IBM User Guide for Java V7 on Windows > IBM SDK for Java > The collector and the allocator > Global garbage collection > Reference objects

Soft, weak, and phantom reference processing

The Garbage Collector (GC) determines if a reference object is a candidate for collection and, if so, performs a collection process that differs for each reference type. Soft references are collected if their referent is not marked and if #get() has not been called on the reference object for a number of garbage collection cycles. Weak and phantom references are always collected if their referent is not marked.

For each element on a list, GC determines if the reference object is eligible for processing and then if it is eligible for collection.

An element is eligible for processing if it is marked and has a non-null referent field. If this is not the case, the reference object is removed from the reference list, resulting in it being freed during the sweep phase.

If an element is determined to be eligible for processing, GC must determine if it is eligible for collection. The first criterion here is simple. Is the referent marked? If it is marked, the reference object is not eligible for collection and GC moves onto the next element of the list.

If the referent is not marked, GC has a candidate for collection. At this point the process differs for each reference type. Soft references are collected if their referent has not been marked for a number of garbage collection cycles. The number of garbage collection cycles depends on the percentage of free heap space. You adjust the frequency of collection with the -Xsoftrefthreshold option. For more information about using -Xsoftrefthreshold, see Garbage Collector command-line options. If there is a shortage of available storage, all soft references are cleared. All soft references are guaranteed to have been cleared before the OutOfMemoryError is thrown.

Weak and phantom references are always collected if their referent is not marked. When a phantom reference is processed, its referent is marked so it will persist until the following garbage collection cycle or until the phantom reference is processed if it is associated with a reference queue. When it is determined that a reference is eligible for collection, it is either queued to its associated reference queue or removed from the reference list.

Parent: Reference objects

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