Circular References in Java

Solution 1:

Only a very naive implementation would have a problem with circular references. Wikipedia has a good article on the different GC algorithms. If you really want to learn more, try (Amazon) Garbage Collection: Algorithms for Automatic Dynamic Memory Management . Java has had a good garbage collector since 1.2 and an exceptionally good one in 1.5 and Java 6.

The hard part for improving GC is reducing pauses and overhead, not basic things like circular reference.

Solution 2:

The garbage collector knows where the root objects are: statics, locals on the stack, etc and if the objects aren't reachable from a root then they will be reclaimed. If they are reachable, then they need to stick around.

Solution 3:

Ryan, judging by your comment to Circular References in Java, you fell into the trap of referencing objects from a class, which was probably loaded by the bootstrap/system classloader. Every class is referenced by the classloader that loaded the class, and can thus be garbage-collected only if the classloader is no longer reachable. The catch is that the bootstrap/system classloader is never garbage collected, therefore, objects reachable from classes loaded by the system classloader cannot be garbage-collected either.

The reasoning for this behavior is explained in JLS. For example, Third Edition 12.7 http://java.sun.com/docs/books/jls/third_edition/html/execution.html#12.7.

Solution 4:

If I remember correctly, then according to the specifications, there are only guarantees about what the JVM can't collect (anything reachable), not what it will collect.

Unless you are working with real-time JVMs, most modern garbage collectors should be able to handle complex reference structures and identify "subgraphs" that can be eliminated safely. The efficiency, latency, and likelihood of doing this improve over time as more research ideas make their way into standard (rather than research) VMs.

Solution 5:

No, at least using Sun's official JVM, the garbage collector will be able to detect these cycles and free the memory as soon as there are no longer any references from the outside.