Here I would be discussing whether explicit nulling of variables helps garbage collection in any significant way. For those who are not up to date with HotSpot GC, let me throw some light on that. The heap is divided into several spaces: Eden space, 2 survivor spaces and an old generation space. The Eden space and the 2 survivor spaces make up the young generation. When an object is created, it is created in the Eden space. Most objects die there (become out of scope) and are collected by the garbage collector. If an object remains in the Eden space till the next minor GC runs, it is promoted to the 1st survivor space and then to the next survivor space. If an object becomes too old or the young generation becomes full, objects get promoted to the old generation space.
Now, lets get back on the topic: Does nulling variables explicitly help garbage collector ?
The answer is it depends. If the Eden space is full and an object is being created then if sufficient space could be created in Eden, then no more work is required. But if sufficient space can't be freed in Eden then existing objects in Eden space have to be promoted to the 1st survivor space before new objects can be created. There might be some objects in the Eden space that the application doesn't need now. But those objects have still not gone out of scope and hence can't be garbage collected. Explicitly nulling the variables holding the references to those objects would make a difference and those objects can now be garbage collected.
One thing that is to be noted here is that compiler can make object references null even if you don't make them null explicitly. Only in cases where the compiler can't prove that the object is no longer used, it doesn't make them null. Explicitly nulling can help GC in rare cases such as the one discussed above. All the following conditions should be true if you are explicitly nulling object references :
Now, lets get back on the topic: Does nulling variables explicitly help garbage collector ?
The answer is it depends. If the Eden space is full and an object is being created then if sufficient space could be created in Eden, then no more work is required. But if sufficient space can't be freed in Eden then existing objects in Eden space have to be promoted to the 1st survivor space before new objects can be created. There might be some objects in the Eden space that the application doesn't need now. But those objects have still not gone out of scope and hence can't be garbage collected. Explicitly nulling the variables holding the references to those objects would make a difference and those objects can now be garbage collected.
One thing that is to be noted here is that compiler can make object references null even if you don't make them null explicitly. Only in cases where the compiler can't prove that the object is no longer used, it doesn't make them null. Explicitly nulling can help GC in rare cases such as the one discussed above. All the following conditions should be true if you are explicitly nulling object references :
- The variable is the only weak reference to the object.
- The variable will stay in scope for an extended period of time.
- The compiler is not able to prove that the object is no longer used but you are able to prove that the object is no longer needed.
In general, explicitly nulling is not required because the Java compiler is smart enough to do that. One very important use case of explicitly nulling is when you have a map having some entries that you no longer need. The objects to which the map entries point to are not eligible for garbage collection. Making them null explicitly does help. But something like the following won't help since the object would anyhow go out of scope and would be garbage collected in the next GC run. We should never do this :
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