Java reference objects basic
0 likes128 views
The document discusses Java reference objects and garbage collection. It explains that the garbage collector's job is to identify objects no longer in use and reclaim their memory. An object is considered reachable if it can be accessed from the program's root set of references or threads. All other unreachable objects are eligible for garbage collection. Reference objects are treated differently and allow programs to control an object's reachability through strong, soft, weak, and phantom references. The document provides details on each type of reference and how they impact an object's eligibility for garbage collection.
Java reference objects basic
- 1. Java Reference Objects Basic Understanding
- 2. GC’s job ● Identity objects that are no longer in use ● Reclaim the memory
- 3. What is “use” ● An object can be reached by a program in its current state Main Thread forked Threads RunnClass# runningMethod.. Root set of reference static reference JNI API registered references Reachable
- 4. Garbage Collection ● All other than reachable objects are unreachable ● Unreachable objects are eligible for garbage collection Red box are unreachable objects
- 5. Reference object ● Reference objects will be treated differently by GC ● Concept: Refernece and Referent SoftReference Image Reference Referent
- 6. Object Reachability ● Reference Objects API gives us strengths of object reachability ○ Strongly Referent ○ Softly Reference ○ Weakly Reference ○ Phantomly Reference ● By this API we can get a limited amount of interaction with GC according to the strength of the object’s reachability
- 7. Strongly reachable ● An object is reachable from the root set of references
- 8. Weak Reachable ● The only way to reach an object involve at least one weak reference object ● Weak referenced objects are eligible for collection ● Weak reference object allow us to refer to and object without keeping it from being collected ● When a referenced object is collected, the value of WeakReference will be set to null ● Weak reference life time won’t be extended
- 9. Softly Reachable ● Not a strongly reachable ● No weak reference ● No Phantom reference ● One or more soft reference ● GC must clear all soft reference before throwing OutOfMemoryError ● GC will clear it when memory is not enough ● GC may not clear it when it’s used recently, life time may be extended ● Often used to implement a cache
- 10. Phantomly Reachable ● When an object has no weak reference and soft reference, or become unreachable by itself. The object will be phantomly referenced ● PhantomReference has a constructor: ReferenceQueue. It can be used to monitor finialization status controlled by GC.
- 11. ReferenceQueue ● A data structure for you to find out when an object becomes softly, weakly, or phantomly reachable.