JVM学习笔记-引用计数收集器(Reference Counting Collectors)

 

Reference Counting Collectors

Reference counting was an early garbage collection strategy. In this approach, a reference count is maintained for each object on the heap. When an object is first created and a reference to it is assigned to a variable, the objectís reference count is set to one. When any other variable is assigned a reference to that object, the object's count is incremented. When a reference to an object goes out of scope or is assigned a new value, the object's count is decremented. Any object with a reference count of zero can be garbage collected. When an object is garbage collected, any objects that it refers to have their reference counts decremented. In this way the garbage collection of one object may lead to the subsequent garbage collection of other objects.

引用计数是垃圾收集的早期策略。在这种方法中，堆中每一个对象都有一个引用计数。一个对象被创建了，并且指向该对象的引用被分配给一个变量，这个对象的引用计数被置为1。当任何其他变量被赋值为对这个对象的引用时，计数加1。当一个对象的引用超过了生存期或者被设置一个新的值时，对象的引用计数减1。任何引用计数为0的对象可以被当作垃圾收集。当一个对象被垃圾收集的时候，它引用的任何对象计数值减1。在这种方法中，一个对象被垃圾收集后可能导致后续其他对象的垃圾收集行动。

 

An advantage of this approach is that a reference counting collector can run in small chunks of time closely interwoven with the execution of the program. This characteristic makes it particularly suitable for real-time environments where the program can't be interrupted for very long. A disadvantage is that reference counting does not detect cycles: two or more objects that refer to one another. An example of a cycle is a parent object that has a reference to a child object that has a reference back to the parent. These objects will never have a reference count of zero even though they may be unreachable by the roots of the executing program. Another disadvantage of reference counting is the overhead of incrementing and decrementing the reference count each time.

这种方法的好处是，引用计数收集器可以很快地执行，交织在程序的运行之中。这个特性对于程序不能被长时间打断的实时环境很有利。坏处就是，引用计数无法检测出循环(即两个或者更多的对象互相引用)。循环的例子如，父对象有一个对子对象的引用，子对象又反过来引用父对象。这些对象永远都不可能计数为0，就算它们已经无法被执行程序的根对象可触及。还有一个坏处就是，每次引用计数的增加或者减少都带来额外开销。

 

 

Because of the disadvantages inherent in the reference counting approach, this technique is currently out of favor. It is more likely that the Java Virtual Machines you encounter in the real world will use a tracing algorithm in their garbage-collected heaps.

 因为引用计数方法固有的缺陷，这种技术现在已经不为人所接受。现实生活中所遇到的Java虚拟机更有可能在垃圾收集堆中使用追踪算法。