Garbage Collection Eligibility in Java

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Example 91
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Garbage Collection Eligibility
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class HeavyItem { int[] itemBody; String itemID; HeavyItem nextItem; HeavyItem(String ID, HeavyItem itemRef) { itemBody = new int[100000]; itemID = ID; nextItem = itemRef; } protected void finalize() throws Throwable { Systemoutprintln(itemID + ": recycled"); superfinalize(); } // (1)
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// (2)
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// (3)
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} //______________________________________________________________________________ public class RecyclingBin { public static HeavyItem createHeavyItem(String itemID) { // HeavyItem itemA = new HeavyItem(itemID + " local item", null); // itemA = new HeavyItem(itemID, null); // Systemoutprintln("Return from creating HeavyItem " + itemID); return itemA; // } (4) (5) (6) (7)
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93: FACILITATING GARBAGE COLLECTION public static HeavyItem createList(String listID) { HeavyItem item3 = new HeavyItem(listID + ": item3", null); HeavyItem item2 = new HeavyItem(listID + ": item2", item3); HeavyItem item1 = new HeavyItem(listID + ": item1", item2); Systemoutprintln("Return from creating list " + listID); return item1; } public static void main(String[] args) { HeavyItem list = createList("X"); list = createList("Y"); HeavyItem itemOne = createHeavyItem("One"); HeavyItem itemTwo = createHeavyItem("Two"); itemOne = null; createHeavyItem("Three"); createHeavyItem("Four"); Systemoutprintln("Return from main()"); } } // // // // (8) (9) (10) (11)
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// (12)
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// (13) // (14) // (15) // // // // // (16) (17) (18) (19) (20)
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Possible output from the program:
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Return from creating list X Return from creating list Y X: item3: recycled X: item2: recycled X: item1: recycled Return from creating HeavyItem Return from creating HeavyItem Return from creating HeavyItem Three local item: recycled Three: recycled Two local item: recycled Return from creating HeavyItem One local item: recycled One: recycled Return from main()
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One Two Three
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Four
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In Example 91, the main() method at (13) in the class RecyclingBin uses the methods createHeavyItem() and createList() It creates a list X at (14), but the reference to its first item is reassigned at (15), making objects in list X eligible for garbage collection after (15) The first item of list Y is stored in the reference list, making this list noneligible for garbage collection during the execution of the main() method The main() method creates two items at (16) and (17), storing their reference values in the references itemOne and itemTwo, respectively The reference itemOne is nulled at (18), making HeavyItem with identity One eligible for garbage collection The two calls to the createHeavyItem() method at (19) and (20) return reference values to HeavyItems, which are not stored, making each object eligible for garbage collection right after the respective method call returns
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CHAPTER 9: OBJECT LIFETIME
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The output from the program bears out the observations made above Objects in list Y (accessible through the reference list) and HeavyItem with identity Two (accessible through the reference itemTwo) remain non-eligible while the main() method executes Although the output shows that HeavyItems with identity Four was never garbage collected, it is not accessible once it becomes eligible for garbage collection at (20) Any objects in the heap after the program terminates are reclaimed by the operating system
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94 Object Finalization
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Object finalization provides an object with a last resort to undertake any action before its storage is reclaimed The automatic garbage collector calls the finalize() method in an object that is eligible for garbage collection before actually destroying the object The finalize() method is defined in the Object class
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protected void finalize() throws Throwable
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An implementation of the finalize() method is called a finalizer A subclass can override the finalizer from the Object class in order to take more specific and appropriate action before an object of the subclass is destroyed Note that the overridden method cannot narrow the visibility of the method and it must be declared either protected or public A finalizer can, like any other method, catch and throw exceptions (see Section 67, p 245) However, any exception thrown but not caught by a finalizer that is called by the garbage collector is ignored, and the finalization of this object is terminated The finalizer is only called once on an object, regardless of whether any exception is thrown during its execution In case of finalization failure, the object still remains eligible for disposal at the discretion of the garbage collector (unless it has been resurrected, as explained in the next subsection) Since there is no guarantee that the garbage collector will ever run, there is also no guarantee that the finalizer will ever be called In the following code, the finalizer at (1) will take appropriate action if and when called on objects of the class before they are garbage collected, ensuring that the resource is freed Since it is not guaranteed that the finalizer will ever be called at all, a program should not rely on the finalization to do any critical operations
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public class AnotherWellBehavedClass { SomeResource objRef; // protected void finalize() throws Throwable { try { if (objRef != null) objRefclose(); } finally { superfinalize(); } } }
// (1) // (2) // (3) // (4)