JDK 18的最新消息

根据openjdk 18发布的JEPS 421, finalize()方法的终结和功能将被标记为已弃用(forRemoval=true)，这意味着永久删除将在jdk 18之后的某个后续版本中进行。

从jdk 18开始，一个新的命令行选项——finalization=disabled禁用了所有地方的终结机制，甚至是jdk内部的声明。

这也与这里的这个问题有关，因为它被计划删除的原因是它包含的一些主要缺陷。其中一个缺陷是，从一个对象变得不可访问到调用它的终结器之间可能会经过很长一段时间。GC也不能保证任何终结器将被调用，这也是事实。

在最近与终结器方法搏斗之后(为了在测试期间处理连接池)，我不得不说终结器缺少很多东西。使用VisualVM来观察以及使用弱引用来跟踪实际的交互，我发现以下事情在Java 8环境中是正确的(Oracle JDK, Ubuntu 15):

Finalize is not called immediately the Finalizer (GC part) individually owns the reference elusively The default Garbage Collector pools unreachable objects Finalize is called in bulk pointing to an implementation detail that there is a certain phase the garbage collector frees the resources. Calling System.gc() often does not result in objects being finalized more often, it just results in the Finalizer getting aware of an unreachable object more rapidly Creating a thread dump almost always result in triggering the finalizer due to high heap overhead during performing the heap dump or some other internal mechanism Finalization seams to be bound by either memory requirements (free up more memory) or by the list of objects being marked for finalization growing of a certain internal limit. So if you have a lot of objects getting finalized the finalization phase will be triggered more often and earlier when compared with only a few There were circumstances a System.gc() triggered a finalize directly but only if the reference was a local and short living. This might be generation related.

最后认为

最后确定方法是不可靠的，但只能用于一件事。您可以确保在垃圾收集之前关闭或释放对象，从而在正确处理涉及生命结束操作的更复杂生命周期的对象时实现故障安全。这是我能想到的一个值得我们去推翻它的原因。

Java allows objects to implement a method called finalize() that might get called. finalize() method gets called if the garbage collector tries to collect the object. If the garbage collector doesn't run, the method doesn't get called. If the garbage collector fails to collect the object and tries to run it again, the method doesn't get called in the second time. In practice, you are highly unlikely to use it in real projects. Just keep in mind that it might not get called and that it definitely won't be called twice. The finalize() method could run zero or one time. In the following code, finalize() method produces no output when we run it since the program exits before there is any need to run the garbage collector.

源

An Object becomes eligible for Garbage collection or GC if its not reachable from any live threads or any static refrences in other words you can say that an object becomes eligible for garbage collection if its all references are null. Cyclic dependencies are not counted as reference so if Object A has reference of object B and object B has reference of Object A and they don't have any other live reference then both Objects A and B will be eligible for Garbage collection. Generally an object becomes eligible for garbage collection in Java on following cases:

该对象的所有引用显式设置为空，例如object = null 对象在块内创建，一旦控件退出该块，引用就会离开作用域。 父对象设置为空，如果一个对象持有另一个对象的引用，并且当您将容器对象的引用设置为空时，子对象或包含对象自动符合垃圾收集的条件。 如果一个对象只有通过WeakHashMap的活动引用，那么它就有资格进行垃圾收集。

最终确定方法是不保证的。当对象符合GC条件时调用此方法。在许多情况下，对象可能不会被垃圾收集。

正如在https://wiki.sei.cmu.edu/confluence/display/java/MET12-J.+Do+not+use+finalizers上指出的那样，

There is no fixed time at which finalizers must be executed because time of execution depends on the Java Virtual Machine (JVM). The only guarantee is that any finalizer method that executes will do so sometime after the associated object has become unreachable (detected during the first cycle of garbage collection) and sometime before the garbage collector reclaims the associated object's storage (during the garbage collector's second cycle). Execution of an object's finalizer may be delayed for an arbitrarily long time after the object becomes unreachable. Consequently, invoking time-critical functionality such as closing file handles in an object's finalize() method is problematic.