guarantee that your data will remain even if it is still needed.
I could be mistaken about this though, as I haven't used this class in a year or two.
On WeakReferences:
http://docs.oracle.com/javase/7/docs/api/java/lang/ref/WeakReference.html

You can declare your objects as WeakReference and add them in ReferenceQueue. In this way , whenever your object will not be further referenced , it will be liable for GC.
/**
Initialize the reference queue , even if you don't do it , no problem . Default reference queue will be taken.
**/
ReferenceQueue<? super Object> testReferenceQueue = new ReferenceQueue<Object>();
Map<String,String> demoHashMap = new HashMap<String,String>();
demoHashMap.put("SomeValue","testValue");
// Declare the object as weak object and put it in reference queue
WeakReference<?> weakObject = new WeakReference<Object>(demoHashMap,testReferenceQueue );
demoHashMap.clear();
demoHashMap = null; // This object is not referenced from anywhere
if(weakObject!=null){
System.out.println("Object is not GCd yet");
}else{
System.out.println("It is already garbage collected");
}

In Java, is it safe to use "String ret = someString.split("\t")[0]"?

Is it safe to use String ret = someString.split("\t")[0] in Java?
I'm worried about someString.split("\t")[0] is temporary variable, so it might be recycled by GC.

Yes, it's safe. And it doesn't matter that a "temporary" array is being created, you got the result you wanted and stored it in the local variable ret, so it won't go away. Garbage collection doesn't work like that, only the objects that are no longer referenced (say, by a local variable) are candidates for being garbage collected.

It is absolutely safe. When
someString.split("\t")
is executed, a reference to the object it returns is put on the stack. It is therefore reachable and cannot be GC'ed. The object can only be GC'ed after
someString.split("\t")[0]
// ^ array access
has been executed.

Absolutely.
If your application can use it, the garbage collector won't delete it1.
However there are a couple of unrelated edge-cases for your example:
if someString is null, you will get a NullPointerException
if someString is the empty string, then split will give you a zero length array, and that will result in an ArrayIndexOutOfBoundsException.
1 - The only exception is when something outside of the pure Java world does something really bad with references. But when that happens, you've already got a corrupted heap, and a JVM crash is imminent.

String ret = someString.split("\t")[0]; will fail if someString is null, so no, it's not safe.
But if you know that someString is not null, then this is perfectly safe. The garbage collector will never remove any values while it is still possible for your code to access them, so this is one thing you never need to worry about.

Finalize() cleanup vs. Garbage Collector removing an object from memory

I was reading about the finalize() method and was curious:
What is the difference between the task of cleaning up objects ( setting them to NULL ) in finalize, and removing an object from memory?

What is the difference between the task of cleaning up objects (
setting them to NULL ) in finialize
setting to null removes ONE reference to the object.
if NO more references to an object exists, the garbage collector is allowed (not required) to remove the object
and removing an object from memory?
there is NO explicit way in java to remove (destroy, delete) an object.
The garbage collector will do it when he likes.
Especially the time from removing the last reference to remove/destroy the object is indefinite
There is NO need to set references to null in finalize method.
when the garbage collector call finalize the objects and its references will gone soon anyway.
I never wrote an own finalize method during my very long java experience.
The rare occasion in which it make sense to wrote an own finalize method appear if your object is dealing with os-resources. However, in general you use standard packages for os accesss

You don't "clean up" an object when you set it to null, you're just setting the reference to null, consider:
Object a = new Object();
Object b = a;
a = null;
System.out.println(b);
Once an object loses all references, it will be collected on the next GC pass. Finalize is a method that gets called when this happens, and you should avoid using it.
Just don't keep extra references around and let the GC do it's job.

finalize() is called by garbage collector when an object has no more references. You can override it and best practice is to use it in a try-catch-finally block to free non java resources like files. Anyway if you use it this way you should also call super.finalize() to ensure class hierarchy finalization.
This method is always for advanced use and shouldn't be used in normal production code. Free your resources in finally clauses in methods using those resources.

Explicit nulling

In what situations in java is explicit nulling useful. Does it in any way assist the garbage collector by making objects unreachable or something? Is it considered to be a good practice?

In Java it can help if you've got a very long-running method, and the only reference to the an object is via a local variable. Setting that local variable to null when you don't need it any more (but when the method is going to continue to run for a long time) can help the GC. (In C# this is very rarely useful as the GC takes "last possible use" into account. That optimization may make it to Java some time - I don't know.)
Likewise if you've got a member field referring to an object and you no longer need it, you could potentially aid GC by setting the field to null.
In my experience, however, it's rarely actually useful to do either of these things, and it makes the code messier. Very few methods really run for a long time, and setting a variable to null really has nothing to do with what you want the method to achieve. It's not good practice to do it when you don't need to, and if you do need to you should see whether refactoring could improve your design in the first place. (It's possible that your method or type is doing too much.)
Note that setting the variable to null is entirely passive - it doesn't inform the garbage collector that the object can be collected, it just avoids the garbage collector seeing that reference as a reason to keep the object alive next time it (the GC) runs.

In general it isn't needed (of course that can depend on the VM implementation). However if you have something like this:
private static final Map<String, String> foo;
and then have items in the map that you no longer need they will not be eligible for garbage collection so you would need to explicitly remove them. There are many cases like this (event listeners is another area that this can happen with).
But doing something like this:
void foo()
{
Object o;
// use o
o = null; // don't bother doing this, it isn't going to help
}
Edit (forgot to mention this):
If you work at it, you should find that 90-95% of the variables you declare can be made final. A final variable cannot change what it points at (or what its value is for primitives). In most cases where a variable is final it would be a mistake (bug) for it to receive a different value while the method is executing.
If you want to be able to set the variable to null after use it cannot be final, which means that you have a greater chance to create bugs in the code.

One special case I found it useful is when you have a very large object, and want to replace it with another large object. For example, look at the following code:
BigObject bigObject = new BigObject();
// ...
bigObject = new BigObject(); // line 3
If an instance of BigObject is so large that you can have only one such instance in the heap, line 3 will fail with OutOfMemoryError, because the 1st instance cannot be freed until the assignment instruction in line 3 completes, which is obviously after the 2nd instance is ready.
Now, if you set bigObject to null right before line 3:
bigObject = null;
bigObject = new BigObject(); // line 3
the 1st instance can be freed when JVM runs out of heap during the construction of the 2nd instance.

From "Effective Java" : use it to eliminate obsolete object references. Otherwise it can lead to memory leaks which can be very hard to debug.
public Object pop(){
if(size == 0)
throw new EmptyStatckException();
Object result = elements[--size];
elements[size] = null; //Eliminate Object reference
return result;
}

If you are nulling an object that is about to go out of scope anyway when your method block closes, then there is no benefit whatsoever in terms of garbage collection. It is not unusual to encounter people who don't understand this who work really hard to set a lot of things to null needlessly.

Explicit nulling can help with GC in some rare situations where all of the following are true:
The variable is the only (non-weak) reference to the object
You can guarantee that the object will no longer be needed
The variable will stay in scope for an extended period of time (e.g. it is a field in a long-lived object instance)
The compiler is unable to prove that the object is no longer used, but you are able to guarantee this though your superior logical analysis of the code :-)
In practice this is quite rare in good code: if the object is no longer needed, you should normally be declaring it in a narrower scope anyway. For example, if you only need the object during a single invocation of a method, it should be a local variable, not a field in the enclosing object.
One situation where explicit nulling is genuinely useful: if null is used to indicate a specific state then setting to a null value is sometimes going to be necessary and useful. Null is a useful value in itself for a couple of reasons:
Null checks are extremely fast, so conditional code that checks for null is typically more efficient than many alternatives (e.g. calling object.equals())
You get an immediate NullPointerException if you try to dereference it. This is useful because it is good Fail Fast coding style that will help you to catch logic errors.

See also WeakReference in J2SE.