java - How can Garbage Collector quickly know which objects do not have references to them any more?

A typical modern JVM uses several different types of garbage collectors.

One type that's often used for objects that have been around for a while is called Mark-and-Sweep. It basically involves starting from known "live" objects (the so-called garbage collection roots), following all chains of object references, and marking every reachable object as "live".

Once this is done, the sweep stage can reclaim those objects that haven't been marked as "live".

For this process to work, the JVM has to know the location in memory of every object reference. This is a necessary condition for a garbage collector to be precise (which Java's is).

Java has a variety of different garbage collection strategies, but they all basically work by keeping track which objects are reachable from known active objects.

A great summary can be found in the article How Garbage Collection works in Java but for the real low-down, you should look at Tuning Garbage Collection with the 5.0 Java[tm] Virtual Machine

An object is considered garbage when it can no longer be reached from any pointer in the running program. The most straightforward garbage collection algorithms simply iterate over every reachable object. Any objects left over are then considered garbage. The time this approach takes is proportional to the number of live objects, which is prohibitive for large applications maintaining lots of live data.

Beginning with the J2SE Platform version 1.2, the virtual machine incorporated a number of different garbage collection algorithms that are combined using generational collection. While naive garbage collection examines every live object in the heap, generational collection exploits several empirically observed properties of most applications to avoid extra work.

The most important of these observed properties is infant mortality. ...

I.e. many objects like iterators only live for a very short time, so younger objects are more likely to be eligible for garbage collection than much older objects.

For more up to date tuning guides, take a look at:

• Java SE 6 HotSpot[tm] Virtual Machine Garbage Collection Tuning
• Java Platform, Standard Edition HotSpot Virtual Machine Garbage Collection Tuning Guide (Java SE 8)

Incidentally, be careful of trying to second guess your garbage collection strategy, I've known many a programs performance for be trashed by over zealous use of System.gc() or inappropriate -XX options.

GC will know that object can be removed as quickly as it is possible. You are not expected to manage this process.

But you can ask GC very politely to run using System.gc(). It is just a tip to the system. GC does not have to run at that moment, it does not have to remove your specific object etc. Because GC is the BIG boss and we (Java programmers) are just its slaves... :(