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How to identify garbage in jvm, in view of this problem, this article introduces the corresponding analysis and solution in detail, hoping to help more partners who want to solve this problem to find a more simple and feasible way.

First, how to determine garbage

1.1. Citation counting method

In Java, references and objects are associated. If you want to manipulate an object, you must use a reference. Therefore, it is obvious that a simple way is to determine whether an object can be recycled by reference counting. To put it simply, if an object has nothing to do with it,

The reference of the union, that is, their reference count is not 0, indicates that the object is unlikely to be used again, so the object is a recyclable object.

1.2. Accessibility analysis

In order to solve the circular reference problem of reference counting, Java uses the method of reachability analysis. Search through a series of "GC roots" objects as a starting point. If there is no reachable path between "GC roots" and an object, the object is said to be unreachable. It should be noted that an unreachable object is not equivalent to a recyclable object, and it takes at least two marking processes for an unreachable object to become a recyclable object. If it is still a recyclable object after being marked twice, it will face recycling.

2.1Marker removal algorithm (Mark-Sweep)

The most basic garbage collection algorithm is divided into two phases, labeling and clearing. The marking phase marks all the objects that need to be recycled, and the cleanup phase reclaims the space occupied by the marked objects. As shown in the picture

From the figure, we can find that the biggest problem of the algorithm is the serious fragmentation of memory, and the problem that large objects can not find available space may occur later.

2.2 replication algorithm (copying)

In order to solve the memory fragmentation defect of Mark-Sweep algorithm, an algorithm is proposed. The memory is divided into two blocks of the same size according to the memory capacity. Use only one of the chunks at a time. When this memory is full, copy the surviving objects to another, and clear the used memory, as shown in the figure:

Although this algorithm is simple to implement, high memory efficiency, and not easy to produce fragments, the biggest problem is that the available memory is compressed to half of the original. And if the number of surviving objects increases, the efficiency of Copying algorithm will be greatly reduced.

2.3Marker finishing algorithm (Mark-Compact)

Combined with the above two algorithms, in order to avoid defects. The marking stage is the same as the Mark-Sweep algorithm, but not clear after marking.

Manage the object, but move the living object to one end of memory. Then clear the objects outside the end boundary. As shown in the figure:

This is the answer to how to identify spam in jvm. I hope the above content can be of some help to you. If you still have a lot of doubts to be solved, you can follow the industry information channel to learn more about it.

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